3.3 Medium Access Control

33

3.3.1.6Synchronization and Beacons

All stations within a single BSS are synchronized to a common clock by maintaining a local timer. by using the Timing Synchronization Function (TSF). To synchronize the stations, a management frame, the beacon, is used.

Beacons are transmitted periodically, hence, every station knows when the next beacon frame will arrive; this time is called Target Beacon Transmission Time (TBTT). The TBTT of each beacon is announced in the previous beacon.

The TSF’s original function is to support various PHYs that require synchronization, and management functions such as a station joining a BSS, and saving power through sleep modes. Local timers are updated by the information received from other stations as part of a beacon. In order to give beacon transmissions highest priority of medium access, stations stop initiating frame exchanges upon reaching a TBTT. However, ongoing frame exchanges are completed, even this means that beacon transmissions are delayed. Note that beacons are transmitted after the channel was idle for PIFS (which is 25 us in 802.11a), and in a BSS without backoff. Thus, if a frame exchange is ongoing at TBTT, then the beacon is delayed. In BSS and IBSS, the synchronization is maintained by broadcasting the TSF timer in the beacon. The decision about if the local timer in a station has to be updated or not upon reception of the beacon, is different for BSS and IBSS.

Figure 3.12, left, illustrates the TSF in an infrastructure BSS3. Only the AP generates beacons in a BSS. At each TBTT, the AP schedules a beacon as the next frame to be transmitted.

														synch.		11 12	1
			11 12		1	Access										10	2		11 12
																9	3				1
			10		2
			9		3	Point			11 12		1					8	4		10		2
			8		4			10			2					7 6	5		9		3	no
				6															8		4
			7		5			9			3											update
	beacon trans-							8			4			time					7	6	5
									7	6	5	TBTT
	mitted by HC								update
														11 12 1
11 12		1				assoc.						to be	10		2					TBTT		time
10			2										9		3
9			3			station						updated	8		4				11 12 1
8			4											7 6	5
7	6	5	assoc.				10	12	1	2								beacon	10	2
																			9	3
							11
update station						assoc.	9			3		TBTT time						holder	8	4
						station	8 7	6	5	4								beacon	7 6 5
						sync.											TBTT	time

Figure 3.12: Left: Stations 2 and 3 change their TSF timers to the value received in the beacon of the AP. Right: In IBSS, station 2 transmits the beacon because of the smaller backoff counter.

3 An infrastructure BSS is referred to as BSS and an independent BSS is referred to as IBSS.

34	3. IEEE 802.11

If the channel has been idle for at least PIFS before TBTT, the AP transmits the beacon right at TBTT, otherwise the beacon is transmitted PIFS after the current transmission, without contention. All stations associated to this AP update their local timers with the information received from the beacon.

Figure 3.12, right, illustrates the TSF in an IBSS. There, the TSF is distributed over all stations. All stations take part in the generation of beacons. The beacon generation is distributed using a mechanism similar to the backoff mechanism.

At the TBTTs, stations that are part of an IBSS attempt to transmit a beacon in contention, with small CWmin and after PIFS. Stations stop attempting to transmit a beacon when they receive a beacon from another station of the IBSS. However, beacons transmitted with contention window may collide, which is allowed as part of the standard. Beacons are not retransmitted; the next beacon will be transmitted at the next TBTT. Note that beacons are not acknowledged by other stations. A station that transmitted a colliding beacon will not detect this collision, as it is not waiting for a subsequent ACK frame.

In BSS and IBSS, upon receiving a beacon, a station updates its local timer with the information from the beacon only if the received value represents an earlier time than the value currently maintained in the local timer. This is indicated in Figure 3.12. This distributed synchronization results in the shared information about the slowest running clock, with which the complete IBSS will synchronize.

Among the timing information needed to synchronize stations, the beacon delivers other parameters related to the protocol and to radio regulations. In addition to the timing information needed to synchronize the BSS, the beacon delivers protocol related parameters, for example

•the Basic Service Set Identification (BSSID)

•the beacon interval (next TBTT)

•PHY depending parameters

•the duration of the Contention Free Period (CFP)

•regulatory and spectrum management information, such as the available channels and the power limits.

Depending on the type of the BSS, not all information may be contained in what is broadcasted across the BSS in the beacon. In case of an infrastructure BSS the AP uses the beacon for instructions to its associated stations and announcements of future transmissions, e.g. delivery of multicast traffic to stations in power save mode. Stations may also use the signal strength of the received beacons to decide when to disassociate, because of channel conditions, and to which AP to associate again.