QUALITY OF SERVICE PERFORMANCES IN AD HOC IEEE

802.11 WIRELESS LANS

Maria Stella Iacobucci

Telecom Italia Learning Services, Via G. Falcone 25,67100 L’Aquila, Italy

Fabio Graziosi, Panfilo Ventresca

Università degli studi dell’Aquila, Località Monteluco di Roio, 67040 L'Aquila, Italy

Keywords: Quality of Service, Ad Hoc Networks, IEEE 802.11, WLANs

Abstract: WLAN is a wireless network which provides connectivity in a limited area. IEEE 802.11 is the most

widespread standard for wireless LANs, but it is not suitable for real time services. The draft standard

IEEE802.11e provides solutions for Quality of Service (QoS), and maintains the compatibility with the

IEEE802.11 standard. The paper, after a brief introduction on the WLAN technology, describes the

IEEE802.e solutions for QoS and provides simulation results in an ad hoc network with different loads. It is

shown that QoS in an ad hoc network can be provided with completely distributed techniques even if the

network is heavy loaded with real time services.

1 INTRODUCTION

WLAN is a wireless network which provides

connectivity in a limited area. IEEE 802.11 is the

most widespread standard for wireless LANs, which

includes two network topologies:

Infrastructure Network, consisting of a Distribution

System (DS) that connects two or more Access

Points (APs). Each AP provides a radio coverage

and each station (STA) is attached to one AP. One

AP with the attached STAs is called Basic Service

Set (BSS). The DS with the connected BSSs is an

Extended Service Set (ESS). The ESS allows the

communication between STAs belonging to

different BSSs.

Ad Hoc Network, where the STAs are connected

peer-to-peer. An Ad Hoc Network self-creates, self-

organizes, self-administrates. The STAs sharing a

radio channel form an Independent Basic Service Set

(IBSS). Performances of an Ad Hoc Network strictly

depend on the STAs number, on the mutual distance,

and on their instantaneous position. All the

algorithms are completely distributed.

IEEE 802.11 is the most widespread standard for

wireless LANs, but it is not suitable for real time

services. The draft standard IEEE 802.11e provides

solutions for Quality of Service (QoS), and

maintains the compatibility with the IEEE 802.11

standard.

The paper is organized as follows. Section 2

describes the IEEE 802.11 WLAN standard, section

3 deals with the required changes to the MAC

protocol for the support of QoS.

In section 4 the simulation scenario is described; in

section 5 simulation results are provided with

comments and conclusions.

2 IEEE 802.11 WLAN

The standard IEEE 802.11 defines the Physical

(PHY) and Medium Access Control (MAC) layer for

Wireless LANs, both for infrastructure and ad hoc

topologies.

The most important physical layers are:

b, which works at 2.4 GHz and provides up to 11

Mb/s

a, which works at 5 GHz and provides up to 54 Mb/s

g, which works at 2.4 GHz and provides up to 54

Mb/s. The g PHY is backwards compatible with the

b standard.

The MAC is unique for each PHY, and is based on

Carrier Sense Multiple Access/Collision Avoidance

(CSMA/CA). The implemented function is called

160

Stella Iacobucci M., Graziosi F. and Ventresca P. (2004).

QUALITY OF SERVICE PERFORMANCES IN AD HOC IEEE 802.11 WIRELESS LANS.

In Proceedings of the First International Conference on E-Business and Telecommunication Networks, pages 160-165

DOI: 10.5220/0001391501600165

 SciTePress

Distributed Coordination Function (DCF), and

works as follows. Before the transmission, each STA

listens the wireless medium: if it is heard free for a

DIFS (Distributed Inter Frame Space) time, the

STA sends the bits, otherwise it launches the backoff

procedure. The backoff procedure calculates a

backoff time through a random function which takes

uniformly distributed values between 0 and CW

(Contention Window), where CW is always:

min

≤ CW ≤ CW

max

Initially CW is initialised at CW

min.

