现代计算机网络：性能与服务质量

计算机网络

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"《Computer Networks: Performance and Quality of Service》是由Ivan Marsic撰写的一本专注于计算机网络性能和服务质量（QoS）的书籍。本书旨在为高级本科和研究生级别的计算机网络课程提供教材，重点关注基本原理和核心概念，而非详尽无遗的技术细节。书中涵盖了未来融合有线和无线/移动网络以及多媒体应用日益重要的场景。作者假设读者具有概率和统计的基础知识，并在附录中进行了回顾。大多数概念不需要超出本科一年级水平的数学技巧。" 在深入探讨计算机网络的性能和QoS时，本书涉及的关键知识点包括： 1. **网络性能**：这部分内容可能会涵盖网络带宽、延迟、丢包率、吞吐量和可扩展性等关键指标。这些性能参数对于评估和优化网络的效率至关重要，特别是在处理大量数据传输或实时通信的环境中。 2. **服务质量（QoS）**：QoS是指网络为满足特定应用程序需求（如视频流、语音通话或在线游戏）而提供的性能等级。QoS策略可能包括流量整形、优先级队列、拥塞控制和带宽预留等，确保关键服务的稳定性和可靠性。 3. **网络架构**：书中可能会讨论不同的网络层次模型（如OSI七层模型和TCP/IP四层模型），以及每层如何影响性能和QoS。 4. **协议和算法**：包括TCP/IP协议栈中的各种协议（如TCP、UDP、IP等）和路由算法（如距离矢量、链路状态等），这些都与网络性能密切相关。 5. **无线和移动网络**：这部分内容将探讨无线网络（如Wi-Fi、蓝牙）和移动网络（如3G、4G、5G）的特点，以及它们在性能和QoS方面的挑战，比如信号衰减、多径传播和频谱效率问题。 6. **多媒体应用**：随着多媒体应用的普及，书中的重点可能包括流媒体、VoIP和视频会议等服务的网络需求，以及如何通过QoS策略确保这些服务的质量。 7. **概率和统计基础**：书中提到读者需要具备概率和统计知识，因为这些工具在分析网络性能和预测网络行为时非常关键，例如在建模和仿真网络流量分布。 8. **实践应用**：除了理论知识，本书也面向实践，帮助读者理解实际网络中遇到的技术背后的原理，这对于网络管理员、系统架构师和开发人员尤其有价值。 9. **学习导向**：这本书不仅适合学术环境，也适合希望深入了解网络工作原理的从业者，它强调理解和应用基础知识，而不仅仅是查找参考信息。《Computer Networks: Performance and Quality of Service》是一本深入浅出的教材，旨在通过讲解基本概念和原则，帮助读者理解和掌握计算机网络性能和QoS的核心议题。

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Ivan Marsic • Rutgers University

single bit of information, then the minimum tolerable separation of pulses determines the

maximum number of bits that can be transmitted over a particular transmission line.

It is common to represent data transmissions on a timeline diagram as shown in Figure 1-5. This

figure also illustrates delays associated with data transmission. Although information bits are not

necessarily transmitted as rectangular pulses of voltage, all transmission lines are conceptually

equivalent, as represented in Figure 1-6, because the transmission capacity for every line is

expressed in bits/sec or bps. The time required to transmit a single bit on a given link is known as

bit time. In this text, we will always visualize transmitted data as a train of digital pulses. The

reader interested in physical methods of signal transmission should consult a communications-

engineering textbook, such as [Haykin, 2006].

Time

Sender Receiver

Communication link

Physical setup:

Timeline diagram:

transmission

delay

propagation

delay

101101

Start of transmission

End of reception

First drop of the fluid

enters the pipe

Fluid packet in transit

Last drop of the fluid

exits the pipe

Fluid flow analogy:

Figure 1-5: Timeline diagram for data transmission from sender to receiver.

Chapter 1 • Introduction to Computer Networking

A common characterization of noise on transmission lines is bit error rate (BER): the fraction of

bits received in error relative to the total number of bits received in transmission. Given a packet

n bits long and assuming that bit errors occur independently of each other, a simple

approximation for the packet error rate is

BERnn

eBERPER

⋅−

−≈−−= 1)1(1 (1.1)

An important attribute of a communication link is how many bitstreams can be transmitted on it

at the same time. If a link allows transmitting only a single bitstream at a time, then the nodes

connected to the link must coordinate their transmissions to avoid different bitstreams corrupting

each other (known as data collision). Such links are known as broadcast links or multiple-access

links. Point-to-point links often support data transmissions in both directions simultaneously. This

kind of a link is said to be

full duplex. A point-to-point link that supports data flowing in only

one direction at a time is called

half duplex. In other words, the nodes on each end of this kind of

a link can both transmit and receive, but not at the same time—they only can do it by taking

turns. It is like a one-lane road with bidirectional traffic. We will assume that all point-to-point

links are full duplex, unless stated otherwise. A full-duplex link can be implemented in two ways:

either the link must contain two physically separate transmission paths, one for sending and one

for receiving, or the capacity of the communication channel is divided between signals traveling

in opposite directions. The latter is usually achieved by time division multiplexing (TDM) or

frequency division multiplexing (FDM).

