WLAN Timers-TSF, SMK, probedelay, NAV, Backoff, MSDU, MMPDU timer

This page on WLAN timers mentions TSF timer, SMK timer, probedelay timer, NAV timer, Backoff timer, MSDU timer and MMPDU timer as defined in IEEE 802.11 WLAN standard.

WLAN Timer type Description
NAV Timer It refers to Network Allocation Vector Timer. When a station sends an RTS frame, it includes the duration of time that it needs to occupy the channel. If one station acquires access, the other will postpone sending data by setting this NAV timer.
Backoff Timer The WLAN backoff timer is decreased as long as the medium is sensed to be idle for a DIFS interval. The timer is frozen when a transmission is detected on the medium. The backoff timer is resumed when the channel is detected as idle again for a DIFS interval. When the backoff timer reaches 0, the WLAN station transmits the packet.
ProbeDelay timer Scanning is used to identify the wifi network. There are two modes active and passive. In active scanning mode, four parameters are important viz.ProbeTimer, ProbeDelay, MaxChannelTime and MinChannelTime. These are manufacturer specific. ProbeDelay is the time taken by the NIC to passively monitor the channel for activities before sending a Probe Request frame. Immediately after a Probe Request is transmitted, the existing ProbeTimer is cleared and restarted. If no Probe Response frame is received when ProbeTimer reaches MinChannelTime, the STATION declares that the channel is empty or no Access Point is in range and proceeds to the next channel. Otherwise, the STATION waits until ProbeTimer reaches MaxChannelTime while collecting other potential Probe Response frames from other APs on the channel.
TSF Timer The timing synchronization function (TSF) timer is a local timer synchronized with the TSF of every other station in the basic service area. The TSF timer is based on a 1 MHz clock and it ticks in unit of microseconds. Beacon frames are used to periodically announce the value of the TSF to other stations in the wlan network. In an infrastructure BSS, STAs shall always adopt the TSF timer value in a Beacon frame or probe response coming from the AP in their BSS by using some algorithm.
SMK Timer SMK stands for station-to-station link (STSL) master key. A random value generated by WLAN access point during an SMK Handshake. It is used for deriving an STSL transient key. The handshake protocol helps to secure the STSLs. If SMK timer has not expired, the STAs will initiate a 4-Way Handshake to create a new STK. If the SMK has expired, the STA_I shall not use an existing STKSA and shall start the SMK Handshake followed by a 4-Way Handshake to create new keys
MSDU Timer/MMDU timer The source STA shall maintain a transmit MSDU timer for each MSDU being transmitted. The attribute dot11MaxTransmitMSDULifetime specifies the maximum amount of time allowed to transmit an MSDU. The timer starts on the initial attempt to transmit the first fragment of the MSDU. If the timer exceeds dot11MaxTransmitMSDULifetime, then all remaining fragments are discarded by the source STA and no attempt is made to complete transmission of the MSDU.

The receive MSDU or MMPDU timer starts on the reception of the first fragment of the MSDU or MMPDU. If the receive MSDU timer exceeds aMaxReceiveLifetime, then all received fragments of this MSDU or MMPDU are discarded by the destination STA. If additional fragments of an individually addressed MSDU or MMPDU are received after its aMaxReceiveLifetime is exceeded, those fragments shall be acknowledged and discarded.

Other System Timers

Engineers and testers working in multimode or multiRAT modem design consisting of multiple wireless technologies need GSM timers and WCDMA timers other than LTE timers mentioned on this page. Refer following links for the same.

•  GSM Timers
•  WCDMA Timers
•  GSM MS Timers
•  GSM BTS Timers

What is Difference between

FIR filter Vs. IIR filter
difference between FDM and OFDM
Difference between SC-FDMA and OFDM
Difference between SISO and MIMO
Difference between TDD and FDD
Difference between 802.11 standards viz.11-a,11-b,11-g and 11-n
Bluetooth vs zigbee

WLAN MAC frames Links

WLAN Association Request and response frame
WLAN authentication Request and deauthentication frame
WLAN beacon frame
WLAN passive vs active scanning frame
WLAN Probe Request and response frame
WLAN reassociation Request and response frame
WLAN RTS and CTS frame

Other Standard MAC layer protocol


RF and Wireless Terminologies