The backoff

time is calculated as follows:

SlotTimeRandomCWtimeBackoff ×

= ()_

Standard values for the SlotTime

are 31µs for

802.11b and 15µs for 802.11a. Because the CW

value is chosen as a power of two minus one, if

Random() assumes integer values the Backoff_time

is:

SlotTimeRandomtimeBackoff

××−= ()])12[(_

where CW=2

-1. When a STA which has launched

the backoff procedure finds the medium as free, it

begins to decrement of a slot time the backoff timer

until it relieves the medium as occupied. When this

timer reaches zero value, the station transmits the

Mac Service Data Unit (MSDU). Each MSDU

which has been correctly received must be

acknowledged with and ACK frame. If the ACK is

not received into an ACKtimeout time, then the

transmission is considered unsuccessful and the

backoff procedure is launched by duplicating the

previous CW value as follows:

1)1(2 −+×=

oldnew

CWCW

The backoff time at the i-th tentative of access is:

SlotTimerandomtimeBackoff

××−=

()])12[(_

When a STA has successful transmitted a MSDU, it

launches a post-backoff procedure in order to allow

other stations to access the medium.

DCF is the basic MAC for both infrastructure and ad

hoc 802.11 networks. An added function, not

mandatory from the standard, is the Virtual Carrier

Sense (VCS), which solves the hidden node problem

with RTS/CTS frames. Finally, the PCF is an

optional access technique which can be implemented

only in infrastructure networks where the Point

Coordinator (PC) regulates the access to the

medium during a time called Contention Free

Period.

3 QOS IN IEEE 802.11

DCF is for best effort services, because it does not

provide QoS. In fact all the stations belonging to a

BSS or an IBSS compete with the same priority to

access the same wireless medium.

In the standardization bodies was accepted that QoS

mechanisms had to be added in the 802.11 standard,

and in 1999 the task group TGe was created, that

later gave birth to the draft standard IEEE 802.11e.

This new MAC maintains the compatibility with

802.11.

In IEEE 802.11e, the stations are named QSTA, the

BSS and IBSS are QBSS and QIBSS. It is worth to

underline that the existence of a QBSS or a QIBSS

does not preclude the good functioning of the non-

QoS stations.

With IEEE 802.11e the MAC is enhanced by the

Enhanced Distributed Coordination Function

(EDCF), which is a completely distributed technique

that provides a service differentiation based on 8

priority levels, named User Priority (UP). Each

QSTA can manage 4 Access Categories (AC), and

each AC has a different value for DIFS, which in

IEEE 802.11e is renamed AIFS (Arbitration

InterFrame Space), CW

min

and CW

max

. In general,

the higher is the priority, the lower are AIFS[AC],

min

[AC] and CW

max

[AC] parameters.

The mapping between UP and AC is represented in

table 1.

Table 1: mapping between UP and AC

User

Priority

(UP)

IEEE Definition Access

Category

(AC)

0 Best Effort (BE) 0

1 Background (BK) 0

2 - 0

3 Excellent Effort (EE) 1

4 Controlled Load (CL) 2

5 Video 2

6 Voice 3

7 Network Control (NC) 3

The backoff interval is given by:

SlotTimeRandomtimeBackoff ×

()_

where

()Random

assumes uniformly distributed

values in (1, CW[AC]+1).

In EDCF there are 4 traffic queues, where are

mapped the eight defined User Priorities.

QUALITY OF SERVICE PERFORMANCES IN AD HOC IEEE 802.11 WIRELESS LANS

161

Each queue is characterized by different access

parameters.

Each time the backoff procedure is relaunched

because of transmission failure of internal collision,

the CW value is updated as follows:

{}

12)1]AC[],AC[min]AC[

max

−

oldnew

CWCWCW

Once a STA has won the competition to access the

medium, it transmits MSDUs for a period whose

maximum depends on the AC and is

TXOPlimit[AC]. Therefore the in EDCF a STA is

allowed to transmit more than a frame without

having to regain access to the channel.

4 SIMULATION SCENARIO

Simulations have been performed with Network

Simulator version2, which is an open source

simulator for different kinds of telecommunication

networks.

The simulated scenario is the ad hoc network

represented in figure 1. The ad hoc network is

composed of 6 wireless stations in fixed positions.

Each STA performs one or more transmissions of

real time (voice, videoconference), multimedia

(video and audio streaming) and best-effort (data

transfer) applications.

Figure 1: Simulation scenario 1

The simulation time has been divided into three sub-

intervals where the different traffic sources start,

until the network load reaches the full load.

As shown in figure 2, in the first time interval, voice

and videoconference start; in the second time

interval, streaming audio and audio-video are added;

in the third time interval, the network is loaded with

best effort data transfers until the saturation point of

the network. The network load is 17% in the first

time interval (30 s), 88% in the second time interval

(30 s), 104% in the third time interval (30 s).