0111001

01 11001

Time

Link 1:

1 Mbps

Link 2:

10 Mbps

100 ns

1 μs

Figure 1-6: Transmission link capacity determines the speed at which the link can transmit

data. In this example, each bit on Link 1 is 1

μs wide, while on Link 2 each bit is 100 ns

wide. Hence, Link 2 can transmit ten times more data than Link 1 in the same time interval.

Ivan Marsic • Rutgers University

Wireless Link

Consider a simple case of a point source radiating electromagnetic waves in all directions (Figure

1-7, left). The received signal strength decreases exponentially with the sender-receiver distance.

As with any other communication medium, the wireless channel is subject to thermal noise,

which distorts the signal randomly according to a Gaussian distribution of noise amplitudes. As

the distance between a transmitter and receiver increases, the received signal strength decreases to

levels close to the background noise floor. At a certain distance from the sender, the signal

strengths will become so weak that the receiver will not be able to discern reliably signal from

noise. This distance, known as

transmission range, is decided arbitrarily, depending on what is

considered acceptable bit error rate. For example, we can define the transmission range as the

sender-receiver distance for which the packet error rate is less than 10 %.

In addition to thermal noise, the received signal may be distorted by parallel transmissions from

other sources (Figure 1-7, right). This phenomenon is known as

interference. Because this

normally happens only when both sources are trying to transmit data (unknowingly of each

other’s parallel transmissions), this scenario is called

packet collision. A key observation is that

collisions occur at the receiver—the sender is not disturbed by concurrent transmissions, but

receiver cannot correctly decode sender’s message if it is combined with an interfering signal. If

the source and receiver nodes are far away from the interfering source, the interference effect at

the receiver will be a slight increase in the error rate. If the increased error rate is negligible, the

source and receiver will be able to carry out their communication despite the interference. Notice,

however, that the interference of simultaneously transmitting sources never disappears—it only is

reduced exponentially with an increasing mutual distance (Figure 1-8). The minimum distance

(relative to the receiver) at which interferer’s effect can be considered negligible is called

interference range. In Figure 1-8, node D is within the interference range of receiver B. Nodes C

and E are outside the interference range. However, although outside the interference range

defined for a single interferer, if nodes C and E are transmitting simultaneously their combined

interference at B may be sufficiently high to cause as great or greater number of errors as a single

interferer within the interference range.

Figure 1-7: Wireless transmission. Left: single point source. Right: interference of two poin

sources.

Chapter 1 • Introduction to Computer Networking

1.1.3 Packets and Statistical Multiplexing

The communication channel essentially provides an abstraction of a continuous stream of

symbols transmitted that are subject to a certain error probability. When interacting with another

person, whether face-to-face or over the telephone, we think of units of communication in terms

of conversational turns: first one person takes a turn and delivers their message, then the other

person takes a turn, and so on. Messages could be thought of as units of communication

exchanged by two (or more) interacting persons. We notice that there are benefits of slicing a

long oration into a sequence of smaller units of discourse. This slicing into messages gives the

other person chance to clarify a misunderstanding or give a targeted response to a specific item.

In computer communication networks, messages are represented as strings of binary symbols (0

or 1), known as bits. Generally, messages are of variable length and some of them may still be

considered too long for practical network transmission. There are several reasons for imposing a

limit on message length. One is that longer messages stand a higher chance of being corrupted by

an error (see equation (1.1)). Another reason is to avoid the situation where a sending application

seizes the link for itself by sending very long messages while other applications must wait for a

long time. Therefore, messages are broken into shorter bit strings known as

packets. These

packets are then transmitted independently and reassembled into messages at the destination. This

allows individual packets to opportunistically take alternate routes to the destination and

interleave the network usage by multiple sources, thus avoiding inordinate waiting periods for

some sources to transmit their information.

Different network technologies impose different limits on the size of data blocks they can handle,

which is known as the

maximum transmission unit (MTU). For example, a regular Ethernet

Transmission

range

Interference

range

Interfering

source

Sender

Receiver

Transmitted

signal power

Received signal

power from an

interfering source

Interfering

source

Threshold

Interfering

source

Threshold

0 0 Distance from receiverDistance from sender

Figure 1-8: Transmission range and interference range for wireless links.

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现代计算机网络：性能与服务质量

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