Methods, devices, and computer programs for obfuscating traffic indication elements in a BSS

Abstract

A communication method for a wireless local area network (WLAN), e.g. Wi-Fi, the method comprising, in an apparatus, e.g. Access Point (AP), transmitting a beacon frame comprising an element indicatin

Description

FIELD OF THE DISCLOSURE The present disclosure relates to wireless communications and more specifically to user privacy during wireless communications. BACKGROUND OF DISCLOSURE 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. Today, the evolution of wireless systems has brought privacy concerns at the forefront, driven by user demand and requirements of the General Data Protection Regulation (GDPR). The global wireless industry is faced with the growing need to protect users’ personally identifiable information from increasingly sophisticated user tracking and user profiling activities, while continuing to improve wireless services and the user experience. In particular, the Media Access Control (MAC) address of a user’s device constitutes a piece of data that can be used to track this user. Indeed, the access points (APs) of wireless networks can monitor the locations of mobile devices (tablets, laptops, mobile phones, etc.) of a user without his / her consent, by means of their MAC addresses. This is because mobile phones are configured to discover surrounding access points to wireless networks. As a user moves, his / her mobile phone sends requests to determine if there are any access points nearby, these requests identifying the mobile phone which sends these requests and including in particular the MAC address of the mobile phone. Access points that hear these requests can respond. In the context of Wi-Fi networks as defined by IEEE 802.11 standards (Wi-Fi is a trademark), this procedure is called Probe Request / Response exchange. So even when a mobile phone is not connected to a Wi-Fi network, surrounding access points may receive its MAC address. It is then possible to track a user by reconstructing his / her trajectory from access points to which his / her mobile phone has sent its MAC address. In addition, if the mobile phone has been associated with one of the access points (i.e., the user has connected to an associated Wi-Fi network through that access point) and the user has provided personal identification information (name, place of residence, etc.) in the past, the access point may have recorded in a database the MAC address of the phone in association with the items of identification information. Therefore, even if the user is not connected to the Wi-Fi network, these items of identity information could be recovered by comparing the MAC address contained in a Probe Request to the MAC address used for the past association. In the context of Wi-Fi networks, a solution has been proposed by the IEEE 802.11 working group to limit the risk of a user being tracked, and consists in dynamically modifying the MAC address of the user device. This mechanism is called Randomized and Changing MAC (RCM) procedure. It has been originally introduced as a privacy enhancing feature in the 802.11aq Pre-Association Service Discovery Task Group and finally included in the standard IEEE Std 802.11-2020. It comprises periodical change of the MAC address of a non-AP station or STA (i.e., a station which is not an access point) to a random value, while the non-AP station is not associated with a network (or, equivalently, with an access point). The non-AP station may construct the randomized MAC address from the locally administered address space as defined in IEEE Std 802®-2014 and IEEE Std 802c™-2017. More specifically, a new Management Information Base (MIB) variable controllable by an external management entity has been specified. This variable is called ‘dotHMACPrivacyActivated’. When dotHMACPrivacyActivated is set to “true”, the non-AP station can apply specific mechanisms for enhancing the privacy at MAC level, including RCM. The MAC address, or EUI-48 address, of a device is an Extended Unique Identifier (EUI) composed of 48 bits. It can be administered universally or locally. A universally administered address is uniquely assigned to the device by the manufacturer. On the contrary, a locally administered address is assigned to the device by software or a network administrator, and replaces the physical burned-in address. The second-least significant bit of the first octet of the MAC address, i.e., the seventh bit of the first octet of the address, also referred to as “U / L bit” (for “Universal / Local bit”), indicates whether it is universally (when set to 0) or locally (when set to 1) administered. The least significant bit of the first octet of the MAC address, i.e., the eighth bit of the first octet of the address, also referred to as “l / G bit” (for “Individual / Group bit”), indicates whether the frame is sent to only one receiving device (when set to 0, indicating unicast transmission) or to a plurality of devices (when set to 1, indicating multicast transmission). When the RCM mechanism is operated in the non-AP station, the MAC address of the non-AP station is randomly changed (for instance periodically). More specifically, the U / L bit is set to 1, the l / G bit is set to 0, and the remaining 46 bits are randomly generated by using a pseudorandom function (PRF), or can be obtained by any other means (for instance received from AP, or taken in a predefined list of addresses). Unfortunately, RCM is not sufficient to preserve privacy of a device in view of advanced technics of data traffic analysis that create a kind of digital fingerprint of the devices. To overcome this issue, a set of Enhanced Data Privacy (EDP) parameters that would allow an eavesdropper to fingerprint a device has been identified. Among these EDP parameters, identifiers like the MAC address or the Association Identifier (AID) are of course the most important ones, but other parameters like the Sequence Number (SN) or the Packet Number (PN) present in the non-encrypted part of the data frames are also listed. The inventors have observed that the MAC address is not the only identifier present in clear in frames exchanged between stations of a wireless LAN, and an eavesdropper may capture other elements that uniquely identify emitting stations. In particular, one of these unique identifiers is the Association Identifier (AID). This identifier, that is assigned by an access point upon association with a station, is communicated in an Association Response frame sent by the access point in the last frame exchange of the association process. The AID is a 16 bit identifier that is locally unique. This means that two stations associated with two different Basic Service Sets (BSSs) of an AP station may have the same AID value, but two stations associated with the same BSS shall not share the same AID value. The AID has been originally introduced to reduce the signalling overhead when an AP wants to identify a station for delivery of buffered frames when power-saving is enabled. The AID is advantageously used in replacement of the MAC address (that is 48 bit long) when an AP indicates in Traffic Indication Messages or in Multi Link Traffic information element that it has some traffic waiting for transmission to a station in power save mode and in sleep state. Since the power save mode in a persistent mode among long period of time, and because only a sub part of the stations associated to an AP are in power save mode, being in power save mode is a clearly differentiating point compared to other stations. Unfortunately, the Traffic Indication Map (TIM) or Multi-Link Traffic (MLT) elements are transmitted in clear in beacon frames sent regularly over the air, and any eavesdropper can analyse its content to determine if a station is in the power save mode or not. Therefore, in addition to the modification of the MAC address and the associated AID to maintain user privacy and avoid tracking, it is necessary to also hide the fact that a given device is in power save mode since this information can be used as an element of a digital fingerprint of a station. While recent solutions have been proposed to solve this issue, relying on complex encryption of TIM element, the associated drawbacks, that