# Performance Evaluation of IEEE 802.11 ah Networks With High-Throughput Bidirectional Traffic
## 3 IEEE 802.11 ah Overview
### Traffic Indication Map (TIM) Segmentation  
IEEE 802.11 ah introduces TIM segmentation, which distributes the stations in a hierarchy, enabling effective management of a large number of stations as well as energy conservation and contention reduction in a structured manner. Each station is assigned a unique 13-bits AID in the range 1-8191 (AID 0 is reserved for group addressed traffic). The AID represents the station in a hierarchical structure as Figure 1 illustrates. The TIM is structured in a three-level hierarchy: Page ID, Block (TIM Group) and Sub-Block (contains station (STA) indexes). This hierarchy enables the AP to indicate in the bitmap whether a station has pending DL data buffered at the AP on multiple levels. For instance, the AP can indicate that there is DL data pending for TIM group \(g\) in the DTIM beacon. All stations will check their own TIM group based on their assigned AID and only stations that belong to TIM group \(g\) will wake up in time for TIM beacon of TIM group \(g\) to hear which station-index has data to expect, whereas all the remaining stations from other TIM groups can resume sleeping until the next AP announcement. This enables longer sleep periods for stations, which conserves energy and reduces contention as illustrated with an example in Figure 2. Moreover, the hierarchy enables TIM segmentation, which reduces the data length of beacons containing TIMs. In highly dense networks, the TIM would be very long (1 bit per station) without such segmentation. All stations would need to listen to the beacon and receive a very long TIM, including the stations that do not have pending DL data at all. Waste of energy is evident in this case. TIM segmentation does not only reduce energy consumption; it also enables connecting up to 8191 stations thanks to the hierarchical organization of stations into groups. Figure 2: An example of TIM segmentation: all stations wake up to receive the Delivery TIM (DTIM) beacon, only paced TIM groups wake up to receive their TIM beacon and only paced stations within paced TIM groups contend for the medium. In the example, 512 stations are organized in 1 page (\(N_{P}=1\)) and 8 TIM groups (\(N_{B}=8\)), where each TIM group has 64 stations. The traffic-indication virtual bitmap consists of \(64N_{P}N_{B}\) bits and is organized into \(N_{P}\) pages where each page consists of \(N_{B}\) blocks, each block consists of eight subblocks, and each subblock consists of eight bits, as illustrated in Figure 1. The 8-bit _bitmap control field_ defines the TIM segmentation. Five bits of this field are reserved for the _Page Slice Number_\(N_{B}\), while two bits are reserved for the _Page Index_\(N_{P}\). Ergo, \(N_{P}\) is at most 4, whereas \(N_{B}\) is at most 32, which implies a maximum of 2048 stations in each page and a maximum of 8192 stations overall. As shown in Figure 1, the arbitrary bit number \(N\) in the traffic-indication virtual bitmap corresponds to the bit number \(N[0:2]\) of the \(N[3:5]\)-th subblock of the \(N[6:5+n_{1}]\)-th block of the \(N[6+n_{1}:12]\)-th page, where \(2^{n_{1}}=N_{B}\), \(N[0]\) is the least significant bit and \(N[a:b]\) represents bits \(a\) to \(b\) of the bit number \(N\), both inclusive. When TIM segmentation is used, all TIM-stations wake up at the Target Beacon Transmission Time (TBTT) to receive the beacon carrying the Delivery Traffic Indication Map (DTIM) that indicates which TIM group (block) has buffered DL data at the AP and needs to wake up to receive it. Further, one TIM beacon is broadcast for each TIM group at every Target Short Beacon Transmission Time (TSBTT). The short beacon interval \(T_{TIM}\) is announced in the DTIM beacon or during association and represents the number of IEEE 802.11 Time Units (TUs) of 1024 us between two subsequent TSBTTs. It is an integer in the range 1-65,535. The beacon interval \(T_{DTIM}=N_{P}N_{B}\cdot T_{TIM}\) is the number of TUs between two consecutive TBTTs.