Figure 2: Source timing

For each traffic source the following models have

been used.

Voice: bi-directional CBR source at 64 kb/s,

packet length 160 bytes, packet interarrival time 20

Videoconference: the traffic is composed of

video and audio, and is modelised as follows:

Video: bi-directional CBR source at 98.4 kb/s,

packet length 1500 bytes, packet interarrival time

122 ms

Audio: bi-directional CBR source at 6.3 kb/s,

packet length 160 bytes, packet interarrival time 203

MPEG-2 video stream: unidirectional CBR

source at 4 Mb/s, packet length 1504 bytes, packet

interarrival time 3 ms

MP3 audio stream: unidirectional CBR source at

128 kb/s, packet length 512 bytes, packet interarrival

time 32 ms

Data traffic: unidirectional CBR source at 128

kb/s, packet length 1500 bytes, packet interarrival

time 9.6 ms

The transport layer implemented for simulations is

UDP for voice, videoconference, MPEG-2 and MP3,

and TCP for data transfer.

Table 2: MAC parameters chosen for DCF and EDCF

simulations

DCF and EDCF common values

SlotTime 20 ms

SIFS 10 ms

PIFS 30 ms

DIFS 50 ms

FragmentationThreshold 2304 byte

RTSThreshold 300 byte

MaxTrasmitMSDULifetime 512*1024 µs

DCF access parameters

min

, CW

max

31, 1023

EDCF access parameters

AIFS[3], CW

min

[3], CW

max

[3] 2, 7, 31

AIFS[2], CW

min

[2], CW

max

[2] 4, 15, 256

AIFS[1], CW

min

[1], CW

max

[1] 8, 31, 1023

AIFS[0], CW

min

[0], CW

max

[0] 16, 63, 1023

DATA

MPEG2

MP3

VOICE

(

)

DATA

(

)

STA1

STA0

STA2

STA3

STA4

STA5

Voice + Videoconference (audio+video)

time (s)

MPEG2+MP3

Data

9060

Traffic

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Table 3: traffics and ACs mapping

Priority Traffic sources AC

6-7 (Very

High Priority)

Voice,

Videoconference (audio

and video)

4-5 (High

Priority)

Streaming audio (MP3)

and streaming video

(MPEG-2)

3 (Excellent

Effort)

- 1

0-2 (Best

Effort)

Data transfer 0

Table 2 shows MAC parameters chosen for DCF

and EDCF simulations. The common values are

default values of the IEEE 802.11b physical level;

EDCF parameters have been chosen in order to

obtain good differentiation among access categories.

The RTSThreshold has been chosen such that the

RTS/CTS mechanism is not applied to voice and

audio of the videoconference traffic.

Table 3 shows the mapping among traffic sources

and ACs; table 4 presents physical parameters used

for simulations. Carrier sense and Rx ranges have

been computed considering antennas height of 1.5

Table 4: PHY parameters chosen for simulations

Definition Value

(dBm) 20

RXThresh (dBm) -50

CSThresh (dBm) -56

Propagation model Two-ray ground

Carrier Sense range (m) 113

Rx range (m) 80

Max Trasmission Rate (Mbit/s) 11

Min Trasmission Rate (Mbit/s) 1

In the simulation scenario, STA1 and STA2, STA3

and STA4, STA5 and STA6 are 50 m away; STA1

and STA3, STA3 and STA5, STA2 and STA4,

STA4 and STA6 are 60 m away.

The magnitudes that have been computed for QoS

performance evaluation are: throughput, delay, jitter,

Packet Loss Ratio (PLR), IP Packet Delay Variation

(IPDV). The IPDV is defined as follows:

minmax

IPTDIPTDIPDV −=

where

max

IPTD

and

min

IPTD

are respectively the

maximum and minimum IP Packet Transfer Delay

(IPTD) measured in the considered interval.

Table 5 shows needed QoS parameters for the

described simulation scenario.