comprise a lack of retro compatibility with legacy stations and a high cost of complexity and overhead, make these solutions unapplicable in practice. There is thus still an ongoing need to improve user privacy and communication efficiency. SUMMARY OF THE DISCLOSURE It is a broad aspect of the present disclosure to provide methods, devices, and computer programs for obfuscating traffic indication elements in a BSS. According to some embodiments, the stations in a power save mode, for which data are buffered in the AP to which they are associated, are hidden among real stations in a power save mode, for which data are buffered in this AP, and real and / or fake stations in a power save mode, for which fake data are buffered in this AP. According to a first aspect, it is provided a communication method for a wireless network, the method comprising, in an apparatus: transmitting a beacon frame comprising an element indicating (i) data buffered in relation to an association identifier, AID, assigned to a real station in a power save mode, for which data are buffered in the apparatus and (ii) data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode, for which no data are buffered in the apparatus. Accordingly, the method of the invention improves user privacy by making station identification more complex, in particular when these stations are in a power save mode and change of identity. According to particular embodiments, indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly. Still according to particular embodiments, indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is based on indicating data buffered in relation to an AID assigned to the real station in a power save mode, for which data are buffered in the apparatus. Still according to particular embodiments, a first AID is assigned to the real station until a transition time and a second AID is assigned to the real station after the transition time, indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus comprises indicating data buffered in relation to the second AID before the transition time and / or comprises indicating data buffered in relation to the first AID after the transition time. Still according to particular embodiments, the AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly among a set of Al Ds not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus. Still according to particular embodiments, the AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly among a set of Al Ds not assigned to a real station. Still according to particular embodiments, the set of AIDs comprises all possible AIDs not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly among AIDs not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus. Still according to particular embodiments, the set of AIDs is a subset of all possible AIDs not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus, the subset being bounded by a lowest and a highest values of AIDs assigned to real stations. Still according to particular embodiments, the method further comprises determining an obfuscation ratio as a ratio of Al Ds for which buffered data are to be indicated, but having no data buffered in the apparatus, to Al Ds assigned to real stations in the power save mode and for which data are buffered in the apparatus. Still according to particular embodiments, the method further comprises selecting at least one AID not assigned to a real station and indicating, in the element of the beacon frame, data buffered in relation to the at least one AID. Still according to particular embodiments, the method further comprises updating a traffic table, the traffic table being representative of indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus. Still according to particular embodiments, the traffic table comprises counters representative of a number of beacon frames data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus are to be indicated. Still according to particular embodiments, the method further comprises updating an internal traffic indication bitmap as a function the traffic table. Still according to particular embodiments, the method further comprises generating a partial virtual bitmap from the internal traffic indication bitmap. Still according to particular embodiments, the element is a Traffic Indication Map, TIM, element ora Multi-Link Traffic, MLT, element. Still according to particular embodiments, the AID assigned to the real station is assigned to the real station at association with an access point, AP, of the apparatus. Still according to particular embodiments, the apparatus is an access point, AP, or a set of affiliated APs. At least parts of the methods according to the disclosure may be computer implemented. Accordingly, the present disclosure 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 disclosure 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 disclosure can be implemented in software, the present disclosure 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 Embodiments of the disclosure 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 network system in which some embodiments of the disclosure may be implemented; Figure 2 illustrates an example of a representation of an internal traffic indication bitmap stored in an access point; Figures 3a and 3b illustrate the frame format of a TIM element as described in the IEEE 802.11 standard family; Figures 4a, 4b, and 4c illustrate the frame format of a Multi-Link Traffic Indication element as described in the IEEE 802.11 standard family; Figure 5 illustrates an example of data that may be stored in an AP, in addition to an internal traffic indication bitmap, to make it possible to obfuscate traffic indication elements according to some embodiments of the disclosure; Figure 6 illustrates an example of a sequence of steps performed by an access point implementing some embodiments of the disclosure, during the generation of a partial bitmap for buffered traffic indication; Figures 7a, 7b, and 7c illustrate examples of a sequence of steps performed by an access point implementing some embodiments of the disclosure, for updating a list of fake power save stations; Figures 8, 9, and 10 illustrate three sequences of values of bits, set into the partial bitmap along time, according to some embodiments of the disclosure; and Figure 10 schematically illustrates an example of a communication device configured to implement at least some embodiments of the present disclosure. DETAILED DESCRIPTION OF THE DISCLOSURE According to some embodiments of the disclosure, the list of stations of a given BSS, that are in a Power Save (PS) mode, is obfuscated by indicating fake buffered data (also referred to as fake buffered traffic) in the traffic indication element sent from an AP station to non-AP stations, e.g., in clear over the air, without modifying the indication for the real buffered data. In other words, some bits of the traffic indication element of a Traffic Indication Map (TIM) element or of a Mult-Link Traffic (MLT) Indication element, for stations that do not have any buffered data on the AP, are set to one to misleadingly indicate buffered traffic. The stations corresponding to these bits set to one, without having real buffered data, are referred to as fake power save stations (or PS stations). These stations may be real stations associated with the AP or virtual stations that do not exists, for which the AP misleadingly indicates buffered data. According to some embodiments of the disclosure, an access point manages a list of the associated stations that are currently in power save mode. This may be done by carrying steps such as the ones described by reference to Figure 7b, by updating the list when a station indicates, to its associated AP, a change of its Power Save (PS) mode. Using this list, the AP manages a table such as traffic table 500 described by reference to Figure 5, containing a second list of stations, e.g., list 505, for which the AP misleadingly indicates a fake power saving activity according to different parameters that may also be included in the table. According to some embodiments, the table comprises a counter associated with each fake PS station (e.g., counters 515 associated with fake PS stations 505), to