802.11ax fundamentals: Target Wake Time (TWT)

The Wi-Fi industry experiences a seismic change approximately every five years – and 802.11ax is the latest generation of Wi-Fi that bridges the performance gap towards ten gigabit speeds. The new Wi-Fi standard will deliver faster network performance, connect more devices simultaneously and transition Wi-Fi from a ‘best-effort’ endeavor to a deterministic wireless technology that is now the de-facto medium for internet connectivity. With an expected four-fold capacity increase over its 802.11ac Wave 2 predecessor, 802.11ax deployed in dense device environments will support higher service-level agreements (SLAs) to more concurrently connected users and devices with more diverse usage profiles. This is made possible by a range of technologies that optimize spectral efficiency, increase throughput and reduce power consumption. These include Target Wake Time (TWT), OFDMA and MU-MIMO, Uplink MU-MIMO, sub-carrier spacing and MAC/PHY enhancements. In this article, we’ll be taking a closer look at Target Wake Time and how 802.11ax wireless access points (APs) can utilize this mechanism to extend the battery life of client devices and optimize spectrum utilization.

Target Wake Time: From IEEE 802.11ah to 802.11ax

Target Wake Time enables devices to determine when and how frequently they will wake up to send or receive data. Essentially, this allows 802.11ax access points to effectively increase device sleep time and significantly conserve battery life, a feature that is particularly important for the IoT. In addition to saving power on the client device side, Target Wake Time enables wireless access points and devices to negotiate and define specific times to access the medium. This helps optimize spectral efficiency by reducing contention and overlap between users. The Target Wake Time mechanism first appeared in the IEEE 802.11ah “Wi-Fi HaLow” standard. Published in 2017, the low-power standard is specifically designed to support the large-scale deployment of IoT infrastructure – such as stations and sensors – that intelligently coordinate signal sharing. The TWT feature further evolved with the IEEE 802.11ax standard, as stations and sensors are now only required to wake and communicate with the specific Beacon(s) transmitting instructions for the TWT Broadcast sessions they belong to. This allows the wireless IEEE 802.11ax standard to optimize power saving for many devices, with more reliable, deterministic and LTE-like performance. As Maddalena Nurchis and Boris Bellalta of the Universitat Pompeu Fabra in Barcelona noted in a recent paper, TWT also “opens the door” to fully maximizing new MU capabilities in 802.11ax by supporting the scheduling of both MU-DL and MU-UL transmissions. In addition, TWT can be used to collect information from stations, such as channel sounding and buffers occupancy in pre-defined periods. Last, but certainly not least, TWT can potentially help multiple WLANs in dense deployment scenarios reach consensus on non-overlapping schedules to further improve Overlapping Basic Service Set (OBSS) co-existence.

Smarter Wi-Fi

Designed for high-density connectivity, the new IEEE 802.11ax standard offers up to a four-fold capacity increase over its 802.11ac Wave 2 predecessor. With 802.11ax, multiple APs deployed in dense device environments can collectively deliver required quality-of-service (QoS) to more clients with more diverse usage profiles. This is made possible by a range of technologies – such as Target Wake Time (TWT) – that reduce power consumption and improve spectral efficiency. TWT is clearly an important part of both the new 802.11ah and 802.11ax standards. From our perspective, TWT will play a critical role in helping Wi-Fi evolve into a collision-free, deterministic wireless technology as the IEEE looks to integrate future iterations of the mechanism into new wireless standards to support the IoT and beyond.