TID-to-link mapping signalling for multi-link devices in WLAN

Abstract

A multi-link device (MLD) sends to a peer MLD a traffic identifier-to-link (TID-to-link) mapping element, 400, for links setup or to be setup between the MLD and the peer. The TID-to-Link mapping elem

Description

The present invention generally relates to wireless communications and more specifically to Multi-Link (ML) communications. BACKGROUND With the development of latency sensitive applications such as online gaming, real-time video streaming, virtual reality, drone or robot remote controlling, better throughput, low latency and robustness requirements and issues need to be taken into consideration. Such problematic issues are currently under consideration by the IEEE 802.11 working group as a main objective to issue the next major 802.11 release, known as 802.11 be or EHT for “Extremely High Throughput”. The IEEE P802.11be / D3.2 draft specification (May 2023, below “D3.2 standard”) introduces the Multi-Link (ML) Operation (MLO) feature. MLO improves data throughput by allowing communications between multi-link devices (MLDs) over multiple concurrent communication links. MLO enables a non-access point (non-AP) MLD to register with an AP MLD, i.e., to discover, authenticate, associate and set up multiple links (hereafter called “setup 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 the association procedure. The D3.2 standard also defines a Traffic IDentifier(TID)-to-link mapping mechanism. The TID-to-link mapping (TTLM) mechanism controls on which link a frame belonging to each TID can be exchanged. By default, all TIDs are mapped on all setup links for both downlink (DL) and uplink (UL). When a non-default TID-to-link mapping is applied, a different mapping may be provided for each TID. There is a need to optimize the signalling of the TID-to-link mapping mechanism between an initiating MLD (e.g., an AP MLD) and a responding MLD (e.g., a non-AP MLD). SUMMARY OF INVENTION It is a broad objective of the present invention to optimize the size of a TID-To-Link Mapping element for different TID-to-link mapping possibilities. According to embodiments of the invention, it is proposed solutions to reduce the overhead of the TID-To-Link Mapping element. Especially, the present invention permits to reduce overhead of the TID-To-Link Mapping element for the signalling of a TID-to-link mapping when all TIDs are mapped to the same link set (where the link set is a subset of the setup links) or when part of the TIDs are mapped to a set links. To reach this goal, the present invention defines TID-to-link mapping modes and the use of these defined modes in the TID-To-Link Mapping element. According to one aspect of the invention, there is provided a method for wireless communication performed by a multi-link device (MLD), the method comprising sending, to a peer MLD, a traffic identifier-to-link (TID-To-Link) Mapping element for links setup or to be setup between the MLD and the peer MLD, wherein the TID-To-Link Mapping element comprises a bitmap for the mapping of at least two TIDs and specifying a set of links on which the at least two TIDs are mapped. This advantageously allows to optimize the size of the TID-To-Link Mapping element and to reduce bandwidth usage. According to an implementation, the TID-To-Link Mapping element is included in a Request frame. The request frame may be for example an (Re)Association Request frame. The MLD may then receive a (Re)Association Response frame comprising a TID-To-Link Mapping element including an updated bitmap resulting from a negotiation between the MLD and the peer MLD. According to an implementation, the TID-To-Link Mapping element is advertised by an AP affiliated with the MLD and is included in a beacon or probe response frame. According to embodiments of the invention, the bitmap maps part of defined TIDs on all the setup links. According to embodiments ofthe invention, the bitmap maps all defined TIDs on a subset of the setup links. According to embodiments ofthe invention, the TID-To-Link Mapping element further includes a Link Mapping Mode subfield capable of indicating the presence ofthe bitmap mapping at least two TIDs. According to an implementation, the Link Mapping Mode subfield signals one or more mapping modes, including at least one of: a mode, referred to as a default mapping mode, for signalling that all TIDs are mapped to all setup links for both uplink (UL) and downlink (DL); a mode for signalling that at least two TIDs are mapped to the same link set; a mode for signalling that all TIDs are mapped to the same link set for both DL and UL, where the link set is a subset of setup links; a mode for signalling the presence of a bitmap for mapping each TID to a set of one or more links; or a mode for signalling that TIDs are mapped by pairs to sets of one or more links. According to an implementation, the MLD is a non-access point (non-AP) MLD and the peer MLD is an AP MLD. In a variant implementation, the MLD is an access point (AP) MLD and the peer MLD is a non-AP MLD. According to an implementation, the MLD is an initiating MLD and the peer MLD is a responding MLD. In a variant implementation, the MLD is a responding MLD and the peer MLD is an initiating MLD. According to an implementation, the links to which the at least two TIDs are mapped are a subset ofthe setup links. According to an implementation, the bitmap maps two TIDs belonging to a same Access Category (AC). According to an implementation, the TID-To-Link Mapping element includes a field comprising the bitmap mapping the at least two TIDs. According to another aspect of the invention, there is provided a multi-link device (MLD) comprising a memory and processing circuitry configured to send, to a peer MLD, a traffic identifier-to-link (TID-To-Link) Mapping element for links setup or to be setup between the MLD and the peer MLD, wherein the TID-To-Link Mapping element comprises a bitmap forthe mapping of at least two TIDs and specifying a set of links on which the at least two TIDs are mapped. According to a further aspect of the invention, there is provided a traffic identifier-to-link (TID-To-Link) Mapping element for links setup or to be setup between a multi-link device (MLD) and a peer MLD, wherein the TID-To-Link Mapping element comprises a bitmap forthe mapping of at least two TIDs and specifying a set of links on which the at least two TIDs are mapped. According to an implementation, the TID-To-Link Mapping element further comprising a Link Mapping Mode subfield for signalling a mode of the mapping. According to an implementation, the TID-To-Link Mapping element further comprising a field carrying the bitmap mapping the at least two TIDs. At least parts of the methods according to the invention may be computer implemented. Accordingly, the present invention 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, the present invention 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 present invention can be implemented in software, the present invention 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 solidstate 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 Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which: Figure 1 illustrates an example of a wireless network involving multi-link (ML) transmissions between MLDs in which the present invention may be implemented: Figures 2a, 2b and 2c illustrate exemplary frames exchanged during TlD-to-link mapping negotiation, TlD-to-link mapping advertising and association procedure, respectively, forTID-to-link mapping; Figure 3a illustrates the TID-To-Link Mapping element format as specified in the IEEE P802.11 be / D3.2 standard; Figure 3b illustrates the TID-To-Link Control field format of the TID-To-Link Mapping element as specified in the IEEE P802.11be / D3.2 standard; Figure 4a illustrates a modified TID-To-Link Mapping element format according to a first embodiment of the invention; Figure 4b illustrates a modified TID-To-Link Control field format of the modified TID-To-Link Mapping element according to the first embodiment of the invention. Figure 5a illustrates a modified TID-To-Link Mapping element format according to a second embodiment of the invention; Figure 5b illustrates a modified TID-To-Link Control field format of the modified TID-To-Link Mapping element according to a second embodiment of the invention; Figure 5c illustrates a modified TID-To-Link Control field format of the modified TID-To-Link Mapping element according to a variant of the second embodiment of the invention; Figure 6 shows a schematic representation of a wireless communication device in accordance with embodiments of the present invention; Figure 7a illustrates a modified TID-To-Link Mapping element format according to a third embodiment of the invention; and Figure 7b illustrates a modified TID-To-Link Control field format of the modified TID-To-Link Mapping element according to the third embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS A Multi-Link Operation (MLO) at media access control (MAC) layer allows multi-link devices (MLDs) to establish or setup multiple links and operate them simultaneously. A Multi-Link Device (MLD) is a logical entity and has more than one affiliated station and has a single MAC service access point (SAP) to logical link control (LLC), which includes one MAC data service. An Access Point Multi-Link Device (or AP MLD) then corresponds to a MLD where each station affiliated with the MLD is an AP, hence referred to as “affiliated AP”. A non-Access Point Multi-Link Device (or non-AP MLD) corresponds to a MLD where each station affiliated with the MLD is a non-AP station (STA), referred to as “affiliated non-AP station” or “affiliated STA”. Depending on the literature, “multilink device”, “ML Device” (MLD), “multilink logical entity”, “ML logical entity” (MLE), “multilink set” and “ML set” are synonyms to designate the same type of ML Device. Multiple affiliated STAs of a non-AP MLD can then setup communication links with multiple affiliated APs of an AP MLD, hence forming a multi-link channel. A communication link or “link” corresponds to a given channel (e.g., Channel 6, Channel 36, Channel 151, and so on) in a given frequency band (e.g., 2.4 GHz, 5 GHz, 6 GHz) between an AP affiliated with the AP MLD and a STA affiliated with the non-AP MLD. The