# Time-Sensitive Networking in IEEE 802.11 be: On the Way to Low-Latency WiFi 7
## 3 IEEE 802.11 be
### MAC Layer  
Many significant MAC features from IEEE 802.11 ax such as MU-MIMO, OFDMA, and spatial reuse will be extended in IEEE 802.11 be. The support of more spatial streams will also enable more flexible MU-MIMO arrangements. However, current explicit channel state information acquisition procedure may not cope well with such high number of antennas and, for that reason, TGbe is currently evaluating several alternatives to enhance explicit sounding, even considering the introduction of an implicit procedure. As for OFDMA, enhanced resource unit (RU) allocation schemes will allow allocating multiple contiguous and non-contiguous RUs to a single STA. Consequently, these novel schemes could significantly increase spectral efficiency and overall network throughput and, even better, satisfy timely data delivery [21]. In fact, whether based on MU-MIMO or OFDMA, MU transmissions are key to reduce the channel access latency, as packets from/to different users can be de-queued simultaneously. Multi-link operation will likely become the most representative feature of IEEE 802.11 be, being able to yield an order-of-magnitude reduction in the worst-case latency experienced by WiFi devices and meet the stringent requirements of real-time applications even under dense traffic conditions [22]. In general, multi-link operation aims to (1) improve throughput by aggregating links [23], (2) enhance reliability by transmitting multiple copies of the same frame in separated links, (3) decrease channel access delay by selecting the first available link in terms of latency [24], and (4) enable isolation of time-sensitive traffic from other network traffic [25]. In short, having two active links operating at different bands/channels between an AP and an STA may increase channel access efficiency by enabling opportunistic link selection, link aggregation, and multi-channel full duplex [26]. Figure 1 shows several of the previously mentioned cases: opportunistic link selection, link aggregation, and multi-channel full duplex. Two different channel access modes can be considered for multi-link operation: asynchronous and synchronous [27; 28]. * Asynchronous mode treats each link individually, allowing both opportunistic link selection (see DL #1 and UL #1 packet transmissions) and simultaneous DL/UL transmissions in a multichannel full duplex fashion (see DL #3 and UL #2 packet transmissions). This mode may create out-of-band emissions, though, resulting in interference between links. Therefore, to operate properly, it requires either large gaps between the selected channels or interference cancellation techniques. * Synchronous mode is an alternative that avoids interference issues by synchronizing more than one link to transmit at the same time and for periods of equal duration (see DL #2 packet transmission). It uses a primary link that counts the back-off, while the other links are secondary. Once the back-off reaches 0, if the secondary channels have been idle for a PIFS interval they can be used as well [29]. Otherwise, the primary link transmits alone. TGbe also considers multi-AP coordination, although it is not yet clear if it will finally be included in the final amendment. It allows neighboring APs to share a transmission  
Figure 1: Multi-link operation techniques at ISM frequency bands. opportunity and coordinate their transmissions in enterprise-like IEEE 802.11 be WLANs, as a way to improve overall performance by means of different techniques:  
* Coordinated spatial reuse (CSR) consists in jointly negotiating the transmission power of potential overlapping APs to reduce overall interference. Access delay to the medium could be then reduced, as CSR allows to increase the number of concurrent transmissions. * Coordinated OFDMA (Co-OFDMA) optimizes the efficiency of the wireless spectrum both in time and frequency, as APs are able to allocate the available RUs to their corresponding STAs in a coordinated way.