802.11 DCF性能分析：无线局域网的竞争与退避策略

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"Bianchi对802.11分布式协调功能(DCF)的性能分析" 本文详细探讨了IEEE 802.11无线局域网标准中的分布式协调功能(DCF)，这是一种基于载波侦听多址访问/冲突避免(CSMA/CA)的协议。DCF的核心在于二进制槽位指数退避策略，它用于解决多个无线站点之间的介质竞争问题。Giuseppe Bianchi在2000年的这篇文章中提出了一种简洁但极其精确的分析模型，以计算有限数量终端和理想信道条件下的802.11 DCF吞吐量。 802.11 DCF采用了两种包传输机制：基本接入和RTS/CTS（请求发送/清除发送）接入机制。基本接入方法允许站点直接发送数据包，而RTS/CTS机制则引入了短的控制包交换来减少冲突的可能性，适用于大数据包的传输。Bianchi的分析模型同时适用于这两种机制，并且能够处理它们的组合使用情况，即根据数据包的长度决定采用基本接入还是RTS/CTS机制。通过这个模型，文章对DCF的吞吐量性能进行了全面评估。它考虑了各种因素，如网络中的终端数量、数据包大小、信道的繁忙程度以及退避算法的参数设置。分析中，作者揭示了这些因素如何影响网络的整体效率，包括传输成功概率、碰撞概率和空闲时间比例等关键指标。在DCF中，每个节点在尝试传输前会先监听信道。如果信道空闲，节点会随机选择一个退避时间，以降低与其它节点同时传输的概率。当两个或更多节点在同一时刻发送数据时，会发生碰撞，所有节点都需要再次退避并重新尝试。二进制槽位退避策略意味着每次碰撞后退避时间会翻倍，直到达到最大重传次数。通过数学建模，Bianchi展示了如何计算在给定网络配置下，预期的平均吞吐量，这对于理解802.11网络的性能极限至关重要。这些结果对于网络设计者和优化者来说非常有价值，因为他们可以预测不同场景下的网络性能，从而做出更有效的决策。此外，该研究还讨论了DCF的公平性问题，即所有节点如何平等分享信道资源。它指出，虽然DCF设计的初衷是防止冲突并提高效率，但在高密度网络中，可能会出现某些节点占据大部分信道时间的情况，导致其他节点的吞吐量下降。这种现象称为“隐含不公”。 Bianchi的工作深入剖析了802.11 DCF的工作原理，提供了理解无线局域网中CSMA/CA机制的关键洞察，有助于进一步改进和优化无线通信系统的性能。

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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