Adaptive deferred EDCA for low latency traffic in communication networks

Abstract

A mechanism to provide adaptive pre-emption of a contention period to various ACs in order to transmit pending data includes the following. The communication device first transmits a Defer signal over

Description

FIELD OF THE INVENTION The present disclosure relates generally to network access and communication therein and more specifically to medium access and communication methods in a wireless network. The proposed mechanisms apply to contention-based channel access methods using backoff values. BACKGROUND OF THE INVENTION The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section. Wireless communication networks are widely deployed to provide various communication services such as voice, video, packet data, messaging, broadcast, etc. These wireless networks may be multiple-access networks capable of supporting multiple users by sharing the available network resources. Examples of such multiple-access networks include Code Division Multiple Access (CDMA) networks, Time Division Multiple Access (TDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA) networks. The 802.11 family of standards adopted by the Institute of Electrical and Electronics Engineers (IEEE) provides a great number of mechanisms for wireless communications between stations. An operating channel of 40, 80,160 or 320MHz bandwidth (as defined in the latest 802.11 be D4.1 standard, but may be wider in future amendments) is usually made of a primary channel and one or more secondary channels (each channel being 20MHz or a multiple thereof). The primary channel is used for signalling (including channel access procedure such as the contention-based channel access method called Enhanced Distributed Channel Access-EDCA) and backwards compatibility while the secondary channels are only used when sending data at full speed. All of the known features for efficient medium usage assumes that the primary channel is idle in order to perform contention and access the wireless medium. QoS (Quality of Service) is provided in 802.11 networks thanks to EDCA which defines traffic categories and four corresponding access categories making it possible to handle differently high-priority traffic compared to low-priority traffic. Implementation of EDCA in the stations can be made using a plurality of traffic queues (known as "Access Categories (AC)") for serving data traffic at respective different priorities, each traffic queue being associated with a respective queue backoff counter. The queue backoff counter of a traffic queue (AC) is initialized with a backoff value randomly drawn from respective queue contention parameters, known as EDCA parameters. The backoff counter is then decremented during a contention period at each time slot the wireless medium is detected as idle. Conventionally, the contention period starts a DIFS (DCF InterFrame Space - equal to SIFS + 2 * Slotime) after the medium is detected as being idle. The decrementing is stopped and deferred when the wireless medium becomes busy. On the other hand, when it reaches zero, the station gains access to the wireless medium, hence can transmit pending data stored in the traffic queue. As the EDCA parameters are specific to each AC queue, packets from different ACs are transmitted according to different priorities mirroring the respective EDCA parameters. To improve latency in 802.11 networks, publication IEEE 802.11-23 / 1065r0 proposed an enhanced wireless medium access method for low latency traffic, by provided a pre-emption or pre-empting mechanism of the wireless medium by some stations (STAs). In this publication, non-AP stations having low latency data to be retransmitted are allowed to transmit the same Defer signal at the beginning of a new contention period to force all STAs that are not sending the Defer signal to have CCA busy and not participate in a subsequent backoff contention. In other words, the Defer signal ensures some STAs (sending it) are isolated for the subsequent backoff contention. Next, a contention can be performed only between the STAs with prioritized access after the Defer signal has been transmitted. The proposed solution therefore provides a prioritized access to the medium compared to the conventional EDCA for the four ACs. It is however not fully satisfactory. On one hand, it is applicable to retransmissions only. The Defer signal can only be transmitted after an initial transmission of the data is unsuccessful (failure or collision). That could be too late with regards to low latency budget. On the other hand, no distinction is made between the ACs once the deferring mechanism is enabled (the number of failed retransmissions before sending the Defer signal can vary from one AC to the other). It means that stations having pending data of different ACs compete one with each other during the pre-empted contention period. This may quickly become detrimental to the data having tight latency requirements such as high-definition video, advanced telemedicine, ultra-low latency gaming, and AR / VR. SUMMARY OF THE INVENTION The present disclosure seeks to overcome the foregoing concerns by introducing an adaptive pre-emption mechanism still based on a Defer or “reservation” signal. In this context, the present disclosure provides a method of accessing a wireless medium to transmit pending data, the method comprising, at a communication device: transmitting a Defer signal over the wireless medium within a contention period. This is to pre-empt the medium for the subsequent backoff contention, and then performing a backoff procedure within the contention period. The deferred contention is performed after the Defer signal has been transmitted, as long as the medium is idle. This is to actually gain access to the medium in order to transmit the pending data, wherein a length (i.e., duration) of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period are determined based on an access class of the pending data. By setting the duration of the Defer signal and / or by shifting the Defer signal over time thanks to the pre-emption delay, it becomes possible to discriminate between multiple Defer signals transmitted by various stations for multiple types (access classes) of data. The discrimination allows defining multiple configurations of lengths or pre-emption delays corresponding to multiple priorities. The proposed solution therefore adapts the deferred contention to the priority of the data to be sent. An improved wireless network is thus obtained. Optional features are defined below with reference to methods, while they can be transposed into device features. In some embodiments, the method further comprises receiving, from an access point with which the communication device is associated, multiple lengths and / or pre-emption delays corresponding to multiple respective access classes, and selecting the length and / or pre-emption delay corresponding to the access class of the pending data. By providing the parameters driving the transmission of the Defer signal, the AP can control each station individually and more generally impose a homogeneous behaviour over the entire network (usually a Basic Service Set). Furthermore, the AP can dynamically adjust the pre-emption parameters overtime, in order to adapt the overall behaviour to changes in the network. In specific embodiments, the lengths and / or pre-emption delays for multiple access classes are included in a management frame transmitted by the access point. Exemplary management frames adapted to carry such parameters for the network include Beacon, Probe Response and (Re)Association Response frames. As an example, the length and / or pre-emption delay for an access category are included in an AC Parameter Record field corresponding to the access category within an EDCA Parameter Set element compliant with the EDCA Parameter Set element format of Fig.9-293 of IEEE Std 802.11-2020 in the management frame. As described below, multiple signalling configurations may be contemplated within the AC Parameter Record fields. In some embodiments, the method comprises performing EDCA (Enhanced Distributed Channel Access) using the pre-emption delay as backoff value, within the contention period to transmit the Defer signal. In other words, a backoff procedure based on the pre-emption delay (backoff counter is decremented from the delay) is launched as soon as the contention period starts. The management of the Defer signal advantageously relies on conventional tools (EDCA). In some embodiments, the method further comprises performing legacy EDCA with legacy EDCA parameters simultaneously to performing the EDCA with the pre-emption delay. Two EDCA procedures are therefore conducted concurrently by the same communication device. The legacy EDCA allows the communication device to directly send its pending data (when the legacy backoff counter reaches zero) while the other EDCA allows it to send the Defer signal in view of the subsequent EDCA to access the medium for actual transmission of the pending data. Alternatively, only the legacy EDCA may be conducted. In such alternative embodiments, the method further comprises performing legacy EDCA with legacy EDCA parameters within the contention period and stopping the legacy EDCA when the pre-emption delay lapses in order to transmit the Defer signal. Again, either the pending data are directly transmitted or the Defer signal is sent before the subsequent EDCA, depending on the length the pre-emption delay compared to the legacy EDCA backoff values. Consequently, the Defer signal is transmitted when the EDCA with the pre-emption delay ends before the legacy EDCA, otherwise the pending data are directly transmitted over the wireless medium. The simultaneous conduct of the two EDCAs allows the stations to use the Defer signal for a first transmission of the pending data, in addition to retransmissions thereof. It means that, in some embodiments, the pending data are transmitted for first time after the communication device accesses the medium through the backoff procedure. In some embodiments, performing the backoff procedure includes resuming legacy EDCA with legacy EDCA parameters for the access class of the pending data, to gain access to the medium. The legacy EDCA has been used during a previous attempt to access the wireless (e.g., concurrently to the EDCA to transmit the Defer signal as proposed above or during a previous contention period). In these embodiments, legacy EDCA parameters are reused. This advantageously constitutes a simple solution. In alternative embodiments, performing the backoff procedure includes setting contention parameters for the backoff procedure with Deferred EDCA parameters specific to a backoff procedure following a Defer signal. Hence, the parameters used are different from the legacy EDCA parameters and require another backoff counter to be used. These alternative embodiments allow shorter backoff periods to be used, which is particularly adapted to the reduced number of stations that are involved in this subsequent backoff procedure. In particular embodiments, the Deferred EDCA parameters are included in a management frame sent by an access point with which the communication device is associated. Again, Beacon, Probe Response and (Re)Association Response frames can be used. As an example, the Deferred EDCA parameters include contention window parameters for an access category that are included in an ECWmin / ECWmax field of the AC Parameter Record field corresponding to the access category. In some embodiments, the length and / or pre-emption delay for high-priority pending data are designed to end the Defer signal later than the length and / or pre-emption delay for low-priority pending data. Since the stations ending their Defer signal earlier can detect a signal over the wireless medium, their CCA is set or held busy. They do not participate to the subsequent backoff procedure. The stations ending their Defer signal later are therefore given high priority to transmit during the subsequent backoff procedure. In some embodiments, the length of the Defer signal is longer for high-priority pending data than for low-priority pending data. In some embodiments, the method further comprises, if the access class of the pending data belongs to a predefined group of access classes, not initiating a mechanism to transmit the Defer signal and performing a legacy backoff procedure within the contention period. In these embodiments, the deferred approach (Defer signal and subsequent backoff procedure) is activated for some types of data (e.g. Low Latency data) while it is disabled for other types of data. This advantageously avoids the pre-emption parameters to be retrieved and used (e.g. decrementing a counter set with the pre-emption delay). Processes at the communication devices are therefore reduced for some types of data. The activation or disabling of the deferring mechanism can be dynamically controlled. In some embodiments, the method further comprises transmitting a Stream Classification Service (SCS) Action frame including an SCS Descriptor element defining a class of data, the SCS Descriptor element having afield orbit set to a first value to enable transmission of a Defer signal before performing a backoff procedure for the class of data, or set to a second value to disable such transmission for the class of data before performing a backoff procedure. A similar frame (e.g. response) may be received, e.g. from an AP, with the same aim to enable or disable the proposed mechanism for a given class of data. In embodiments, the backoff procedure starts a SIFS after an end of the Defer signal transmission or starts immediately after an end of the Defer signal transmission. This defines a behaviour different from a conventional one where a contention period starts a DIFS after the wireless medium is detected as idle. Advantage is taken from the pre-emption (thanks to the Defer signal) to reduce the waiting time since the stations have priority to access the medium. In some embodiments, the pre-emption delay is