affiliated APs and STAs operate on their respective channels in accordance with one or more of the IEEE 802.11 standards (a / b / g / n / ac / ad / af / ah / aj / ay / ax / be) or other wireless communication standards. Thanks to the multi-link aggregation, traffic associated with a single MLD can theoretically be transmitted across multiple parallel communication links, thereby increasing network capacity and maximizing utilization of available resources. Figure 1 illustrates an example of a wireless network involving multi-link (ML) transmissions between MLDs in which the present invention may be implemented. The figure illustrates a wireless communication network 100 comprising an AP MLD 110 and two non-AP MLDs 120 and 130. It is assumed that the two non-AP MLDs have declared their corresponding capabilities to the AP MLD 110 during an association procedure. Of course, another number of non-AP MLDs registering to the AP MLD 110 and then exchanging frames with it may be contemplated, as well as another (greater) number of setup links between the non-AP MLDs and the AP MLD. AP MLD 110 has two affiliated APs 111 and 112 (also referenced AP1, AP2 respectively) in the exemplary Figure 1, each of which behaves as an AP over its operating channel within one frequency band. For example, frequency bands may include the 2.4 GHz band, the 5 GHz band and the 6 GHz band. Of course, other frequency bands may be used in replacement or in addition to these three bands. The non-AP MLDs 120,130 have multiple affiliated non-AP STAs, each of which behaves as a non-AP station in a BSS (managed by an affiliated AP 111 or 112) to which it registers. In the exemplary Figure 1, two non-AP STAs 121 and 122 (also referenced A1 and A2 respectively) are affiliated with non-AP MLD 120 and two non-AP STAs 131 and 132 (also referenced B1 and B2 respectively) are affiliated with non-AP MLD 130. There is a possibility to establish a link between each affiliated AP of the AP MLD 110 and an affiliated STA of a non-AP MLD (120 or 130). Hence, the links for each non-AP MLD can be merely identified with the identifiers ofthe respective affiliated APs. In this context, each of the affiliated APs 111 and 112 can be identified by an identifier referred to as “link ID”. The link ID of each affiliated AP is unique and may not change during the lifetime ofthe AP MLD. AP MLD may assign the link ID to its affiliated APs by incrementing the IDs from 0 (for the first affiliated AP). Of course, other wording, such as “AP ID”, could be used in a variant. To perform multi-link communications, each non-AP MLD 120, 130 has to discover, authenticate, associate and set up multiple links with the AP MLD 110, through ML discovery, authentication, association and setup procedures. Each link being established between an affiliated AP ofthe AP MLD 110 and an affiliated STA ofthe non-AP MLD. Each such link is referred to as a setup link. For illustrative purpose, in wireless communication network 100, during ML setup procedures, two candidate setup links have been requested by non-AP MLD 120 and accepted by AP MLD 110: a first link 151 between affiliated AP 111 (AP1) and affiliated non-AP STA 121 (A1), a second link 152 between affiliated AP 112 (AP2) and affiliated non-AP STA 122 (A2). Similarly, two candidate setup links have been requested by multi-radio non-AP MLD 130 and accepted by AP MLD 110: a first link 161 between affiliated AP 111 (AP1) and affiliated non-AP STA 131 (B1), a second link 162 between affiliated AP 112 (AP2) and affiliated non-AP STA 132 (B2). It is to be pointed out that the links 151 and 161 share the same frequency band and same channel provided by the affiliated AP 111 (AP1) and so, share the same Link ID. Similarly, the links 152 and 162 share the same frequency band and same channel provided by the affiliated AP 112 (AP2) and so, share the same Link ID. A setup link of a non-AP MLD to which at least one TID is mapped either in downlink (DL) or in uplink (UL) is referred to as enabled link. By default, all TIDs are mapped to all setup links for both DL and UL; hence, the setup links become enabled links. The enabled links permit individual channel access and frame exchanges between the non-AP MLD and the AP MLD based on supported capabilities exchanged during association procedure. The TID-to-link mapping mechanism allows an AP MLD and a non-AP MLD, that performed or are performing multi-link setup, to determine how data frames belonging to TIDs 0-7 and management frames will be assigned for transmission, on the setup links between the two MLDs in DL and UL. The Standard IEEE P802.11be / D3.2 specifies several procedures to apply a TID-to-link mapping between an AP MLD and a non-AP MLD. These procedures include: Negotiation of TID-to-link mapping, Advertised TID-to-link mapping in Beacon and Probe Response frames, and Association procedures for TID-to-link mapping. All these procedures rely on the use of a TID-To-Link Mapping element holding the necessary information to determine the mapping of the TIDs on the links setup between the two MLDs in DL and UL. TID-to-link mapping negotiation An MLD may initiate a TID-to-link mapping negotiation with a peer MLD by sending the TID-To-Link Mapping element. During a ML (re)setup procedure, a non-AP MLD may initiate a TID-to-link mapping negotiation by including the TID-To-Link Mapping element in the (Re)Association Request frame. During the ML (re)setup procedure, the non-AP MLDs declare part or all of their capabilities. For instance, they may declare their TID-to-link mapping Negotiation Support value in the “MLD Capabilities And Operations” subfield. For this, appropriate fields are provided in the management frames. In particular, some of the management frames exchanged during the ML discovery and ML setup procedures contain a new Information Element specific to the Multi-Link Operation (MLO), referred to as Basic Multi-Link element. After the ML (re)setup is successful, to negotiate a new TID-to-link mapping, an initiating MLD may send an individually addressed TID-to-link Mapping Request frame to a responding MLD. Upon receiving the individually addressed TID-to-link Mapping Request frame, the responding MLD may send an individually addressed TID-to-link Mapping Response frame to the initiating MLD. An MLD may suggest a preferred TID-to-link mapping to a peer MLD by sending an unsolicited TID-to-link Mapping Response frame that includes the TID-to-link Mapping element. A TID-to-link mapping negotiation is successful if a MLD successfully transmits or receives a TID-to-link Mapping Response frame with the value of the Status Code field equal to 0 (SUCCESS). Once an MLD has successfully negotiated the TID-to-link mapping with a peer MLD, both the MLD and the peer MLD update uplink and / or downlink TID-to-link mapping information according to the negotiated TID-to-link mapping. Figure 2a illustrates a frame exchange during a TID-to-link mapping negotiation between an initiating MLD (e.g., between affiliated non-AP STA 122 of the initiating MLD) and a responding MLD (e.g., affiliated AP 112 of the responding MLD) as discussed above. The frame 201 transmitted from the initiating MLD to the responding MLD may be a request frame such as a (Re)Association Request frame or a TID-to-link Mapping Request frame. In a variant, frame 201 may be an advertisement frame such as a beacon frame or a probe response frame. The response frame 202 transmitted form the responding MLD to the initiating MLD may be a TID-to-link Mapping Response frame. Each of the request, advertisement and response frames may include a TID-To-Link Mapping element used to reach an agreement between the initiating MLD and the responding MLD on a TID-to-link mapping to be applied. Advertised TID-to-link mapping in Beacon and Probe Response frames An AP MLD may advertise a mandatory TID-to-link mapping by including a TID-To-Link Mapping element in the Beacon and Probe Response frames that the APs affiliated with the AP MLD transmit. An AP that advertises a TID-to-link mapping maps all TIDs to the same link set in the advertised TID-to-link mapping, both for DL and UL. The Direction field (see 341 in Figure 3b) of an advertised TID-To-Link Mapping element is for example set to 2. The advertised TID-to-link mapping will be established by the MLDs at a time and fora duration specified in the TID-to-link Mapping element. Figure 2b illustrates an advertisement frame 221 transmitted for a TID-to-link mapping advertisement from an initiating MLD to a responding MLD as discussed above. The advertisement frame 221 may be a beacon frame or a probe response frame. The advertisement frame includes a TID-To-Link Mapping element to be applied by both the initiating and the responding MLDs. In this context, the advertisement frame is for example broadcasted by an affiliated AP of the initiating MLD (e.g., affiliated AP 111) to a plurality of responding MLDs. Affiliated non-AP STAs of these responding MLDs (e.g., affiliated non-AP STAs 121 and 131), recipient of the advertisement frame, apply then the TID-to-link mapping advertised in the TID-To-Link Mapping element. Association procedure for TID-to-link mapping During a ML (re)setup procedure, a non-AP MLD may initiate a TID-to-link mapping negotiation by including the TID-to-link Mapping element in a (Re)Association Request frame if an AP MLD has indicated a support of TID-to-link mapping negotiation. After receiving the (Re)Association Request frame, the AP MLD enters a TID-to-link mapping negotiation procedure by accepting, updating or rejecting the requested TID-to-link mapping using a (Re)Association Response frame. Figure 2c illustrates the (Re)Association Request frame 231 from an initiating MLD (e.g., from affiliated non-AP STA 131 of the initiating MLD) and the (Re)Association Response frame 232 from a responding MLD (e.g., from affiliated AP 111 of the responding MLD) exchanged during the association procedure for applying a TID-to-link mapping. The (Re)Association Request frame 231 and / or the (Re)Association Response frame 232 may include the TID-to-link Mapping element to support agreeing and applying the TID-to-link mapping. Besides the type of frames described above in which the TID-To-Link Mapping element may be embedded, any other type of frames may be considered according to embodiments of the invention. Figure 