provide the number of beacons that misleadingly indicates traffic for this fake station. The fake buffered traffic of a fake station may be determined in several ways, for example randomly or by reproducing the buffered traffic of a station which is really in the PS mode. In such a case, the table may indicate the AID (e.g., AID 510 in Figure 5) of the station that is really in PS mode and which behaviour is mimic by the fake PS station (in such a case the fake PS station indicates the same buffered traffic than the station having the given AID). This table may be updated according to the steps described by reference to Figures 6 and 7a to 7c. As described hereafter, different strategies may be used to obfuscate the list of stations in PS mode, for example to update a traffic table containing a list of stations for which the AP misleadingly indicates a fake power saving activity. During the process of creating the Partial bitmap (i.e., a portion of an internal traffic indication bitmap, that is encoded and sent to associated stations to inform them about buffered data), the AP updates its internal traffic indication bitmap according to the traffic table to set virtual buffered traffic (or fake buffered traffic) to the fake PS stations. Then, using this updated internal bitmap, the AP encode a portion of it and creates the elements such as the ones described by reference to Figures 3a and 4a, that are to be included in the next transmitted beacon. It is important to note here that according to the IEEE 802.11 family, there exist several different ways to encode the content of a portion of an internal traffic indication bitmap, as described by reference to Figure 5, into a field transmitted thru different frames (beacon frames, or dedicated action frames for instance). All the different ways of encoding a portion of the internal traffic indication bitmap into a TIM element, that are well known and out of the scope of the present disclosure, are not disclosed. For the sake of clarity, the disclosure is directed to managing a partial virtual bitmap as described by reference to Figure 3a, it being noted that as mention by reference to step 615 in Figure 6, all other encoding methods (hierarchical, or by block for instance), as described in the TIM element specification of the IEEE 802.11 standard, being compatible with the disclosure since the latter updates the internal traffic indication bitmap, for example at step 610 in Figure 6, before encoding apportion of it for transmission. Figure 1 illustrates an example of a network system in which some embodiments of the disclosure may be implemented. For the sake of illustration, Figure 1 represents an 802.11 network (i.e., a Wi-Fi network) system 100 comprising four wireless devices: an access point station (AP) 105 and three non-AP stations (STAs) 110a, 110b, and 110c. The AP station and the non-AP stations may be an AP multi-link device (MLD) and non-AP MLDs, respectively. Of course, the number of non-AP stations 110a, 110b, and 110c may be different from three. AP station 105 provides wireless connections between non-AP stations 110a, 110b, 110c and a wider network, such as the Internet (not represented). The connection of one of non-AP station 110a, 110b, and 110c to AP 105 may be performed by a standardized process called association. Once a non-AP station is associated with the AP station, the non-AP station can send data to the network and receive data from the network through the AP station. AP station 105 may comprise, be implemented as, or known as a Node B, Radio Network Controller (RNC), evolved Node B (eNB), 5G Next generation base station (gNB), Base Station Controller (BSC), Base Transceiver Station (BTS), Base Station (BS), Transceiver Function (TF), Radio Router, Radio Transceiver, Basic Service Set (BSS), Extended Service Set (ESS), Radio Base Station (RBS), or some other terminology. It can be a standalone product or it may be integrated in a device, for instance in a broadband remote access server (BRAS). Non-AP stations 110a, 110b, and / or 110c may comprise, be implemented as, or known as a subscriber’s station, a subscriber unit, a mobile station (MS), a remote station, a remote terminal, a user terminal (UT), a user agent, a user’s device, a user equipment (UE), a user station (STA), or some other terminology. In some implementations, a non-AP station may be or may comprise a cellular telephone, a cordless telephone, a Session Initiation Protocol (SIP) phone, a wireless local loop (WLL) station, a personal digital assistant (PDA), a handheld device having wireless connection capability, or some other suitable processing device connected to a wireless modem. Accordingly, one or more aspects taught herein may be incorporated into a phone (e.g., a cellular phone or a smartphone), a computer (e.g., a laptop), a tablet, a portable communication device, a portable computing device (e.g., a personal data assistant), an entertainment device (e.g., a music or video device, or a satellite radio), a global positioning system (GPS) device, or any other suitable device that is configured to communicate via a wireless or wired medium. In some aspects, some of non-AP stations 110a, 110b, and 110c may be wireless nodes. Such a wireless node may provide, for example, connectivity for or to a network (e.g., a wide area network such as the Internet or a cellular network) via a wired or wireless communication link. AP station 105 manages a set of stations that together organize their accesses to the wireless medium for communication purposes. All the stations (AP station 105 and non-AP stations 110a, 110b, and 110c) form a service set, which may be referred to as basic service set, BSS (although other terminology can be used). It is noted that AP station 105 may manage more than one BSS: each BSS is thus uniquely identified by a specific basic service set identifier (BSSID) and managed by a separate virtual AP station implemented in physical AP station 105. Figure 2 illustrates an example of a representation of an internal traffic indication bitmap 200, stored in and managed by an access point, that reflects the presence of buffered data (or buffered traffic) for stations in PS mode. According to this representation, each bit represents the status of a possible AID. Therefore, the internal traffic indication bitmap is composed of a number of bits corresponding to the maximum number of possible AID values. For example, the first AID, corresponding to index 205, may have the value 0 and the last AID, corresponding to index 210, may have the value 2007. The status of an AID, within internal traffic indication bitmap 200, indicates whether or not traffic is buffered for the corresponding station (i.e., the station having this AID). For example, the value 0 may indicate that there are no buffered data and the value 1 may indicate that there are buffered data. A station having buffered data is a station in a PS mode. For the sake of illustration, the station having the AID with index 215 is in the PS mode. A representation of a portion of internal traffic indication bitmap 200 may be sent by the AP to the associated stations so that each station may determine whether there are buffered data intended for it, by using its AID. Since only a subset of the possible AIDs is used, a simple way of transmitting a representation of internal traffic indication bitmap 200 comprises determining the portion of internal traffic indication bitmap 200 starting from the first index that bit is set to 1 and ending with the last index that bit is set to 1. Such portion may be referred to as the partial traffic indication bitmap. Since the internal traffic indication bitmap is generally handled as a set of bytes of contiguous bits representing the status of the AIDs, the partial traffic indication bitmap may start from the first byte of internal traffic indication bitmap comprising a bit set to 1 (e.g., the byte starting at index 220) and ends with the last byte comprising a bit set to 1 (e.g., the byte ending at index 225). According to some embodiments of the disclosure, the internal traffic indication bitmap is updated by the AP before the transmission of a TIM element (e.g., at step 610 in Figure 6), during the preparation phase of this transmission, and