defined as a number of time slots (conventionally 9ps slots) after the start of the contention period. In other embodiments, the Defer signal includes legacy 802.11 preamble fields followed by padding bits. This allows Defer signals simultaneous sent by devices to be fully aligned one to the other at least along their preamble fields, to allow a correct decoding by any station receiving them. Furthermore, the communication device can easily adjust the length of the Defer signal to a desired length, by merely adapting the amount of padding bits accordingly. In some embodiments, the access class includes one from an access category, a traffic class, a traffic identifier (TID), a user priority (UP), TCLAS information element containing parameters to identify frames of a traffic stream, a Stream Classification Service identifier (SCSID) identifying a stream of data. In some embodiments, the communication device is a non-access point station associated with an access point. In that case, the pending data may be peer-to-peer data. Alternatively, the pending data may be uplink data to be addressed to an access point with which the communication device is associated. In other embodiments, the communication device is an access point. In that case, the pending data may be downlink data to be addressed to one or more non-access point station that are associated with the communication device. Further to the medium access method, the present disclosure also provides a method of communication in a wireless network comprising, at a communication device: accessing a wireless medium of the wireless network using any method defined above, and transmitting the pending data over the accessed wireless medium. As mentioned above, the access point also plays a role in the overall management of the deferring mechanism. In this perspective, the disclosure also provides a method of communication in a wireless network comprising, at an access point, AP, transmitting a management frame to non-AP stations associated with it, wherein the management frame includes multiple lengths and / or pre-emption delays for multiple access classes to drive a non-AP station willing to transmit pending data to transmit a Defer signal within a contention period (before initiating the deferred backoff procedure still within the contention period), a length of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period being based on the length and / or pre-emption delay corresponding to the access class of the pending data. Similarly, a management frame to be sent by an access point, AP, is also proposed that comprises multiple lengths and / or pre-emption delays for multiple access classes to drive a non-AP station willing to transmit pending data to transmit a Defer signal within a contention period in such a way a length of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period are based on the length and / or pre-emption delay corresponding to the access class of the pending data. The management frame is preferably a Beacon frame or a Probe Response frame or a (Re)Association Response frame. The particulars described above of the management frame, of the signalling of the lengths and delays as well as of the behaviours of the AP still apply to this method from AP perspective and to the management frame. Correlatively, the disclosure also provides a wireless communication device comprising at least one microprocessor configured for carrying out the steps of any of the above methods. The wireless communication device may be either of an AP station and a non-AP station. In particular, the communication device may comprise: a communication interface to a wireless medium, a memory storing pending data to be transmitted, a deferring unit configured to transmit a Defer signal over the wireless medium within a contention period, and a contention unit configured to perform, after the Defer signal has been transmitted, a backoff procedure within the contention period, wherein a length of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period are determined based on an access class of the pending data. Another aspect of the disclosure relates to a non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a wireless device, causes the wireless device to perform any method as defined above. At least parts of the methods according to the disclosure may be computer implemented. Accordingly, it may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit", "module" or "system". Furthermore, it may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium. Since the proposed mechanisms can be implemented in software, they can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible, non-transitory carrier medium may comprise a storage medium such as a floppy disk, a CD-ROM, a hard disk drive, a magnetic tape device or a solid-state memory device and the like. A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g., a microwave or RF signal. BRIEF DESCRIPTION OF THE DRAWINGS Some embodiments of the present disclosure are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements and in which: Figure 1 illustrates a typical wireless communication system in which embodiments of the disclosure may be implemented; Figure 2a illustrates the IEEE 802.11e EDCA involving access categories; Figure 2b illustrates an example of mapping between eight priorities of traffic class and the four EDCA ACs; Figure 3 illustrates 802.11e mechanism for the backoff counter countdown in a conventional channel access scheme; Figure 4a shows a schematic representation of a communication device; Figure 4b illustrates schematically the architecture of the communication device of Figure 4a; Figure 5 illustrates, using a flowchart, general steps of a medium access method and of a communication method at a communication device, according to embodiments of the present disclosure; Figure 5a illustrates, using a flowchart, steps of medium access and communication methods at a communication device, according to various embodiments of the disclosure; Figures 6a, 6b, 6c illustrate, using timelines, various exemplary deferred mechanisms according to the disclosure, depending on variations of the pre-emption parameters, such as the WSN and DSD parameters; Figures 7a and 7b illustrate exemplary formats of a pre-emption EDCA Parameter Set element according to various embodiments of the disclosure; and Figure 8 illustrates an exemplary enhanced Stream Classification Service, SCS, Descriptor element format for requesting / allowing the deferred mechanism according to the disclosure. DETAILED DESCRIPTION The invention will now be described by means of specific non-limiting exemplary embodiments and by reference to the figures. In the following description, the term legacy refers devices that may operate in accordance with one or more of IEEE 802.11 a / b / g / n / ac / ad / af / ah / aj / ay / ax / be, or another legacy wireless communication standard. The legacy devices may be STAs, IEEE STAs or Wi-Fi STAs. An AP may communicate with legacy devices in accordance with legacy IEEE 802.11 communication techniques. Figure 1 illustrates a communication system in which several communication devices or stations (or “nodes”) 101-107 exchange data frames over a radio transmission channel 100 of a wireless local area network (WLAN), under the management of a central station, or access point (AP) 110, also seen as a station of the network. The radio transmission channel 100 is defined by an operating frequency band constituted by a single channel ora plurality of channels forming a composite channel. In the following, the word “station” refers to any kind of station. The wording “access point station”, or in short “access point” (AP), refers to the station playing the role of access point 110. The wording “non-access point station”, or in short “non-AP station”, or client station (STA) refers to the other stations 101-107. Access to the shared radio medium to send data frames is primarily based on the CSMA / CA technique, for sensing the carrier and avoiding collision by separating concurrent transmissions in space and time. Carrier sensing in CSMA / CA is performed by both physical and virtual mechanisms. Virtual carrier sensing is achieved by transmitting control frames to reserve the medium prior to transmission of data frames. Next, a source or transmitting station, including the AP, first attempts through the physical mechanism, to sense a medium that has been idle for at least one DIFS (standing for DCF InterFrame Spacing) time period, before transmitting data frames. However, if it is sensed that the shared radio medium is busy during the DIFS period, the source station continues to wait until the radio medium becomes idle. The wireless communication system of Figure 1 comprises physical access point 110 configured to manage the WLAN BSS (Basic Service Set), i.e., a group of non-AP stations which have previously registered to the AP. Such BSS managed by the AP is called an infrastructure BSS. In the following, the term BSS will be used as an equivalent of infrastructure BSS. Once the BSS is established, the Access Point can bridge traffic inside the BSS or from other networks (e.g., wired networks) into the BSS (or vice and versa). Thus, the stations of the BSS originally talked to the AP only, which is in charge of relaying data frames if the data frames are targeted to another station of the BSS. Recent developments in the 802.11 family of standards have given the opportunity to the non-AP stations to send data directly to another non-AP station, referred to as peer-to-peer (P2P) or Direct Link communications. To access the medium, any station, including the AP, starts counting down a backoff counter from the start of a contention period, which is a DIFS afterthe medium is sensed as idle (e.g. when a previous communication ends). The backoff counter is designed to expire after a number of timeslots, chosen randomly in a so-called contention window [0, CW], where CW is an integer. This backoff mechanism or procedure, also referred to as channel access scheme, is the basis of the collision avoidance mechanism that defers the transmission time for a random interval, thus reducing the probability of collisions on the shared channel. Afterthe backoff time expires (i.e., the backoff counter reaches zero), the source station may send data or control frames if the medium is still idle. Management of quality of service (QoS) has been introduced at station level in the wireless networks, through well-known EDCA mechanism defined in the IEEE 802.11e standard. Indeed, in the original DCF standard, a communication station includes only one transmission queue / buffer. However, since a subsequent data frame cannot be transmitted until the transmission / retransmission of a preceding frame ends, the delay in transmitting or retransmitting the preceding frame prevented the communication from having QoS. Figure 2a illustrates the IEEE 802.11e EDCA mechanism which is a channel access scheme involving access categories, in order to improve quality of service (QoS), to make more efficient use of the wireless medium. The 802.11e standard relies on a coordination function, called hybrid coordination function (HCF), which has two modes of operation: enhanced distributed channel access (EDCA) and HCF controlled channel access (HCCA). EDCA enhances or extends functionality of the original access DCF method: EDCA has been designed to support prioritized traffics similar to DiffServ (Differentiated Services), which is a protocol for specifying and controlling network traffic by class so that certain types of traffic get precedence. EDCA is the dominant channel access scheme or mechanism in WLANs because it features a distributed and easily deployed mechanism. The scheme contends for access to the wireless medium (or communication channel) of the communication network using EDCA contention parameters, in order for the station to transmit data stored locally over the medium that is accessed. QoS support in EDCA is achieved with the introduction of four Access Categories (ACs), and thereby of four corresponding transmission / traffic queues or buffers (210). Hence, “traffic queue” and “access category” may be considered as being synonymous in the present disclosure. Usually, the four ACs are the following in decreasing priority order: voice (or “AC_VO”), video (or “AC_VI”), best effort (or “AC_BE”) and background (or “AC_BG”). In embodiments, QoS could be extended to support another - higher - number of access categories. Each AC has its own traffic queue / buffer to store corresponding data frames to be transmitted on the network. The data frames, namely the MSDUs, incoming from an upper layer of the protocol stack are mapped onto one of the AC queues / buffers and thus input in the mapped AC buffer. Each AC has also its own set of queue contention parameters, known as EDCA parameters. Each AC is associated with a priority value, hence defining traffics of higher or lower priority of MSDUs. Thus, there is a plurality of traffic queues for serving data traffic at different priorities. The EDCA parameters usually include CWmin, CWmax, AIFSN and TXOP_Limit parameters for each traffic queue (or AC). CWmin and CWmax are the lower and higher boundaries of a selection range from which the EDCA contention window CW is selected for a given traffic queue. AIFSN stands for Arbitration Inter-Frame Space Number and defines a number of time slots (each usually being 9 ps-long), additional to a SIFS interval (the total defining the AIFS period - see Figure 3), the station must sense the medium as idle at the beginning of the contention period, before decrementing the queue backoff counter associated with the traffic queue considered. TXOP_Limit defines the maximum size of a TXOP the station may request. That means that each AC (and corresponding buffer) acts as an independent DCF contending entity including its respective queue backoff engine 211. Thus, each queue backoff engine 211 is associated with a respective traffic queue 210 for using respective queue contention parameters (EDCA parameters) and drawing