3a illustrates the TID-To-Link Mapping element format as specified in the IEEE P802.11 be / D3.2 standard. The TID-To-Link Mapping element 300 indicates links on which frames belonging to each TID can be exchanged. The Element ID field 310 and the Element ID Extension field 330 are used to identify the TID-To-Link Mapping element 300, they are set to the values 255 and 109, respectively. The Length field 320 indicates the number of octets in the TID-To-Link Mapping element excluding the Element ID and Length fields. The format of the TID-To-Link Mapping Control field 340 is defined in Figure 3b. The Mapping Switch Time field 350 is present when the TID-To-Link Mapping element 300 is transmitted by an AP affiliated with an AP MLD in a Beacon or Probe Response frame and the indicated TID-to-link mapping is not yet established; otherwise, it is not present. The absence of Mapping Switch Time field 350 in the TID-To-Link Mapping element in a Beacon or Probe Response frame transmitted by an AP affiliated with an AP MLD indicates that the indicated TID-to-link mapping is already established. The 2 octets Mapping Switch Time field 350 has units of Time Units (1 TU = 1024ps) and is set to the time at which the indicated new mapping is established using as a time base the value of the Timing Synchronization Function (TSF) corresponding to the Basic Service Set (BSS) identified by the BSSID of the transmitted frame containing the TID-To-Link Mapping element: i.e., bits 10 to 25 of the TSF. The Expected Duration field 360 indicates the duration for which the proposed TID-to-link mapping is expected to be effective in units of TUs when the Mapping Switch Time field 350 is present, and the remaining duration for which the proposed TID-to-link mapping is expected to be effective in units of TUs when the Mapping Switch Time field 350 is not present. The Expected Duration field 360 is present if the TID-To-Link Mapping element 300 is carried in a Beacon or a Probe Response frame transmitted by an AP affiliated with an AP MLD, and is not present otherwise. The “Link Mapping Of TID ri’ field (where n = 0, 1, .... 7) 370-377 indicates the link(s) on which frames belonging to TID n are allowed to be sent (i.e., carries a bitmap of the links to which the TID n is mapped to). A value of 1 in bit position i (where / = 0,1,.... 7 if the Link Mapping Size subfield 345 is set to 1, and / = 0,1, ..., 14 otherwise) of the Link Mapping Of TID n field indicates that TID n is mapped to the link associated with the link ID i for the direction specified in the Direction subfield 341. A value of 0 in bit position i indicates that the TID n is not mapped to the link associated with the link ID i for the direction specified in the Direction subfield 341. When the Default Link Mapping subfield 342 is set to 1, no Link Mapping Of TID n field 370-377 is present. Figure 3b illustrates the TID-To-Link Mapping Control field format of the TID-To-Link Mapping element 300 as specified in the IEEE P802.11be / D3.2 standard. The Direction subfield 341 is set to 0 if the TID-To-Link Mapping element 300 provides the TID-to-link mapping information for frames transmitted on the downlink (DL). It is set to 1 if the TID-To-Link Mapping element 300 provides the TID-to-link mapping information for frames transmitted on the uplink (UL). It is set to 2 if the TID-To-Link Mapping element 300 provides the TID-to-link mapping information for frames transmitted both on the downlink and the uplink. The value of 3 is reserved. The Default Link Mapping subfield 342 is set to 1 if the TID-To-Link Mapping element 300 represents the default TID-to-link mapping. Otherwise, it is set to 0. The Mapping Switch Time Present subfield 343 is set to 1 if the Mapping Switch Time field 350 is present and 0 otherwise. The Expected Duration Present subfield 344 is set to 1 if the Expected Duration field 360 is present and 0 otherwise. The Link Mapping Size subfield 345 is set to 1 if the length of the Link Mapping Of TID n field 370-377 is 1 octet and is set to 0 if the length of the Link Mapping Of TID n field 370-377 is 2 octets. The Reserved subfield 346 corresponds to the reserved bits not assigned, it is 2 bits length. The Link Mapping Presence Indicator (also referred to as "Link Mapping Presence Bitmap") subfield 347 indicates which of the Link Mapping Of TID n fields are present in the TID-To-Link Mapping element 300 (i.e., the subfield identifies the TID(s) for which the mapping 370-377 is provided in the element). A value of 1 in bit position n of the Link Mapping Presence Bitmap subfield 347 indicates that the Link Mapping Of TID n field is present in the TID-To-Link Mapping element 300. Otherwise, the Link Mapping Of TID n field is not present in the TID-To-Link Mapping element 300. If the Default Link Mapping subfield 342 is set to 1, this subfield is not present. From the above description of Figures 3a and 3b, it can be noticed that the current TID-To-Link Mapping element 300 is optimized for signalling a Default TID-to-link mapping (no bitmap provided) and for the case where the mapping is different between all the TIDs (a bitmap for each TID is provided in fields 370-377). It is not optimized for example in the TID-to-link mapping case where all TIDs are mapped to a same link set which is a subset of the setup links. Indeed, for such a case, the 8 bitmaps in fields 370-377 are present in the TID-To-Link Mapping element 300 and they all concern a same link set (i.e., same link bitmap is repeated 8 times). It is also not optimized for example in the TID-to-link mapping case where part of the TIDs are mapped to a same link set. Indeed, for such a case, the 8 bitmaps in fields 370-377 are present in the TID-To-Link Mapping element 300 and part of these bitmaps concern a same link set (i.e., same link bitmap is repeated for the TIDs mapped to the same link set). Consequently, for these different examples, redundant information is transmitted and unnecessary overhead is added. For illustration, in an implementation where TID-To-Link Mapping Control field 340 has a size of 2 octets, the Mapping Switch Time field 350 is present (i.e., advertised TID-To-Link mapping is not yet established), the Expected Duration field 360 is present and the Link Mapping fields 370-377 have a size of 2 octets each (as signalled in Link Mapping Size subfield 345), the signalling method according to Figures 3a and 3b requires a transmission of 10+8*2=26 octets. This represents a non-negligible overhead which is especially costly when the TID-To-Link Mapping element 300 is included in a Beacon frame. The signalling method is thus not optimized in terms of bandwidth usage, in particular for the situations where all TIDs are mapped to a subset of setup links and / or part of the TIDs are mapped to a same set or subset of the setup links. The overhead of the advertised TID-To-Link Mapping is especially damageable when it is used for the link disablement procedure specified in IEEE P802.11be / D3.2 standard. The reason is that an advertised TID-To-Link Mapping that disables at least one link maps all TIDs to a same subset of the setup links, and hence a same bitmap is repeated for all TIDs. Indeed, according to the standard, a link is disabled when no TID is mapped to this link, neither in UL nor in DL. The link disablement procedure is an important feature that permits to stop all communications on a link, for example to handle the maintenance or the power saving of the corresponding affiliated AP. Embodiments of the invention are aimed to optimized the size and reduce overhead of the TID-To-Link Mapping element. In particular, embodiments of the invention aim to leverage transmission of a single bitmap for signalling the mapping for at least two TIDs. Non-limitative example embodiments are provided hereinafter. According to embodiments of the invention, TID-to-link mapping modes are defined and the use of these defined modes in the TID-To-Link Mapping element is described. For the TID- to-link mapping mode corresponding to the TID-to-link mapping where all TIDs are mapped to the same link set (where the link set may be a subset of setup links), embodiments of the present invention introduce and describe a signalling method where a common link mapping applies to all TIDs reducing the size of the transmitted TID-To-Link Mapping element. The TID-to-link mapping modes may be defined as follows according to embodiments of the invention. The indexing (0, 1,2 and 3) used here below is a preferred indexing; however, other indexing or different ordering may be contemplated. The TID-To-Link mapping mode 0 corresponds to the Default mapping mode. Under this mode, all TIDs are mapped to all setup links for both DL and UL (= Default TID-to-link mapping) and all setup links are enabled. TID-To-Link mapping mode 0 is supported by an MID that has set the TID-To-Link Mapping Negotiation Support subfield in the MLD Capabilities And Operations subfield of the Basic Multi-Link element to any value. A non-AP MLD associated with an AP MLD may operate under this mode if the following two conditions are met: — a TTLM is not advertised by the AP MLD, — a TTLM negotiation for a different mapping did not occur, was unsuccessful while having no successfully negotiated non-default TTLM before or was torn down. The TID-To-Link mapping mode 1 corresponds to a mapping where all TIDs are mapped to the same link set for both UL and DL, where the link set is a subset of setup links. According to this definition, the TID-To-Link mapping modes 1 and 0 are exclusive (TTLM modes defined exclusively). In a variant, the TID-To-Link mapping mode 1 corresponds to a mapping where all TIDs are mapped to the same link set for both UL and DL, regardless of whether the link set corresponds to the setup links or a subset of the setup links. In other words, in this variant, the TID-To-Link mapping mode 1 includes the TID-To-Link mapping mode 0 (TTLM modes defined inclusively). TID-To-Link mapping mode 1 is supported by an MLD that has set the TID-To-Link Mapping Negotiation Support subfield in the MLD Capabilities And Operations subfield of the Basic Multi-Link element to a non-zero value. A non-AP MLD associated with an AP MLD may operate under this mode if one of the following two conditions is met: — a TTLM corresponding to this mode 1 is advertised by the AP MLD, — a