after the transmission of the TIM element (e.g., at step 640 in Figure 6). Figures 3a and 3b illustrate the frame format of a TIM element as described in the IEEE 802.11 standard family, a TIM element being used to signal the timing and availability of buffered data for associated stations. As illustrated in Figure 3a (corresponding to Figure 9-149 of IEEE Std 802.11™-2020, Part 11), TIM element 300 comprises an element identifier (Element ID having the value 5), the length of the TIM element, a DTIM Count, a DTIM Period, a Bitmap Control, and a Partial Virtual Bitmap, DTIM standing for delivery traffic indication map. As set forth in the IEEE 802.11 standard, the DTIM Count field indicates how many Beacon frames (including the current frame) appear before the next DTIM. A DTIM count of 0 indicates that the current TIM is a DTIM. The DTIM Period field indicates the number of beacon intervals or short beacon intervals between successive DTIMs. If all TIMs are DTIMs, the DTIM Period field has the value 1. According to a particular configuration (in particular when TIM is carried in a non-S1G PPDU), the first bit (Bit 0) of the Bitmap Control contains the traffic indication virtual bitmap bit associated with AID 0 and the remaining 7 bits of the field form the Bitmap Offset, as shown in Figure 3b (corresponding to Figure 9-150 of IEEE Std 802.11™-2020, Part 11). When the TIM is carried in a non-S1G PPDU, the internal traffic indication bitmap consists of 2008 bits, organized into 251 octets, each bit representing the traffic status of a possible AID, as described by reference to Figure 2. The Partial Virtual Bitmap is derived from the traffic indication virtual bitmap, and sent to the associated stations in TIM elements, to inform each of the associated stations about the corresponding traffic status. As described in the IEEE 802.11 standard (e.g., IEEE Std 802.11™-2020, Part 11), the Partial Virtual Bitmap may be encoded using known methods to reduce its size. Figures 4a, 4b, and 4c illustrate the frame format of a Multi-Link Traffic (MLT) Indication element as described in the IEEE 802.11 standard family, that is used to signal the timing and availability of buffered data for associated stations, per link. As illustrated in Figure 4a (corresponding to Figure 9-1074ay of IEEE Std 802.11be™ / D7.0, Part 11, Amendment 2), Multi-Link Traffic Indication element 400 comprises an element identifier (Element ID), the length of the Multi-Link Traffic Indication element, an element identifier extension (Element ID Extension), a Multi-Link Traffic Indication Control, and a Per-Link Traffic Indication List. Multi-Link Traffic Indication element 400 provides information similar to the one of TIM element 300 in Figure 3a, for each link. The Multi-Link Traffic Indication Control field is illustrated in Figure 4b (corresponding to Figure 9-1074az of IEEE Std 802.11be™ / D7.0, Part 11, Amendment 2) and the Per-Link Traffic Indication List field is illustrated in Figure 4c (corresponding to Figure 9-1074ba of IEEE Std 802.11be™ / D7.0, Part 11, Amendment 2). IEEE Std 802.11be™ / D7.0, Part 11, Amendment 2 describes in detail the Multi-Link Traffic Indication element. Figure 5 illustrates an example of data that may be stored in an AP, in addition to an internal traffic indication bitmap, to make it possible to obfuscate traffic indication elements according to some embodiments of the disclosure. Such additional data may be stored in a traffic table 500. According to some embodiments, traffic table 500 comprises a first row 505 storing Fake Traffic Al Ds. Each cell of the Fake Traffic AID row comprises an AID of a station (also referred to as a Fake AID) that indicates an AID corresponding to a fake station (i.e., an AID value corresponding to a virtual station that does not correspond to a real device) with fake traffic or that indicates an AID corresponding to a real station with fake traffic. Still according to some embodiments, traffic table 500 may comprise another row 510 comprising Real Traffic AID. Each cell of the Real Traffic AID row comprises an AID of a station (also referred to as a Real AID) corresponding to real station associated with the AP with real traffic. Still according to some embodiments, traffic table 500 may comprise another row comprising a beacon counter 515 associated with each Fake Traffic AID and / or Real Traffic AID, indicating the remaining number of beacons to be transmitted indicating a fake traffic for the corresponding Al Ds 505 or indicating a real traffic for the corresponding AIDs 510. According to some particular embodiments, and as illustrated with references 520, 525, and 530, a Real AID is set to 0 to indicate that a fake traffic is associated with an AID non assigned to an associated station or is associated with an associated station not in the power save mode. This is especially useful in some cases such as the one described by reference to Figure 8, to hide real stations, in the crowd of fake stations, that are in PS mode, or in some cases such as the one described by reference to Figure 9, to hide the change of the AID of a station with buffered traffic (at transition 900 in Figure 9). Still according to some particular embodiments, and as illustrated with references 535 and 540, both a Fake AID and a corresponding Real AID are set to a non-zero value. In these specific embodiments, one or more Fake Al Ds (two in the illustrated example) may correspond to a single Real AID, the Real AID being used as a model to create one or more doppelgangers of this real station in power save mode from a buffered traffic point of view. Similarly, several Real AIDs may correspond to a single Fake AID. In such a case, the Fake AID indicates traffic that is a combination, for example a logical OR operation, of the traffic of all the corresponding real stations. Still according to some particular embodiments, and as illustrated with reference 545, a Fake AID is set to 0 to indicate that this AID corresponding to the fake traffic is a Real AID. This makes it possible for the AP to signal a fake traffic to an existing station. This is especially useful in some cases such as the one described by reference to Figure 8 to hide the stations in PS mode in the crowd of stations. It is observed that all these embodiments can be carried out independently or in combination. Figure 6 illustrates an example of a sequence of steps performed by an access point implementing some embodiments of the disclosure, during the generation of a partial bitmap for buffered traffic indication. For the sake of illustration, these steps may be executed in preparation of the transmission of a beacon, especially a beacon containing a Traffic Indication Map (TIM) information element, or a basic Multi-Link (ML) information element including a Multi-Link Traffic (MLT) element, or containing both TIM and MLT elements. In a first step (step 600a), the AP updates a list of Fake PS stations to optimize the obfuscation of the list of Real stations in PS mode. This list of Fake PS stations corresponds, for example, to row 505 in traffic table 500 in Figure 5, that comprises the AIDs of all the Fake PS stations. The AP then applies its policy to obfuscate its internal traffic indication bitmap in prevision of the encoding of this internal traffic bitmap and its transmission, for instance in a partial virtual bitmap as illustrated in Figure 3a, or in one of the Per-Link Traffic Indication Bitmap illustrated in Figure 4c. Applying policies to obfuscate an internal traffic indication bitmap may comprise updating a list of Real PS stations, for example updating row 510 in traffic table 500 in Figure 5, that comprises the AIDs of all the Real PS stations in relation to Fake PS stations. According to some embodiments, and as illustrated in Figure 8, the AP determines an Obfuscation Ratio (OR) of the number of fake stations having a real AID (e.g., an AID set to 0 in row 505 in table 500 in Figure 5), that are already present in the list of Fake PS stations (i.e., the number of fake stations that are present to hide the real stations in PS mode, in the crowd of associated stations) to the number of associated stations (Real stations), in PS mode, having buffered traffic. Still according to some embodiments, the