arXiv:1912.06086v2 [cs.NI] 26 Nov 2020 0.1x[]adtecr ic fnx ii7 IEEE ecosystem 7, Gbps WiFi WiFi 30 the next of in throughput and solutions of disruptive peak PHY piece incorporate a IEEE new achieve and core of to a the aspires successor of 802.11be and the design [4] as the 802.11ax Considered address amendment. to MAC 2019 May making in now are networks. guaranteed [2], wireless and latency to failure, way end-to-end their equipment on to bounds extremely due congestion, upper buffer loss to packet due low loss whic packet sub-standards, zero TSN ensure Ethernet, framework for (TSN) intended networking Originally time-sensitive capabil- the new medium under as and Group well ities as (PHY) enhancements, Interest physical (MAC) control Technical promoting access are Application [1] Time initiative TIG) (RTA Real networks, the 802.11-based like IEEE within plications traf latency. time-sensitive low manage abl bounded to been with solution yet similar not a produce peak has to it constantly improve efficiency, to and been capacity, amendments throughput, high has an successive efficiency, WiFi its through cost evolving on Although better complexity. substantiated devices, reduced mainly of been mobility flexibility, has success wide operation. successful their communica- and, for latency bandwidth-demanding tions low high extremely applica very on occasionally, real-time depend strongly cutting-edge are which of tions vehicles, emergence unmanned intel- the and artificial fostering technological robotics, computing, latest automation, cloud ligence, the multimedia, by on advances motivated sectors, productive h otrpeettv o-aec s ae o ii7are 7 WiFi for cases of be requirements use could and low-latency reviewed. functionalities benefits representative TSN Finally, most how 7. then the discuss are WiFi to which in basis 802.11be, implemented the IEEE main of as articl features used its This key bands. of the ul spectrum introduces and one license-exempt time-sensi latency low include in support as to reliability to aims capabilities amendment 7 (TSN) WiFi networking 802.11be 7. future WiFi parts, IEEE ecosystem: constituent (WLAN) the network area With local succes wireless its designing the already in are groups working 802.11 IEEE EEP0.1eTs ru Tb)[]wscreated was [3] (TGbe) Group Task P802.11be IEEE ap- real-time emerging of requirements the address To world- WiFi 2000s, early the in emergence its Since heterogeneous of number increasing an years, recent In Abstract hr ieatrteofiilluc fWF 6, WiFi of launch official the after time Short — eateto nomto n omncto Technologies Communication and Information of Department .I I. ieSniieNtokn nIE 802.11be: IEEE in Networking Time-Sensitive NTRODUCTION Email: nteWyt o-aec ii7 WiFi Low-latency to Way the On oiAae acCracs,adBrsBellalta Boris and Carrascosa, Marc Adame, Toni { oiaae accracs,boris.bellalta marc.carrascosa, toni.adame, first e tive sor tra fic h d e s - . raiga pnevrnetfraysain(T)willing (STA) station any for environment open an creating eto Ipeet h bandcnlsosaddiscusses and conclusions The challenges. 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Section presents Section VI in in Section reviewed provided low are leverage are could communications that 7 latency cases use WiFi 7 enhancements WiFi in representative potential most the it and support TSN layers. to of MAC and description PHY brief of ma terms A the to in 802.11be describes WLANs IEEE III of current features Section of traffic. limitations time-sensitive access the handle wireless overviews II leading Section the of era. 6G one the as in technology concepts WiFi TSN keep delay of license- integration in bounded would and operation adoption offer and the nature to bands, own exempt able their whereas to be due Hence, never guarantees mechanisms. will TSN networks support of to WiFi used adoption be seamless can features a new its how discussing ment, IEEE in introduced profiles MAC-layer 802.15.4e. resu of specialized a spotlight as the the particularly of in networks, ago sensor deterministic long wireless low-power to since also support 5G. are of and communications capabilities techniques enhanced reduction the Latency for the areas of application ultra- one main defined as also (URLLC) In has communications ecosystem. low-latency body WiFi reliable standards the mobile in 3GPP only the not fact, and years, upcoming the while devices. 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MTTOSOF IMITATIONS TIME nvria opuFabra Pompeu Universitat , } @upf.edu - ESTV TRAFFIC SENSITIVE EE802.11 IEEE OHANDLE TO local of in s, lt - - l t to associate to the network. But at the same time, the wireless cusses to what extent they would help to satisfy low-latency medium is precisely the main cause that hinders proper requirements. In general terms, and following the traditional delivery of time-sensitive traffic, due to its variable capacity IEEE 802.11 evolution, IEEE 802.11be will adopt IEEE (which depends on the link quality) and typically higher 802.11ax contributions, further refining and extending them, PER (due to the stochastic properties of the channel and and adding some new features [10]. the presence of interference) [6]. As for the MAC layer, IEEE 802.11 has traditionally relied A. PHY layer on the distributed coordination function (DCF): a contention- The ongoing release of the 6 GHz band throughout the based random access scheme based on carrier sense and world will be of great benefit to WiFi dense scenarios, not exponential backoff rules. The main drawback of DCF, only due to the additional 1.2 GHz of available