A COMPARATIVE STUDY OF 802.11 AND 802.11E WIRELESS

LAN STANDARDS

Fedoua Didi

University Abou Bekr Belkaid of Tlemcen Algeria

Houda Labiod

ENST of Paris France

Guy Pujolle

LIP6 of University Pierre and Marie Currie France

Keywords: IEEE 802.11, Medium Access Control (MAC), Quality of Service (QoS), Distributed Coordination

Function (DCF), Point Coordination Function (PCF), Hybrid Coordination Function (HCF), IEEE 802.11e,

Network Simulator (NS).

Abstract: Quality of service (QoS) is a key problem in wireless environments where bandwidth is scarce and channel

conditions are time varying and sometimes highly loss. Although IEEE 802.11 wireless LAN (WLAN) is

the most widely used WLAN standard today, and the upcoming IEEE 802.11e QoS enhancement standard

exists and introduces the QoS for supporting multimedia applications. This paper compares the propositions

of standard IEEE 802.11e with the standard IEEE 802.11 without QoS, a simulation of these standards is

performed by using the NS simulator. A discussion is presented in detail using simulation-based evaluations

and we let us confirm the QoS of IEEE 802.11e compared to IEEE 802.11, but we have detected some

weaknesses of 802.11e. It starves the low priority traffic in case of high load, and leads to higher collision

rates, and did not make a good estimate of weight of queues, so there is an unbalance enters the flows with

high priorities. We finish with a conclusion.

1 INTRODUCTION

IEEE 802.11 wireless LAN (WLAN) (IEEE 802.11

WG, 1999) is one of the most deployed wireless

technologies all over the world and is likely to play a

major role in next generation wireless

communications networks. The main characteristics

of 802.11WLAN technology are simplicity,

flexibility and cost effectiveness. This technology

provides people with a ubiquitous communications

and computing environment in offices, hospitals,

campuses, factories, airports, stock markets, etc.

Simultaneously, multimedia applications have

experienced an explosive growth. People are now

requiring to receive high speed video, audio, voice

and Web services even when they are moving in

offices or travelling around campuses. However,

multimedia applications require some quality of

service support such as guaranteed bandwidth,

delay, jitter and error rate. Guaranteeing those QoS

requirements in 802.11 WLAN is very challenging

due to the QoS unaware functions of its medium

access control (MAC) layer and the noisy and

variable physical (PHY) layer characteristics. In this

paper we compare the two standards 802.11 and

802.11e by using a simulation with Network

Simulator (NS) and present a detailed discussion of

results. The paper is organized as follows. Section 2

introduces an overview of IEEE 802.11 WLAN and

section 3 introduces the QoS enhancement standard

802.11e. In section 4, we present the model of

simulation with its parameters and a detailed

discussion of results. We finish with a conclusion.

133

Didi F., Labiod H. and Pujolle G. (2006).

A COMPARATIVE STUDY OF 802.11 AND 802.11E WIRELESS LAN STANDARDS.

In Proceedings of WEBIST 2006 - Second International Conference on Web Information Systems and Technologies - Internet Technology / Web

Interface and Applications, pages 133-139

DOI: 10.5220/0001238601330139

 SciTePress

2 DESCRIPTION OF 802.11

STANDARD

2.1 Introduction

The IEEE 802.11 WLAN standard covers the MAC

sub-layer and the physical (PHY) layer of the open

system interconnection (OSI) network reference

model (IEEE 802.11 WG, 1999). Logical link

control (LLC) sub-layer is specified in the IEEE

802.2 standard. This architecture provides a

transparent interface to the higher layer users:

stations (STAs) may move, roam through an 802.11

WLAN and still appear as stationary to 802.2 LLC

sub-layer and above. This allows existing TCP/IP

protocols to run over IEEE 802.11 WLAN just like

wired Ethernet deployed. We can show (Aad I,

Castelluccia C., 2001) different standardization

activities done at IEEE 802.11 PHY and MAC

layers.

The standard comprises three PHY layers, which

are an InfraRed (IR) base band PHY; a frequency

hopping spread spectrum (FHSS) radio and direct

sequence spread spectrum (DSSS) radio. These

entire choices support both 1 and 2Mbps PHY rate.

