Computer Networks and IntComputer Networks and Internetsernets

《计算机网络与因特网》课件《计算机网络与因特网》课件

林坤辉 2004-02-6

PART II Packet TransmissionPART II Packet Transmission

Chapter 15 Network CharacteristicChapter 15 Network Characteristics:s:

Ownership, Service Paradigm, and Ownership, Service Paradigm, and PerformancePerformance

网络所有权、服务模式和性能网络所有权、服务模式和性能

15.2 Network Ownership15.2 Network Ownership网络所有权

Network hardware and software can be Network hardware and software can be owned by a single company or individual owned by a single company or individual ,or it can be owned by a communication ,or it can be owned by a communication company.company.

A network is said to be private if use of A network is said to be private if use of the network is restricted to the corporate the network is restricted to the corporate or individual owner.or individual owner.

15.2.1 Private Networks 15.2.1 Private Networks 私有网络私有网络

15.2.2 Public Networks 15.2.2 Public Networks 公有网络公有网络

A public network is owned and operated A public network is owned and operated by a service provider.by a service provider.

Any subscriber can use a public network Any subscriber can use a public network to communication with other subscriber.to communication with other subscriber.

15.3 Privacy and Public Networks15.3 Privacy and Public Networks私有与公有网络

When applied to a network, the term public When applied to a network, the term public refers to availability of the service, not the refers to availability of the service, not the data transferred.data transferred.

Most public networks provide private Most public networks provide private communication.communication.

15.4 Advantages and 15.4 Advantages and DisadvantagesDisadvantages

The chief advantage of a private network The chief advantage of a private network is that the owner has complete control is that the owner has complete control over both the technical decision and over both the technical decision and policies.policies.

An owner can guarantee that the network An owner can guarantee that the network is isolated from computers outside the is isolated from computers outside the organization.organization.

A large private network can be expensive A large private network can be expensive to install and maintainto install and maintain

The chief advantage of a public network The chief advantage of a public network are flexibility and the ability to use state-are flexibility and the ability to use state-of-the–art network without maintaining of-the–art network without maintaining technical expertise.technical expertise.

15.5 Virtual Private Networks15.5 Virtual Private Networks虚拟私有网络

Known as a Virtual Private Network Known as a Virtual Private Network (VPN),the technology allows a company (VPN),the technology allows a company with multiple sites to have a private with multiple sites to have a private network,but use a public network as a network,but use a public network as a carrier.carrier.VPN technology restricts traffic so that VPN technology restricts traffic so that packets can travel only between the packets can travel only between the company’s sites.company’s sites.To build a VPN, a company buys a To build a VPN, a company buys a special hardware and software system for special hardware and software system for each of its sites.each of its sites.

15.6 Guaranteeing Absolute 15.6 Guaranteeing Absolute Privacy Privacy

VPN system use encryption(VPN system use encryption( 加密加密 ) to gua) to guarantee absolute privacy.rantee absolute privacy.

Even if an outsider does manage to obtaiEven if an outsider does manage to obtain a copy of a packet, the outsider will be n a copy of a packet, the outsider will be unable to interpret the contents.unable to interpret the contents.

15.7 Service Paradigm15.7 Service Paradigm服务模式

Connection-oriented service(Connection-oriented service( 面向连面向连接的服务接的服务 ), connection-oriented servi), connection-oriented service is analogous to telephone commuce is analogous to telephone communication.nication.

Connectionless service(Connectionless service( 无连接服务无连接服务 ), ), connectionless service is analogous tconnectionless service is analogous to the postal mail system.o the postal mail system.

15.8 Connection-oriented 15.8 Connection-oriented Service ParadigmService Paradigm

15.8.1 Continuous and Bursty Traffic.15.8.1 Continuous and Bursty Traffic.

15.8.2 Simplex and Duplex Connections.15.8.2 Simplex and Duplex Connections.

15.8.3 Connection Duration and Persisten15.8.3 Connection Duration and Persistence ce 连接期限与保持 ..

15.8.4 Service Guarantees15.8.4 Service Guarantees

15.8.5 Stream or Message Interface15.8.5 Stream or Message Interface

15.11 A Comparison of Service 15.11 A Comparison of Service ParadigmsParadigms

advantages.advantages.

disadvantage.disadvantage.