AP determines the Minimum Continuous (MC) Range of the values encompassing all the Real stations in PS mode, having buffered traffic. If the smallest AID value assigned to a station in PS mode having buffered traffic is AID1 (e.g., an AID value set to 51), and the largest AID value assigned to an associated station in PS mode having buffered traffic is AID2 (e.g., an AID value set to 125), the MC Range is [AID1, AID2] (e.g. [51,125]). This range can be determined as an octet range ([7,15] in our previous example), each boundaries of the MC range being equal to the Octet index including the value (7*8 <= 51 <8*8 , 15*8<= 125 <16*8, so in our example, the MC Range in octet is [7,15]), to prepare the partial bitmap creation. If the Obfuscation Ratio (OR = Nb fake ID / Nb STA in PS mode having buffered traffic) is lower than a given threshold (for example a threshold set to 3), the AP creates new Fake stations, for example by populating traffic table 500 in Figure 5 with additional column to reach the desired threshold. For each added column, the AP selects a nonassigned AID, set the Fake AID to this value, set the real AID to 0, and set the counter to a random value (determined in a predefined range defined by the system administrator (e.g., the range [3,9]). The selection of a new fake AIDs may be done from a list of non-assigned AIDs, from a list of non-assigned Al Ds included in the MC Range, or from a list of non-assigned AIDs of an AID range wider than the MC Range, including the MC range, if the MC range is not large enough to contain all the AIDs of the Real stations in PS mode having buffered traffic and of the required Fake AIDs. This variant using an AID range of potential values enables a more efficient encoding of the Partial bitmap (less bits are required), but makes easier the characterization of the Real stations in PS mode. Still according to some embodiments, the selection of Fake PS stations is carried out in a range of AIDs in the current virtual bitmap (i.e., the partial bitmap of the last transmitted TIM element). Such a range may be defined as follows: - for a TIM element, as illustrated in Figures 3a and 3b: [Bitmap Offset *8, 8*(Bitmap Offset + length)] for a MLT element, as illustrated in Figures 4a, 4b, and 4c: [AID Offset, AID Offset + 8 * Bitmap size] Next, in a following step (step 610), the AP obfuscates the internal bitmap by setting some bits to 1 even if there is no buffered traffic for the corresponding AIDs. This will drive the transmitted TIM element to contain false indication of buffered traffic that will mislead a potential eavesdropper while not impacting the normal power saving operations. As described above, the AP never set to 0 a bit that is set to 1, but only change to 1 a bit set to 0. This means that the AP can indicate additional fake buffered data, but that it always indicates the real buffered data. As a consequence, stations in PS mode having buffered data can be informed of this buffered traffic (the other ignoring the TIM elements). In some embodiments, the fake traffic is indicated on real stations that are not in PS mode. In this case, since these stations do not take into account the TIM elements, there is no impact on these stations. In some embodiments, the fake traffic is indicated in relation to virtual stations that are associated with non-assigned AID (meaning that no real stations are associated with the fake traffic). In such a case, there is no impact on the associated stations. In some embodiments, the fake traffic is indicated in relation to stations having AIDs corresponding stations in PS mode. In such a case, this creates an unnecessary action from these stations to try to recover data indicated as buffered data, and thus causes a slight loss of power, but does not cause any failure in the station behaviour. Still according to some particular embodiments, the AP reads, at step 610, a traffic table like traffic table 500 in Figure 5, to determine the AIDs that should indicate a fake traffic. For each non-empty column of the traffic table (i.e., for each corresponding cells of the Fake station and Real stations rows for which at least one cell is not empty), the AP set the bit of the corresponding index of the virtual bitmap to 1. According to different embodiments, this index may be: - the AID value of the Fake AID (e.g., row 505 in Figure 5) if the corresponding Real AID (e.g., row 510 in Figure 5) is set to 0. For the sake of illustration and considering reference 530 in Figure 5, the index is the value of AID3, - the AID value of the Real AID if the corresponding Fake AID is set to 0. Still for the sake of illustration and considering reference 545 in Figure 5, the index is the value of AID2. Still at step 610, and according to some other embodiments, the AP duplicates the buffered data status of a given real station in PS mode to one or more doppelganger stations. To do so, the AP may read a traffic table, for example traffic table 500 in Figure 5, and for a given column, if both the Fake AID and the Real AID are non-zero values, the AP set the bit corresponding to the AID value of the fake AID to the value of the bit corresponding to the Real AID value. Once the internal traffic bitmap is obfuscated, for example according to one or a combination of the previously described embodiments, the AP encodes the internal traffic indication bitmap into a TIM element to be transmitted (step 615). Such a step may be carried out according to different embodiments, following the different encoding options of the TIM as described within the 802.11 standard documents. While the encoding methods are out of scope of this disclosure, steps 620 and 630, corresponding to a common encoding scheme based on a Partial bitmap transmission, are described, for sake of completeness. Other encoding methods including, but not limited to, the ones described in the IEEE 802.11 REVme D6.0 document, in chapter 9.4.2.5 (TIM element), are also compatible with the present disclosure. Based on the internal traffic indication bitmap, as described by reference to Figure 2, the AP determines a Bitmap Offset (e.g., bitmap offset 220 in Figure 2), that corresponds to the lowest octet index associated with a non-0 value (step 620). According to the example illustrated in Figure 2, the lowest bit index associated with a non-0 value is the bit 215, and the lowest octet index 220 is the octet comprising the bit 215. Next (step 630), the AP creates a Partial Virtual Bitmap, as described by reference to Figure 3a or 4a, and store the bitmap offset in the corresponding field, as illustrated in Figures 3b and 4b. Next (step 640), the AP decrements by one all the counters (e.g., counters 515 in Figure 5) representing the number of beacons to be transmitted with a fake traffic indication (bit set to 1) for a given AID. When a counter of a given column elapses (i.e., when it reaches 0), the corresponding bit of the internal bitmap is set to 0. The corresponding bit is the bit of index equal to the Fake AID value if this value is non zero value. If the AID value of the column is equal to 0, the bit index is equal to the Real AID. The column is then removed, and no more fake traffic is signalled for the corresponding Real AID or Fake AID. Figures 7a, 7b, and 7c illustrate examples of a sequence of steps performed by an access point implementing some embodiments of the disclosure, for updating a list of fake PS stations. These sequences may be carried out in lieu of step 600 in Figure 6 or according to any combination (i.e., steps 600, 600a, 600b, and 600c may be combined according to any combination). Figure 7a illustrates an example of a sequence of steps performed by an access point implementing some embodiments of the disclosure when preparing an epoch transition toward a new epoch. According to some embodiments of the disclosure, the AP prepares a new MAC address, a new AID value, and potentially other new obfuscation values for a set of parameters used to obfuscate the real identity of one of its associated stations. These new values may be prepared in advance (for example with several beacon intervals) of a transition, from a previous identity to the new identity. Still according to some embodiments, and after having determined this new set of parameter