spectrum, however, is its non-predictable behavior and lack of traffic but also to the resulting interference reduction among net- prioritization techniques. In fact, in presence of multiple works/BSSs. The incorporation of the 6 GHz band into IEEE STAs, DCF may lead to channel saturation by contending 802.11be will also encompass channels as wide as 320 MHz, packets, thus being unable to guarantee timely data delivery. hence enabling higher transmission rates. The alternative point coordination function (PCF), based on As for the maximum number of spatial streams, it is a centralized polling system, has never been widely adopted. expected to double its number from 8 in IEEE 802.11ac/ax The enhanced distributed channel access (EDCA) was to 16 in IEEE 802.11be, thus further benefiting from funda- envisioned as part of the IEEE 802.11e amendment to extend mental advantages of predominantly indoor WiFi operation: DCF and provide quality of service support according to rich scattering, higher angular spreads, lower correlation, and 4 differentiated access categories (ACs): background, best diversity of channels with good propagation conditions. effort, video, and voice. Prioritization is then implemented The maximum supported size in IEEE by allocating different contention-related parameters to each 802.11be is likewise expected to be boosted with the adop- AC. Nevertheless, the low number of ACs, the lack of mech- tion of the 4096-QAM modulation, whose practical use, how- anisms for the prioritization of different streams belonging ever, will only be feasible in combination with . to the same AC, and (in some hardware devices) the use of All in all, new IEEE 802.11be PHY features favor low- a single buffer to store packets with different priorities are latency operation, as (1) wider available bandwidth results among the main EDCA shortcomings. in faster transmissions and (2) more spatial streams turn into To outperform IEEE 802.11e operation for real-time mul- higher rates in the single-user (SU) mode and into more timedia content delivery, IEEE 802.11aa introduced the intra- parallel transmissions (with less waiting time in the buffer) AC traffic differentiation functionality, with the definition in the multi-user (MU) mode. of two new time-critical voice and video ACs. However, in general, none of IEEE 802.11 mechanisms guarantee the quality of service of heterogeneous real-time streams B. MAC layer when a WLAN is overloaded [7]. In such cases, flexible Many significant MAC features from IEEE 802.11ax such scheduling policies and/or admission control algorithms are as MU-MIMO, OFDMA, and spatial reuse will be extended highly required to effectively manage different traffic flows. in IEEE 802.11be. The support of more spatial streams will Neighboring networks represent a key limitation to provide also enable more flexible MU-MIMO arrangements. How- low-latency guarantees in all the aforementioned channel ever, current explicit channel state information acquisition access methods. In dense scenarios, overlapping of basic procedure may not cope well with such high number of service set (BSS) coverage areas turns into large delays for antennas and, for that reason, TGbe is currently evaluating STAs waiting to access the channel. IEEE 802.11ax partially several alternatives to enhance explicit sounding, even con- addresses this issue by allowing concurrent transmissions sidering the introduction of an implicit procedure. under the spatial reuse scope, showing a clear gain for As for OFDMA, enhanced resource unit (RU) allocation time-sensitive communication [8]. A gain that could be schemes will allow to allocate multiple contiguous and non- remarkably boosted by means of coordination mechanisms contiguous RUs to a single STA. Consequently, these novel among neighboring APs. schemes could significantly increase spectral efficiency and And lastly, when it comes to the transport layer, the overall network throughput, and even better satisfy timely bufferbloat problem may prevent IEEE 802.11 networks from data delivery [11]. In fact, whether based on MU-MIMO or delivering time-sensitive traffic in presence of TCP flows, OFDMA, MU transmissions are key to reduce the channel due to the high latency produced by excessive buffering access latency, as packets from different users can be de- of packets. In fact, well-known techniques to mitigate this queued simultaneously. problem in wired networks (e.g., decreasing buffer sizes Multi-link operation will likely become the most repre- and/or applying modern queue management algorithms) have sentative feature of IEEE 802.11be, aiming to 1) improve proven low success in WiFi [9]. throughput by aggregating links, 2) enhance reliability by transmitting multiple copies of the same frame in separated III. IEEE 802.11BE links, and 3) decrease channel access delay by selecting the This section introduces the main technologies under dis- first available link in terms of latency [12]. As it can be seen cussion in TGbe for both PHY and MAC layers and dis- in Figure 1, having two active links operating at different bands between an AP and an STA may increase channel higher levels of determinism, thus facilitating the manage- access efficiency by enabling opportunistic link selection, ment of real-time deterministic traffic and the inclusion of , and multi-channel full duplex. TSN mechanisms. TGbe also considers multi-AP coordination, which con- sists in the cooperative use of neighboring APs in enterprise C. Standardization status IEEE 802.11be WLANs, as a way to improve overall per- The standardization process