TTLM negotiation corresponding to this mode 1 occurred, was successful and is not torn down. - The TID-To-Link mapping mode 2 may or may not be defined according to embodiments of the invention. The TID-To-Link mapping mode 3 corresponds to a mapping where all TIDs are not all mapped to a same link set. In other words, the TID-To-Link mapping mode 3 corresponds to a mapping where at least one TID is mapped to a link set that is different of at least another TID. According to this definition, the TID-To-Link mapping modes 3, 1 and 0 are exclusives. In a variant, the TID-To-Link mapping mode 3 corresponds to a mapping where each TID is mapped to the same or different link set. In other words, in this variant, the TID-To-Link mapping mode 3 includes the TID-To-Link mapping modes 0 and 1. TID-To-Link mapping mode 3 is supported by an MLD that has set the TID-To-Link Mapping Negotiation Support subfield in the MLD Capabilities And Operations subfield of the Basic Multi-Link element to the value 3. A non-AP MLD associated with an AP MLD may operate under this mode if the following condition is met: — a TTLM negotiation corresponding to this mode 3 occurred, was successful and is not torn down. These modes are advantageously relied upon for describing exemplary embodiments of the invention. In particular, exemplary embodiments comprise a first embodiment of the invention described with reference to Figures 4a and 4b, a second embodiment of the invention described with reference to Figures 5a and 5b and a third embodiment of the invention described with reference to Figures 7a and 7b. It is to be noted that optimizing the size (reducing overhead) of the TID-To-Link Mapping element for the TID-to-link mapping mode 1 is particularly advantageous since many redundant information is present in this mode. In addition, TID-to-link mapping modes 0 and 1 are more likely to be used relatively to mode 3, in particular for TID-to-link mapping advertisement in Beacon and Probe response frames. Indeed, it is likely that advertisement frames comprise simple and similar mappings to suit a great number of responding MLDs. Thus, modes 0 and 1 may be the only modes supported for the TID-to-link mapping advertisement in Beacon and Probe response frames. It may also be mandated for a non-AP MLD to support TID-To-Link mappings corresponding to at least TID-to-link mapping modes 0 and 1. It is also to be noted that the “bit cost” in a beacon frame is higher than other type of frames as its content is regularly broadcasted and it contains many legacy fields that must be present. Decreasing redundant information in beacon frames in particular and reducing their size is thus important. Figure 4a illustrates a modified TID-To-Link Mapping element format according to a first embodiment of the invention. The TID-To-Link Mapping element 400 described in this Figure 4a is based on the TID-To-Link Mapping element 300 described previously in Figure 3a. The field and subfields identified with a same reference number in both figures remain unchanged and will not be described again in this Figure 4a. The format of the TID-to-link mapping Control field 440 is defined in reference to Figure 4b. The Link Mapping Of All TIDs / Link Mapping Of TID 0 field 470 behaves as follows: - if the Link Mapping Mode subfield 442 is set to 3: the Link Mapping Of All TIDs / Link Mapping Of TID 0 field 470 corresponds to the Link Mapping Of TID 0 field and indicates the link(s) on which frames belonging to TID 0 are allowed to be sent (i.e., carries a bitmap of the links to which the TID 0 is mapped to). A value of 1 in bit position i (where / = 0,1.....7 if the Link Mapping Size subfield 445 is set to 1, and / =0,1,..., 14 otherwise) of the Link Mapping Of TID 0 field indicates that TID 0 is mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. A value of 0 in bit position i indicates that the TID 0 is not mapped to the link associated with the link ID / for the direction specified in the Direction subfield 341. - If the Link Mapping Mode subfield 442 is set to 1: the Link Mapping Of All TIDs I Link Mapping Of TID 0 field 470 corresponds to the Link Mapping Of All TIDs field and indicates the link(s) on which frames belonging to any TID are allowed to be sent (i.e., carries a bitmap of the links to which all TIDs are mapped to). A value of 1 in bit position i (where / =0,1,..., 7 if the Link Mapping Size subfield 445 is setto 1, and i= 0, 1,.... 14 otherwise) of the Link Mapping Of All TIDs field indicates that all TIDs are mapped to the link associated with the link ID i for the direction specified in the Direction subfield 341. A value of 0 in bit position / indicates that no TID is mapped to the link associated with the link ID / for the direction specified in the Direction subfield 341 (i.e., if the Direction subfield is set to 2, the link is disabled). In the embodiment where the TID-To-Link mapping modes are defined exclusively, the bitmap of the links to which all TIDs are mapped to should be set to a value where at least one of the bits corresponding to one of the setup links is set to 0. The latter requirement is not needed in the variant where the TID-To-Link mapping modes are defined inclusively. - if the Link Mapping Mode subfield 442 is set to 0 (i.e., corresponding to the Default TID-to-link mapping), the Link Mapping Of All TIDs I Link Mapping Of TID 0 field 470 is not present. The Link Mapping Of TID nfield (where n = 1,..., 7) 471-477 behaves as the Link Mapping Of TID n field (where n = 1, ..., 7) 371-377 described previously in Figure 3a with the exception indicated hereafter. If the Link Mapping Mode subfield 442 is setto 0 (i.e., corresponding to the Default TID-to-link mapping) or 1, the Link Mapping Of TID n field (where n = 1, .... 7) 471-477 are not present. Figure 4b illustrates a modified TID-To-Link Control field format of the modified TID-To-Link Mapping element 400 according to the first embodiment of the invention. The TID-To-Link Control field 440 described in this Figure 4b is based on the TID-To-Link Control field 340 described previously in Figure 3b. The field and subfields identified with a same reference number in both figures remain unchanged and will not be described again in this Figure 4b. The Link. Mapping Mode subfield 442 corresponds to the TID-to-link mapping mode to be applied by the TID-To-Link Mapping element 400. As a variant naming, this subfield 442 may also be called TID-to-link mapping Mode. The indexing used here below for this Link Mapping Mode subfield 442 is a preferred indexing; however, other indexing or different ordering may be contemplated. - The Link Mapping Mode subfield 442 is set to 0 if the TID-To-Link Mapping element 400 represents the TID-to-link mapping mode 0 (Default TID-to-link mapping). If this setting is used, the Link Mapping Presence Bitmap subfield 447, Link Mapping Of All TIDs / Link Mapping Of TID 0 470 and Link Mapping Of TID n (with n = 1,..., 7) 471-477 are not present. - The Link Mapping Mode subfield 442 is set to 1 if the TID-To-Link Mapping element 400 represents the TID-to-link mapping mode 1. If this setting is used, the Link Mapping Of All TIDs I Link Mapping Of TID 0 470 is present and corresponds to Link Mapping of All TIDs. If this setting is used, the Link Mapping Presence Bitmap subfield 447 and Link Mapping Of TID n (with n = 1, ..., 7) 471-477 are not present. - The value 2 of the TID-To-Link Mode subfield 442 is not used in this embodiment. - The Link Mapping Mode subfield 442 is set to 3 if the TID-To-Link Mapping element 400 represents the TID-to-link mapping mode 3. If this setting is used, the Link Mapping Presence Bitmap 447 is present. The Link Mapping Size subfield 445 is set to 1 if the length of the Link Mapping Of All TIDs I Link Mapping Of TID 0 field 470 and the length of Link Mapping Of TID n field (with n = 1, ..., 7) 471-477 is 1 octet and is set to 0 if the length of the Link Mapping Of All TIDs I Link Mapping Of TID 0 field 470 and the length of Link Mapping Of TID n field (with n = 1, .... 7) 471-477 is 2 octets. The Reserved subfield 446 corresponds to the reserved bit not assigned, it is 1 bit length. The Link Mapping Presence Bitmap subfield 447 behaves as the Link Mapping Presence Bitmap subfield 347 describes previously in Figure 3b with the exception indicated hereafter. If the Link Mapping Mode subfield 442 is set to 0 (i.e., corresponding to the Default TID-to-link mapping) or 1, this subfield is not present. As a summary, the following features may be considered in the first embodiment: a new proposed signalling applies to both Advertised and Negotiated TTLM procedures; in the TTLM element, 2 fields / subfields are modified: o in TID-To-Link Control field, the Default Link Mapping subfield (1 bit) is replaced by a Link Mapping Mode subfield (2 bits); o The Link Mapping Of TID 0 field is replaced by a Link Mapping of All TIDs / Link Mapping Of TID 0 field; the new proposed signalling is based on the value set for the new Link Mapping Mode subfield. From the above description of the first embodiment of the present invention according to Figures 4a and 4b, it can be noticed that the new TID-To-Link Mapping element 400: - provides a signalling method well adapted for a Default TID-to-link mapping case (TID-to-link mapping mode 0); - provides a signalling method well adapted for a TID-to-link mapping case where all TIDs are not all mapped to a same link set (TID-to-link mapping mode 3); - and furthermore, provides an appropriate and efficient signalling method for a TID-to-link mapping where all TIDs are mapped to a same link set (TID-to-link mapping mode 1). Indeed, with this optimized signalling, only Link Mapping of All TIDs subfield 470 is present in the TID-To-Link Mapping element 400 and indicates a same link set for all TIDs at a time (i.e., bitmap is no more repeated). In an implementation where the Mapping Switch Time field 350 is present (i.e., advertised TID-To-Link mapping is not yet established), the Expected Duration field 360 is present and the size of a Link Mapping field is 2 Octets (as signalled in Link Mapping Size subfield 445), and considering that here the TID-To-Link Mapping Control field 440 has a size of 1 octet (subfield 447 not present), the signalling method according to Figures 4a and 4b requires a transmission of only 9+1*2=11 octets. This