values, the AP also updates a traffic table, for example traffic table 500 in Figure 5. As illustrated, the AP determines, in a first step (step 700), the list of Al Ds corresponding to associated stations in PS mode having actual buffered traffic. This step may be carried out by reading the internal traffic indication bitmap, for example internal traffic indication bitmap 200 in Figure 2, and by listing the index of the bits set to 1. Since these stations, that currently have buffered traffic, are likely to still have buffered traffic indicated after the transition, the AP may apply obfuscation means, during step 710, on the future new AID value to indicate buffered traffic before the new AID is actually assigned to the station (as illustrated in Figure 9 with transition 900). Such an anticipation of the presence of buffered traffic on a future assigned AID avoid an obvious correlation of an old identity and a new identity thru an easy correlation of an old Al D and a new Al D value at the transition, as described in the example of Figure 9, with reference to lines 905 and 906. Next (still during step 710), the AP may add a column to the traffic table, to indicate fake traffic, for each station listed at step 700. The added fake AID is set to the future AID value (e.g., AID1 indexed n+1 for AID1, according to the example provided in Figure 5, with reference 520), that is currently not yet assigned to a real station, and set the Real AID to 0. The initial value of the corresponding counter (e.g. counters 515 in Figure 5), upon creation of the column, may be set to a fixed value (e.g., 3), or advantageously to a random value selected in a specific range (e.g., [1,5]). This allows the AP, as illustrated with line 922 or 923 in Figure 9, to add a fake buffered traffic to the AID (n+1) before the transition happens. Figure 7b illustrates an example of a sequence of steps performed by an access point implementing some embodiments of the disclosure, when a power management mode negotiation occurs with a station. When a station associated with an AP wants to change its power management mode, to enter into the Power Save (PS) mode or to go back into the Active mode, the station should indicate it to the AP by setting the power management bit accordingly, in a frame transmitted to the AP. Upon acknowledgment by the AP of the change of Power Save mode, the AP updates the internal list of stations in Power save mode (step 720), by adding or removing the station from this list, and, according to some embodiments, updates the list of Fake PS stations (step 730). Updating the list of Fake PS stations may comprise removing all the doppelganger stations of the Real station going back to the Active mode. This may comprise modifying a traffic table, for example removing the columns of the traffic table wherein the AID of the Real station is set in the Real AID row, and wherein a non-zero value is set in the Fake AID row). Conversely, updating the list of Fake PS stations may comprise adding some doppelganger stations of the Real stations entering in the PS mode by adding a set of columns to the traffic table, for example by adding columns with the AID of the real station (for each concerned Real station) and setting the AID of the Real station in the Real AID row and setting an AID value, selected among the nonassigned AID values, in the Fake AID row. The selection of a new Fake AID value may be done according to the selection process described in step 600 in Figure 6. Figure 7c illustrates an example of a sequence of steps performed by an access point implementing some embodiments of the disclosure upon epoch transition. According to some embodiments of the disclosure, upon an epoch transition (e.g., transition 900 in Figure 9), the AP applies the new parameters (MAC address, AID values, and other parameters values) and next, updates the traffic table, for example traffic table 500 in Figure 5, according to the illustrated steps. As illustrated, the AP determines, in a first step (step 740), the list of Al Ds corresponding to the associated stations in the PS mode having actual buffered data. This step may be carried out by reading the internal traffic indication bitmap, for example internal traffic indication bitmap 200 in Figure 2, and by listing the index of the bits set to 1. Since it is likely that these stations, that have currently buffered traffic, also had previously buffered traffic (before the transition occurred), the AP applies obfuscation means, during step 750, on the previous Al D value, to indicate buffered traffic for a period of time after the new AID is assigned to the station. This continuation of the presence of traffic on the previous AID avoids an obvious correlation of an old identity and a new identity thru an easy correlation of an old AID and a new AID value at the transition, as described in the example illustrated in Figure 9, with lines 911 and 913. Next (still during step 750), the AP adds a column to the traffic table, to indicate fake traffic, for each station listed during step 740, wherein the Fake AID row is set to the previous AID value (e.g., AID1 indexed n for AID1 in the example of Figure 5, with reference 525), that is currently not assigned anymore to a real station, and the corresponding Real AID row is set to 0. The initial value of the corresponding counter (e.g., counters 515 in Figure 5), upon creation of the column, may be a fixed value (e.g., 4), or advantageously a random value selected in a specific range (e.g., [1,5]). This allows the AP to add a fake buffered traffic into the AID (n), after the change transition, as illustrated in Figure 9 with line 911 or 913. According to some embodiments, if some doppelganger stations have been created (e.g., as described with reference to Figure 7b), the AP removes the column corresponding to the old AID value, and creates new columns corresponding to doppelganger stations of the new AID value. In some non-specific embodiments, the old AID value is part of the created doppelganger stations (like in the example illustrated in Figure 10). Figure 8 illustrates a first sequence of values of bits, set into the partial bitmap, along time, according to some embodiments of the disclosure. For the sake of illustration, a transition, from an old identity to a new identity of a given device, occurs at time 800. As illustrated with references 805 and 806, the MAC address of the given device changes from MAC1(n) to MAC1(n+1) at time 800. Naturally, other parameters may change when the transition occurs, for example the AID of the given device may also change from AID1(n) to AID1(n+1) at time 800. In other words, the MAC address and the AID assigned to the given device, among other parameters, are MAC1(n) and AID1(n) before time 800 and MAC1(n+1) and AID1(n+1) after. As described above, an obfuscation is advantageously implemented to prevent a link being easily established by an eavesdropper between the device having the MAC1(n) and AID1(n) parameters and the device having the MAC1(n+1) and AID1(n+1) parameters. For the sake of illustration, references 810, 820, 830, and 840 and references 811, 821, 831, and 841 represent the values of the bits in the internal traffic indication bitmap with an index corresponding respectively to the AID1(n), AID2, AID1(n+1), and AID3, indicating with a black full rectangle the bits set to 1 (they are set to 0 otherwise), for two different situations. The black full rectangles indicate Real buffered traffic, Fake buffered traffic being indicated with hatched rectangles. References 810, 820, 830, and 840 represent the values of the bits according to the prior art while references 811, 821, 831, and 841 represent the values of the same bits in the internal traffic indication bitmap just before encoding it in a TIM element according to a particular embodiment of the disclosure, applied to hide the traffic assignment during the transition period 800. References 820 and 840 correspond to Al Ds that are not assigned to real stations. Therefore, and according to the prior art, there is no indication of any buffered traffic in relation to these Al Ds. Still for the sake of illustration, references 821 and 841 illustrate the result obtained by carrying out the steps described by reference