of IEEE 802.11be, initiated formance by means of different techniques: by TGbe in May 2019, consists of two stages: Release 1 • Coordinated spatial reuse (CSR) consists in jointly and 2, and it is expected to be completed in May 2024 with negotiating the transmission power of potential overlap- the publication of the final amendment. Release 1 is aimed to ping APs to reduce overall interference. Access delay prioritize the development of a small distinctive set of IEEE to the medium could be then reduced, since CSR allows 802.11be candidate features, such as the 320 MHz channels, to increase the number of concurrent transmissions. the 4096-QAM modulation, and the multi-link operation, • Coordinated OFDMA (Co-OFDMA) optimizes the ef- becoming available by 2022. Release 2 shall contain the rest ficiency of the wireless spectrum both in time and of the features (including a low-latency operation mode) as frequency, as APs are able to allocate the available RUs well as the potential extensions and/or modifications of the to their corresponding STAs in a coordinated way. In already introduced ones in Release 1. consequence, time-sensitive and best-effort traffic could be provided with differentiated RUs to meet timely IV. SUPPORTING TSN IN WIFI 7 delivery requirements. TSN consists of a set of sub-standards defined by the • Coordinated beamforming (CBF) enables simultaneous IEEE 802.1 TSN Task Group [13] to support deterministic transmissions within the same coverage area while en- messaging on standard Ethernet. Essentially, TSN technology suring spatial radiation nulls to non-targeted devices. relies on a central management that uses time scheduling • Distributed MU-MIMO allows APs to perform joint data to ensure reliable packet delivery with bounded latency and transmissions to multiple STAs by reusing the same low packet delay variation (jitter) in deterministic real-time time/frequency resources. Spatial diversity can then be applications.1 The coexistence of different traffic classes exploited to increase frame reception probability. is guaranteed by two TSN sub-standards: whereas IEEE Thanks to the multi-AP coordination, multiple overlapping 802.1Qbu implements frame preemption to interrupt any BSSs (OBSSs) can turn channel contention in our favor, ongoing operation if a time-sensitive frame is selected for resulting in a better use of shared resources. Beyond the transmission, IEEE 802.1Qbv creates exclusive time slots for latency reduction obtained by using the spectrum more time-sensitive frames managed by a time-aware shaper. efficiently, new solutions to protect time-critical traffic across A careful design of WiFi 7 technologies taking into the cooperating BSSs may be enabled. For instance, APs account the TSN principles could certainly contribute to dealing with best-effort traffic may agree on reducing trans- reduce WiFi latency issues, yet at the present time that mission power to provide spatial reuse opportunities, so that potential integration is neither straightforward nor exempted STAs from other BSSs can successfully transmit their short- from uncertainties and incompatibilities. The approach that duration, time-sensitive packets at the same time. could be followed by TGbe encompasses both adaptations Advanced transmission schemes such as hybrid automatic from TSN sub-standards and proposals of new solutions in repeat request (HARQ) offer notable performance gains in several areas, as compiled in Table I. varying channels compared to the traditional stop & wait This section elaborates on the time synchronization and approach, but it is not yet clear if such gains will also be the traffic shaping and scheduling components of TSN, achieved in WLANs due to the severity of collisions. Be analyzes the most suitable IEEE 802.11be enhancements to that as it may, HARQ still retains the prospect of improving support them, points out the main challenges involved in performance in terms of latency, because of its ability to the integration process, and sheds some light on possible reduce the number of required retransmissions per packet. solutions and open research directions. In short, the new IEEE 802.11be MAC functionalities will help to use more efficiently the spectrum resources and allo- A. Time synchronization cate them in a more flexible way to optimize throughput, la- TSN sub-standard IEEE 802.1AS includes a version of the tency, or reliability, depending on the scenario requirements. (PTP), which enables the distribution Furthermore, these functionalities can be better exploited of a single reference clock across network devices in a for low-latency purposes if some core TSN features (e.g., master/slave basis. The availability of a common clock is admission control and scheduled operation) are integrated likewise a key requirement for WiFi 7, as it would permit to on top of them, as we will see in Section IV. successfully schedule MU transmissions in both uplink and Last but not least, the lack of legacy devices operating downlink, as well as to establish coordination mechanisms in the upcoming 6 GHz band also offers the possibility among APs. of rethinking channel access for future WiFi 7 adopters. 1Ultra reliability in TSN is responsibility of IEEE 802.1CB, which In this sense, traditional channel access schemes based on basically sends duplicate copies of each frame over disjoint wired paths contention might be partially replaced by others able to offer to provide proactive seamless redundancy. DL 1 6 11 36 40 ... 1651 ... 17 ... 233 Channel #17 6 GHz band 2.4 GHz 5 GHz 6 GHz f band channels band channels band channels