represents an overhead reduction of (10+8*2) - (9+1*2) = 15 Octets compared to the current TID-To-Link Mapping element 300 described previously in Figures 3a and 3b. Such overhead reduction is of interest, especially when the TID-To-Link Mapping element 400 is included in a Beacon frame. These overhead gains could be even higher when 2 TTLM elements are sent in a Beacon or Probe response frame and when the advertised TTLM is transmitted by several affiliated APs. In the description of Figures 4a and 4b, the TID-To-Link Mapping Element 400 and the TID-To-Link Mapping Control field 440 have been described considering the TTLM modes defined exclusively. A similar description may be contemplated considering the TTLM modes defined inclusively. The Figures 5a, 5b and 5c described hereafter represent a second embodiment of the present invention. In this second embodiment, it is assumed that the TID-to-link mapping mode 3 is no more considered. As example, this second embodiment is well adapted for a TID-to-link mapping advertised in a Beacon or Probe response frame. In draft IEEE P802.11 be / D3.2, an affiliated AP MLD that advertises a TID-to-link mapping shall map all TIDs to the same link set in the advertised TID-to-link mapping, both for DL and UL. It is thus advantageous to consider only TID-to-link mapping modes 0 and 1 according to embodiments of the invention for signalling an advertised TID-to-link mapping. This second embodiment can then be dedicated for an advertised TID-to-link mapping in a Beacon or Probe response frame, thus addressing the overhead problem raised in the present invention which is critical for such frames. In addition, for a negotiated TID-to-link mapping, the embodiments described previously with reference for example to Figures 3a, 3b or to Figures 4a, 4b may still be applied. Figure 5a illustrates a modified TID-To-Link Mapping element format according to a second embodiment of the invention. The TID-To-Link Mapping element 500 described in this Figure 5a is based on the TID-To-Link Mapping element 300 described previously in Figure 3a. The field and subfields identified with a same reference number in both figures remain unchanged and will not be described again in this Figure 5a. The format of the TID-to-link mapping Control field 540 is defined according to two implementation variants 540a and 540b described with reference to Figures 5b and 5c, respectively. The Link Mapping Of All TIDs / Link Mapping Of TID 0 field 570 corresponds to the Link Mapping Of All TIDs field when the TID-To-Link Mapping element 500 is transmitted by an AP affiliated with an AP MLD in a Beacon or Probe Response frame. Otherwise, the Link Mapping Of All TIDs / Link Mapping Of TID 0 field 570 corresponds to the Link Mapping Of TID 0 field. When the TID-To-Link Mapping element 500 is transmitted in a Beacon or Probe Response frame, the Link Mapping Of All TIDs field 570 behaves as follows: - If the Default Link Mapping subfield 542b is set to 0 or, in a variant, if the Link Mapping Mode subfield 542a is set to 1, the Link Mapping Of All TIDs field 570 indicates the link(s) on which frames belonging to any TID are allowed to be sent (i.e., carries a bitmap of the links to which all TIDs are mapped to). A value of 1 in bit position i (where / = 0,1,.... 7 if the Link Mapping Size subfield 445 is set to 1, and / = 0,1, ..., 14 otherwise) of the Link Mapping Of All TIDs field indicates that all TIDs are mapped to the link associated with the link ID / for the direction as specified in the Direction subfield 341. A value of 0 in bit position i indicates that no TID is mapped to the link associated with the link ID / forthe direction as specified in the Direction subfield 341 (i.e., if the Direction subfield is set to 2, the link is disabled). In the embodiment where the TID-To-Link mapping modes are defined exclusively, the bitmap of the links to which all TIDs are mapped to should be set to a value where at least one of the bits corresponding to one of the setup links is set to 0, meaning that at least one of the setup links is disabled. The latter requirement is not needed in the variant where the TID-To-Link mapping modes are defined inclusively. When the Link Mapping Mode subfield 542b is set to 0 or, in a variant, when the Default Link Mapping subfield 542a is set to 1, no Link Mapping Of TID n field (with n= 1,..., 7) 371-377 is present. - If the Default Link Mapping subfield 542b is set to 1 or, in a variant, when the Link Mapping Mode subfield 542a is set to 0 (i.e., corresponding to the Default TID-to-link mapping), the Link Mapping Of All TIDs field 570 and the Link Mapping Of TID n fields (with n = 1, ..., 7) 371-377 are not present. When the TID-To-Link Mapping element 500 is transmitted in other frames than Beacon or Probe Response frame, the Link Mapping Of TID n field (where n = 0, 1, .... 7) 570, 371-377 indicates the link(s) on which frames belonging to TID n are allowed to be sent (i.e., carries a bitmap of the links to which the TID n is mapped). A value of 1 in bit position i (where / = 0,1,..., 7 if the Link Mapping Size subfield 545 is set to 1, and / =0, 1, ..., 14 otherwise) of the Link Mapping Of TID n (where n = 0, 1, .... 7) field 570, 371-377 indicates that TID n is mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. A value of 0 in bit position i indicates that the TID n is not mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. When the Default Link Mapping subfield 542b is set to 1 or, in a variant when the Link Mapping Mode subfield 542a is set to 0, no Link Mapping Of TID n (where n = 0, 1, ..., 7) field 570, 371-377 is present. Figure 5b illustrates a modified TID-To-Link Mapping Control field format of the modified TID-To-Link Mapping element 500 according to a second embodiment of the invention. The TID-To-Link Mapping Control field 540a described in this Figure 5b is based on the TID-To-Link Mapping Control field 340 described previously in Figures 3b. The field and subfields identified with a same reference number in both figures remain unchanged and will not be described again in this Figure 5b. The Link Mapping Mode subfield 542a corresponds to the TID-to-link mapping mode to be applied by the TID-To-Link Mapping element 500. As a variant naming, this subfield 542a could be also called TID-To-Link Mapping Mode. The indexing used here below for this Link Mapping Mode subfield 542a is a preferred indexing; however, other indexing could be contemplated. When the TID-To-Link Mapping element 500 is transmitted in a Beacon or Probe Response frame, the Link Mapping Mode subfield 542a behaves as follows: - The Link Mapping Mode subfield 542a is set to 0 if the TID-To-Link Mapping element 500 represents the TID-to-link mapping mode 0 (Default TID-to-link mapping). If this setting is used, the Link Mapping Of All TIDs field 570, the Link Mapping Of TID n field (with n = 1, ..., 7) 371-377 and the Link Mapping Presence Bitmap subfield 347 are not present. - The Link Mapping Mode subfield 542a is set to 1 if the TID-To-Link Mapping element 500 represents the TID-to-link mapping mode 1. If this setting is used, the Link Mapping Of All TIDs field 570 is present and the Link Mapping Of TID n field (with n = 1, .... 7) 371-377 is not present. If this setting is used, the Link Mapping Presence Bitmap subfield 347 may be not present or may be present with a predefined value (e.g., value 10000000 (x80) or 00000001 (x01)) to indicate the presence of Link Mapping Of All TIDs field 570. When the TID-To-Link Mapping element 500 is transmitted in other frames than Beacon or Probe Response frame, the Link Mapping mode subfield 542a is set to 0 if the TID-To-Link Mapping element represents the default TID-To-Link Mapping (TTLM). Otherwise, the subfield is set to 1. The Link Mapping Size subfield 545 is set to 1 if the length of the Link Mapping Of All TIDs I Link Mapping Of TID 0 field 570 and the length of Link Mapping Of TID n field (with n = 1, ..., 7) 371-377 is 1 octet and is set to 0 if the length of the Link Mapping Of All TIDs / Link Mapping Of TID 0 field 570 and the length of Link Mapping Of TID n field (with n = 1, .... 7) 371-377 is 2 octets. When the TID-To-Link Mapping element 500 is transmitted in other frames than Beacon or Probe Response frame, the Link Mapping Presence Bitmap subfield 347 indicates which of the Link Mapping Of TID n fields (with n = 0, ..., 7) 570, 371-377 are present in the TID-To-Link Mapping element 500 (i.e., the subfield identifies the TID(s) for which the mapping is provided in the element). A value of 1 in bit position n of the Link Mapping Presence Bitmap subfield 347 indicates that the Link Mapping OfTIDnfield 570, 371-377 is present in the TID-To-Link Mapping element 500. Otherwise, the Link Mapping Of TID n field is not present in the TID-To-Link Mapping element. When the TID-To-Link Mapping element 500 is transmitted in a Beacon or Probe Response frame, the Link Mapping Presence Bitmap subfield 347 may not be present or may be present with a predefined value (e.g., x80) to indicate the presence of the Link Mapping Of All TIDs field 570. If the Link Mapping mode subfield 542a is set to 0, the Link Mapping Presence Bitmap subfield 347 is not present. Figure 5c illustrates a modified TID-To-Link Mapping Control field format of the modified TID-To-Link Mapping element 500 according to a variant of the second embodiment of the invention. The TID-To-Link Mapping Control field 540b described in this Figure 5c is based on the TID-To-Link Mapping Control field 340 described previously in Figure 3b. The field and subfields identified with a same reference number in both figures remain unchanged and will not be described again in this Figure 5c. When the TID-To-Link Mapping element 500 is transmitted in a Beacon or Probe Response frame, the Default Link Mapping subfield 542b behaves as follows: The Default Link Mapping subfield 542b is set to 1 if the TID-To-Link Mapping element 500 represents the default TID-to-link mapping, i.e., corresponding to TID-to-link mapping mode 0. If this setting is used, the Link Mapping Of All TIDs field 570, the Link Mapping Of TID n field (with n = 1,..., 7) 371-377 and the Link Mapping Presence Bitmap subfield 347 are not present. The Default Link Mapping subfield 542b is set to 0 if the TID-To-Link Mapping element 500 represents a TlD-to-link mapping where all TIDs are mapped to the same link set (where the link set may be a subset of setup links), i.e., corresponding to TID-to-link mapping mode 1. If this setting is used, the Link Mapping Of All TIDs field 570 is present and the Link Mapping Of TID n field (with n = 1,..., 7) 371-377 is not present. If this setting is used, the Link Mapping Presence Bitmap subfield 347 may not be present or may be present with a predefined value (e.g., value 10000000 (x80) or 00000001 (x01)) to indicate the presence of Link Mapping Of All TIDs field 570. When the TID-To-Link Mapping element 500 is transmitted in other frames than Beacon or Probe Response frame, the Default Link Mapping subfield 542b is set to 1 if the TID-To-Link Mapping element represents the default TTLM. Otherwise, the subfield is set to 0. The Link Mapping Size subfield 545 is set to 1 if the length of the Link Mapping Of All TIDs / Link Mapping Of TID 0 field 570 and the length of Link Mapping Of TID n field (with n = 1, .... 