a backoff value (from CW) to initialize a respective queue backoff counter that is used to contend for access to the wireless medium in order to transmit data stored in the respective traffic queue over the medium. The contention window CW and the backoff value are known as being EDCA variables. It results that the ACs within the same communication station compete one with each other to access the wireless medium and to obtain a transmission opportunity, using the conventional EDCA access scheme as explained above for example. Service differentiation between the ACs (hence QoS) is achieved by setting different EDCA parameters between the ACs, such as different CWmin, CWmax, AIFSN and / or different transmission opportunity duration limits (TXOP_Limit). This contributes to adjusting QoS. The use of lower AIFSN values (to defer the decrementing of the backoff counters), additional to the use of an on-average lower CW for high priority ACs compared to low priority ACs makes that traffic of a high priority AC has a higher chance to be transmitted than traffic from a low priority AC: a station with high priority AC traffic statistically waits less, on average, before it sends its packet than a station with low priority AC traffic. Referring to the four AC buffers (210) shown in Figure 2a, buffers AC3 and AC2 are usually reserved for real-time applications (e.g., voice AC_VO or video transmission AC_VI). They have, respectively, the highest priority and the second highest priority. Buffers AC1 and ACO are reserved for best effort (AC_BE) and background (AC_BG) traffic. They have, respectively, the second lowest priority and the lowest priority. Hence, the ACs in decreasing priority order are the following ones: AC_VO (AC=3), AC_VI (AC=2), AC_BE (AC=1) and AC_BG (AC=0). Each data unit, MSDU, arriving at the MAC layer from an upper layer (e.g., Link layer) with a priority is mapped to an AC according to mapping rules. Figure 2b shows an example of mapping between eight priorities of traffic class (also called Traffic Identification (TID), referring to values of User Priorities or UP between 0-7 according to IEEE 802.1 d) and the four ACs. The data frame is then stored in the buffer corresponding to the mapped AC. As a result, each of the four Access Categories includes two User Priority (UP) levels and has its own independent transmit queue. Figure 3 illustrates the channel access scheme conducted independently for each AC. Each station waits a fixed amount of time to ensure the medium is clear before attempting transmission. After detecting the medium as idle for a minimum DIFS duration (e.g. 34 ps), a contention period starts for all stations. The stations keep sensing the medium (listening) for an additional random time called backoff time within the contention period. A backoff counter is decremented to that end. A station initiates its transmission only if the medium remains idle for this additional random time, obtained when the backoff counter expires. As mentioned above, the duration of this random time, hence the initializing value of the backoff counter, is randomly drawn from the contention window [0, CW] as a multiple of a slot time (9 ps). With DCF, the waiting time, here DIFS, is constant for all types of traffic. With 802.11e the fixed amount of time that a station has to wait before starting decrementing its backoff counter depends on the access category and is referred to as the Arbitration Interframe Space (AIFS). Using AIFS, each traffic queue T awaiting transmission (i.e., that has pending data to transmit) waits until the medium is declared to be available (idle) through Clear Channel Assessment (CCA) and the Network Allocation Vector (NAV), not discussed here for brevity. Once the medium is available after the DIFS, each traffic queue T waits the corresponding AIFS[i] period (that includes a SIFS period deferring access to the medium) before decrementing its associated queue backoff counter. SIFS is the “shortest Interframe space”. It is used to separate packets within a single transmission and defines the time a receiving station has to wait after receiving data before sending an acknowledgement thereof. Thus, each of the four traffic queues has a defined interframe space value corresponding to the priority assigned to the queue. For example, the AC_VO queue is the highest priority and as such has the lowest interframe space timer. The AIFS timers (250) assigned by IEEE 802.11e are all defined as one Short InterFrame Spacing (SIFS) value plus a variable number (AIFSN[i]) of slots times which is defined by the physical layer encoding method in-use (CCK, DSSS, OFDM). The values of EDCA parameter AIFS Number (AIFSN) are administrator configurable, with default values defined as the following: AC_VO 1 SIFS + 2 * slot time (AIFSN[3] = 2) AC_VI 1 SIFS + 2 * slot time (AIFSN[2] = 2) AC_BE 1 SIFS + 3 * slot time (AIFSN[1] = 3) AC_BG 1 SIFS + 7 * slot time (AIFSN[0] = 7) The AIFSN values may be provided by the AP within a so-called EDCA Parameter Set information element (provided for example in the beacon frames sent by the AP). The AIFSN field in the Information Element is four bits long, with a minimum value of 2 defined in the standard and a maximum value of 15 based on the field length limitation. In this manner, arbitration inter-frame spacing allows a statistical advantage for frames in higher priority traffic queues because those frames are not required, relatively to the other queues, to wait too long prior to decrementing their random backoff counters. The Figure shows two AIFS corresponding to two different traffic queues ‘i’ and ‘j’. One can see that, due to this prioritizing difference, one prioritized traffic queue ‘j’ starts decrementing its backoff value earlier than the other less-prioritized traffic queue T. This situation is repeated after each new medium access by any station in the network (i.e., upon sensing anew the medium as being free). To initiate transmission of data, a traffic queue in a station first randomly selects a backoff value for its backoff counter from [0, CW] defined for the traffic queue. Similar to the AIFS parameter, the differences between the contention windows of the various traffic queues serve to prioritize traffic in higher priority queues by allowing them to wait shorter time intervals before being allowed to transmit over the air. Once the appropriate AlFS[i] period expires, each traffic queue can begin decrementing its queue backoff counter (251) by one at every slot time that passes. If the medium gets busy due to interference or other transmissions while a station is decrementing a backoff counter (i.e., waiting until it expires), the station stops decrementing and defers from medium access until the medium becomes idle fora DIFS + AIFS again. When the EDCA backoff procedure for a traffic queue (or an AC) ends (at least one backoff counter reaches zero), the MAC controller (reference 422 in Figure 4b below) of the transmitting station transmits a data frame from this traffic queue to the physical layer for transmission onto the wireless communication network. Since the traffic queues operate concurrently in accessing the wireless medium, it may happen that two traffic queues of the same communication station have their backoff ending simultaneously. In such a situation, a virtual collision handler (212) of the MAC controller operates a selection of the AC having the highest priority (as shown in Figure 2b) between the conflicting ACs, and gives up transmission of data frames from the ACs having lower priorities. Then, the virtual collision handler commands those ACs having lower priorities to start again a backoff operation using an increased CW value. Developments in the 802.11 standards seek to enhance efficiency and usage of the wireless channel. In this perspective, and addressing dense environments, the 802.11ax standard introduced multi-user (MU) transmission features, allowing multiple simultaneous transmissions to / from different users in both downlink (DL) and uplink (UL) directions with an access point. In the uplink, multi-user transmissions can be used to mitigate the collision probability by allowing multiple non-AP stations to simultaneously transmit to the AP. Contrary to MU downlink OFDMA wherein the AP can directly send multiple data to multiple stations (supported by specific indications inside the PLCP header), a trigger mechanism has been adopted for the AP to trigger MU uplink communications from various non-AP stations. The trigger mechanism includes a trigger frame sent by the APto obtain a Transmission Opportunity TXOP on the wireless medium, which frame defines a plurality of resource units (RUs) splitting the medium usually in the frequency domain. RUs may be scheduled to specific non-AP stations. Some RUs (known as random RUs) may be kept opened to contention by the non-AP stations willing to transmit uplink data. Hence, contention-based medium access schemes also exist for RUs in trigger frames. Nowadays, the RUs can be used to transmit uplink data and / or P2P data. Recently, the IEEE P802.11be / D4.1 version (below the “D4.1 standard”) introduced the Multi-link (ML) operation (MLO). MLO improves data throughput by allowing communications between stations over multiple concurrent and non-contiguous communication links. Multi-Link Operation enables a non-AP (access point) MLD (ML device) to register with an AP MLD, i.e., to discover, authenticate, associate and set up multiple links with the AP MLD. Each link enables channel access and frame exchanges between the non-AP MLD and the AP MLD based on supported capabilities exchanged during association. A MLD is a logical entity that has more than one affiliated station (AP or non-AP) and has a single medium access control (MAC) service access point (SAP) to logical link control (LLC), which includes one MAC data service. An AP MLD is thus made of multiple affiliated APs whereas a non-AP MLD is made of multiple affiliated non-AP stations. The affiliated stations in both AP MLD and non-AP-MLD can use 802.11 mechanisms to communicate with affiliated stations of another MLD over each of the multiple communication links that are set up. Hence, contention-based medium access schemes also exist for each affiliated station within the AP MLD and non-AP MLDs. Also, Stream Classification Service (SCS) as defined in IEEE 802.11-2020 was extended in the D4.1 standard to support latency sensitive traffics. SCS is a service that may be provided by an AP to its associated STAs that support SCS. In SCS, the AP classifies incoming individually addressed MSDUs based upon parameters provided by the non-AP STA, hence defining SCS flows or streams. The classification allows the UP, drop eligibility, and EDCA transmit queue to be selected for all MSDUs matching the classification. A non-AP STA that supports SCS may request use of SCS by sending an SCS Request frame that includes an SCS Descriptor element with the Request Type field set to “Add” or “Change.” The SCS Descriptor List field in the SCS Descriptor element identifies how MSDUs are classified and the priority to assign to MSDUs that match this classification. Each SCS stream is identified by an SCSID. This SCSID is used by a non-AP STA to request creation, modification, or deletion of an SCS stream. The SCSID is used by an AP to identify an SCS stream in SCS responses. In the D4.1 standard, SCS can be used to additionally specify QoS rules for certain SCS flows (e.g., assign flows to the desired Access Categories), and therefore allows traffic characteristics (e.g., traffic data rate and burst size) and QoS expectations or KPIs (e.g., latency bound) to be specified. An activation of SCS rules is always initiated by the non-AP STA (acting as client device) sending the SCS Request frame to the AP: the request from the STA explicitly provides the AP with classification parameters for each SCS flow (as example, UP can be used but also other elements included in various TCP / UDP / IP headers of the frames forming the flow). For downlink SCS rules (i.e., regarding downlink flows from the AP), a QoS Characteristics element can be optionally included within the SCS Descriptor element, and indicates the traffic characteristics and QoS KPIs for the downlink flow. The AP is expected to schedule the packets of the downlink flow such that the delay bound and minimum data rate parameters are met. For uplink SCS rules (i.e., regarding uplink flows to the AP), a QoS Characteristics element is always included and indicates the traffic characteristics, QoS KPIs and required cadence of trigger frames that schedule resources for that uplink flow. The AP is expected to schedule trigger frames with RU allocation for the client device and the corresponding Preferred AC such that the delay bound, minimum data rate and minimum / maximum service interval parameters in the QoS Characteristics element are met. The QoS Characteristics element can only be included when both the AP and client device support SCS Traffic Description. Hence, if this is not the case, uplink SCS rules cannot be established. With SCS, the client device needs to explicitly request a rule for each downlink / uplink flow individually. The complexity and overhead of SCS signalling can be high if a large number of flows need to be managed simultaneously and / or the flows are short-lived. As a result of this large complexity and also of the inefficiency of the AP to schedule the non-AP STAs in an appropriate manner, the non-AP STAs may desire to transmit their flows by their own. However, the EDCA medium access scheme has not evolved during the years. The 802.11 networks give equal priority access to all connected devices and data flows. Hence, they cannot provide the latency, throughput and stability required when traffic demands exceed the available bandwidth and collisions occur. Recently-created IEEE 802.11 bn (the successor of IEEE 802.11 be) Task group works on ultra-reliable low-latency communication (URLLC) requirements, focusing on some aspects such as tail latency and jitter, and high priority access for latency-sensitive