PERFORMANCE ANALYSIS OF CSMA/CA PROTOCOL IN IEEE

802.11 NETWORKS USING BACKOFF MECHANISM

Amith M. N.

Mascon Global Limited, Bangalore, Karnataka, India

Keywords: Backoff, CSMA/CA, IEEE 802.11, Wireless LAN.

Abstract: The distributed coordination function (DCF) in the IEEE 802.11 standard for wireless LAN is based on the

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) like medium access control protocol.

This collision avoidance is implemented by means of backoff procedure, which uses a rotating window

mechanism. This paper presents simulation studies along with the analytical approach towards the slot

selection probabilities and backoff mechanism. The obtained simulations allow us to determine the effective

throughput versus offered load for different values of the contention window parameter and the number of

the contending stations. The choice of different CWmin and CWmax parameters are analyzed.

1 INTRODUCTION

With the spread of high performance portable

computers, data terminals, and devices such as

personal digital assistants, Wireless LANs (WLANs)

have become an emerging technology for today’s

computer and communication industries. WLANs

have also been used in environments where cable

installation is expensive or impractical. Two

organizations have been involved in standardizing

the physical layer and the medium access control

layer for WLANs and the standards are IEEE 802.11

wireless LAN and ETSI HIPERLAN. This paper

considers only the IEEE 802.11 Medium Access

Control (MAC) layer.

The backoff mechanism has been intensively

studied in the literature since the beginning of 70’s.

The idea of using the backoff mechanism in the

MAC layer of the IEEE 802.11 standard has brought

a new interest in such a mechanism (Natkaniec and

Pach, 2000). The proper selection of backoff

parameters is an essential issue for the network

performance. For example, the problem of unequal

slot selection probabilities was considered in

(Woesner et al., 1995).

2 IEEE 802.11 MAC

The IEEE 802.11 standard covers the MAC sub-

layer and the physical layer of the Open System

Interconnection (OSI) reference model. This paper

mainly focuses on the MAC part of the standard.

IEEE 802.1l standard supports two services:

Distributed Coordination Function (DCF) and Point

Coordination Function (PCF). DCF is particularly

made for asynchronous data traffic, where users with

data to transmit have an equally fair chance of

accessing the network. PCF is the second MAC

service. The PCF is based on Polling that is

controlled by an access point (AP). The PCF is

primarily designed for the transmission of delay-

sensitive traffic. This paper considers DCF MAC

scheme with Carrier Sense Medium Access with

Collision Avoidance (CSMA/CA) protocol as the

medium access control protocol.

3 BASIC ACCESS METHOD: DCF

The Basic access method in the 802.11 standard is

DCF, which is essentially CSMA/CA. In CSMA, a

node with a packet to transmit first senses the

medium to detect any on going transmission. If the

medium is busy (i.e. some other node is

transmitting), the node differs its transmission to

later time. If the medium is sensed to be free for

duration greater than the DCF interframe space

(DIFS) interval, the packet is transmitted

478

M. N. A. (2006).

PERFORMANCE ANALYSIS OF CSMA/CA PROTOCOL IN IEEE 802.11 NETWORKS USING BACKOFF MECHANISM.

In Proceedings of the Third International Conference on Informatics in Control, Automation and Robotics, pages 478-481

DOI: 10.5220/0001205004780481

 SciTePress

immediately. The sending station will recognize a

successful transmission by means of an

acknowledgement (ACK) packet from the receiving

station. The receiving station, on successful

reception of packet delivers an ACK packet to

source station after short inter frame space interval

(SIFS). All stations except the source station in the

BSS use the duration field of the data packet to

adjust their Network Allocation Vector (NAV),

which indicates the amount of time that must elapse

until the end of the current transmission, for

deferring access to the medium. CSMA is very

effective when the medium is lightly loaded since

the protocol allows nodes to transmit with minimum

delay. Due to finite propagation delay along the

transmission medium, there is a probability of two or

more nodes simultaneously sensing the medium as

being free and transmitting at the same time, thereby

causing a collision. Clearly, when the network is

heavily loaded, such collisions will occur often

(Woesner et al., 1995) (Crow et al., 1997).