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TABLE I: Potential enhancements to support TSN in WiFi 7.

Targeted feature Component Subcomponent Potential enhancement Resource Latency Jitter Reliability management IEEE 802.1AS over IEEE 802.11 x x Time synchronization IEEE 802.11mc FTM x x Traffic EDCA operation enhancements x prioritization Frame IEEE 802.1Qbu-based frame preemption x x Traffic preemption shaping Admission IEEE 802.11e/aa admission control x x and control Multi-band admission control x x scheduling IEEE 802.1Qbv-based time-aware shaper x x x Scheduled Trigger-based access x x x operation TWT mechanism x x x Multi-link operation x x HARQ x x Ultra reliability Rate adaptation in trigger-based access x IEEE 802.1CB adaptations x CSR x x x Multi-AP Resource Co-OFDMA x x x x resource management CBF x x x coordination Distributed MU-MIMO x x x

Indeed, IEEE 802.1AS can already be operated over opportunities (TXOPs) are used (for instance, to allow using IEEE 802.11 by means of the timing measurement (TM) any TXOP, regardless the AC that has obtained it, to send procedure defined in IEEE 802.11v, which takes wireless link time-sensitive traffic when available [10]). asymmetric delay into consideration. Time is propagated in In addition, the adaptation of the IEEE 802.1Qbv time- private action frames between a master (i.e., the AP) and a aware shaper on top of one of the IEEE 802.11 MAC slave (i.e., the STA), being the latter able to compute the modes would allow devices to control how traffic arrives clock offset and adjust its own time accordingly. to the different EDCA ACs according to new rules yet to Furthermore, the next revision of the IEEE 802.1AS stan- be defined. By shaping the traffic that arrives to the MAC dard (IEEE 802.1AS-Rev, still in draft version) will containa layer, devices will be able to reduce inter-AC contention, as novel synchronization method by using the IEEE 802.11mc well as better control how and when they content for the fine timing measurement (FTM) procedure. FTM provides channel. On this basis, Figure 2 exemplifies the hypothetical 0.1 ns of timestamp resolution, far more accurate than TM, integration and joint operation of a TSN-based time-aware whose timestamp resolution is 10 ns [14]. shaper with EDCA.