applications. Document IEEE 802.11-23 / 1065r0 submitted to the Task group provides some adaptations to the EDCA rules in order to actually favour channel access for those STAs who failed or collided, in order to reduce the maximum delay bound and minimize any additionally penalty a STA will incur if there is a collision. The proposal seeks to give prioritized access to STAs that have failed to transmit pending data, in order to re-transmit its packets. The proposal isolates STAs with prioritized access from the rest of the STAs at the beginning of the contention period (a DIFS after the medium is sensed idle). The STAs with prioritized access are those having experienced one or more failed retransmissions, which number may vary from one AC to the other. Isolation is obtained by allowing these STAs to transmit a Defer Signal at the beginning of the contention period to force all other STAs that are not sending the Defer signal to have CCA busy and not participate in the contention period. Indeed, the otherSTAs will considerthe medium as busy and will likely have to wait for an EIFS duration before re-counting down its backoff counter or counters. EIFS stands for “extended interframe space” and is used whenever there is an error in transmission. If a previously received frame contains an error then a station has to defer EIFS duration instead of DIFS before transmitting a frame. This is because, although this station was not able to decode the frame, it could be that the intended receiver could decode the frame. It should have the opportunity to return an Acknowledgment frame; the EIFS ensures the transmission of the Ack can proceed without interference from those not able to decode the frame. The EIFS is derived from the SIFS and the DIFS and the length of time it takes to transmit an Ack frame at the lowest PHY mandatory rate: EIFS = SIFS + AckTxTime + DIFS where AckTxTime is the time expressed in microseconds required to transmit an Ack frame, including preamble, PHY header and any additional PHY dependent information, at the lowest PHY mandatory rate or MCS. The Defer signal is thus a reservation signal pre-empting the contention for the isolated STAs (having transmitted the Defer signal). If multiple of these STAs have prioritized access (i.e., failed in a previous transmission of the pending data), they transmit the same Defer Signal at the same time with the same result on other STAs (CCA busy). Next, a contention immediately follows the transmission of the Defer signal, which contention is performed only between the STAs with prioritized access during the pre-empted contention period. The STA with prioritized access that wins the contention gains access to the medium to transmit its pending data. By reducing the number of STAs participating in the contention, the proposal indeed contributes to reducing latency for data that have to be retransmitted. It is however not fully satisfactory. On one hand, it is applicable to retransmissions only. The Defer signal can only be transmitted after an initial transmission of the data is unsuccessful (failure or collision). That could be too late with regards to low latency budget. On the other hand, no substantial distinction is made between the ACs once the deferring mechanism is enabled (the number of failed retransmissions before sending the Defer signal can however vary from one AC to the other). It means that stations having pending data of different ACs compete one with each other during the pre-empted contention period. This may quickly become detrimental to the data having tight latency requirements such as high-definition video, advanced telemedicine, ultra-low latency gaming, and AR / VR. In this context, the present disclosure seeks to introduce adaptive pre-emption of the contention period to the ACs. This is to apply prioritized access to latency sensitive traffic while ensuring relative fairness for the legacy stations (i.e., not implementing the innovative mechanisms of the present disclosure), and to apply prioritized access in between the latency sensitive traffics (e.g., distinct traffic queues). The proposed mechanisms are suitable for implementation within a standard environment, and especially in the transmission state machine of an 802.11 communication device. Figure 4a schematically illustrates a communication device 400, which may be any stations of radio network 100 of Figure 1, configured to implement at least one embodiment of the present disclosure. The communication device 400 may preferably be a device such as a micro-computer, a workstation or a light portable device. The communication device 400 comprises a communication bus 413 to which there are preferably connected: a central processing unit 401, such as a processor, denoted CPU; a memory 403 for storing an executable code of methods or steps of the methods according to embodiments of the disclosure as well as the registers adapted to record variables and parameters necessary for implementing the methods; and at least one communication interface 402 connected to a wireless communication network, for example a communication network according to one of the IEEE 802.11 family of standards and / or Wireless-Fidelity (Wi-Fi) specifications, via transmitting and receiving antennas 404. Preferably the communication bus 413 provides communication and interoperability between the various elements included in the communication device 400 or connected to it. The representation of the bus is not limiting and in particularthe central processing unit 401 is operable to communicate instructions to any element of the communication device 400 directly or by means of another element of the communication device 400. The executable code may be stored in a memory that may either be read only, a hard disk or on a removable digital medium such as for example a disk. According to an optional variant, the executable code of the programs can be received by means of the communication network, via the interface 402, in order to be stored in the memory of the communication device 400 before being executed. In an embodiment, the device is a programmable apparatus which uses software to implement embodiments of the disclosure. However, alternatively, embodiments of the present disclosure may be implemented, totally or in partially, in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC). Figure 4b is a block diagram schematically illustrating the architecture of the communication device 400, adapted to carry out, at least partially, some embodiments of the disclosure. As illustrated, device 400 comprises a physical (PHY) layer block 423, a MAC layer block 422, and an application layer block 421. The PHY layer block 423, here a plurality of 802.11 standardized PHY layer modules in case the device is a MLD (however a single PHY layer module may be contemplated when the device is single link), has the task of formatting, modulating on or demodulating from any 20MHz channel or composite channel or resource unit. The PHY layer thus sends or receives frames over the radio medium NETW, such as 802.11 frames. These frames may be for instance medium access trigger frames to reserve a transmission slot, MAC data and management frames based on a 20MHz width to interact with legacy 802.11 stations and with legacy Wi-Fi Direct specification, as well as of MAC data frames of OFDMA type having smaller width than 20MHz legacy (typically 2 or 5 MHz) to / from that radio medium. The MAC layer block or controller 422 preferably comprises a MAC 802.11 layer 424 implementing conventional 802.11 MAC operations. It may comprise additional block 425 for carrying out, at least partially, embodiments of the disclosure. MAC layer block 422 may optionally be implemented in software, which software is loaded into RAM 403 and executed by CPU 401. MAC 802.11 layer 424 may implement an Upper-MAC stack 424a along with a series of Lower-MAC modules 424b in case the device is a MLD. Of course, a single-link architecture is supported (whereas not illustrated here) Preferably, additional block 425, referred to as Enhanced EDCA medium access module, implements, in collaboration with MAC 802.11 layer 424, embodiments of the present disclosure to perform enhanced medium access operations for pending data to transmit, such as data of latency sensitive traffic streams. As an example, block 425 performs the operations of the methods illustrated on Figures 5-6. In that respect, block 425 acts as a deferring unit configured to transmit a Defer signal over the wireless medium within a contention period, and also as a contention unit configured to perform, after the Defer signal has been transmitted, a backoff procedure within the contention period. On top of the Figure, application layer block 421 runs an application that generates and receives data packets, for example data packets such as a video stream. Application layer block 421 represents all the stack layers above MAC layer according to the ISO standardization. A mechanism to provide adaptive pre-emption of the contention period to various classes of data is now proposed. This is to allow a communication device to access the wireless medium to transmit pending data. Through the proposed mechanism, the communication device first transmits a Defer signal over the wireless medium within a contention period. This is to pre-empt the medium for the subsequent backoff contention. Any other name may be given to the Defer signal, such as a pre-empting or pre-emption frame, a reservation frame, and so on. The communication device then continues by performing a backoff procedure within the contention period. This is a deferred contention because it takes place after the Defer signal has been transmitted. The deferred contention is running as long as the medium is idle. This is to actually gain access to the medium in order to transmit the pending data. The adaptive pre-emption is obtained by having a length (i.e., duration) of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period that are determined based on an access class of the pending data. An “access class” is meant to designate any classification criterion allowing distinguishing between various data, hence to apply different strategies. Of course, the access classes may be access categories, traffic classes, traffic identifiers (TIDs), user priorities (UPs), TCLAS information element containing parameters to identify frames of a traffic stream, SCSIDs identifying streams of data, and so on. The proposed approach thus provides a pre-emption mechanism wherein the preemption frame, here the DS Signal, is emitted after a predetermined delay counted from when the medium is sensed as free, and wherein the pre-emption frame lasts a predetermined duration, both predetermined delay and predetermined duration being dependent on the access class of the pending data. By setting the duration of the Defer signal and / or by shifting the transmission of the Defer signal over time thanks to the pre-emption delay, the STAs having prioritized access transmit different Defer signals that end at different time instants. It turns that some of them detect the medium as being busy after they finish their Defer signal transmission, due to the other Defer signals. In that case, they cannot participate to the subsequent contention period, although they have prioritized access. In other words, the pre-emption parameters (predetermined delays and predetermined durations) allow setting relative priorities on a data class basis between the STAs having prioritized access. Although the pre-emption parameters for the access classes may be predefined within the STAs, preference is given, as described in the embodiments below, to a determination thereof based on parameters advertised by the AP. In embodiments, the STAs receive, from the AP with which they are associated, multiple lengths and / or pre-emption delays corresponding to the multiple access classes. This allows the AP to dynamically adjust the pre-emption parameters over time. The predetermined pre-emption delays may be chosen to adapt fairness with the legacy stations (legacy EDCA contention-based access scheme) and the predetermined durations or lengths may be chosen to provide relative priority (e.g., per AC queue or Traffic class) in between reduced subsets of stations for transmitting their latency sensitive traffics. In any case, the STAs have to select the length and / or pre-emption delay corresponding to the access class of the pending data they are about to transmit. Figure 5 illustrates, using a flowchart, general steps of a medium access method and of a communication method at a communication device, according to embodiments of the present disclosure. These steps are for instance performed by the MAC layer 422 of station 400 according to embodiments, for instance upon receiving new data to transmit (e.g., an MSDU packet) from an upper layer (e.g., from the application layer 421) and when it intends to access the medium based on the EDCA medium access scheme. The methods start (step 500) when the communication device has pending data (PPDU) to transmit. These may be data to retransmit because a previous (re)transmission failed. They may also be data newly stored in a transmission buffer. In that case, the pending data are about to be transmitted for first time after the communication device accesses the medium through the (subsequent) backoff procedure. The pending data have an access class providing them a priority. For ease of illustration, the description below mainly uses the Access Category as an access class for the data, although multiple other classifying criteria may be used alternatively. AC below may thus indifferently refer to an Access Category or any access class. At step 505, the communication device obtains the pre-emption parameter or parameters for the AC of the pending data. This may include receiving the sets of pre-emption parameters for all ACs from the AP as mentioned above and then selecting the set corresponding to the AC of the pending data. The pre-emption parameters such as the lengths and / or pre-emption delays mentioned above, for multiple