In 1999, the IEEE define two high rate: 802.11b in

the 2.4GHz band with 11Mbps, based on DSSS

technology; and 802.11a in the 5GHz band with

54Mbps, based on orthogonal frequency division

multiplexing (OFDM) technology. Recently,

802.11g is finalized to be an extension of 802.11b

with 54Mbps in the 2.4GHz band.

2.2 The MAC Sub-Layer of 802.11

It defines two medium access coordination

functions, the basic Distributed Coordination

Function (DCF) and the optional Point Coordination

Function (PCF) (IEEE 802.11 WG, 1999).

Asynchronous transmission is provided by DCF

which operate in contention-based period, and

synchronous transmission is provided by PCF that

basically implements a polling-based access which

operate in contention free period. A group of STAs

coordinated by DCF or PCF is formally called a

basic set (BSS). The area covered by BSS is the

basic service area (BSA), like a cell in a cellular

mobile network.

Two modes exist: ad-hoc mode and

infrastructure mode. The first mode forms an Independent

BSS (IBSS) where the STAs can directly communicate

with each other by using only the DCF, without any

connectivity to any wired backbone. In the second mode,

the STAs communicate with the wired backbone through

the bridge of access point (AP), which can use both DCF

and PCF.

2.2.1 Distributed Coordination Function

DCF is a distributed medium access scheme based

on carrier sense multiple accesses with collision

avoidance (CSMA/CA) protocol. In this mode, the

STAs must sense the medium before transmitting a

packet, if the medium is found idle for an interval of

time longer than a Distributed InterFrame Space

(DIFS); the STA can transmit the packet

immediately (IEEE 802.11 WG, 1999), meanwhile

other STAs defer their transmission and adjusting

their Network Allocation Vector (NAV) which is a

local timer. Then the backoff process starts, the STA

compute a random Backoff_timer=rand [0,

CW]*slot time, where CWmin ≤ CW (window

contention parameter) ≤ CWmax and slot time

depends on the PHY layer type. The backoff timer is

decreased only when the medium is idle. Each time

the medium becomes idle, the STA waits for a DIFS

and continuously decrements the backoff timer. As

soon as the backoff expires, the STA is authorized to

access the medium. Obviously, a collision occurs if

two or more STAs start transmission simultaneously.

If the acknowledgement, used to notify that the

transmitted frame has been successfully received

(see Figure 1), is not received, the sender assumes

that a collision was occurred, so it schedules a

retransmission and enters the backoff process again.

To reduce the probability of collisions, after each

unsuccessful transmission attempt, the CW is

doubled until a predefined maximum value CWmax

is reached. But after each successful transmission,

the CW is reset to a fixed minimum value CWmin.

Two carrier sensing mechanisms are possible, PHY

carrier sensing at air interface and virtual carrier

sensing at PHY MAC layer. Virtual carrier sensing

can be used by an STA to inform all other STAS in

the same BSS how long the channel will be reserved

for its frame transmission. On this purpose, the

sender can set a duration field in the MAC header of

data frames. Then other STAS can update their

NAVS to indicate this duration, and will not start

transmission before the updated NAV timers reach

zero.

Figure 1: Basic DCF CSMA/CA.

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2.2.2 PCF: Point Coordination Function