B. Traffic prioritization C. Frame preemption The four access categories (ACs) employed by EDCA are In the case a node is transmitting multiple traffic flows, insufficient for fine control of real-time applications, as they placing the time-sensitive traffic in the highest priority queue cannot provide hard bounds on latency/jitter, especially under may not be enough to mitigate the residual delay caused congestion. For that reason, TGbe is considering possible by large ongoing low-priority transmissions, which may enhancements to EDCA, such as the incorporation of a new include many aggregated packets and last up to the maximum AC with the highest priority for time-sensitive traffic [15], physical protocol data unit (PPDU) duration (i.e., ∼ 5 ms). as well as some modifications to how obtained transmission As shown in Figure 3, a possible solution to that issue could be based on the adaptation of the IEEE 802.1Qbu frame TABLE II: WiFi 7 low-latency use cases. preemption mechanism, which would also foster the use of Sector and Requirements use case Latency Reliability Throughput packet aggregation even in presence of time-sensitive traffic, (ms) (%) (Mbps) thus improving overall throughput. MULTIMEDIA Real-time high-quality Integrating frame preemption into WiFi would require, 3 - 10 >99.9 5 - 25 video streaming however, several changes in the physical and link layers, Virtual Reality 10 - 20 >99.9 25 - 500 such as the format of preemptable frames and the methods Augmented Reality 1 - 50 >99.99 1 - 200 Real-time pro graming 5 - 50 >99.9 >3 to fragment frames while preserving integrity of preemptable Cloud gaming 5 - 50 >99.9 10 - 35 traffic. In any case, it seems reasonable to only support this HEALTH CARE Telediagnosis, telemonitoring, 50 - 200 >99.9 0.5 - 5 new feature when aggregate MAC protocol data units (A- and telerehabilitation MPDUs) are transmitted, and so extend the service field used Telesurgery 1 - 10 >99.9999 ∼10 to identify the different MPDUs. Exoskeletons and prosthetic hands 5 - 20 >99.999 0.2 - 1 INDUSTRIAL Despite the fact that frame preemption may well be applied Process automation 1 - 50 >99.99 0.1 - 5 on outgoing transmissions from a same node, its extension Human machine interface 50 - 200 >99.9 ∼1 Tactile / Haptic technology 1 - 5 >99.999 ∼1 to incorporate transmissions from other APs/STAs would re- TRANSPORT quire a complex channel access mechanism. In that situation, Real-time traffic information 40 - 500 >99 0.1 - 1 Autonomous vehicle, and in presence of a time-sensitive traffic flow, it would be automated guided vehicle, 10 - 100 >99.9999 1 - 5 advisable to simply avoid the use of packet aggregation in and drone control Remote-controlled vehicle 10 - 100 >99.99 ∼10 the OBSS, even if that implied a severe throughput loss. with video

D. Admission Control • By using the target wake time (TWT) mechanism, STAs A traditional approach to protect time-sensitive traffic is to adopt a wake time schedule that makes them wake up avoid channel overloading (i.e., to limit the traffic load, the on a periodic basis to transmit/receive data. number of traffic flows, and/or the number of STAs allowed Whereas the two aforementioned methods just determine to transmit data) in a given band and time period: the very moment in which the channel is accessed, the new • Whereas IEEE 802.11e admission control mechanisms MAC features fostered by TGbe could empower scheduled limit the number of traffic flows per service class in operation, especially if, as discussed before, only devices a BSS, IEEE 802.11aa extends this capability to an supporting those mechanisms are admitted in the 6 GHz entire OBSS. Both could be used by WiFi 7 alone or in band. In consequence, multi-link operation and OFDMA combination with the traffic shaping solution previously could play an important role by allocating devices’ and introduced in subsection Traffic prioritization to control network resources together with the computed schedule in how traffic arrives to the transmission buffer. function of the existing time-sensitive traffic load. • Multi-link operation and the incorporation of the 6 GHz In brief, such a scheduled operation is key in terms of band in IEEE 802.11be foresee the emergence of traffic- delay. However, the main obstacle that hinders its precise aware multi-band admission control systems. For in- operation in the wireless domain continues to be the con- stance, depending on the network conditions and exist- tention in the context of several OBSSs, which can only be ing load, the 6 GHz band could be fully and exclusively effectively handled in combination with a proper multi-AP dedicated to time-sensitive traffic. resource coordination strategy (as in Figure 4, for instance). Future admission control mechanisms may also be re- quired to support scheduled operation. Thus, in the aforemen- V. USE CASES tioned example, the 6 GHz band would become even more The ability of WiFi 7 to support low-latency operation exclusive, by only accepting time-sensitive traffic coming would open the door to multiple use cases. This section from devices able to operate in contention-free mode. groups them into a set of productive sectors, details their performance requirements in Table II, and discusses the E. Scheduled operation suitability of using WiFi 7 with respect to other alternatives. Transmission of time-sensitive and non time-sensitive traf- fic could be performed on a periodic basis to isolate one A. Multimedia from another. In this sense, two methods of scheduled access WiFi is nowadays the predominant Internet access tech- facilitating collision-free operation are already available in nology for mobile devices in home and office environments IEEE 802.11ax: running multimedia applications. The short-term evolution • The trigger-based access allows the AP to schedule of this sector foresees the consolidation of more advanced uplink MU transmissions. Future improvements could time-sensitive services such as real-time high-quality 4K/8K include a rate adaptation mechanism to increase the de- audio and video streaming, virtual reality, augmented reality, livery ratio of time-sensitive frames as well as persistent cloud gaming, and interactive applications which will not be allocation schemes to reduce control overhead caused by only targeted for entertainment, but also for educational and trigger signaling. instructive purposes.