Table 5: QoS parameters for the described simulation

scenario

QoS Parameters

Applications

Data

Rate

Delay

end-

to-end

IPDV PLR%

Voice

Kbit/s

<50ms <50ms < 5%

Real-time audio

(videoconference)

6.3

Kbit/s

<50ms <50ms < 3%

Real-time video

(videoconference)

98.4

Kbit/s

<50ms <50ms < 1%

Streaming audio

MP3

128

Kbit/s

<10 s 1 s < 1%

Streaming video

(MPEG-2)

Mbit/s

<10 s 1 s < 1%

Data transfer

128

Kbit/s

<60 s - zero

5 SIMULATION RESULTS

Simulations compare ad hoc network performances

between DCF and EDCF MAC in terms of

throughput, average and instantaneous delay, jitter,

IPDV.

Figure 3: MPEG-2 throughput

Figure 4: MP3 throughput

Figures 3, 4 and 5 show a comparison of MPEG-2,

MP3 and voice instantaneous and average

QUALITY OF SERVICE PERFORMANCES IN AD HOC IEEE 802.11 WIRELESS LANS

163

throughputs between DCF and EDCF functions.

EDCF maintains good throughputs even at high

network loads.

Figures 6 and 7 show the delay of voice and

videoconference traffics, and highlight that delay

limits are not guarantied with DCF even in the

second simulation interval. Table 6 shows the

videoconference average delay, jitter, and IPDV

with DCF and EDCF functions. EDCF guarantees

QoS limits even in the third simulation interval,

when the network is high loaded. In tables 7 is

shown that the probability that the delay of real time

traffic is lower than 50 ms is always higher than

99% with EDCF, and is sometimes lower than 50%

with DCF.

Finally, table 8 shows PLR for EDCF and DFC,

highlighting EDCF better performances.

Figure 5: Voice throughput

Figure 6: Voice delay

Figure 7: Videoconference (audio) delay

Table 6: Average delay, jitter, IPDV of videoconference

traffic

Application s MAC

Avera

delay

(ms)

Jitter

(ms)

IPDV

(ms)

DCF 6.61 25.8 58.95

Videoconf.

(audio)

STA0→STA3

60-

EDCF 1.77 3.48 4.19

DCF 446.43 46.1 555

30-

EDCF 1.61 4.47 13.17

DCF 618.58 121. 285.7

Videoconf.

(audio)

STA3→STA0

60-

EDCF 1.64 3.46 5.89

DCF 6.97 25.2 67.14

Videoconf.

(video)

STA0→STA3

60-

EDCF 3.21 3.94 8.81

DCF 446.12 47.8 559.3 30-

EDCF 2.93 4.17 8.89

DCF 666.73 149. 282.5

Videoconf.

(video)

STA3→STA0

60-

EDCF 3.18 3.92 6.13

Table 7: probability that the delay of real time traffic is

lower than 50 ms

REFERENCES

ANSI/IEEE Std 802.11, 1999. In IEEE Standard for

Information Technology - between system - Local and

metropolitan area network - Specific requirements –

Part 11: Wireless LAN Medium Access Control (MAC)

and Physical Layer (PHY) Specifications.

Application MAC P(delay<50 ms)

DCF 82.34 % Voice

STA1→STA2

EDCF 99.88 %

DCF 99.64 % Voice

STA0→STA4

EDCF 100 %

DCF 100 % Voice

STA4→STA0

EDCF 100 %

DCF 99.77 % Videoconference

(audio)

STA0→STA3

EDCF 100 %

DCF 40.72 % Videoconference

(audio)

STA3→STA0

EDCF 100 %

DCF 99.59 % Videoconference

(video)

STA0→STA3

EDCF 100 %

DCF 39.03 %

Videoconference

(video)

STA3→STA0

EDCF 100 %

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164

IEEE Std 802.11b, 1999. In Supplement to IEEE Standard

for Information technology – Telecommunication and

information exchange between system – Specific

requirements – Part 11: Wireless LAN Medium Access

Control (MAC) and Physical Layer (PHY)

specifications: Higher-Speed Physical Layer

Extension in the 2.4 GHz Band.

IEEE 802.11e/D3.3, 2002. In Draft Supplement to

STANDARD FOR Telecommunications and

Information Exchange Between System – LAN/MAN

Specific Requirements – Part 11: Wireless Medium

Access Control (MAC) and physical layer (PHY)

specifications: Medium Access Control (MAC)

Enhancements for Quality of Service (QoS)

http://www.isi.edu/nsnam/ns/

P. Coverdale, 2001. In Workshop on QoS and user

perceived transmission quality in evolving networks

ITU-T Recommendation Y.1541, 2002.

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