to Figure 6, indicating the presence of Fake buffered data (as illustrated with hatched rectangles) for Al Ds that are not assigned to real stations. According to this embodiment, and as illustrated, the AP signals the presence of buffered data for the AID values AID2 and AID3, to hide the real traffic of AID(n) and AID (n+1) in the crowd of the stations. Figure 9 illustrates a second sequence of values of bits, set into the partial bitmap, along time, according to some embodiments of the disclosure. For the sake of illustration, and similarly to Figure 8, a transition, from an old identity to a new identity of a given device, occurs at time 900. As illustrated with references 905 and 906, the MAC address of the given device changes from MAC1(n) to MAC1(n+1) at time 900. Naturally, other parameters may change when the transition occurs, for example the AID of the given device may also change from AID1(n) to AID1(n+1) at time 900. In other words, the MAC address and the AID assigned to the given device, among other parameters, are MAC1(n) and AID1(n) before time 900 and MAC1(n+1) and AID1(n+1) after. For the sake of illustration, references 910 and 920, 911 and 921, 912 and 922, and 913 and 923 represent the values of the bits in the internal traffic indication bitmap with an index corresponding respectively to the AID1(n) and AID1(n+1), indicating with a black full rectangle the bits set to 1 (they are set to 0 otherwise), in four different situations. The black full rectangles indicate Real buffered traffic, Fake buffered traffic being indicated with hatched rectangles). References 910 and 920 represent the values of the bits according to the prior art while references 911 and 921,912 and 922, and 913 and 923 represent the values of the same bits in the internal traffic indication bitmap just before encoding it in a TIM element according to different embodiments of the disclosure, applied to hide the traffic assignment during the transition period 900. Still for the sake of illustration, reference 911 illustrates the result obtained by carrying out the steps described by reference to Figure 7c, indicating the presence of Fake buffered traffic for the old AID value AID1 (n) after the given device changed its AID value to the new values AID (n+1). According to this particular embodiment, the Fake buffered traffic is indicated in relation to the old AID value for an additional duration of consecutive beacons, denoted P and referenced 950, as illustrated with a hatched rectangle. It is noted that optionally, and to avoid issues with potential real traffic, the AP may refrain assigning the AID1(n) value to a new station for an exclusion duration of a number of consecutive beacons, denoted E and referenced 951. As illustrated, the buffered traffic 921 that is associated with AID1 (n+1) is similar to the one of the prior art (referenced 920). Still for the sake of illustration, reference 922 illustrates the result obtained by carrying out the steps described by reference to Figure 7a, according to which the AP indicates, when preparing the change of identity, a Fake buffered traffic for the future AID value AID1(n+1), for an Anticipated duration denoted A and referenced 960, as illustrated with a hatched rectangle. According to particular embodiments, and to avoid issues with real traffic, the AP may refrain to assign the AID1(n+1) to another station for an exclusion period denoted E and referenced 961, before the effective assignment of this AID1(n+1) value at time 900. As illustrated, the buffered traffic 912 that is associated with AID1(n) is similar to the one of the prior art (referenced 910). Still for the sake of illustration, references 913 and 923 illustrate the result obtained by carrying out the steps described by reference to Figures 7a and 7c, to indicates the presence of a Fake buffered traffic for the old AID value (AID1(n)) and a Fake buffered traffic for the future AID value (AID1(n+1)). Figure 10 illustrates a third sequence of values of bits, set into the partial bitmap, along time, according to some embodiments of the disclosure. For the sake of illustration, and similarly to Figure 8, a transition, from an old identity to a new identity of a given device, occurs at time 1000. As illustrated with references 1005 and 1006, the MAC address of the given device changes from MAC1(n) to MAC1(n+1) at time 1000. Naturally, other parameters may change when the transition occurs, for example the AID of the given device may also change from AID1(n) to AID1(n+1) at time 1000. In other words, the MAC address and the AID assigned to the given device, among other parameters, are MAC1(n) and AID1(n) before time 1000 and MAC1(n+1) and AID1(n+1) after. For the sake of illustration, the same change of identity from AID(n) to AID(n+1) is similar to the one illustrated in Figure 8, with the same Real buffered traffic. Still for the sake of illustration, and before the change of identity occurs at time 1000, the AP creates two doppelgangers, having here the AID values 26 and 28 (and referenced 1020 and 1030, respectively), of the AID (n) (having the AID value 24 in this example, referenced 1010). According to this example, the next AID value of the device (i.e., AID(n+1)) would correspond to doppelganger 1020 before the change. After the change of identity at time 1000, the AP creates two new doppelgangers of the new AID (n+1) that have respectively the values 24 and 28 in this example (referenced 1010 and 1030, respectively). According to this example, the old AID value AID(n), i.e., the value 24, is part of the doppelgangers 1010 and 1030 of the new AID value AID (n+1) 26, referenced 1020. Figure 11 schematically illustrates an example of a communication device that may correspond any of the stations described by reference to Figure 1, of a wireless network, configured to implement at least some embodiments of the disclosure. The communication device, referenced 1100, may preferably be a device such as a microcomputer, a workstation, or a light portable device. Communication device 1100 may comprise a communication bus 1113 to which may be connected: - a central processing unit 1101, such as a processor, denoted CPU; - a memory 1103, denoted MEM, 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 two communication interfaces 1102 and 1102’ 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 1104 and 1104’, respectively. Preferably, communication bus 1113 may provide communication and interoperability between the various elements included in the communication device 1100 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 1100 directly or by means of another element of the communication device 1100. 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 1102 or 1102’, in order to be stored in the memory 1103 of communication device 1100 before being executed. In some embodiments, communication device 1100 may be a programmable apparatus which uses software to implement embodiments of the disclosure. However, alternatively, some embodiments of the disclosure may be implemented, totally or in partially, in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC). Embodiment(s) of the disclosure can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a “non-transitory computer-readable storage medium”) to perform the functions of one or more of the above-described embodiment(s) and / or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and / or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard-disk, a random-access memory (RAM), a read-only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), etc.), a flash memory device, a memory card, and the like. Expressions such as “comprise”, “include”, “incorporate”, “contain”, “is” and “have” are to be construed in a non-exclusive manner when interpreting the description and its associated claims, namely construed to allow for other items or components which are not explicitly defined also to be present. Reference to the singular is also to be construed in be a reference to the plural and vice versa. A person skilled in the art will readily appreciate that various parameters disclosed in the description may be modified and that various embodiments disclosed may be combined without departing from the scope of the disclosure.

