IEEE 802.11 DCF性能分析

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"DCF模型性能分析英文原文.pdf" 这篇文章源自IEEE期刊《Selected Areas in Communications》第18卷第3期，2000年3月发布。文章主要关注的是IEEE 802.11分布式协调功能（Distributed Coordination Function, DCF）的性能分析。DCF是无线局域网（WLAN）中的主要介质访问控制（MAC）技术，它基于载波侦听多路访问/冲突避免（Carrier Sense Multiple Access with Collision Avoidance, CSMA/CA）策略，并采用二进制槽定时指数退避算法。作者Giuseppe Bianchi在论文中提出了一个简单但极其精确的分析模型，用于计算有限数量终端和理想信道条件下的802.11 DCF吞吐量。DCF支持两种包传输方式：基本访问机制和请求发送/清除发送（Request To Send/Clear To Send, RTS/CTS）访问机制。此外，模型还适用于这两种机制的组合，即对于长度超过特定阈值的包，采用RTS/CTS机制进行传输。通过这个模型，文章对这两种访问机制的吞吐量性能进行了详尽的评估。这不仅包括了独立的基本访问和RTS/CTS机制，也考虑了它们在实际网络环境中的交互作用和性能影响。这种分析对于理解802.11标准在实际无线网络环境中的行为至关重要，有助于优化网络配置，提高无线通信效率和资源利用率。在MATLAB环境中，这些理论模型可以被实现和模拟，以验证分析结果并进一步探索不同参数（如终端数量、包大小、退避策略等）对DCF性能的影响。MATLAB是一种强大的数学计算软件，广泛应用于科研和工程领域，可以方便地进行数值计算、符号计算以及图形可视化，因此是进行这种类型性能分析的理想工具。这篇论文对于无线网络研究人员、工程师和学生来说具有很高的参考价值，因为它提供了一种量化分析DCF性能的方法，并为改善无线网络性能提供了理论基础。通过深入理解DCF的工作原理和性能边界，我们可以更好地设计和优化无线网络，以适应不断增长的数据传输需求。

BIANCHI: PERFORMANCE ANALYSIS OF THE IEEE 802.11 DCF 537

Fig. 1. Example of basic access mechanism.

mitting stations employ the RTS/CTS mechanism, collision oc-

curs only on the RTS frames, and it is early detected by the

transmitting stations by the lack of CTS responses. A quanti-

tative analysis will be carried out in Section VII.

III. M

AXIMUM AND SATURATION

THROUGHPUT PERFORMANCE

In this paper we concentrate on the “Saturation Throughput”.

This is a fundamental performance figure defined as the limit

reached by the system throughput as the offered load increases,

and represents the maximum load that the system can carry in

stable conditions.

It is well known that several random access schemes exhibit

an unstable behavior. In particular, as the offered load increases,

the throughput grows up to a maximum value, referred to as

“maximum throughput.” However, further increases of the

offered load lead to an eventually significant decrease in the

system throughput. This results in the practical impossibility to

operate the random access scheme at its maximum throughput

for a “long” period of time, and thus in the practical mean-

ingless of the maximum throughput as performance figure

for the access scheme. The mathematical formulation and

interpretation of this instability problem is the object of a wide

and general discussion in [13].

Indeed, the 802.11 protocol is known to exhibits some form

of instability (see, e.g., [5], and [11]). To visualize the unstable

behaviour of 802.11, in Fig. 3 we have run simulations in which

the offered load linearly increases with the simulation time. The

general simulation model and parameters employed are summa-

rized in Section V. The results reported in the figure are obtained

with 20 stations. The straight line represents the ideal offered

load, normalized with respect of the channel capacity. The sim-

ulated offered load has been generated according to a Poisson

arrival process of fixed size packets (payload equal to 8184 bits),

where the arrival rate has been varied throughout the simulation

to match the ideal offered load. The figure reports both offered

load and system throughput measured over 20 s time intervals,

and normalized with respect to the channel rate.

From the figure, we see that the measured throughput follows

closely the measured offered load for the first 260 s of sim-

ulation, while it asymptotically drops to the value 0.68 in the

second part of the simulation run. This asymptotic throughput

value is referred to, in this paper, as saturation throughput, and

represents the system throughput in overload conditions. Note

than, during the simulation run, the instantaneous throughput

temporarily increases over the saturation value (up to 0.74 in

Fig. 2. RTS/CTS Access Mechanism.

Fig. 3. Measured Throughput with slowly increasing offered load.

the example considered), but ultimately it decreases and stabi-

lizes to the saturation value. Queue build-up is observed in such

a condition.

IV. T

HROUGHPUT ANALYSIS

The core contribution of this paper is the analytical evalu-

ation of the saturation throughput, in the assumption of ideal

channel conditions (i.e., no hidden terminals and capture [6]).

In the analysis, we assume a fixed number of stations, each al-

ways having a packet available for transmission. In other words,

we operate in saturation conditions, i.e., the transmission queue

of each station is assumed to be always nonempty.

The analysis is divided into two distinct parts. First, we study

the behavior of a single station with a Markov model, and we

obtain the stationary probability

that the station transmits a

packet in a generic (i.e., randomly chosen) slot time. This prob-

ability does not depend on the access mechanism (i.e., Basic

or RTS/CTS) employed. Then, by studying the events that can

occur within a generic slot time, we express the throughput of

both Basic and RTS/CTS access methods (as well as of a com-

bination of the two) as function of the computed value

A. Packet Transmission Probability

Consider a fixed number

of contending stations. In satura-

tion conditions, each station has immediately a packet available

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