ACs may be included in a management frame transmitted by the AP, such as a Beacon, Probe Response or (Re)Association Response frame. They may have been received by the communication device prior to the methods of Figure 5 and stored in local memory of the communication device until it is used for the purposes of these methods. Once the communication device knows its pre-emption parameters, it waits for a new contention period (step 510) which starts when the medium becomes idle and remains free for a predetermined duration, usually a DIFS. Once such contention period starts, the communication device transmits a Defer signal according to the selected pre-emption parameters. This is step 515. If a pre-emption delay is defined, the communication device waits for the corresponding time after the start of the contention period to transmit the Defer signal. This may be done by performing a conventional medium access scheme such as the DCF or EDCA, using however the pre-emption delay to initialize a dedicated “pre-emption” backoff counter. In other words, the communication device performs a “pre-emption” EDCA using the pre-emption delay as backoff value, within the contention period to transmit the Defer signal. If no pre-emption delay is defined, the transmission may be performed immediately after the start. The pre-emption delay may be the same for all ACs, in which case only the lengths of the Defer signal define the priorities between the ACs. This is illustrated by the scenario of Figure 6a discussed below. Alternatively, the pre-emption delays may be different between the ACs, hence shifting the start of the Defer signals as illustrated for instance by the scenarios of Figures 6b and 6c discussed below. The Defer signal lasts the length predetermined for the AC. The signal length may be the same for all ACs, in which case only the pre-emption delays to transmit the Defer signal define the priorities between the ACs. This is illustrated by the scenario of Figure 6b. Alternatively, the signal lengths may be different between the ACs, hence modifying the end time of the transmission as illustrated for instance by the scenarios of Figures 6a and 6c. From these figures and the explanations above, it is clear that, in embodiments, the later the Defer signal transmission ends, the higher priority the transmitting STA has to access the medium. In this perspective, the length and / or pre-emption delay for high-priority pending data are designed to end the Defer signal later than the length and / or pre-emption delay for low-priority pending data. For instance, in the case of Figure 6a, the length of the Defer signal is longer for high-priority pending data than for low-priority pending data. Once the Defer signal has been transmitted, the communication device configures itself to start contending within the same contention period. Due to the Defer signal, such contention has been deferred compared to conventional EDCA. However, benefits of the present disclosure are that the number of STAs participating to this deferred contention is reduced. This allows the communication device to perform a backoff procedure to access the medium. This is step 520. Of course, should the communication device senses activity on the medium just after it ends transmitting its Defer signal (meaning e.g. that another device is still transmitting a Defer signal, which other device has therefore more priority than the communication device), it has CCA busy meaning it cannot participate to the backoff procedure. The process therefore ends until a new contention period is detected. To allow the communication device to efficiently sense the medium after its Defer signal transmission (hence to determine whether another device transmits a more prioritized Defer signal), the backoff procedure may start a further delay after the end of the Defer signal transmission. This is an alternative to a start immediately after such end. The further delay may merely be a SIFS. One may envisage other values for the further delay such as the aSlotTime delay, or any delay composed of one of: aRxPHYStartDelay (delay from the start of the PPDU at the receiver’s antenna to the issuance of the PHY-RXSTART.indication primitive), aRxPHYStartDelay (delay that the PHY takes to generate the PHY-RXEND.indication for a received PPDU). If the wireless medium remains idle after the Defer signal, the backoff procedure can be launched. A conventional medium access scheme such as the DCF or EDCA can be used for the backoff procedure. In some embodiments, the EDCA parameters used may be the legacy EDCA parameters conventionally used by the communication station for the relevant AC to gain access to the medium without the proposed deferring mechanism (i.e., without transmitting a Defer signal). This means for the communication device to only resume legacy EDCA with legacy EDCA parameters for the AC of the pending data, to gain access to the medium. The existing backoff counter for the relevant AC is decremented again. In alternative embodiments, specific contention parameters can be used for the backoff procedure following a Defer signal. This particularly allows shorter backoff periods to be used compared to the legacy EDCA, which shorter backoff periods are more adapted to the limited number of STAs that compete during this subsequent (pre-empted) backoff procedure. Those specific contention parameters are referred to as Deferred EDCA parameters below. Result of the deferred contention (EDCA) of step 520, the communication device may gain access to the medium if its backoff counter elapses. This ends the medium access method. Once access to the medium is obtained through the backoff procedure of step 520, the communication device can send the pending data over the medium. This is step 525 that ends the communication method. In other words, a method of communication in a wireless network may comprise, at the communication device, accessing the wireless medium using the medium access method above, and then transmitting the pending data over the accessed wireless medium. Figure 5a illustrates, using a flowchart, steps of medium access and communication methods at a communication device, according to various embodiments of the disclosure. As for Figure 5, these steps are for instance performed by the MAC layer 422 of station 400 according to embodiments, for instance upon receiving new data to transmit (e.g., an MSDU packet) from an upper layer (e.g., from the application layer 421) and when it intends to access the medium based on the EDCA medium access scheme. Similarly, the methods are performed for an initial transmission of pending data, and possibly also when a retransmission is required. At step 500a, a latency sensitive frame is selected in the traffic queue. According to embodiments, this frame is the oldest frame of a latency sensitive traffic stored in the traffic queue, which runs according to a conventional FIFO queue, meaning that the oldest packets of the queue are transmitted first. According to embodiments, this frame matches an Uplink QoS rule, indicating that it pertains to a latency sensitive traffic. Here are provided exemplary Uplink QoS rules, which can work independently or in combination: Traffic classification (TCLAS): TCLAS element(s) can specify the IP classifier (in term of IP Addresses and Ports), and can be used together with a TCLAS Processing element. Stream Classification Service (SCS): SCS enables a client device to manage AP treatment of uplink traffic data flows based on TIDs. The SCSID field indicates an index value, selected determined by the client device, that is a unique identifier of the SCS rule between the AP and the client device. DSCP mapping Policy: DSCP marking policies as part of network wide QoS management by the AP may be envisaged in order to manage the mapping between DSCP values and User Priorities on both APs and client devices to achieve differentiated QoS. The non-AP STA uses the DSCP-to-UP Mapping table to classify its incoming traffic. The DSCP Policy feature allows finer-grained QoS management for uplink IP flows. For example, it enables configuration of policies that cause IP flows that would otherwise be marked with the same DSCP value (e.g., Default Forwarding) to instead be marked with different DSCP values, and therefore be assigned to different User Priorities. Those three exemplary mechanisms are used to obtain a User Priority (UP) for a given MSDU, and select an appropriate queueing (typically an existing AC queue 211). While not addressed here, it is envisaged that an alternative queue could be determined (such as an additional one to the existing four EDCA AC queues, or a dedicated one for each SCSID). Preferably, this new queue may be seen as an alternate one to one of the legacy four EDCA queues 211, meaning that the EDCA parameters for medium access contention are the same for the two queues (new one and corresponding legacy EDCA AC queue) but used separately depending on the MSDU to be transmitted. According to embodiments, the physical layer protocol data unit (PPDU, hence pending data) that contains the desired MSDU can aggregate other MSDUs. In that case, the PPDU is processed with the priority (QoS rule) of the most urgent (most prioritized) MSDU. The procedure is presented below for a communication device to support transmission of a PPDU to an AP (i.e., uplink direction). It means the communication device is a non-access point station associated with an access point. Furthermore, the pending data are uplink data to be addressed to the AP. Alternatively, it could be used also for P2P communication in between two non-AP STAs. In that case, the communication device may still be a non-AP station associated with the AP, and the pending data are P2P data. It is worth noticing that the AP itself can implement the deferring mechanism of the disclosure to gain prioritized access, e.g. to transmit downlink low latency traffic. In this respect, the communication device is an access point, and the pending data are downlink data to be addressed to one or more non-AP stations that are associated with the communication device. In embodiments that give priority to the AP compared to the non-AP stations, the length of the Defer signal for the AP may be higher than the lengths of the Defer signals used by the non-AP stations. The deferred mechanism or scheme of the present disclosure may be used for any pending data or for some types of data to give priority to those types. A black list of ACs without the deferred mechanism may be defined. Alternatively, a white list of ACs for which the deferred mechanism is active can be defined. For example, if the access class or AC of the pending data belongs to a predefined group of access classes, the deferred mechanism to transmit the Defer signal may be disabled or not initiated. In that case, the legacy backoff procedure (EDCA) is performed within the contention period. Similarly, the proposed deferred mechanism may be enabled / activated or disabled / deactivated on demand, by the AP possibly upon request of a non-AP STA. Any management frame may be used although the SCS Action frame sounds suitable for such signalling. As an example, the AP or the STA may transmit (to the other) a Stream Classification Service (SCS) Action frame including an SCS Descriptor element defining a class of data, the SCS Descriptor element having a field or bit set to a first value to enable transmission of a Defer signal before performing the deferred backoff procedure for the class of data or set to a second value to disable such transmission for the class of data before performing a (legacy) backoff procedure. An illustration of such signalling is shown in Figure 8 discussed below. In embodiments, the use of the proposed deferred mechanism for an access class of data is allowed by the AP upon admitting traffic having such access class for a given STA in its BSS. This could be result from a STA-initiated negotiation of uplink (or direct-link) QoS by the SCS mechanism, where the traffic characteristics (e.g. low latency characteristics) and TID / UP used by the STA to transmit the stream are specified. As an example, the SCS Request frame transmitted by the STA may contain a QoS Characteristics element within an SCS Descriptor element that requests use of the deferred mechanism, while the SCS Response frame from the AP may contain an AC Descriptor with a QoS Characteristics element indicating that the use of the deferred mechanism is accepted. Again, Figure 8 illustrates a signalling for such request and response. The AP is therefore free to accept or not new access classes (e.g. new incoming SCS streams) in the proposed deferred mechanism, depending for example on the activity within its BSS. Similarly, it is also free to adjust the pre-emption parameters of the proposed deferred mechanism depending on its BSS, for example according to an evolving number of SCSID indexes (per STA, as new SCS streams are accepted) and the number of STAs and / or on the load and the collision rates. Back to Figure 5a, step 505a comprises obtaining the medium access parameters corresponding to the incoming MSDU (pending data), and then determining how to get medium access based on the AC (more generally access class) of this MSDU. The communication device first determines whether the deferred mechanism applies to the pending data. If not, conventional medium access scheme is used. If it applies, the preemption parameter or parameters are retrieved. By knowing the UP of the MSDU (according to the QoS Rules presented before), the communication device determines which AC queue 211 to consider. This allows the communication device to retrieve the legacy EDCA parameters as well as the pre-emption parameters forthat AC including: a number of waiting slots (WSN, Waiting Slot Number) representing a pre-emption delay before emitting the Defer Signal on the wireless medium. The pre-emption delay can thus be defined as a number of time slots (conventionally 9ps slots) after the start of the contention period; and / or a duration or length of the Defer Signal, referred to as