15.12 Examples of Service Para15.12 Examples of Service Paradigms digms 服务模式实例

15.13 Addresses and Connecti15.13 Addresses and Connection Identifierson Identifiers地址与连接标识

In a connectionless network, each packet In a connectionless network, each packet must contain the address of the remote must contain the address of the remote destination.destination.In a connection-oriented network, the In a connection-oriented network, the destination address is only used when destination address is only used when creating a new connection, packets creating a new connection, packets flowing over the connection contain a flowing over the connection contain a connection identifier that is smaller than connection identifier that is smaller than a full address.a full address.

15.14 Network Performance Ch15.14 Network Performance Characteristics aracteristics 网络性能特性

Delay Delay 延迟 ..

Throughput Throughput 吞吐量 .

Jitter Jitter 抖动抖动 ..

15.14.1 Delay15.14.1 Delay延迟

The delay of a network specifies how The delay of a network specifies how long it takes for a single bit of data to long it takes for a single bit of data to travel across the network from one travel across the network from one computer to another.computer to another.

delay is measured in seconds or delay is measured in seconds or fractions of seconds.fractions of seconds.

Engineers usually report both the Engineers usually report both the maximum and average delay.maximum and average delay.

The delay is divided into several parts:The delay is divided into several parts:Propagation delay(Propagation delay( 传播延迟传播延迟 ))switching delay(switching delay( 交换延迟交换延迟 ))access delay(access delay(访问延迟访问延迟 ))queueing delay(queueing delay( 排队延迟排队延迟 ))

15.14.2 Throughput 15.14.2 Throughput 吞吐量Throughput is a measure of the rate at which Throughput is a measure of the rate at which data can be sent through the network.data can be sent through the network.

Throughput is usually specified in bits per Throughput is usually specified in bits per second(bps).second(bps).

The throughput capability of the underlying The throughput capability of the underlying hardware is called bandwidth.hardware is called bandwidth.

Effective throughput < hardware bandwidthEffective throughput < hardware bandwidth

Hardware bandwidth is often used as an Hardware bandwidth is often used as an approximation of the network’s throughput.approximation of the network’s throughput.

Throughput is a measure of the network Throughput is a measure of the network capacity,not speed.capacity,not speed.

The throughput specifies how many bits The throughput specifies how many bits can enter the network per unit time.can enter the network per unit time.

Imagine a network to be a road between Imagine a network to be a road between two places and packets traveling across two places and packets traveling across the network to be analogous to cars the network to be analogous to cars traveling down the road.traveling down the road.

15.14.3 The Relationship 15.14.3 The Relationship Between Delay and ThroughputBetween Delay and ThroughputThroughput and delay are not completely Throughput and delay are not completely independent.independent.

As traffic in a computer network increaseAs traffic in a computer network increases, delays increase.s, delays increase.

Excessive traffic in a network is called coExcessive traffic in a network is called congestion(ngestion( 拥塞拥塞 ))

A network that operates at close to 100% A network that operates at close to 100% of its throughput capacity experiences seof its throughput capacity experiences severe delay.vere delay.

D=DD=D00/(1-U); /(1-U);

D: the effective delay;D: the effective delay;

DD0 0 :: the delay when network is idle; the delay when network is idle;

U(0<U<1): the current utilization.U(0<U<1): the current utilization.

15.14.4 Delay-Throughput Pro15.14.4 Delay-Throughput Product duct 延迟 -吞吐量的乘积

The product of delay and throughput The product of delay and throughput measures the volume of data that can measures the volume of data that can be present on the network.be present on the network.A network with throughput T and delay A network with throughput T and delay D has a total of T D has a total of T X X D bits in transit at D bits in transit at any timeany time

15.15 Jitter 15.15 Jitter 抖动抖动

The network’s jitter represents the The network’s jitter represents the variance in delay.variance in delay.

Jitter is becoming important as networks Jitter is becoming important as networks are used for transmission of real-time are used for transmission of real-time voice and video.voice and video.

作业作业1.1.实现实现 VPNVPN 的意义何在的意义何在2.2.网络时间延迟和网络吐量的关系如何网络时间延迟和网络吐量的关系如何3.3.简述抖动对音频和视频的网上播放的影响简述抖动对音频和视频的网上播放的影响