Many of the consumer IT devices we use in our homes, at work, or elsewhere are battery-powered. We expect our smartphones, laptops, AR/VR and more to run on their battery power for as long as possible.
Older, existing Wi-Fi client power-saving methods were putting the device in sleep mode between Access Point (AP) beacons and would wake up just when they had data to transmit. Now with Wi-Fi 6/6E (the IEEE 802.11ax standard), there is a 4x capacity increase over the previous standard versions. Multiple APs, deployed in dense device environments, can together deliver the required quality-of-service (QoS) to more clients along with more diverse usage profiles.
Enter ‘Target Wait Time’ (TWT) which has the goal to reduce power consumption and improve spectral efficiency. TWT enables devices to negotiate when and how frequently they will wake up to send or receive data. This Wi-Fi 6/6E feature increases device sleep time and greatly improves battery life; this is critical in the Internet of Things (IoT).
TWT meets the power-saving objectives of 802.11ax
The 802.11ax standard goals include the following:
1. To increase performance of the Wi-Fi network by a factor of 4 while improving/not impacting power requirements.
2. To provide power saving mechanisms for new/emerging IoT devices with a more efficient network.
TWT facilitates these goals:
1. Use the spectrum more efficiently by saving time and power
2. Enable less contention between clients with transmissions in the shared medium
TWT for the IoT
New IoT devices are continuously being created to make our daily tasks more efficient. Often those devices run on batteries. Examples include smart locks, thermostats, various sensors and surveillance devices. TWT will assist multiple WLANs in a heavy deployment environment to reach a consensus on non-overlapping schedules. This extends battery life, provides better power savings, and enables reduced network congestion.
With TWT, the AP has more control over the network and decides who is going to talk and when. The AP decides which Resource Unit (RU) will be used and how many (an RU is a contiguous set of subcarriers). The TWT extends this since it provides for more efficient scheduling of transmissions. The AP has more control now in sending frames according to a specific schedule.
Clients may stay asleep even longer if they wish. Before 11ax, TWT clients would sleep for some milliseconds and then wake up, exchange data and go back to sleep for some milliseconds and repeat. With TWT, clients may sleep for seconds, minutes, or even hours. Some IoT devices only need to communicate back to the network once per day. In theory, they could sleep for 23 hours and 59 minutes and just wake up and do a transmission, then go back to sleep for another day. These efforts greatly improve battery life.
TWT modes of operation
There are three modes of operation for TWT:
1. Individual: The client chooses when to wake up and when to sleep. The client will negotiate agreement with the AP so all other clients will know when other clients will wake up and send data,
2. Broadcast: The AP is in charge here and will provide the schedule to all clients that support broadcast TWT.
3. Opportunistic power savings: This mode allows a peer-to-peer (P2P) Group Owner to opportunistically save power when all its associated clients are sleeping.
Some key specific applications
Wi-Fi 6/6E has TWT which will benefit such efforts as commercial or residential Smart Buildings with remotely-powered sensors for temperature, humidity, motion, contact/status, air quality, electric current monitoring, as well as portable devices such as mobile phones and tablets. These devices are battery operated (or will need battery backup in the event of a power loss in the building) and TWT will enable these remote sensors to operate longer. TWT enables devices to negotiate when and how frequently they will wake up to send or receive data. This Wi-Fi 6/6E feature increases device sleep time and greatly improves battery life.
Real-time mobile apps, whether giving alerts, or uploading updates on location, will be dependent on the excellent wireless connectivity of Wi-Fi. TWT will play a critical role, going forward, in helping Wi-Fi evolve into a collision-free, deterministic wireless technology. TWT can potentially assist multiple WLANs in a heavy deployment environment to reach a consensus on non-overlapping schedules.
Smart buildings are challenged to operate stably in the presence of growing interference from overlapping networks, and increasing number of devices driving network congestion. Wi-Fi 6/6E with TWT will help mitigate these problems.
In summary, TWT in Wi-Fi 6/6E will enhance user services by extending battery life of devices, improving spectral efficiency and also minimizing contention between clients. The IoT greatly benefits from TWT with power-savings and reduced network congestion so that all clients are properly accommodated.