PCF uses a centralised polling scheme, which

requires the AP as a point coordinator (PC) in a

BSS. The channel access time is divided into

periodic intervals named beacon intervals, see

Figure 2. The beacon interval is composed of a

contention-free period (CFP) and a contention

period (CP). During the CP, the PC maintains a list

of registered STAs and polls each STA according to

its list. Then, when a STA is polled, its gets the

permission to transmit data frame. Since every STA

is permitted a maximum length of frame to transmit,

the maximum CFP duration for all the STAs can be

known and decided by the PC, which is called

CFP_max_duration. The time used by the PC to

generate beacon frames is called target beacon

transmission time (TBTT). In the beacon, the PC

denotes the next TBTT and broadcast it to all the

others in the BSS. In order to ensure that no DCF

STAs are able to interrupt the operation of the PCF,

a PC waits for a PCF InterFrame Space (PIFS),

which is shorter than DIFS, to start the PCF. Then,

all the others STAs set their NAVs to the values of

CFP_max_duration time, or the remaining duration

of CFP in case of delayed beacon. During the CP,

the DCF scheme is used, and the beacon interval

must allow at least one DCF data frame to be

transmitted. A typical medium access sequence

during PCF is shown in Figure 2. When a PC polls

an STA, it can piggyback the data frames to the STA

together with the CF-poll, then the STA sends back

data frame piggybacked with an ACK after a SIFS

interval. When the PC polls the next STA, it

piggybacks not only the data frame to the

destination, but also an ACK to the previous

successful transmission. Note that almost all packet

transmissions are separated by the SIFS except for

one scenario: if the polled STA does not respond the

AP within a PIFS period, the AP will poll the

following STA. Silent STAs are removed from the

polling list after several periods and may be polled

again at beginning of the next CFP. At any time, the

PC can terminate the CFP by transmitting a CF-end

packet, then all the STAs in the BSS should reset

their NAVs and attempt to transmit during the CP.

Normally, PCF uses a round robin scheduler to poll

each STA sequentially in the order of polling list,

but priority based polling mechanisms can also be

used if different QoS levels are requested by

different STAs.

3 DESCRIPTION OF 802.11E

STANDARD

3.1 HCF: Hybrid Coordination

Function

There are many new features in 802.11e draft 4.2

(IEEE 802.11 WG, 2003). In this section, we will

briefly describe HCF. HCF is composed of two

access methods: contention-based channel access

(called EDCF) and controlled channel access

mechanisms. One main feature of HCF is to

introduce four access category (AC) queues and

eight traffic stream (TS) queues at MAC layer.

When a frame arrives at MAC layer, it is tagged

with a traffic priority identifier (TID) according to

its QoS requirements. Which can take the values

from 0 to 15. The frames with TID values from 0 to

7 are mapped into four AC queues using EDCF

access rule. On the other hand, frames with TID

values from 8 to 15 are mapped into eight TS queues

using HCF controlled channel access rule. The

reason of separating TS queues from AC queues is

to support strict parameterized QoS at TS queues

while prioritized QoS is supported at AC queues.

Another main feature of the HCF is the concept of

transmission opportunity (TXOP), which is the time

interval permitted, for a particular STA to transmit

packets. During the TXOP, there can be a series of

frames transmitted by an STA separated by SIFS.

The TXOP is called either EDCF-TXOP, when it is

obtained by winning a successful EDCF contention;

or polled-TXOP, when it is obtained by receiving a

QoS CF-poll frame from the QoS-enhanced AP

(QAP). The maximum value of TXOP is called

TXOPLimit, which is determined by QAP.

3.2 Enhanced Distributed

Coordination Function (EDCF)

The EDCF is designed for the contention-based

prioritized QoS support. Each QoS-enhanced STA

(QSTA) has 4 queues (ACs), to support 8 user

priorities (UPs). Therefore, one or more UPs are

mapped to the same AC queue. This comes from the

observation that usually eight kinds of applications

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do not transmit frames simultaneously, and using

less ACs than UPs reduces the MAC layer

overheads. Each AC queue works as an independent

DCF STA and uses its own backoff parameters. In

EDCF, two main methods are introduced to support

service differentiation: The first one uses different

InterFrame Space (IFS) sizes for different ACs. A

new kind of IFS called Arbitrary IFS (AIFS) is used

in EDCF, instead of DIFS in DCF. AIFS[AC] =

AIFSN[AC] * SlotTime + SIFS, where the default

value of the arbitration inter frame spacing number

(AIFSN) is defined as either 1 or 2 (IEEE 802.11

WG, 2003).When AIFSN=1, high priority queues

AC1, AC2 and AC3 have AIFS value equal to PIFS.

When AIFSN=2, the low priority queue AC0 has

AIFS value of DIFS. When a frame arrives at an

empty AC queue and the medium has been idle

longer than AIFS [AC]+SlotTime, the frame is

transmitted immediately .If the channel is busy, the

arriving packet in each AC has to wait until the

medium becomes idle and then defer for

AIFS+SlotTime. So the AC with the smaller AIFS

has the higher priority. For example, the earliest

transmission time for high priority queue is to wait

for PIFS+SlotTime=DIFS, while the earliest

transmission time for best effort queue is to wait for

DIFS+ SlotTime. The second method consists in

allocating different CW sizes for different ACs.