7) 371-377 is 1 octet and is set to 0 if the length of the Link Mapping Of All TIDs / Link Mapping Of TID 0 field 570 and the length of Link Mapping Of TID n field (with n = 1, .... 7) 371-377 is 2 octets. When the TID-To-Link Mapping element 500 is transmitted in other frames than Beacon or Probe Response frame, the Link Mapping Presence Bitmap subfield 347 indicates which the Link Mapping Of TID n fields (with n = 0, ..., 7) 570, 371-377 are present in the TID-To-Link Mapping element 500 (i.e., the subfield identifies the TID(s) for which the mapping is provided in the element). A value of 1 in bit position n of the Link Mapping Presence Bitmap subfield 347 indicates that the Link Mapping Of TID n field 570, 371-377 is present in the TID-To-Link Mapping element 500. Otherwise, the Link Mapping Of TID n field is not present in the TID-To-Link Mapping element. When the TID-To-Link Mapping element 500 is transmitted in a Beacon or Probe Response frame, the Link Mapping Presence Bitmap subfield 347 may not be present or may be present with a predefined value (e.g., x80) to indicate the presence of the Link Mapping Of All TIDs field 570. When the Default Link Mapping subfield 542b is set to 1, the Link Mapping Presence Bitmap subfield 347 is not present. As a summary, the following features may be considered in the second embodiment: a new proposed signalling applies to Advertised TTLM procedure only, no change for the Negotiated TTLM procedure; in the TTLM element, one field is modified: o the Link Mapping Of TID 0 field is replaced by a Link Mapping of All TIDs I Link Mapping Of TID 0 field; the new proposed signalling is based on a specific signalling operation for the Advertised TTLM procedure, i.e., when the TTLM element is transmitted in a Beacon or Probe Response frame by an AP affiliated with an AP MLD (this signalling operation relies on the fact that, in an advertised TTLM, all TIDs are mapped to the same link set); the definition of TID-To-Link Mapping modes 0, 1, 2, 3 is not mandatory in this second embodiment. To illustrate advantages of embodiments of the invention, the table below highlights the gain in terms of overhead reduction. 802.1 Ibe draft 3.2 (2B / 1B Link Mapping Size) First embodiment (2B / 1B Link Mapping Size) Second embodiment (2B / 1B Link Mapping Size) Comments Advertised TTLM mode 1 10B+ 8*2B / 1OB + 8*1B 9B + 1*2B / 9B + 1*1B 10B+ 1*2B / 10B+ 1*1B 1st emb. gain = 15B / 8B 2nd emb. gain = 14B / 7B Negotiation TTLM mode 1 5B + 8*2B / 5B + 8*1B 4B + 1*2B / 4B + 1*1B 5B + 8*2B / 5B + 8*1B 1st emb. gain = 15B / 8B 2nd emb. gain = 0B / 0B The following assumptions are considered in constructing the above table: TTLM mode 1 corresponds to the TTLM case where all TIDs are mapped to the same link set; In the Advertised TTLM mode 1 row, an advertised TTLM not yet established is considered (i.e., Mapping Switch Time and Expected Duration fields are present); In the table, the gains are estimated for a Beacon or Probe Response frame including one TTLM element and transmitted by one affiliated AP. These overhead gains could be even higher when 2 TTLM elements are sent in a Beacon or Probe response frame and when the advertised TTLM is transmitted by several affiliated APs. In the description of Figures 5a, 5b and 5c, the TID-To-Link Mapping Element 500 and the TID-To-Link Mapping Control fields 540a and 540b have been described considering the TTLM modes defined exclusively. A similar description may be contemplated considering the TTLM modes defined inclusively. Figures 7a and 7b described hereafter represent a third embodiment of the present invention. This third embodiment is similar to the second embodiment described previously in reference to the Figures 4a and 4b except that a TID-to-link mapping mode 2 is defined. According to the third embodiment of the invention, TID-to-link mapping mode 2 is defined and the use of this defined mode in the TID-To-Link Mapping element is described. Forthe TID-To-Link Mapping mode 2, the third embodiment introduces and describes a signalling method where the TIDs belonging to a same Access Category (AC) are mapped to a same link mapping bitmap subfield, thus reducing the size of the TID-To-Link Mapping element. It is recalled that an IEEE 802.11 station gathers TIDs within ACs as follows: - TID1 and TID2 are mapped in ACO used typically for background traffic; - TIDO and TID3 are mapped in AC1 used typically for best effort traffic; - TID4 and TID5 are mapped in AC2 used typically for video traffic; and - TID6 and TID7 are mapped in AC3 used typically for voice traffic. The TID-To-Link Mapping mode 2 is defined as follows. The TID-To-Link Mapping mode 2 corresponds to the mapping of at least the TIDs belonging to a same AC to at least one of the setup links, for both DL and UL. The TID-To-Link mapping mode 2 is supported by an MLD that has set the TID-To-Link Mapping Negotiation Support subfield in the MLD Capabilities And Operations subfield of the Basic Multi-Link element to a value >2. A non-AP MLD associated with an AP MLD may operate under this mode if the following condition is met: a TTLM negotiation corresponding to this mode 2 occurred, was successful and is not tom down. In this third embodiment, the TID-To-Link Mapping modes 0, 1 and 3 remain the same as defined previously according to embodiments of the present invention. Figure 7a illustrates a modified TID-To-Link Mapping element format according to a third embodiment of the invention. The TID-To-Link Mapping element 700 described in Figure 7a is based on the TID-To-Link Mapping element 300 described previously in Figure 3a. The field and subfields identified with a same reference number in both figures remain unchanged and will not be described again in Figure 7a. The format of the TID-to-link mapping Control field 740 is defined in reference to the Figure 7b. The Link Mapping Of All TIDs / Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 field 770 behaves as follows: - if the Link Mapping Mode subfield 742 is set to 3: the Link Mapping Of All TIDs / Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 field 770 corresponds to the Link Mapping Of TID 0 field and indicates the link(s) on which frames belonging to TID 0 are allowed to be sent (i.e., carries a bitmap of the links to which the TID 0 is mapped to). A value of 1 in bit position i (where 7=0, 1, ..., 7 if the Link Mapping Size subfield 745 is set to 1, and / = 0, 1, ..., 14 otherwise) of the Link Mapping Of TID 0 field indicates that TID 0 is mapped to the link associated with the link ID / forthe direction as specified in the Direction subfield 341. A value of 0 in bit position i indicates that the TID 0 is not mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. - If the Link Mapping Mode subfield 742 is set to 2: the Link Mapping Of All TIDs I Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 field 770 corresponds to the Link Mapping Of TIDs 1,2 field and indicates the link(s) on which frames belonging to TID 1,2 are allowed to be sent (i.e., carries a bitmap of the links to which the TIDs 1,2 are mapped to). A value of 1 in bit position i (where / =0,1, ..., 7 if the Link Mapping Size subfield 745 is set to 1, and / = 0, 1, ..., 14 otherwise) of the Link Mapping Of TIDs 1,2 field indicates that TIDs 1,2 are mapped to the link associated with the link ID / forthe direction as specified in the Direction subfield 341. A value of 0 in bit position i indicates that the TIDs 1,2 are not mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. - If the Link Mapping Mode subfield 742 is set to 1: the Link Mapping Of All TIDs I Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 field 770 corresponds to the Link Mapping Of All TIDs field and indicates the link(s) on which frames belonging to any TIDs are allowed to be sent (i.e., carries a bitmap of the links to which all TIDs are mapped to). A value of 1 in bit position i (where / = 0, 1, ..., 7 if the Link Mapping Size subfield 745 is set to 1, and / = 0, 1, ..., 14 otherwise) of the Link Mapping Of All TIDs field indicates that all TIDs are mapped to the link associated with the link ID iforthe direction as specified in the Direction subfield 341. A value of 0 in bit position i indicates that no TID is mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. The bitmap of the links to which all TIDs are mapped to should be set to a value where at least one of the bits corresponding to one of the setup links is set to 0. - If the Link Mapping Mode subfield 742 is set to 0 (i.e., corresponding to the Default TID-to-link mapping), the Link Mapping Of All TIDs I Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 field 770 is not present. The Link Mapping Of TIDs 0,3 (or 4,5 or 6,7) / Link Mapping Of TID 1 (or 2 or 3) field 771 (or 772 or 773) behaves as follows: - if the Link Mapping Mode subfield 742 is set to 3: the Link Mapping Of TIDs 0,3 (or 4,5 or 6,7) / Link Mapping Of TID 1 (or 2 or 3) field 771 (or 772 or 773) corresponds to the Link Mapping Of TID 1 (or 2 or 3) field and indicates the link(s) on which frames belonging to TID 1 (or 2 or 3) are allowed to be sent (i.e., carries a bitmap of the links to which the TID 1 (or 2 or 3) is mapped to). A value of 1 in bit position i (where i=0,1,..., 7 if the Link Mapping Size subfield 745 is set to 1, and / =0,1,..., 14 otherwise) of the Link Mapping Of TID 1 (or 