Fig. 2: TSN-based traffic classification and scheduling over EDCA. § Best e ort Time-sensi¦ ve frame arrival frame arrival characteristics with the multimedia sector (e.g., telediagnosis, Time-triggered slot telemonitoring, and telerehabilitation). Due to their criti- cality, some others additionally impose extremely stringent Frame transmission network requirements in terms of end-to-end reliability, (non-preemp ve) latency, and security (e.g., telesurgery). And lastly, a third t BE TS group involving remote motion control with relatively low tBE traffic load requires a fully deterministic approach (e.g., t TS exoskeletons and prosthetic hands). Frame transmission (non-

Guard band preemp© ve with guard band) C. Industrial TS BE t During the next years, wireless networks will have in-

tBE creasing weight in the industry, leading a trend towards more tTS flexible production sites and consolidating the Industry 4.0

Frame transmission concept. Connected factories will then become a reality,

(preemp ve + fragmenta on) involving monitoring, management, and direct control of

Reduced Preemp¨ on machines, robots, and other industrial assets. guard band overhead Future industrial communications will probably rely on the t BE (1) TS BE (2) coexistence among wired (e.g., Fieldbus-based and Industrial tBE Ethernet), wireless (from RFID to LoRa, to cite two exam- tTS ples), and 5G/6G-based cellular technologies. WiFi 7 is also Fig. 3: Frame preemption mechanism, being BE: Best- expected to get a foothold in this sector, not only because of effort frame, TS: Time-sensitive frame, tBE: Best-effort frame its inherited features (namely, flexibility, ease of installation, latency, and tTS: Time-sensitive frame latency. scalability, and interoperability), but also thanks to its new enhancements, particularly in terms of improved resource management and support to deterministic communications. The already generalized adoption of WiFi technology in- doors, its backward compatibility, and its distinctive features D. Transport with respect to wired alternatives (that is, essentially, flexi- Transport is experiencing such profound changes that bility, simplicity, and mobility) suggest that the emergence future mobility will certainly be substantiated by automa- of a low-latency operation mode for WiFi 7 would position tion, sustainability, road/air/sea safety, and energy efficiency. it as a preferential option for upcoming multimedia use cases Real-time traffic information is starting to be served on a together with 5G enhanced Mobile Broadband. regular basis to drivers, using for instance city-wide WiFi deployments. Yet the upcoming revolution is being led by autonomous vehicles and automated guided vehicles, which B. Health care will be able to transport people and goods thanks to their Latest IT advances such as ultra high video resolution, WiFi/5G connections without any human intervention. Big Data and artificial intelligence will take health care to Next-generation vehicle communication and processing a next level, enabling a plethora of innovative applications systems, such as vehicle-to-everything communication or in remote diagnosis (telediagnosis), treatment (telesurgery), advanced driver-assistance systems, will assist future trans- and recovery (telemonitoring, telerehabilitation, exoskele- port systems on the basis of TSN and artificial intelligence. tons, and prosthetic hands) for a wide set of diseases. WiFi 7 Hence, ensuring very high reliability and low latency in and 5G will play here again an important role as enablers future transport applications will become crucial regardless of novel medical wearables and devices intended for use in the employed technology, due to the high relative speeds smart health care and home environments. among end devices, and the continuous dynamism and low As for the specific use cases, some of them share common predictability of the outdoor environment. AP 1 wins the conten on and shares part of the AP 1 has no more data to send, so that STA channel with AP 2 by using Co-OFDMA

2,1 wins the conten on and transmits using BE 80 60 the whole 80 MHz channel width MT BE BACK AP 1 STA 1,1 80 MHz AP 1 bandwidth MT BE BACK TS STA 1,1 STA 2,1 20 80 MT BT TS ACK TS ACK Primary Primary AP 2 tTS tTS AP 1 AP 2 STA 2,1 20 P1 P2 MT BT TS ACK TS ACK

AP 2 tells STA 2,1 that it can AP 2 Time-sensive Time-sensive 80 MHz channel 20 MHz frame arrival transmit using channel frame arrival Fig. 4: Multi-AP resource coordination based on Co-OFDMA, being MT: Multi-AP trigger, BT: Basic trigger, BE: Best-effort frame, TS: Time-sensitive frame, (B)ACK: (Block) acknowledgement, and tTS: Time-sensitive frame latency.