Assigning a short CWsize to high priority AC

ensures that in most cases, high-priority AC is able

to transmit packets ahead of low-priority one. If the

backoff counters of two or more parallel ACs in one

QSTA reach zero at the same time, a scheduler

inside the QSTA will avoid the virtual collision by

granting the EDCF-TXOP to the highest priority

AC. At the same time, the other colliding ACs will

enter a backoff process and double the CW sizes as

if there is an external collision. In this way, EDCF is

supposed to improve the performance of DCF under

congested conditions. The default values of AIFSN

[AC], CWmin [AC], CWmax [AC] and TXOPLimit

[AC] are announced by the QAP in beacon frames,

and the 802.11e standard also allows the QAP to

adapt these parameters dynamically depending on

network conditions (IEEE 802.11 WG, 2003). But

how to adapt to the channel has not been defined by

the standard and remains an open research issue.

3.3 HCF Controlled Channel Access

The HCF controlled channel access mechanism is

designed for the parameterized QoS support, which

combines the advantages of PCF and DCF. HCF can

start the controlled channel access mechanism in

both CFP and CP intervals, whereas PCF is only

allowed in CFP. A typical 802.11e beacon interval ,

is composed of alternated modes of optional CFP

and CP. During the CP, a new contention-free period

named controlled access phase (CAP) is introduced.

HCF can start a CAP by sending downlink QoS-

frames or QoS CP-Poll frames to allocate polled-

TXOP to different QSTAs after the medium remains

idle for at least PIFS interval. Then the remaining

time of the CP can be used by EDCF. This flexible

contention-free scheme makes PCF and CFP useless

and thus optional in the 802.11e standard. For

example, in order to support audio traffic with a

maximum latency of 20 millisecond (ms) using PCF,

the beacon interval should be no more than 20 ms

since the fixed portion of CP forces the audio traffic

to wait for the next poll. On the other hand, the HCF

controlled channel access can increase the polling

frequency by initiating CAP at any time, thus

guarantee the delay bound with any size of beacon

interval. So there is no need to reduce the beacon

interval size that increases the overheads. In HCF

controlled channel access mechanism, QoS

guarantee is based on the traffic specification

(TSPEC) negotiation between the QAP and the

QSTAs. Before transmitting any frame that requires

the parameterized QoS, a virtual connection called

traffic stream (TS) is established. In order to set up a

TS, a set of TSPEC parameters (such as mean data

rate, nominal frame size, maximum service interval,

delay bound, etc.) are exchanged between the QAP

and the corresponding QSTAs. Based on these

TSPEC parameters, the QAP scheduler computes the

duration of polled-TXOP for each QSTA, and

allocates the polled-TXOP to each QSTA. Then the

scheduler in each QSTA allocates the TXOP for

different TS queue according to the priority order. A

Figure 2: PCF and DCF cycles.

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simple round-robin scheduler is proposed in the

IEEE 802.11e draft 4.2 (IEEE 802.11 WG, 2003).

The simple scheduler uses the following mandatory

TSPEC parameters: mean data rate, nominal MAC

frame size and maximum service interval or delay

bound. Note that the maximum service interval

requirement of each TS corresponds to the

maximum time interval between the start of two

successive TXOPs. If this value is small, it can

provide low delay but introduce more CF-Poll

frames. If different TS have different maximum

service interval requirements, the scheduler will

select the minimum value of all maximum service

interval requests of all admitted streams for

scheduling. Moreover, the QAP is allowed to use an

admission control algorithm to determine whether or

not to allow new TS into its BSS. During a CFP, the

medium is fully controlled by QAP. During a CP, it

can also grab the medium whenever it wants (after a

PIFS idle time). After receiving a QoS CF-poll

frame, a polled QSTA is allowed to transmit

multiple MAC frames denoted by contention-free

burst (CFB), with the total access time not exceeding

the TXOPLimit.

4 SIMULATION-BASED

EVALUATIONS OF

OS-ENHANCED SCHEMES

In (Benveniste M. et al., 2001), (Qiang Ni et al.,

2004), different simulations have been conducted

with different topology and parameters of EDCF. To

evaluate the performance of DCF and EDCF

schemes, we use NS-2 (Anelli A et al.), there is no

mobility in the system, each station operates at IEEE

802.11b PHY and transmits three types of traffic

(audio, video and data traffic) to each other. The

DCF MAC parameters are listed in Table 1 and

Table 1: DCF parameters.