2 or 3) field indicates that TID 1 (or 2 or 3) is mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. A value of 0 in bit position i indicates that the TID 1 (or 2 or 3) is not mapped to the link associated with the link ID i for the direction as specified in the Direction subfield 341. - If the Link Mapping Mode subfield 742 is set to 2: the Link Mapping Of TIDs 0,3 (or 4,5 or 6,7) I Link Mapping Of TID 1 (or 2 or 3) field 771 (or 772 or 773) corresponds to the Link Mapping Of TIDs 0,3 (or 4,5 or 6,7) field and indicates the link(s) on which frames belonging to TIDs 0,3 (or 4,5 or 6,7) are allowed to be sent (i.e., carries a bitmap of the links to which the TIDs 0,3 (or 4,5 or 6,7) are mapped to). A value of 1 in bit position i (where / =0,1,..., 7 if the Link Mapping Size subfield 745 is set to 1, and / = 0, 1, ..., 14 otherwise) of the Link Mapping Of TIDs 0,3 (or 4,5 or 6,7) field indicates that TIDs 0,3 (or 4,5 or 6,7) are mapped to the link associated with the link ID / for the direction as specified in the Direction subfield 341. A value of 0 in bit position / indicates that the TIDs 0,3 (or 4,5 or 6,7) are not mapped to the link associated with the link ID / for the direction as specified in the Direction subfield 341. - If the Link Mapping Mode subfield 742 is set to 0 (i.e., corresponding to the Default TID-to-link mapping) or 1, the Link Mapping Of TIDs 0,3 (or 4,5 or 6,7) I Link Mapping Of TID 1 or 2 or 3) field 771 is not present. The Link Mapping Of TID n field (where n = 4.....7) 774-777 behaves as the Link Mapping Of TID n field (where n = 4, ..., 7) 374-377 described previously in Figure 3a with the exception indicated hereafter; if the Link Mapping Mode subfield 742 is set to 0 (i.e., corresponding to the Default TID-to-link mapping), 1 or 2, the Link Mapping Of TID n fields (where n = 4, ..., 7) 774-777 not present. Figure 7b illustrates a modified TID-To-Link Control field format of the modified TID-To-Link Mapping element 700 according to the third embodiment of the invention. The TID-To-Link Control field 740 described in this Figure 7b is based on the TID-To-Link Control field 340 described previously in Figure 3b. The field and subfields identified with a same reference number in figures remain unchanged and will not be described again in this Figure 7b. The Link Mapping Mode subfield 742 corresponds to the TID-to-link mapping mode to be applied by the TID-To-Link Mapping element 700. As a variant naming, this subfield 742 may also be called TID-to-link mapping Mode. The indexing used here below for this Link Mapping Mode subfield 742 is a preferred indexing; however, other indexing or different ordering may be contemplated. - The Link Mapping Mode subfield 742 is set to 0 if the TID-To-Link Mapping element 700 represents the TID-to-link mapping mode 0 (Default TID-to-link mapping). If this setting is used, the Link Mapping Presence Bitmap subfield 747, Link Mapping Of All TIDs / Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 770, Link Mapping Of TIDs 0,3 / Link Mapping Of TID 1 771, Link Mapping Of TIDs 4,5 / Link Mapping Of TID 2 772, Link Mapping Of TIDs 6,7 / Link Mapping Of TID 3 773 and Link Mapping Of TID n (with n = 4, ..., 7) 774-777 are not present. - The Link Mapping Mode subfield 742 is set to 1 if the TID-To-Link Mapping element 700 represents the TID-to-link mapping mode 1. When this setting is used, Link Mapping Of All TIDs / Link Mapping Of TID 1,2 / Link Mapping Of TID 0 770 is present and corresponds to Link Mapping of All TIDs. When this setting is used, the Link Mapping Presence Bitmap subfield 747, the Link Mapping Of TIDs 0,3 I Link Mapping Of TIDs 1 771, Link Mapping Of TIDs 4,5 / Link Mapping Of TID 2 772, Link Mapping Of TID 6,71 Link Mapping Of TID 3 773 and Link Mapping Of TID n (with n = 4, ..., 7) 774-777 are not present. - The Link Mapping Mode subfield 742 is set to 2 if the TID-To-Link Mapping element 700 represents the TID-to-link mapping mode 2. When this setting is used, the Link Mapping Presence Bitmap 747 is present. When this setting is used, the Link Mapping Of All TIDs I Link Mapping Of TID 1,2 I Link Mapping Of TID 0 770, the Link Mapping Of TID 0,3 / Link Mapping Of TID 1 771, Link Mapping Of TID 4,5 / Link Mapping OfTID2 772, Link Mapping Of TID 6,7 / Link Mapping Of TID 3 773 may be present and correspond respectively to Link Mapping of TIDs 1,2, Link Mapping of TIDs 0,3, Link Mapping of TIDs 4,5 and Link Mapping of TID 6,7. When this setting is used, the Link Mapping of TID n (with n = 4,..., 7) fields 774-777 are not present. - The Link Mapping Mode subfield 742 is set to 3 if the TID-To-Link Mapping element 700 represents the TID-to-link mapping mode 3. When this setting is used, the Link Mapping Presence Bitmap 747 is present. When this setting is used, the Link Mapping Of All TIDs I Link Mapping Of TID 1,2 I Link Mapping Of TID 0 770, the Link Mapping Of TID 0,3 / Link Mapping Of TID 1 771, Link Mapping Of TID 4,5 I Link Mapping Of TID 2 772, Link Mapping Of TID 6,7 / Link Mapping Of TID 3 773 may be present and correspond respectively to Link Mapping of TID 0, Link Mapping of TID 1, Link Mapping of TID 2 and Link Mapping of TID 3. When this setting is used, the Link Mapping of TID n (with n = 4, ..., 7) fields 774-777 may be present. The Link Mapping Size subfield 745 is set to 1 if: the length of the Link Mapping Of All TIDs I Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 field 770, the length of the Link Mapping Of TIDs 0,3 / Link Mapping Of TID 1 field 771, the length of the Link Mapping Of TIDs 4,5 / Link Mapping Of TID 2 field 772, the length of the Link Mapping Of TIDs 6,7 I Link Mapping Of TID 3 field 773, and the length of Link Mapping Of TID n field (with n = 4, ..., 7) 774-777 is 1 octet. The Link Mapping Size subfield 745 is set to 0 if: the length of the Link Mapping Of All TIDs I Link Mapping Of TIDs 1,2 / Link Mapping Of TID 0 field 770, the length of the Link Mapping Of TIDs 0,3 I Link Mapping Of TID 1 field 771, the length of the Link Mapping Of TIDs 4,51 Link Mapping Of TID 2 field 772, the length of the Link Mapping Of TIDs 6,7 I Link Mapping Of TID 3 field 773, and the length of Link Mapping Of TID n field (with n = 4, ..., 7) 774-777 is 2 octets. If the Link Mapping Mode subfield 742 is set to 3, the Link Mapping Presence Bitmap subfield 747 indicates which the Link Mapping Of TID n fields are present in the TID-To-Link Mapping element 700 (i.e., the subfield identifies the TID(s) for which the mapping 770-777 is provided in the element). A value of 1 in bit position n of the Link Mapping Presence Bitmap subfield 747 indicates that the Link Mapping Of TID n field is present in the TID-To-Link Mapping element 700. Otherwise, the Link Mapping Of TID n field is not present in the TID-To-Link Mapping element 700. If the Link Mapping Mode subfield 742 is set to 2, the Link Mapping Presence Bitmap subfield 747 indicates which the Link Mapping Of TIDs 1,2, Link Mapping Of TIDs 0,3, Link Mapping Of TIDs 4,5 and Link Mapping Of TIDs 6,7 fields 770, 771, 772 and 773 are present in the TID-To-Link Mapping element 700 (i.e., the subfield identifies the TIDs of an AC for which the mapping 770-773 is provided in the element). A value of 1 in bit position 0 (or 1 or 2 or 3) of the Link Mapping Presence Bitmap subfield 747 indicates that the Link Mapping Of TIDs 1,2 (or 0,3 or 4,5 or 6,7) field 770 (or 771 or 772 or 773) is present in the TID-To-Link Mapping element 700. Otherwise, the Link Mapping Of TIDs 1,2 (or 0,3 or 4,5 or 6,7) field is not present in the TID-To-Link Mapping element 700. If the Link Mapping Mode subfield 742 is set to 2, the bits positions 4, 5, 6 and 7 of the Link Mapping Presence Bitmap 747 may be set to 0. If the Link Mapping Mode subfield 742 is set to 0 (i.e., corresponding to the Default TID-to-link mapping) or 1, the Link Mapping Presence Bitmap 747 subfield is not present. Figure 6 schematically illustrates a communication device 600, typically any of the MLDs discussed above, of a wireless network, configured to implement at least one embodiment of the present invention. The communication device 600 may preferably be a device such as a microcomputer, a workstation or a light portable device. The communication device 600 comprises a communication bus 613 to which there are preferably connected: a central processing unit 601, such as a processor, denoted CPU; a memory 603 for storing an executable code of methods or steps of the methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing the methods; and at least two communication interfaces 602 and 602’ connected to the wireless communication network, for example a communication network according to one of the IEEE 802.11 family of standards, via transmitting and receiving antennas 604 and 604’, respectively. Preferably the communication bus 613 provides communication and interoperability between the various elements included in the communication device 600 or connected to it. The representation of the bus is not limiting and in particular the central processing unit is operable to communicate instructions to any element of the communication device 600 directly or by means of another element of the communication device 600. 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 602 or 602’, in order to be stored in the memory of the communication device 600 before being executed. In an embodiment, the device is a programmable apparatus which uses software to implement embodiments of the invention. However, alternatively, embodiments of the present invention may be implemented, totally or in partially, in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC). Although the present invention has been described hereinabove with reference to specific embodiments, the present invention 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 invention. 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 invention, 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 for wireless communication performed by a multi-link device (MLD), the method comprising:sending, to a peer MLD, a traffic identifier-to-link (TID-To-Link) Mapping element for links setup or to be setup between the MLD and the peer MLD;wherein the TID-To-Link Mapping element comprises a bitmap for the mapping of at least two TIDs and specifying a set of links to which the at least two TIDs are mapped.

