# Time-Sensitive Networking in IEEE 802.11 be: On the Way to Low-Latency WiFi 7
## 2 Limitations of IEEE 802.11 to Handle Time-Sensitive Traffic  
IEEE 802.11 offers great accessibility and ease of use, creating an open environment for any station (STA) willing to associate to the network. However, at the same time, the wireless medium is precisely the main cause that hinders proper delivery of time-sensitive traffic, due to its variable capacity (which depends on the link quality) and typically higher packet error rate (PER; due to the stochastic properties of the channel and the presence of interference) [9].  
As for the MAC layer, IEEE 802.11 has traditionally relied on the distributed coordination function (DCF): a contention-based random access scheme based on carrier sense and exponential back-off rules. The main drawback of DCF, however, is its non-predictable behavior and lack of traffic prioritization techniques. In fact, in the presence of multiple STAs, DCF may lead to channel saturation by contending packets, thus being unable to guarantee timely data delivery. The alternative point coordination function (PCF), based on a centralized polling system, has never been widely adopted.  
The enhanced distributed channel access (EDCA) was envisioned as part of the IEEE 802.11e amendment to extend DCF and provide quality of service support according to four differentiated access categories (ACs): background, best effort, video, and voice [10]. Prioritization is then implemented by allocating different contention-related parameters to each AC. Nevertheless, the low number of ACs, the lack of mechanisms for the prioritization of different streams belonging to the same AC, and (in some hardware devices) the use of a single buffer to store packets with different priorities are among the main EDCA shortcomings.  
To outperform IEEE 802.11e operation for real-time multimedia content delivery, IEEE 802.11 aa introduced the intra-AC traffic differentiation functionality, with the definition of two new time-critical voice and video ACs [11]. The capabilities of IEEE 802.11e/aa enhancements to improve the performance of delay-sensitive traffic in WLANs has been widely considered in the literature [12; 13; 14]. However, in general, none of the IEEE 802.11 mechanisms guarantee the quality of service of heterogeneous real-time streams when a WLAN is overloaded [15]. In such cases, flexible scheduling policies and/or admission control algorithms are highly required to effectively manage different traffic flows.  
Neighboring networks represent a key limitation to provide low-latency guarantees in all the aforementioned channel access methods. In dense scenarios, overlapping of basic service set (BSS) coverage areas turns into large delays for STAs waiting to access the channel. IEEE 802.11 ax partially addresses this issue by allowing concurrent transmissions under the spatial reuse scope, showing a clear gain for time-sensitive communication [16]. A gain that could be remarkably boosted by means of coordination mechanisms among neighboring APs.  
Finally, when it comes to the transport layer, the bufferbloat problem may prevent IEEE 802.11 networks from delivering time-sensitive traffic in presence of TCP flows, due to the high latency produced by excessive buffering of packets. In fact, well-known techniques to mitigate this problem in wired networks (e.g., decreasing buffer sizes and/or applying modern queue management algorithms) have had low success in WiFi [17].