IEEE 802.11ah

IEEE 802.11ah is a wireless networking standard from the IEEE 802.11-2007 wireless networking standard. Its purpose is at creating large groups of stations that cooperate to share air medium while minimizing energy consumption [1].

Description

This is one of the IEEE 802.11 technologies which is the most stranger to the LAN model. The purpose of this amendment is to allow low rate 802.11 wireless stations to be used in sub gigahertz spectrum. The most prominent aspects of 802.11ah is the behaviour of stations that are grouped to minimize contention on the air media, use relay to extend their reach, use little power thanks to predefined wake/doze periods, are still able to send data at high speed under some negotiated conditions and use sectored antennas. It uses 802.11a/g specification that is down sampled to provide 26 channels, each of them able to provide 100 kbit/s throughput. It can cover a one-kilometer area. It aims at providing connectivity to thousands of devices under an access point. The market is about M2M, like smart metering.

Relay Access Point (AP)

A Relay is an entity that logically consists of an Relay AP and a STA, The relay function allows an AP and stations to exchange frames with one another by the way of a relay. The introduction of a relay allows stations to use higher MCSs (Modulation and Coding Scheme) and reduce the time stations will stay in Active mode. This improves battery life on stations. Relay stations may also provide connectivity for stations located outside the coverage of the AP. There is an overhead cost on overall network efficiency and increased complexity with the use of relay stations. To limit this overhead, the relaying function shall be bi-directional and limited to two hops only.

Grouping of stations

Grouping allows partitioning of the stations within a BSS into groups and restricting channel access only to stations belonging to a given group at any given time period. It helps to reduce contention and to avoid simultaneous transmissions from a large number of stations hidden from each other. Grouping can also reduce the signaling overhead where stations receive management frames that is meaningless to them. Grouping of stations may be used to reduce network energy consumption, as stations that are not in the group assigned to the current slot can enter a doze state until their slot time or assigned TWT arrives.

Target Wake Time

Target Wake Time (TWT) is a function that permits an AP to define a specific time or set of times for individual stations to access the medium. The STA and the AP exchange information that includes an expected activity duration to allow the AP to control the amount of contention and overlap among competing STA. The AP can protect the expected duration of activity with various protection mechanisms. The use of TWT is negotiated between an AP and a STA.

Speed Frame Exchange

Speed frame exchange provides the functionality that enables an AP and non-AP STA to exchange a sequence of uplink and downlink frames during a reserved time(TXOP).This operation mode is intended to reduce the number of contention-based channel accesses, improve channel efficiency by minimizing the number of frame exchanges required for uplink and downlink data frames, and enable stations to extend battery lifetime by keeping Awake times short. This continuous frame exchange is done both uplink and downlink between the pair of stations.

Sectorization

The partition of the coverage area of a BSS into sectors, each containing a subset of stations, is called sectorization. This partitioning is achieved through a set of antennas or a set of synthesized antenna beams to cover different sectors of the BSS. The goal of the sectorization is to reduce medium contention or interference by the reduced number of stations within a sector and/or to allow spatial sharing among OBSS APs or stations.