DSD below, standing for Defer Signal Duration. A table may be stored locally at the communication device, which table comprises the pre-emption parameters for each relevant access class (e.g. AC). The table may be set locally in each station of the BSS. Preferably, the table is filled in based on pre-emption parameters sent by the AP in management frames, such as the Probe Response and (Re)Association Response frames. In embodiments, the pre-emption parameters (length and / or pre-emption delay) for an access category are included in an AC Parameter Record field corresponding to the access category within an EDCA Parameter Set element compliant with the EDCA Parameter Set element format of Fig.9-293 of IEEE Std 802.11-2020 in the management frame. Figures 7a and 7b illustrates possible EDCA Parameter Set formats to convey the preemption parameters for one or more access classes. Typically, the EDCA Parameter Set element, referred to as “pre-emption EDCA Parameter Set element” (or LL EDCA Parameter Set element in case the deferred mechanism specifically targets LL traffic), is included by the AP in its Beacon frames (or Probe Response frames or (Re)Association Response frames) and can be updated over time in new frames. In embodiments, the pre-emption EDCA Parameter Set element can also be transmitted in a Stream Classification Service, SCS, descriptor 800 as shown in Figure 8, typically in Optional Subelements subfield 826. Although the proposed format is built on the known EDCA Parameter Set element format that advertises about four ACs (hence four profiles), a number of parameter profiles different from the AC queue number can be provided. They could be applied per UP, per SCSID, or any identifier able to discriminate different types of data. Figure 7a illustrates a first exemplary format of the pre-emption EDCA Parameter Set element when the deferred contention (subsequent backoff procedure) is a continuation of the legacy EDCA contention. Indeed, the legacy EDCA is only resume that is based on legacy parameters (such as CWmin / max) already known by the communication device from previous Beacon frames (in EDCA Parameter Set element). Therefore, the legacy EDCA parameters do not need to be included in the present pre-emption EDCA Parameter Set element. As in the conventional format of Fig.9-293 of IEEE Std 802.11-2020, the pre-emption EDCA Parameter Set element 700 includes QoS Info field 720 and four AC Parameter Record fields 730 corresponding to the four ACs and denoted “LL_AC_xx Parameter Record fields” where “xx” corresponds to the AC. QoS Info field 720 contains (not shown) a pre-emption (or LL) EDCA Parameter Set Update Count subfield, which indicates when the pre-emption parameters have changed (same way as for the conventional EDCA Parameter Set element). The formats of LL_AC_BE, LL_AC_BK, LL_AC_VI, and LL_AC_VO Parameter Record fields 730 are identical. Of course, it is still possible to include more records according to the number of queues (for example, there could have eight records corresponding to the eight possible User priorities UPs). As shown, the records are based on the ACI / AIFSN field format of Fig.9-296 of IEEE Std 802.11-2020, but slightly modified to include a WSN field and a DSD field, one in replacement of the AIFSN field 731 (no longer useful) and the other as a new field 734. The value of the AC index (ACI, 733) references the AC to which all parameters in this record correspond. The ACM (admission control mandatory) subfield 732 indicates that admission control is required for the AC. For example, such an admission control mandates the use of an SCS Request by the client STA. If the ACM subfield is equal to 0, then there is no admission control for the corresponding AC. If the ACM subfield is set to 1, admission control has to be used prior to transmission using the pre-emption parameters specified for this AC. The WSN (Waiting Slot Number) subfield 731 indicates the pre-emption delay for this AC as mentioned above, e.g. the number of time slots after a DIFS (hence after a start of the contention period) the STA defers before starting its transmission of the DS signal. The DSD (Defer Signal Duration) subfield 734 indicates the duration or length of the Defer signal specified forthis AC. In embodiment, the subfield indicates the number of time slots to add to a duration corresponding to the legacy preamble (L-STF 8ps, L-LTF 8ps, and L-SIG 4ps). The minimum value of the DSD subfield is 0, meaning the shortest DS signal corresponds to the transmission of the legacy preamble alone. It is up to the AP to determine the maximum value, but one may consider incrementing by one slot up to the maximum number of considered queues (0 to 4 for AC queues, or 0 to 7 if UP is considered (not shown in the figure), etc.). In other embodiments, the DSD subfield 734 can be coded as a number of slot-time intervals (e.g., multiple of 9ps), starting from a non-null value for the shortest duration (because it may include the PHY preamble duration). In other words, the DSD field is specified as an unsigned integer, in units of given number of ps (e.g., 9ps). Figure 7b illustrates a second exemplary format of the pre-emption EDCA Parameters Set, which advantageously allows defining and advertising Deferred EDCA parameters specific to the proposed deferred mechanism (i.e., different from the legacy EDCA parameters). The formats of LL_AC_BE, LL_AC_BK, LL_AC_VI, and LL_AC_VO Parameter Record fields are identical and follow a new format 750. Advantage is to follow the legacy AC_BE, AC_BK, AC_VI, and AC_VO Parameter Record field format (Fig. 9-295 of IEEE Std 802.11-2020), with the difference that its last subfield 780 no longer represents a TXOP Limit but the DSD duration 734 (or alternatively the WSN value). Its first field, ACI / AIFSN field, 760 is quite similar as the legacy ACI / AIFSN field format, but contains the WSN subfield 731 (or alternatively the DSD duration) in bits B0-B3 instead of the AIFSN value. Back to Figure 5a, at step 505a, the communication device takes into account the most recently received pre-emption parameters. Next, at step 510a, the communication device waits fora contention period as described above with reference to step 510. When a new contention period starts, the communication device may start a legacy EDCA process simultaneously to waiting for the pre-empting delay (hence WSN time slots) to elapse. An implementation of the waiting process includes performing a pre-emption EDCA process that uses the pre-emption delay (WSN) to initialize a pre-emption backoff counter. Hence, the communication device performs a legacy EDCA with the legacy EDCA parameters simultaneously to performing the pre-emption EDCA with the pre-emption delay. Alternatively, the communication device may perform the legacy EDCA with the legacy EDCA parameters and stop the legacy EDCA when the pre-emption delay WSN (counted from when the legacy EDCA is started) lapses in order to transmit the Defer signal. As shown in the Figure, the legacy EDCA process is launched at steps 550 (retrieval of legacy EDCA parameters) and 555 (start of backoff contention), and the counting of the WSN slots is conducted in parallel. Steps 550 and 555 perform the legacy EDCA process to access a wireless medium for transmission of the PPDU based on the legacy EDCA parameters. It is recalled that an AP advertises the EDCA Parameter Set element in its management frames, to provide information needed by STAs for proper operation of the QoS feature. Among parameters, the ECWmin and ECWmax subfields encode the contention values of CWmin and CWmax to be used by the non-AP STAs to compute their contention window CW (initial value of CWmin, CW being an integer within the range of values of the PHY characteristics CWmin and CWmax, CWmin <CW <CWmax). The STAs randomly compute their initial backoff value from a uniform distribution over the interval [0,CW], and prior to attempting to transmit again immediately after a successful transmission, the STAs initialize the backoff counter with another backoff value randomly drawn from [0, CW], The STAs decrement the backoff counter once per time slot while the medium is idle. If the medium becomes busy in the meanwhile, the process stops through test 560. Correlatively, if the legacy backoff counter counts down to zero before the pre-emption delay of 515a elapses (i.e., before all the time slots corresponding to WSN have been decremented), test 570 is positive meaning an access to the medium is obtained, and the pending data (PPDU) can be transmitted (step 525a). Step 555 is suspended when the legacy backoff counter is decremented up to the number of slots corresponding to WSN value: the pre-emption delay has elapsed. In embodiments, this means the pre-emption EDCA process obtains medium access before the legacy EDCA process. In that case, a Defer signal is emitted during a duration corresponding to DSD during step 515a. The Defer signal is thus transmitted when the EDCA with the pre-emption delay ends before the legacy EDCA, otherwise the pending data are directly transmitted over the wireless medium. It is then followed by a new backoff contention corresponding to the deferred contention at step 520a. A deferred backoff counter is decremented. By providing adaptive WSN and DSD in between AC queues, the mechanism provides a simple way to prioritize medium access in between the still prioritized AC for low-latency traffic. The Defer signal aims at pre-empting medium access against legacy devices, but also, by varying the duration or pre-emption delays of the Defer signals per queues, at arbitrating between the ACs to designate which is / are allowed to perform the subsequent contention for medium access. Therefore, at the end of Defer signal, there remain only stations having pending data of the same AC. Adapting the duration of the DS signal makes possible to reduce the competing stations to a limited number of stations, that is profitable for contention (reduce risk of collision). Again, if the deferred backoff counter counts down to zero, test 570 is positive meaning an access to the medium is obtained, and the pending data (PPDU) can be transmitted (step 525a). In some embodiments, the deferred contention may resume the legacy EDCA process (that was running at step 550 but suspended upon the WSN slots elapsing). In that case, the legacy EDCA backoff is re-used and an EDCA process is performed to access the wireless medium for the pending data based on initial legacy EDCA parameters. This option is quite simple to implement since it consists only in continuing the decrement of the legacy EDCA backoff. Due to the possibly large value of EDCA backoff, the backoff period can however take long time. That is why in other embodiments, other Deferred EDCA parameters (specific to the proposed deferred mechanism) than the legacy EDCA parameters can be used to draw a new separate (deferred) backoff value. This would allow the contention window [0, CW] to be reduced because of theoretically less competing stations, resulting in faster contentions. This second option requires the new Deferred EDCA parameters to be defined, for example to be advertised by the AP. The Deferred EDCA parameters are for example included in a management frame sent by the AP, such as a Beacon, Probe Response or (Re)Association Response frame. The AP may for instance use the pre-emption EDCA Parameters Set format illustrated by Figure 7b because it reuses the conventional fields for conveying ECWmin and ECWmax values. Of course, other formats may be used to convey in particular new ECWmin and ECWmax values. In the format of Figure 7b, field 770 follows the legacy format of ECWmin / ECWmax field, that is re-used for the purpose of providing deferred contention window parameters LL_CWmin and LL_CWmax for the deferred contention of step 520a. In other words, the Deferred EDCA parameters include contention window parameters for an access category that are included in an ECWmin / ECWmax field of the AC Parameter Record field corresponding to the access category. ECWmin and ECWmax subfields encode the values of LL_CWmin and LL_CWmax, respectively, in an exponent form. The ECWmin and ECWmax values may be defined so that LL_CWmin = 2ECWmin - 1 LL_CWmax = 2ECWmax - 1 Hence the minimum encoded value of LL_CWmin and LL_CWmax is 0, and the maximum value is 32 767. In practice, the AP will set lower value for ECWmax than in conventional EDCA parameters due to the lower number of STAs per AC participating to the deferred contention. As an alternative, field 770 may represent an integer number representing the deferred contention window CWdeferred that defines the range [0, CWdeferred] from which the deferred backoff value (for deferred contention of step 520a) is randomly drawn. In that case, no adaptation of the contention window is performed through the use of the contention window, but a fixed CW is used overtime for the AC. Still referring to the second option using a dedicated deferred backoff counter, a new backoff value may be drawn each time step 520a (i.e., deferred contention) is invoked. This is an alternative to keeping the last backoff value and decrementing it, in case the deferred backoff counter having been suspended due to a previous step 520a where another station gained access to the medium and is resumed with a next invocation of step 520a within a next contention period. The second option gives the communication device a higher probability of fast access to the wireless medium for pending priority data compared to an use of the legacy EDCA parameters of the first option. In addition, the second option allows the AP to tune the pre-emption EDCA parameters according to the number of prioritized streams (e.g. SCS streams) that it has admitted in its BSS for a given AC. Figures 6a, 6b, 6c illustrate, using timelines, various exemplary deferred mechanisms according to the disclosure, depending on variations of the pre-emption parameters, such as the WSN and DSD parameters. The figures show the behaviour of two STAs having prioritized access (hence using the deferred mechanism