SIFS 16µs MAC header 28bytes

DIFS 34µs PLCP header

length

4µs

ACK

size

14bytes Preamble length 20µs

PHY

rate

36Mbps CWmin 15

Slot

time

9µs WCmax 1023

EDCF parameters are: for audioPCM (Wmin=7,

Wmax=15, AIFSN=1, Packet size in bytes=160,

Packet interval in ms=20, Sending rate in KB/s=8),

for Video MPEG4 (15,35,1,1280,16,80), for Video

VBV(15,31,2, 660,26,25), for Data (31,1023, 2,

1600, 12.5,128).We use CBR/UDP traffic sources.

We vary the load rate by increasing the number of

STAs from 0 to 6.

Figure 2 shows the simulation results for the

bandwidth, and latency. We can see that average

throughput of three kinds of flows per STA are

stable and sufficient as long as the channel load rate

is less than 70% at the 25th second, after all flows

degrade themselves dramatically in DCF, but not in

EDCF. And we let us notice, that there is a high rate

loss of packets in DCF, and a low rate loss of

packets in EDCF. We see also that latency is good

for all flows, but at the 25

second, it increases

significantly in DCF. On the other hand, in EDCF

only data suffer by a high latency. The evolution of

latency in DCF, in function of channel load rate is

dramatic for all flows after 70% rate, but in EDCF

after 60% only data flow degrade themselves. Figure

3 shows the advantages of HCF controlled channel

access mechanism compared to EDCF, we simulate

an topology with 13 STAs (STA 0 is the AP), six

STAs transmit each one a audio flow, and the six

others transmit a video flow (CBR MPEG4) at

AP.We notice that the throughput (D) is stable and

distributed well on all the STAs by HCF, which is

not the case for EDCF, where D fluctuate too much

quickly, what indicates a bad management of the

bandwidth. For EDCF, the latency increases all

gently when the channel load rate increases but only

for audio flows, for the video flows, the latency

increase brutally. For HCF, the evolution of latency

is the same for all flows. Figure 4 shows the

limitations of HCF by a simulation of 19 STAs (the

STA 0 is the AP) and STA1 to STA6 transmits a

PCM Audio flows with inter arrival time of 4.7ms,

Packet size of 160bytes, Sending rate of 64Kbps and

a priority of 6. STA7 to STA12 transmits a VBR

(variable bit rate) video flows with Arrival period

almost equal to 26, Packet size almost equal to 660,

Sending rate almost equal to 200 and a priority of 5.

STA13 to STA18 transmits a MPEG4 video flows

with Arrival period=2, Packet size=800, Sending

rate=3200 and a priority of 4. Let us notice that

latency of VBR flows fluctuate and increase

dramatically, what is not the case of the other flows.

This is with the fact that the AP is unable to make a

good estimate of the size of the queues for a good

scheduling.

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5 CONCLUSION

The results of simulation show that the protocol

DCF can only support best-effort services, not any

QoS guarantees, all the STAs in one BSS compete

for the resources and channel with the same

priorities. There is no differentiation mechanism to

guarantee bandwidth, packet delay and jitter for high

priority STAs or multimedia flows. The EDCF

protocol show to be the best choice for high priority

Figure 3: Throughput and latency performance for DCF and EDCF.

Figure 4: Throughput and channel load for EDCF and HCF Controlled channel access.

Figure 5: Throughput for EDCF and HCF Controlled channel access.

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traffic, but it starves the low priority traffic in case

of high load, and leads to higher collision rates.

Furthermore, when channel is 90% loaded, the

throughput of audio and video start to decrease,

which means that admission control for audio and

video is required during very high load. The HCF

protocol has a drawback, that AP did not make a

good estimate of weight of queues, so there is an

unbalance (il y a un désequilibre dans le partage de

la bp entre les flux multimedia) enters the flows with

high priorities. A HCF protocol which mitigates the

disadvantages of HCF was developed, and we intend

to evaluate it in future research. We can also propose

a new mechanisms of QoS, which can fill the faults

of the standard and evaluates their effectiveness by a

simulation.

ACKNOWLEDGMENTS

We would like to thank mister Salissou for his

valuable help in performing a large set of simulation.

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