The CSMA protocol incorporates a Collision

Avoidance (CA) scheme that introduces a random

interframe space which is called as backoff between

successive packet transmissions. Collision avoidance

part is performed to reduce the high probability of

collision immediately after a successful packet

transmission. If the medium is detected to be busy,

the node must first delay until the end of the DIFS

interval and further wait for a random number of

time slots i.e. the backoff interval, before attempting

transmission. The backoff mechanism is explained

in detail in the later sections.

In wireless scenario, it cannot be guaranteed that

all station will be able to listen to each other. Some

station will be hidden to other simply because they

are out of the range of this station. Carrier sensing is

difficult due to the hidden terminal scenario where

collisions cannot be securely detected at the sending

stations. The DCF protocol is enhanced further by

provision of a virtual carrier sense indication called

Network Allocation Vector, which is based on

duration information transferred in special RTS/CTS

frames before the data exchange. It allows stations to

avoid transmission in time intervals in which the

medium is surely busy.

4 EXPONENTIAL BACKOFF

MECHANISM

IEEE 802.11 backoff mechanism follows rotating

window mechanism for the backoff procedure as

follows: whenever a station gets ready with a packet

for transmission and senses the medium busy it has

to choose a random number between (0,CW) (CW:

Contention Window) and wait for this number of

slots before accessing the medium. Contention

window is the range of slots within which a station

selects random slots during backoff process. For

every station, for a new packet, the value of

contention window will be equal to minimum value:

CWmin, and it will increase exponentially as it

experiences collision until a maximum value:

CWmax, and sets back CWmin on every successful

transmission (Natkaniec and Pach, 2000).

At each backoff time slot, carrier sensing is

performed to determine if there is any activity on the

medium. The station that selected the short random

time will gain access for transmission, the others

freeze their backoff timers until the winning

station’s transmission is finished and wait for the

remaining time in the following cycle. In this case,

when the medium becomes idle again for a period

greater then the DIFS, the backoff procedure

continues decrementing from the time slot, which

was previously disrupted. Hence a packet that was

delayed while performing the backoff procedure has

a high probability of being transmitted earlier than a

newly arrived packet. The process is repeated until

the backoff interval reaches zero and once it reaches

zero the packet is transmitted immediately. That way

station that has been waiting for long to gain access

is more likely to win this competition than another

that just entered it. The probability of gaining access

to medium increases with the time waited. If two or

more stations complete their backoff procedure at

the same time, or else if the stations select the same

slot then collision will occur.

When retransmission is necessary, the backoff

interval increases exponentially up to a certain

threshold. Conversely, the backoff interval reduces

to minimum value when packets are transmitted

successfully. Now, each station will have to increase

its CW exponentially (until the maximum CWmax)

and then select again a new random slot between 0

and CW. For every retransmission attempt, the

backoff time grows as a function of 2

where i is the

number of collision encountered. And whenever

there is a successful transmission for a station its

contention window is brought back to minimum CW

value-CWmin. The effect of this procedure is that

when multiple stations are deferring and go into

random backoff, then the station selecting the

smallest backoff time slot will win the contention

PERFORMANCE ANALYSIS OF CSMA/CA PROTOCOL IN IEEE 802.11 NETWORKS USING BACKOFF

MECHANISM

479

(Natkaniec and Pach, 2000). The situation is best

presented in the fig 1.

Figure 1: Backoff Mechanism.

This paper also discusses about the slot selection

probabilities of the backoff mechanism. Considering

the initial state it appears that each slot is selected

with the same probability. In the next cycle all

stations that competed for access, reduce their

backoff times. The new value is reduced by the time

that elapsed until the wining station started its

transmission. Within this reduced contention

window all slots are selected with the same

probability by the remaining stations. So, if a new

station enters the competition or stations that

collided in the previous cycle will return back into it,

then they will choose slots within the whole range of

contention window with the same probability

(Woesner et al., 1995). Assuming a situation in a

wireless LAN under high load, i.e. there are always

stations left in the competition as well as there are

always stations entering the competition, and in an

equilibrium state, it is seen that slots positioned early

in the contention window have a much higher

probability to be chosen. The result is a decreasing

staircase function for the slot selection probability

equilibrium state.

