Abstract
In this work we present a model for the Carrier Sense
Multiple Access with Collision Avoidance
(CSMA/CA) protocol according to IEEE 802.11
standard, approved in June 1997, for Wireless Local
Area Networks. The model has been developed using
the simulation tool SES/Workbench. Our purpose is to
analyze the access protocol performance in terms of
available throughput, access delay and packet
dropping. The evaluation has been done to make a
comparison between the three physical specifications
standardized: InfraRed (IR), Direct Spread Spectrum
Sequence (DSSS), and Frequency Hopping Spread
Spectrum (FHSS). In this work, we also have
compared CSMA/CA performance versus CSMA/CD
used in wired LANs that are based on IEEE802.3
standard.
The results show different behavior for each
implementation, due basically to the physical
parameters specified for all three implementation.
Using ideal propagation delay conditions, DSSS
physical implementation shows the best throughput
results while introducing more realistic conditions the
results are not so clear.
Introduction
In the last years a great growth deployment and
acceptation of mobile communications has been
produced, in the particular field of Wireless Local Area
Networks (WLANs). WLANs allow connectivity and
access to traditional wired LANs, as WLANs can be
seen as an extension of wired LANs but with the
flexibility and mobility that characterizes wireless
systems. The last release of IEEE 802.11, approved in
June 1997, has been definitive for the introduction and
development of WLAN systems in the mass-market.
This standard specifies the physical and MAC layers
following OSI (Open Systems Interconnection)
architecture.
In this extended abstract we briefly present the
simulation details and some of results obtained. Finally
we present some of the conclusions.
IEEE 802.11 MAC Layer
As IEEE 802.3 standard, the MAC layer defined by
IEEE 802.11 standard is the lower part of the link layer
and is placed between the dependent sublayer of the
physical layer and LLC sublayer of the link layer. The
MAC architecture is composed by two basic
coordination functions: Punctual Coordination
Function (PCF) and Distributed Coordination (DCF).
Each of these functions defines an operation mode for
the stations that want to access the wireless medium.
Coordination Function is defined as the function that
determines, within a Basic Services Set (BSS), when a
station is enabled to transmit and/or receive Protocol
data Units at MAC level (MPDUs) through the
wireless channel.
DCF is a basic and compulsory mode for all stations
and is located at lower part of MAC architecture. The
DCF functionality is based on random techniques and
is used by asynchronous traffic that does not require a
severe bounded time. The IEEE 802.11 standard
specifies the CSMA/CA access algorithm for this level.
PCF is located over DCF and the access algorithm for
this level is based on circular polling from an access
point, that is, deterministic access. This mechanism
allows transmission of traffic that does not tolerate
random and unbounded delays or contention free
asynchronous traffic.
Two coordination modes operates in the same network
over a structure called the superframe: during the first
part of the superframe, the network operates under
DCF mode allowing random access. When the
contention period finishes then the access point, called
central coordinator, takes the medium and a contention
free period begins.
In this paper we analyze the Distributed Coordination
Function according to IEEE 802.11 standard, that is,
CSMA/CA access algorithm.