VI. CONCLUSIONS [6] D. Cavalcanti, J. Perez-Ramirez, M. M. Rashid, J. Fang, M. Galeev, and K. B. Stanton, “Extending accurate time distribution and time- A new world of technological possibilities could make its liness capabilities over the air to enable future wireless industrial way in a very varied range of sectors thanks to the integration automation systems,” Proceedings of the IEEE, vol. 107, no. 6, pp. 1132–1152, 2019. of TSN and the well-established IEEE 802.11 technology. [7] R. Costa, P. Portugal, F. Vasques, C. Montez, and R. Moraes, “Limita- The most solid and promising exponent of this trend is tions of the IEEE 802.11 DCF, PCF, EDCA and HCCA to handle real- IEEE 802.11be, actual precursor of future WiFi 7, which time traffic,” in 2015 IEEE 13th International Conference on Industrial Informatics (INDIN), pp. 931–936, IEEE, 2015. should be accompanied with a well-defined and backward [8] F. Wilhelmi, S. B. Mu˜noz, C. Cano, I. Selinis, and B. Bellalta, “Spatial compatible time-sensitive operation mode to support low- Reuse in IEEE 802.11ax WLANs,” arXiv preprint arXiv:1907.04141, latency communications. 2019. [9] T. Høiland-Jørgensen, M. Kazior, D. T¨aht, P. Hurtig, and A. Brun- Although WiFi will never be able to guarantee fully strom, “Ending the anomaly: Achieving low latency and airtime deterministic communications because of its operation in fairness in WiFi,” in 2017 {USENIX} Annual Technical Conference, license-exempt bands, there is still room to reduce the impact pp. 139–151, 2017. [10] E. Khorov, I. Levitsky, and I. F. Akyildiz, “Current Status and of all manageable causes, both internal and external, that Directions of IEEE 802.11 be, the Future Wi-Fi 7,” IEEE Access, may increase latency. On the one hand, contention with 2020. external networks may be minimized by considering dynamic [11] E. Avdotin, D. Bankov, E. Khorov, and A. Lyakhov, “Enabling Massive Real-Time Applications in IEEE 802.11 be Networks,” in 2019 IEEE spectrum access such as non-contiguous channel bonding and 30th Annual International Symposium on Personal, Indoor and Mobile multi-link operation, as well as cooperative AP strategies. Radio Communications (PIMRC), pp. 1–6, IEEE, 2019. On the other hand, prioritization and scheduling mechanisms [12] D. L´opez-P´erez, A. Garcia-Rodriguez, L. Galati-Giordano, M. Kasslin, and K. Doppler, “IEEE 802.11 be Extremely High Throughput: The inside the same WLAN may provide an effective solution to Next Generation of Wi-Fi Technology Beyond 802.11 ax,” IEEE reduce the latency of time-sensitive traffic in the presence of Communications Magazine, vol. 57, no. 9, pp. 113–119, 2019. large packets from best-effort flows. [13] “IEEE Time-Sensitive Networking Task Group.” http://www.ieee802.org/1/pages/tsn.html. Accessed: 2020-11-24. [14] A. Mahmood, R. Exel, H. Trsek, and T. Sauter, “Clock synchronization ACKNOWLEDGMENT over IEEE 802.11—A survey of methodologies and protocols,” IEEE Transactions on Industrial Informatics, vol. 13, no. 2, pp. 907–922, This work received funding from the Spanish government 2016. under the projects PGC2018-099959-B-100 and TEC2016- [15] E. Genc and L. F. Del Carpio, “Wi-Fi QoS Enhancements for Down- 79510-P, and from the Catalan government through projects link Operations in Industrial Automation Using TSN,” in 2019 15th IEEE International Workshop on Factory Communication Systems SGR-2017-1188 and SGR-2017-1739. (WFCS), pp. 1–6, IEEE, 2019.

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