5 SIMULATION RESULTS

In order to reduce the complexity of the wireless

LAN scenario, some assumptions are made in the

simulation of wireless LAN model. They are as

follows:

• All network nodes are assumed to be in radio

contact with each other.

• The channel is assumed to be ideal with no

transmission errors.

• The propagation delay between the

transmissions is neglected.

• It is assumed that a transmitted MAC protocol

data unit (MPDU) will immediately be received

by the destination station without being

buffered.

• All stations are assumed to be symmetric users.

• Packets are assumed to be of fixed length.

In the simulation, the wireless LAN was

simulated with DCF access scheme using the

CSMA/CA protocol. The gathered simulation results

allow us to determine the slot selection probabilities

in the equilibrium state and the throughput analysis

versus offered load for different CW parameters.

The next group of simulation results presents the

realized throughput versus the number of stations for

different values of the CW parameters. The offered

load is the inverse of the packet inter-arrival rate. As

the inter-arrival rate increases the load decreases and

vice versa.

The result plot in fig. 2 presents slot selection

probability at the equilibrium state obtained for 25

stations with CWmin = 32 and CWmax = 256. By

the analysis of this result it can be inferred that as

the system reaches equilibrium state the higher

numbered slots are selected with lower probability

and the low numbered slots are selected with higher

probability leading to an increase in the number of

collisions.

5000

10000

15000

20000

25000

Number of slot

selections

1 27 53 79 105 131 157 183 209 235

Slot Number

Slot Selection Probability

Figure 2: Slot selection probability at the equilibrium state

obtained for 25 stations.

The analysis of the different backoff parameters with

respect to the window size is presented in the figures

3, 4 and 5.

• The realized throughput for different number of

stations versus offered load for four different

values of CWmin and CWmax.

• The realized throughput versus the number of

stations for different values of CWmin and

CWmax.

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Figure 3: Throughput versus load for 25 stations.

Figure 4: Throughput versus load for 100 stations.

Figure 5: Throughput versus number of stations.

The results obtained for 25 contending stations

are presented in fig 3. Almost best results are

obtained for the CWmin =128 with CWmax = 1024

and CWmin = 512 with CWmax = 4096. The

performance of the system is heavily degraded for

CWmin = 8 and CWmax = 64 with a steep decrease

in the throughput under heavily loaded conditions.

As the window size decreases and the number of

participating stations increases the number of

collisions increases and the performance of the

system degrade under high load conditions. The

results obtained for the 100 participating stations are

shown in the fig 4 with best results obtained for

CWmin = 512 and CWmax = 4096. A very small

degradation in performance brings the choice of

CWmin = 128 and CWmax = 1024 parameters. In

all other parameters the degradation of the

performance is seen. The dependency between the

number of stations and the throughput for different

values of CWmin and CWmax is as shown in fig 5.

By these plots it can be inferred that it is better to

select less value of CWmin for less number of

stations and large value of CWmin parameters for

large number of participating stations.

6 CONCLUSION

The results reported in this paper clearly shows that

the CSMA/CA basic protocol suffers from several

performance drawbacks. DCF is the fundamental

access method with CSMA/CA protocol used to

support the asynchronous data transmission. To

avoid the collisions the random backoff mechanism

is employed. The slot selection probability obtained

is in exponentially decreasing nature from which this

paper concludes that collision increases due to this

characteristic of slot selection.

The analysis of the backoff parameters led to

some important conclusions presented below:

• Using very small values of CWmin at large

number of stations brings large collisions and

degrades the network performance.

• Using very large CWmin at small number of

stations degrades the performance due to

wastage of bandwidth.

Hence the choice of the CWmin is dependent on the

number of participating stations.

REFERENCES

Marek Natkaniec and Andrzej R. Pach, “An Analysis of

the Backoff Mechanism used in IEEE 802.11

Networks”, IEEE: 2000.

Woesner H., Weinmiller J., Wolisz A., Modified Backoff

Algorithms for DFWMAC DCF, IEEE 802.11

Working Group paper: JULY 1995.

Brian P. Crow, I Widjaja, “IEEE 802.11 Wireless Local

Area Networks”, IEEE Communications Magazine:

September 1997.

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MECHANISM

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