3.3 Medium Access Control

29

After an unsuccessful transmission, the next backoff is performed with a doubled size of the contention window. This reduces the collision probability in case there are multiple stations attempting to access the channel. The stations that deferred from channel access during the channel busy period do not select a new random backoff time, but continue to count down the time of the deferred backoff in progress after sensing the channel as being idle again. In this way, stations, that deferred from medium access because their random backoff time was larger than the backoff time of other stations, are given a higher priority when they resume the transmission attempt. Figure 3.8 illustrates the increase of the contention window upon unsuccessful transmissions. Note that a station cannot differentiate between collision and failed transmission due to errors on the wireless channels. A missed ACK frame will always be interpreted as collision.

3.3.1.2Post-Backoff

After each successful transmission, it is mandatory that another random backoff is performed by the transmission-completing station, even if there is no other MSDU to be delivered, as indicated in Figure 3.9.This is referred to as “postbackoff,” as this backoff is done after, not before, a transmission. This can be interpreted as the backoff for the next MSDU Delivery. By using this postbackoff, it is guaranteed that any frame (with the exception of the first MSDU in

			random start
first	previous	DIFS		new frame		backoff
attempt	frame exchange			exchange		stage 1
			CW = CWmin -1	random start
			(= 15 in 802.11a)
					time
second	previous	DIFS		repeated frame		backoff
attempt	frame exchange			exchange		stage 2
			CW = 22-1 x CWmin -1 (= 31 in 802.11a)
				random start
3rd	previous	DIFS		repeated frame exchange		backoff
attempt	frame exchange					stage 3

CW = 23-1 x CWmin -1 (= 63 in 802.11a)

(upon unsuccessful attempts, the CW continues to grow, up to CWmax)

random start

6th and	previous	DIFS		repeated frame
following
attempts	frame exchange			exchange
(until retry counter exceeds			CW = 26-1 x CWmin -1 = CWmax (= 1023 in 802.11a)	backoff
limit, or, until ACK reception)				stage 6

Figure 3.8: Increase of contention window size after unsuccessful frame exchanges. The size is doubled for each new attempt of collided or erroneously received MSDUs, up to a certain limit. The actual numbers vary with the PHY specifications.

30					3. IEEE 802.11
NAV (timer)				post backoff, without any pending data
transmission		random	S	to transmit: station 2 draws random
				backoff (here 10 slots, after 2 slots new
		backoff	I
station 1				transmission, keeps backoff counter =8)
		(6 slots)	F ACK
			S	D
station 2	D	DATA			station
	I			I	defers
	F			F
	S	random		S
				remaining
station 3		backoff		backoff
		(8 slots)		(2 slots)
station 4	D	station 4 defers,		D	DATA
	I	keeps backoff		I
	F	counter (=2)		F
	S			S
busy			STA 3 and 4 set NAV upon
					time
			receiving first frame of transmission
channel

Figure 3.9: Post-backoff. A post-backoff is performed after each successful frame exchange, regardless of the station has MSDUs to deliver or not.

a burst, arriving at an empty queue and during an idle phase) will be delivered with backoff. An MSDU arriving at the station from the higher layer may be transmitted immediately without waiting any time, if the transmission queue is empty, the latest post-backoff has been finished already, and at the same time, the channel has been idle for a minimum duration of DIFS. This helps to reduce the delivery delay in lightly loaded systems.

3.3.1.3Recovery Procedure and Retransmissions

When a frame exchange is not successful, i.e., when a transmitting station does not receive an ACK frame immediately after the frame transmission, the frame size of the transmitted frame is compared against a threshold value before retransmission.

All unsuccessful transmissions of frames with a frame size shorter than the threshold value, and all failed RTS transmissions, increment the Short Retry Counter (SRC). If the SRC reaches a limit (default: 7), the frame is discarded.

All unsuccessful transmissions of frames with a frame size larger than the given threshold, increment the Long Retry Counter (LRC). Again, no more retransmission attempts are made, when LRC is equal to a limit (default: 4). Whenever an MSDU is successfully transmitted, SRC and LRC are reset. The actual value of the threshold is implementation dependent.

3.3.1.4Fragmentation

To reduce the duration the channel is occupied when frames collide, data frames (MSDUs) can be transmitted in more than one MPDU, if their length exceeds a certain threshold. The process of partitioning an MSDU into smaller MPDUs is called fragmentation. See Figure 3.10 for an illustration of fragmentation, where

3.3 Medium Access Control

31

also the complicated protection of frames by the NAV vectors is illustrated. An MPDU protects the subsequent transmissions of its ACK responses within its duration field, see Figure 3.4, and in addition, when fragmentation is used, the following MPDU.