of the disclosure) for two distinct AC queues: STA 1 for a lower priority AC and STA 2 for a higher priority AC. However, there may be more than two STAs, wherein some may try to transmit pending data belonging to the same AC. Also, there may be STAs trying to transmit pending data belonging to more than two ACs, meaning that more than two Defer signals are transmitted that overlap but end at different time instants. For the sake of illustration, this timeline is illustrated for only one stream per STA, but of course several streams belonging to different priorities can exist in a given STA. In that case, an optimization can be performed at the STA to arbitrate in between the streams and apply the deferred mechanism for the higher priority stream. Various phases are illustrated wherein: 601 references a phase where the medium is sensed as busy due to an on-going communication; 602 references the DIFS period (step 510a) taking place afterthe medium becomes free, that precedes (hence defines) the start of a contention period; 603 references the simultaneous legacy EDCA process together with the pre-emption delay based on WSN (steps 550, 555, 515a). Legacy backoff time is obtained and decremented as long as medium is idle and the number of waiting slots WSN is not reached. For sake of illustration, WSN = 2 in the Figure but any other value can be considered (the determination may be done by the AP and advertised for its BSS as explained above). During phase 603, a STA is able to gain access to the medium if one of its backoff counts down to zero in the meanwhile (not shown); 604 references the transmission of the Defer signal by STA 1 and STA 2. The duration of the Defer signal is tuned per access class (or AC), as provided by the AP for its BSS. This corresponds to step 515a; 605 references the deferred contention taking place when the Defer signal ends. This corresponds to step 520a. As mentioned previously, only a subset of the STAs sending the Defer signal are expected to perform contention because low priority ones have deferred their medium access as they sensed the medium busy due to longer Defer signals sent by other STAs. Also, phase 605 may start a SIFS (or any other delay) afterthe end of the Defer signal transmission (604) instead of a DIFS. Optionally, no additional time is considered to start phase 605 (hence decrementing the deferred backoff counter) after the end of the Defer signal transmission. Figure 6a illustrates a scenario where WSN is identical for all ACs, whereas the length DSD for the Defer signal is different from one AC to the other. Hence, the WSN may be preset and not transmitted by the AP to reduce signalling costs. The priority for the stations is therefore provided by the length of the Defer signal only wherein the length of the Defer signal is longer for high-priority pending data than for low-priority pending data. In the scenario, STA2 has higher priority (e.g., its pending data fall into a UP corresponding to VOICE AC queue) than STA1 (e.g., its pending data fall into a UP corresponding to VIDEO AC queue). As shown, the more prioritized stream corresponds to a longer duration of the Defer signal in order that, at the end of phase 604, the remaining STAs that has not sensed the medium busy are the prioritized STAs. This is the case here for STA 2, contrary to STA 1 which senses the medium as busy due to the Defer signal transmitted by STA 2. The Defer signals 610 and 611 overlap each other at least for the beginning (that means the duration of shortest Defer signal, 610 in the present case). The aim of the Defer signal is to trigger symbol detection on other legacy STAs participating in the same contention period (and enter in an EIFS period as they will further see no acknowledgment for the signal). Therefore, the same first symbols are emitted by the two STAs having prioritized access (i.e., using the deferred mechanism). In embodiments, the Defer signal comprises a legacy 802.11 preamble fields (L-STF 8ps, L-LTF 8ps, and L-SIG 4ps) followed by padding bits, the amount of which is adjusted such as the total length of the Defer signal matches the length for the AC concerned. In embodiments, the legacy 802.11 preambles are the same and indicates a non-HT format along with a data length set to the shortest Defer signal’s padding duration. In other embodiments, the duration for Defer signal is made of a multiple of slot time durations (as in Figure 6a, Defer signal 611 is 2 slot-time duration longer than Defer signal 610). At the end of transmitting its Defer signal 610, STA 1 senses the medium as busy (due to Defer signal 611) and therefore suspends its medium access. It does not participate to the deferred contention 605, contrary to STA 2. In the scenario shown STA 2 gains access to the medium through the deferred contention 605. Hence, it sends its pending data over the medium during phase 606. Figure 6b illustrates an alternative scenario to Figure 6a, where WSN is different from one AC to the other while the length DSD of the Defer signal is the same for all ACs. Hence, the length DSD may be preset and not transmitted by the AP to reduce signalling costs. This scenario may provide more probabilities for legacy stations to win the medium, because the Defer signal first sent may be shifted in time. Preferably, the possible values for WSN are lower than DSD so that all the Defer signal at least partially overlap (so that Defer signal 621 does not start after the end of Defer signal 620). In the scenario, STA2 detects that Defer signal 620 was emitted (by decoding it to know it is a Defer signal) in order to continue considering the medium as being idle and then emit its own Defer signal even if the medium is busy. With this scenario, a single station (e.g. STA 2) requesting medium access is offered more possibilities to gain access, in particular it provides an access through the deferred mechanism that may be shorter than legacy EDCA (due to a shorter contention window within the deferred contention 605). Figure 6c illustrates another alternative scenario to Figures 6a and 6b, where both the WSN and DSD values are different from one AC to the other. As above, the STAs that last ends their Defer signal transmission (here STA 2) participate to the deferred contention 605. The scenario shown proposes higher WSN and shorter DSD for low priority ACs. This is the case for low priority STA 1 that is liable to transmit a shorter Defer signal 630 after high priority STA 2 transmits its longer Defer signal 631. One advantage is that legacy EDCA is loosen for low priority (e.g. for STA 1) when no higher priority STA uses the deferred mechanism (e.g. STA 2 is absent). Indeed, theoretical period 633a forSTA 1 to perform legacy EDCA is longer than for high priority STA such as STA 2 (period 633b). To be noted that, since STA 1 is able to sense Defer signal 631 from higher priority STA (hence it senses the medium as being busy), it may avoid transmitting Defer signal 630. Figure 8 illustrates an exemplary enhanced Stream Classification Service, SCS, Descriptor element format for requesting / allowing the deferred mechanism according to the disclosure. SCS streams, because they are used to define separate streams with different QoS requirements (especially low-latency traffics), are privileged candidates for negotiating the activation of the deferred mechanism described above. Following the format provided in the D4.1 Standard, the SCS Descriptor element 800 includes: SCSID field 820 carrying an identifier for the traffic stream described / defined by element 800. The ID is assigned by a STA requesting classification of the stream and is unique across the STA (or non-AP MLD device, in case of multiple links as illustrated by 424); Request Type field 821 takes a value to identify the type of SCS request: Add, Remove, or Change; Optional Intra-Access Category Priority element 822 provides information to the AP on the relative priorities of the SCS traffic streams within an AC. It corresponds to the optional introduction of two alternate queues proposed by the IEEE 802.11aa standard, compared to the four primary queues of EDCA; TCLAS Elements 823 and TCLAS Processing Element 824, if present, describe criteria the STA requests for traffic classification, which criteria the AP has to apply to identify the data or MSDUs forming the corresponding SCS stream; and QoS Characteristics Element 825 providing a Traffic Specification. QoS Characteristics Element field 825 contains zero or one QoS Characteristics element to describe the traffic characteristics and QoS expectations of traffic flows that belong to this SCS traffic stream. The QoS Characteristics are considered by the AP to properly schedule the STA to transmit the local SCS stream. As an example, the AP may enable the transmission of frames from the STA with an interval that falls between the requested minimum and maximum service intervals and the AP may meet the minimum data rate requested if the Direction subfield of the QoS Characteristics element indicates uplink or direct-link. As shown in the Figure, various fields are provided in QoS Characteristics element 825 that define various QoS transmission parameters for the local SCS stream. Control Info field 840 within QoS Characteristics element 825 is defined as follows: Direction subfield 841 specifies the direction of data forming the stream: Uplink (field set to 0), Downlink (1) or Direct-link (2); TID subfield 842 contains the target TID value of the MSDUs belonging to the local SCS stream (i.e., as described by the SCS Descriptor 800). In practice, this TID can then be used by the AP to schedule (e.g., polling Buffer status reports, allocating resources in emitted trigger frames) the SCS stream, rather than using the SCSID; User Priority subfield 843 contains the UP (value 0-7) of the data frames that are described by this element. When the TCLAS element 824 is present in the SCS frame containing this element, the User Priority subfield 843 is set to the user priority value specified in the TCLAS element; and LinkID subfield 844 contains the link identifier of the link (in case of MLD) for which the P2P or direct link transmissions are going to occur (therefore only considered when Direction subfield 841 specifies Direct-link). Other subfields of QoS Characteristics element 825 are of less importance for the present disclosure; they represent the set of parameters defining the characteristics and QoS expectations for the SCS stream (e.g., Minimum and Maximum Service Intervals, Minimum Data Rate, Delay Bound). As mentioned above, the definition of the SCS stream may be used to request AP’s assistance to deliver uplink traffic, in particular through the proposed deferred mechanism. It is thus envisaged that the non-AP STA indicates its wish to take benefit of the deferred mechanism. To do so, a bit (or more generally a field) is provided in SCS Descriptor element 800, more particularly in QoS Characteristics element 825, more particularly in Control Info field 840, more particularly in one of the reserved bits B29-B31, such as bit B29, to signal a request for deferred mechanism or not for the SCS stream concerned. The request for deferred mechanism may be in replacement to conventional EHT AP’s scheduling (i.e., trigger-based MU UL transmission), or in complement thereto. Hence, in embodiments where it replaces EHT AP’s scheduling, the QoS Characteristics element 825, in which the Direction subfield is equal to uplink or direct link and with ’Deferred Scheme” subfield 850 (bit b29 of Control Info) is set to 1, is no longer taken into account when the AP schedules RUs for the STAs using trigger frames (EHT AP’s scheduling). In that case, a non-AP EHT STA may transmit an SCS Request frame with SCS Descriptor element(s) containing a QoS Characteristics element if the Request Type field in the frame is set to “Add” or “Change”. The QoS Characteristics element, in which the Direction subfield is equal to uplink or direct link, describes the traffic characteristics of the requested SCS stream and requests (bit B29) the use of deferred mechanism for enhanced EDCA access. The AP responds with an SCS Response frame. In an SCS Response frame with a Status field value set to REJECTED_WITH_SUGGESTED_CHANGES, an AP may include an SCS Descriptor element containing a QoS Characteristics element with bit B29 set to a different value than the QoS Characteristics element contained in the SCS Descriptor element of the corresponding SCS Request frame. For example, if the STA has requested the use of the deferred mechanism (B29 = 1 in the request), the AP can refuse it (B29 = 0 in the response). Similarly, if the STA has not requested the use of the deferred mechanism (B29 = 0 in the request), the AP can impose it (B29 = 1 in the response). If an AP is not able to answer QoS expectations for the SCS stream proposed in the SCS Request frame requesting the use of the deferred mechanism (e.g., any of Minimum and Maximum Service Intervals, Minimum Data Rate, Delay Bound), it may send an SCS Response frame with a Status field value set to REJECTED_WITH_SUGGESTED_CHANGES, including an SCS Descriptor element containing a QoS Characteristics element with bit B29 set to 1. Keeping bit B29 to 1 with a rejection response indicates the non-AP STA will not get assistance from the AP, and can use the deferred mechanism of the present disclosure. In a variant, when bit B29 is set to 1, the LinkID subfield (844) contains a link bitmap to identify the link(s) for which the deferred mechanism is active. Although the present disclosure has been described herein above with reference to specific embodiments, it is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present disclosure. Many further modifications and variations will suggest themselves to those versed in the art upon referring to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the disclosure, that being determined solely by the appended claims. In particular the different features from different embodiments may be interchanged, where appropriate. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims

1. A method of accessing a wireless medium to transmit pending data, the method comprising, at a communication device:transmitting a Defer signal over the wireless medium within a contention period,and then performing a backoff procedure within the contention period,wherein a length of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period are determined based on an access class of the pending data.

2. The method of Claim 1, further comprising receiving, from an access point with which the communication device is associated, multiple lengths and / or pre-emption delays corresponding to multiple respective access classes, and selecting the length and / or pre-emption delay corresponding to the access class of the pending data.

3. The method of Claim 2, wherein the lengths and / or pre-emption delays for multiple access classes are included in a management frame transmitted by the access point.

4. The method of Claim 3, wherein the length and / or pre-emption delay for an access category are included in an AC Parameter Record field corresponding to the access category within an EDCA Parameter Set element compliant with the EDCA Parameter Set element format of Fig.9-293 of IEEE Std 802.11-2020 in the management frame.

5. The method of Claim 1, comprising performing EDCA (Enhanced Distributed Channel Access) using the pre-emption delay as backoff value, within the contention period to transmit the Defer signal.

6. The method of Claim 5, further comprising performing legacy EDCA with legacy EDCA parameters simultaneously to performing the EDCA with the pre-emption delay.

7. The method of Claim 1, further comprising performing legacy EDCA (Enhanced Distributed Channel Access) with legacy EDCA parameters within the contention period and stopping the legacy EDCA when the pre-emption delay lapses in order to transmit the Defer signal.

8. The method of Claim 6, wherein the Defer signal is transmitted when the EDCA with the pre-emption delay ends before the legacy EDCA, otherwise the pending data are directly transmitted over the wireless medium.

9. The method of Claim 1, wherein the pending data are transmitted for first time after the communication device accesses the medium through the backoff procedure.

10. The method of Claim 1, wherein performing the backoff procedure includes resuming legacy EDCA with legacy EDCA parameters for the access class of the pending data, to gain access to the medium.

11. The method of Claim 1, wherein performing the backoff procedure includes setting contention parameters for the backoff procedure with Deferred EDCA parameters specific to a backoff procedure following a Defer signal.

12. The method of Claim 11, wherein the Deferred EDCA parameters are included in a management frame sent by an access point with which the communication device is associated.

13. The method of Claims 4 and 12, wherein the Deferred EDCA parameters include contention window parameters for an access category that are included in an ECWmin / ECWmax field of the AC Parameter Record field corresponding to the access category.

14. The method of Claim 1, wherein the length and / or pre-emption delay for high-priority pending data are designed to end the Defer signal later than the length and / or pre-emption delay for low-priority pending data.

15. The method of Claim 1, wherein the length of the Defer signal is longer for high-priority pending data than for low-priority pending data.

16. The method of Claim 1, further comprising, if the access class of the pending data belongs to a predefined group of access classes, not initiating a mechanism to transmit the Defer signal and performing a legacy backoff procedure within the contention period.

17. The method of Claim 1, further comprising transmitting a Stream Classification Service (SCS) Action frame including an SCS Descriptor element defining a class of data, the SCS Descriptor element having a field or bit set to a first value to enable transmission of a Defer signal before performing a backoff procedure for the class of data, or set to a second value to disable such transmission for the class of data before performing a backoff procedure.

18. The method of Claim 1, wherein the backoff procedure starts a SIFS after an end of the Defer signal transmission or starts immediately after an end of the Defer signal transmission.

19. The method of Claim 1, wherein the pre-emption delay is defined as a number of time slot after the start of the contention period.

20. The method of Claim 1, wherein the Defer signal includes legacy 802.11 preamble fields followed by padding bits.

21. The method of Claim 1, wherein the access class includes one from an access category, a traffic class, a traffic identifier (TID), a user priority (UP), TCLAS information element containing parameters to identify frames of a traffic stream, a Stream Classification Service identifier (SCSID) identifying a stream of data.

22. The method of Claim 1, wherein the communication device is a non-access point station associated with an access point.

23. The method of Claim 1, wherein the pending data are peer-to-peer data.

24. The method of Claim 1, wherein the pending data are uplink data to be addressed to an access point with which the communication device is associated.

25. The method of Claim 1, wherein the communication device is an access point.

26. The method of Claim 1, wherein the pending data are downlink data to be addressed to one or more non-access point station that are associated with the communication device.

27. A method of communication in a wireless network comprising, at a communication device:accessing a wireless medium of the wireless network using the method of Claim 1, andtransmitting the pending data over the accessed wireless medium.

28. A method of communication in a wireless network comprising, at an access point, AP, transmitting a management frame to non-AP stations associated with it,wherein the management frame includes multiple lengths and / or pre-emption delays for multiple access classes to drive a non-AP station willing to transmit pending data to transmit a Defer signal within a contention period, a length of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period being based on the length and / or pre-emption delay corresponding to the access class of the pending data.

29. A management frame to be sent by an access point, AP, comprising multiple lengths and / or pre-emption delays for multiple access classes to drive a non-AP station willing to transmit pending data to transmit a Defer signal within a contention period in such a way a length of the Defer signal and / or a pre-emption delay prior to transmitting the Defer signal after a start of the contention period are based on the length and / or pre-emption delay corresponding to the access class of the pending data.

30. A wireless communication device comprising at least one microprocessor configured for carrying out the steps of the method of Claim 1 or 27 or 28.

31. A communication device comprising:a communication interface to a wireless medium,a memory storing pending data to be transmitted,a deferring unit configured to transmit a Defer signal over the wireless medium within a contention period, anda contention unit configured to perform, after the Defer signal has been transmitted, a backoff procedure within the contention period,wherein a length of the Defer signal and / or a pre-emption delay prior to transmitting the Defersignal aftera start of the contention period are determined based on an access class of the pending data.

32. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a wireless device, causes the wireless device to perform the method of Claim 1 or 27 or 28.37

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