2. The method of claim 1, wherein the TID-To-Link Mapping element is included in a Request frame.

3. The method of claim 2, wherein the Request frame is an (Re)Association Request frame.

4. The method of claim 3, further comprising receiving a (Re)Association Response frame comprising a TID-To-Link Mapping element including an updated bitmap.

5. The method of claim 1, wherein the TID-To-Link Mapping element is advertised by an AP affiliated with the MLD and is included in a beacon or probe response frame.

6. The method of claim 1, wherein the bitmap maps part of defined TIDs on all the setup links.

7. The method of claim 1, wherein the bitmap maps all defined TIDs on a subset of the setup links.

8. The method of claim 1, wherein the TID-To-Link Mapping element further includes a Link Mapping Mode subfield capable of indicating the presence of the bitmap mapping at least two TIDs.

9. The method of any one of claims 1 to 8, wherein the MLD is a non-access point (non-AP) MLD and the peer MLD is an AP MLD.

10. The method of any one of claims 1 to 8, wherein the MLD is an access point (AP) MLD and the peer MLD is a non-AP MLD.

11. The method of any one of claims 1 to 10, wherein the MLD is an initiating MLD and the peer MLD is a responding MLD.

12. The method of any one of claims 1 to 10, wherein the MLD is a responding MLD and the peer MLD is an initiating MLD.

13. The method of claim 1, wherein the links to which the at least two TIDs are mapped are a subset of the setup links.

14. The method of claim 1, wherein the bitmap maps two TIDs belonging to a same Access Category (AC).

15. The method of claim 8, wherein the Link Mapping Mode subfield signals one or more mapping modes, including at least one of:a mode, referred to as a default mapping mode, for signalling that all TIDs are mapped to all setup links for both uplink (UL) and downlink (DL);a mode for signalling that at least two TIDs are mapped to the same link set;a mode for signalling that all TIDs are mapped to the same link set for both DL and UL, where the link set is a subset of setup links;a mode for signalling the presence of a bitmap for mapping each TID to a set of one or more links; ora mode for signalling that TIDs are mapped by pairs to sets of one or more links.

16. The method of any one of claims 1 to 15, wherein the TID-To-Link Mapping element includes a field comprising the bitmap mapping the at least two TIDs.

17. A computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to any one of claims 1 to 16, when loaded into and executed by the programmable apparatus.

18. A non-transitory computer-readable storage medium storing instructions of a computer program for implementing a method according to any one of claims 1 to 16.

19. A multi-link device (MLD) comprising a memory and processing circuitry configured to send, to a peer MLD, a traffic identifier-to-link (TID-To-Link) Mapping element for links setup or to be setup between the MLD and the peer MLD, wherein the TID-To-Link Mapping element comprises a bitmap for the mapping of at least two TIDs and specifying a set of links on which the at least two TIDs are mapped.

20. A traffic identifier-to-link (TID-To-Link) Mapping element for links setup or to be setup between a multi-link device (MLD) and a peer MLD, wherein the TID-To-Link Mapping element comprises a bitmap for the mapping of at least two TIDs and specifying a set of links on which the at least two TIDs are mapped.

21. The TID-To-Link Mapping element of claim 20 further comprising a Link Mapping Mode subfield for signalling a mode of the mapping.

22. The TID-To-Link Mapping element of claim 20 or 21 further comprising a field carrying the bitmap mapping the at least two TIDs.