# Time-Sensitive Networking in IEEE 802.11 be: On the Way to Low-Latency WiFi 7
## 4 Supporting TSN in WiFi 7
### Scheduled Operation  
IEEE 802.1 Qbv TSN sub-standard creates a time division multiple access (TDMA) scheme that splits communication time on an Ethernet network into repetitive cycles of fixed length. A time-aware scheduler defines the time period in which gates corresponding to each outbound queue (each one linked to a different traffic class) are opened or closed. In consequence, time-sensitive frames can be transmitted with the certainty that they will not be interfered by other best-effort traffic [42]. The potential adaptation of the IEEE 802.1 Qbv time-aware scheduler on top of one of the IEEE 802.11 MAC modes would allow devices to control how traffic arrives to the different EDCA ACs according to new rules yet to be defined. By scheduling the traffic that arrives to the MAC layer, devices will be able to reduce inter-AC contention, as well as better control how and when they content for the channel. On this basis, Figure 3 exemplifies a hypothetical integration and joint operation of a wireless TSN AP using a time-aware scheduler in combination with EDCA. Isolated transmission of time-sensitive traffic on a periodic basis has already been explored by IEEE 802.11. In this sense, two methods of scheduled access facilitating collision-free operation are available in IEEE 802.11 ax:  
* The trigger-based access allows the AP to schedule uplink MU transmissions by means of MU-MIMO, OFDMA, or both joint techniques [43]. In any case, unfairness is likely to appear in heterogeneous networks with coexisting IEEE 802.11 ax and legacy devices, as the former can access the medium only after the AP gets the TXOP. Even so, this problem could be alleviated by applying EDCA appropriately and setting different transmission priorities to nodes [44]. Last, as a potential direction to reduce signaling overhead and support lower latency bounds in trigger-based access, TGbe has identified the use of persistent UL allocation schemes [45]. * By using the target wake time (TWT) mechanism, STAs adopt a wake time schedule that makes them wake up on a periodic basis to transmit/receive data [46]. However, when a new TWT service period starts, there is no guarantee another device that have gained a TXOP just before overlap in time with the TWT service period. To mitigate this situation, and further protect scheduled traffic, IEEE 802.11 be will include an extension of TWT called Restricted TWT, which simply forces all other IEEE 802.11 be-compatible devices to finish their transmissions before the TWT service period begins. Whereas the two aforementioned methods just determine the very moment in which the channel is accessed, the new MAC features fostered by TGbe could empower scheduled operation, especially if, as discussed before, only devices supporting those mechanisms are  
Figure 3: Diagram of the integration of several time-sensitive enhancements into a potential wireless TSN AP. admitted in the 6 GHz band. In consequence, multi-link operation and OFDMA could play an important role by allocating devices' and network resources together with the computed schedule in function of the existing time-sensitive traffic load. In brief, such a scheduled operation is key in terms of delay. However, the main obstacle that hinders its precise operation in the wireless domain continues to be the contention in the context of several OBSSs, which can only be effectively handled in combination with a proper multi-AP resource coordination strategy. By way of an example, Figure 4 shows how the use of Co-OFDMA may help make a more efficient use of the available channel resources while meeting time-sensitive requirements. In Figure 4, two different frames originated in the same BSS (a best-effort one from AP #1 and a time-sensitive one from STA #2,1) are simultaneously transmitted by splitting the total 80 MHz spectrum bandwidth into two non-overlapping channels of 60 MHz and 20 MHz, respectively. To make this coordination possible, once AP #1 wins the contention, it signals the RU allocation information by means of a multi-AP trigger.