Fragmentation creates MPDUs smaller than the original MSDU length to limit the probability of long MPDUs colliding and being transmitted more than once. With fragmentation, a large MSDU can be divided into several smaller data frames, i.e., fragments, which can then be transmitted sequentially as individually acknowledged frames. The benefit of fragmentation is, that in case of failed transmission, the error is detected earlier and there is less data to retransmit. It also increases the probability of successful transmission of the MSDU in scenarios where the radio channel characteristics cause higher error probabilities for longer frames than what can be expected for shorter frames. Each fragment can be transmitted sequentially as individually acknowledged data frame. The obvious drawback is the increased overhead. The process of recombining MPDUs into a single MSDU is called defragmentation, which is accomplished at each receiving station. Only MPDUs with a unicast receiver address may be fragmented. Broadcast/multicast frames may not be fragmented even if their length exceeds the implementation dependent threshold. Note the maximum length of an MSDU is limited to 2346 byte.

3.3.1.5Hidden Stations and RTS/CTS

In wireless communication systems that use carrier sensing, the so-called hidden station problem can occur, depending on the locations of the stations. This problem arises when a station is able to successfully receive frames from two different stations but the two stations cannot detect each other.

station 1 station 2

random

S

S

S

S

I

ACK

I

ACK

I

ACK

I

ACK

backoff (6 slots)

F

F

F

F

S

1

S

2

S

3

S

4

D

DATA 1

S

DATA 2

S

DATA 3

S

DATA

I

I

I

I

4

NAV

F

F

F

F

S

S

S

S

reset

random backoff (8 slots)

NAV ACK 3

station 3	D
	I
	F	NAV ACK 1	NAV ACK 2	NAV DATA 3	NAV
	S
		station defers, but NAV DATA 1	NAV DATA 2		DATA 4
NAV (timer)
transmission		keeps backoff		time
		counter (=2)

Figure 3.10: Fragmentation. Data frames protect the subsequent transmissions of their ACK responses and the following data frame with the NAV.

32	3. IEEE 802.11

When stations cannot detect each other, a station may sense the channel as idle even when other hidden stations are transmitting. It may initiate a transmission while the other station is transmitting already. This may result in a collisions and severely interfered frames at stations that can detect coinciding transmissions of hidden stations.

To reduce throughput reduction owing to hidden stations, 802.11 allows the optional use of a Request-to-Send/Clear-to-Send (RTS/CTS) mechanism. Before transmitting a frame, a station has the option to transmit a short RTS frame, which must be followed by a CTS frame transmission by the receiving station. Between two consecutive frames in the sequence of RTS, CTS, data, and ACK, a Short Interframe Space (SIFS), which is 16 us for 802.11a, gives transceivers time to turn around. It is a decision made locally by the transmitting station, if or if not RTS/CTS is used. Upon receiving an RTS frame, the receiving station has to reply with a CTS frame. The RTS and CTS frames include the information of how long it does take to transmit the next data frame, e.g., the first fragment, and the corresponding ACK frame. Hence, other stations close to the transmitting station and hidden stations close to the receiving station will not start any transmissions; their NAV timer is set. A hidden stations close to the receiving station might not receive the RTS due to the large distance, but will in most cases receive the CTS frame.

See Figure 3.11 for an example of the DCF using RTS/CTS. It is important to note that SIFS is shorter than DIFS, which gives CTS and ACK always the highest priority for access to the radio channel.

				S		S	new random
			random					backoff
				I		I
station 1								(10 slots)
			backoff	F CTS		F ACK
			(7 slots)	S		S	D
station 2		D	RTS	S	DATA			station
		I		I			I	defers
		F		F			F
							S
	NAV	S	random	S				remaining
station 3	reset		back-off					backoff	ACK
			(9 slots)					(2 slots)
station 4		D					D	DATA	S
		I					I		I
	S	F			stations set NAV upon		F		F
		S			receiving RTS		S		S
station 5	I
	F ACK
	S
station 6	DATA		station 3
			defers, but
			keeps backoff	station 6 sets NAV upon receiving CTS,		NAV			NAVs
			counter (=2)
						updates
NAV (timer)				this station is hidden to station 1
transmission								time

Figure 3.11: Timing of frame exchanges and NAV settings of the 802.11 DCF. Station 6 cannot detect the RTS frame of the transmitting station 2, but the CTS frame of station 1. Although station 6 is hidden to station 1, it refrains from channel access because of NAV.