802.11 DCF协议性能分析与优化策略

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"本文主要探讨了网络协议效率分析，特别是针对IEEE 802.11分布式协调功能（DCF）的性能分析。通过对有限数量的终端和理想信道条件下的DCF进行深入研究，建立了简单而精确的分析模型，以计算其吞吐量。该分析不仅适用于基本访问方式，还适用于RTS/CTS访问机制，以及两者的组合策略。" 在无线局域网（WLAN）中，网络协议的设计和效率对网络性能有着至关重要的影响。网络协议的效率决定了数据传输的速度、网络冲突的频率以及整体网络的稳定性和可靠性。"网络协议效率分析"这一主题旨在理解和优化这些协议，以减少数据堵塞，提高网络资源的利用率。 IEEE 802.11 是一个广泛采用的标准，用于定义无线局域网的物理层和媒体访问控制（MAC）层。DCF 是 IEEE 802.11 协议中的主要 MAC 技术，它采用了载波检测多路访问/冲突避免（CSMA/CA）策略，以避免多个设备同时发送数据导致的冲突。CSMA/CA 结合了二进制槽时间的指数退避算法，确保在网络空闲时才允许设备传输数据，从而降低冲突发生的可能性。文章中提到的分析模型专注于在有限终端数量和理想信道条件下的 DCF 吞吐量计算。吞吐量是衡量网络性能的关键指标，它表示在网络中可传输数据的速率。通过这个模型，研究者能够评估 DCF 在不同场景下的性能，包括基本访问方式（直接数据传输）和 RTS/CTS 访问机制（Request To Send/Clear To Send，用于预分配信道，减少冲突）。RTS/CTS 机制通常用于长数据包传输，以减少冲突概率。此外，该分析还考虑了两种访问机制的混合策略，即对于超过一定长度的包使用 RTS/CTS 机制。这种策略可以优化不同大小数据包的传输效率，进一步提升网络的整体性能。通过提出的模型，文章提供了对 DCF 访问机制的详尽吞吐量性能评估，这为网络设计者和管理员提供了有价值的参考，有助于他们在实际部署中选择和调整合适的网络协议策略，以最大程度地减少冲突并提高网络效率。理解并优化这些网络协议是提升无线网络性能的关键步骤，对于现代无线通信系统的设计和发展具有重要意义。

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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802.11 DCF协议性能分析与优化策略

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