Claims

1. A communication method for a wireless network, the method comprising, in an apparatus:transmitting a beacon frame comprising an element indicating (i) data buffered in relation to an association identifier, AID, assigned to a real station in a power save mode, for which data are buffered in the apparatus and (ii) data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode, for which no data are buffered in the apparatus.

2. The method of claim 1, wherein indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly.

3. The method of claim 1 or claim 2, wherein indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is based on indicating data buffered in relation to an AID assigned to the real station in a power save mode, for which data are buffered in the apparatus.

4. The method of any one of claims 1 to 3, wherein a first AID is assigned to the real station until a transition time and a second AID is assigned to the real station after the transition time, indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus comprises indicating data buffered in relation to the second AID before the transition time and / or comprises indicating data buffered in relation to the first AID after the transition time.

5. The method of any one of claims 1 to 3, wherein the AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly among a set of AIDs not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus.

6. The method of any one of claims 1 to 3, wherein the AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly among a set of AIDs not assigned to a real station.

7. The method of claim 5 or claim 6, wherein the set of AIDs comprises all possible AIDs not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus is determined randomly among AIDs not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus.

8. The method of claim 5 or claim 6, wherein the set of AIDs is a subset of all possible AIDs not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus, the subset being bounded by a lowest and a highest values of AIDs assigned to real stations.

9. The method of any one of claims 1 to 8, further comprising determining an obfuscation ratio as a ratio of AIDs for which buffered data are to be indicated, but having no data buffered in the apparatus, to AIDs assigned to real stations in the power save mode and for which data are buffered in the apparatus.

10. The method of claim 9, further comprising selecting at least one AID not assigned to a real station and indicating, in the element of the beacon frame, data buffered in relation to the at least one AID.

11. The method of claim 9 or claim 10, further comprising updating a traffic table, the traffic table being representative of indicating data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus.

12. The method of claim 11, wherein the traffic table comprises counters representative of a number of beacon frames data buffered in relation to an AID not assigned to a station in the power save mode or assigned to a station in the power save mode for which no data are buffered in the apparatus are to be indicated.

13. The method of claim 11 or claim 12, further comprising updating an internal traffic indication bitmap as a function the traffic table.

14. The method of claim 13, further comprising generating a partial virtual bitmap from the internal traffic indication bitmap.

15. The method of any one of claims 1 to 14, wherein the element is a Traffic Indication Map, TIM, element or a Multi-Link Traffic, MLT, element.

16. The method of any one of claims 1 to 15, wherein the AID assigned to the real station is assigned to the real station at association with an access point, AP, of the apparatus.

17. The method of any one of claims 1 to 16, wherein the apparatus is an access point, AP, or a set of affiliated APs.

18. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a wireless apparatus, causes the wireless apparatus to perform the method according to any one of claims 1 to 17.

19. A wireless communication device comprising at least one microprocessor configured to carry out the method according to any one of claims 1 to 17.

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