Traffic indication map
Encyclopedia
The Traffic indication map (TIM) information element is covered under section 7.3.2.6 of 802.11-1999.pdf. The IEEE 802.11
standards chose to use a bitmap
to indicate to any sleeping listening stations if the Access Point (AP) has any buffered frames present for it. Because stations should listen to at least one beacon before the listen interval, the AP periodically sends this bitmap on its beacons as an information element. The bit mask is called the Traffic Indication Map and consists of 2008 bits, each bit representing the Association Id (AID) of a station. For example, the TIM information element allows you to transfer 1 byte
up to the entire 251 bytes (2008 bits) of the TIM. You are allowed to transmit a smaller TIM bitmap as it is expected that only a few number of stations will be asleep. Because of this the bitmap values passed in the TIM information element is called a partial virtual bitmap. To allow you to transmit only a partial bitmap you must make use of the bitmap control and length fields of the TIM information element.
This is what the TIM information element look like
|Element ID|Length|DTIM count|DTIM period|Bitmap control|PVM|
PVM: Partial Virtual Bitmap
Note: all fields are 1-byte long except the PVM which can be 1-256 bytes long
IEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
standards chose to use a bitmap
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
to indicate to any sleeping listening stations if the Access Point (AP) has any buffered frames present for it. Because stations should listen to at least one beacon before the listen interval, the AP periodically sends this bitmap on its beacons as an information element. The bit mask is called the Traffic Indication Map and consists of 2008 bits, each bit representing the Association Id (AID) of a station. For example, the TIM information element allows you to transfer 1 byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
up to the entire 251 bytes (2008 bits) of the TIM. You are allowed to transmit a smaller TIM bitmap as it is expected that only a few number of stations will be asleep. Because of this the bitmap values passed in the TIM information element is called a partial virtual bitmap. To allow you to transmit only a partial bitmap you must make use of the bitmap control and length fields of the TIM information element.
This is what the TIM information element look like
|Element ID|Length|DTIM count|DTIM period|Bitmap control|PVM|
PVM: Partial Virtual Bitmap
Note: all fields are 1-byte long except the PVM which can be 1-256 bytes long