The current article attempts to explain how a partial virtual Bitmap is computed with an example – refer <Understanding Traffic Indication Map> to understand the TIM Element before going through this article.
Consider that an 802.11 Station with Association ID 26 has unicast data buffered and no other station has buffered unicast data at the Access point. The Access Point does not have any multicast/unicast data buffered as well
Hence,
– Bit zero of the bitmap control field is zero (No buffered Multicast/Unicast Data)
– N1 is calculated as given below
Bit 26 AID is in the 3rd byte (starting from byte 0). Since N1 has to be an even number N1 has to be 2
– The Bitmap offset from the above calculation will be N1/2 = 2/2 = 1
– N2 is the “smallest number x 8” beyond which all Association ID bits are zero. Hence, N2 will be 3 since 3 is the smallest number which conforms to the formula
(N2+1) * 8 = (3 + 1) * 8 = 32.
Hence, the partial virtual bitmap will start from byte 2 (bit 16) and will end at the last bit of byte 3 (bit 31). The bitmap offset indicates that the first byte transmitted in the Partial Virtual Bitmap has valid data
The Length of the entire TIM Information Element will be
Length = (N2 – N1) + 4 = (3 – 2) + 4 = 5 bytes (starting from DTIM count)
The Entire Partial Virtual Bitmap for the above example along with the Bitmap Control is provided pictorially below
On the receiving station which has Association ID 26 receiving the bitmap offset and Partial Virtual bitmap in the TIM Information Element can deduce N1 from the Bitmap Offset
N1 = Bitmap Offset *2 = 2. Other parameters of the Partial virtual bitmap can also be recovered via N1, bitmap offset and length. This allows the Partial Virtual Bitmap to be properly decoded. The N2 calculation is shown below for the example case we are considering
Length = (N2 – N1) + 4
5 = (N2 – 2) + 4
N2 = (5 – 4) + 2 = 3.
So the receiving station will now know that the partial virtual bitmap starts at bit 16 and then is able to check if its Association ID is set in the Partial Virtual Bitmap or not.
If an Access Point does not have any buffered data to transmit to any Station – then the Partial Virtual Bitmap will be 1 octet with all zeroed bits.
The TIM Information is used by a WLAN station in Power save mode to determine if the station needs to wake up and get the data from the Access Point. The Legacy PS-POLL mechanism is one mechanism that a WLAN station can use to retrieve data
Legacy Power Save – PS-POLL Mechanism