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广西医科大学. Computer Networking. 网络课件. 双语教学. 模拟实验. 计算机网络教研室. DI. 1. 2. 3. CHAPTER 3 Network Architetures and Standarts. The first section. Exercises. Online lecture. 3-7. 3-4. 3-1. 3-13. 3-10. A caveat about. Standards making org. Advs and disadvs of standards.

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Computer networking

广西医科大学

ComputerNetworking

网络课件

双语教学

模拟实验

计算机网络教研室


Computer networking

DI

1

2

3

CHAPTER 3

Network Architetures and Standarts

The first section

Exercises

Online lecture


Chapter 3 network architetures and standarts

3-7

3-4

3-1

3-13

3-10

A caveat about

Standards making org

Advs and disadvs of standards

Introduction

Manufactures Architecture

Why have Architecture

ISO Reference Model

The V.and X.standards

3-11

3-5

3-14

3-2

3-8

3-6

3-3

3-9

3-12

Communication Architecture

Definition Architecture

Internet Architecture

Layered architecture

Definition Architecture

Chapter 3 Network Architetures and Standarts

TCP/IP Architecture

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Objectives

Chapter 3 Network Architetures and Standarts

OBJECTIVES

Explain the difference between architectures and standards.

Explain the need for network architecture and standards.

Identify a number of communications standards-making org.

Describe the OSI model architecture.

Describe TCP/IP architecture.

Describe the architecture of the Internet.

Discuss the ADV and DISADV of layered architecture.

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3 1 introduction

Chapter 3 Network Architetures and Standarts

3.1 Introduction

Definitions of architectures and standards.

The difference among them.

The need of them

Specific example of them

Examine adv & disadv

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3 2 definition of architectures and standards

Chapter 3 Network Architetures and Standarts

3.2 DEFINITION OF ARCHITECTURES AND STANDARDS

Architecture: is a plan or direction that is oriented toward the needs of the user.

Network architecture is a set of principles used as the basic for the design and implementation of a communications network.

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Chapter 3 network architetures and standarts1

Chapter 3 Network Architetures and Standarts

It includes the organization of functions that must be performed by the network and the description of data formats and procedures.

Network architecture=layer + protocols

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Network architecture features

Chapter 3 Network Architetures and Standarts

Network architecture features

May or may not conform to standards.

Don’t provides enough detail how network to be implemented.

Provides a set of generic concepts .

Most architecture (plan) are designed for the long term use.

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Single layer model

Hardware

&

Software

Hardware

&

Software

1

DEVICE A

DEVICE B

Chapter 3 Network Architetures and Standarts

Single Layer Model

First Communication Between Computer Devices

The communication and application software be intermingled (To mix).

The early networks were not flexible.

Single specification for all aspects of communication

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Ibm sna system network architecture

Chapter 3 Network Architetures and Standarts

IBM SNA(System Network Architecture)

IBM : 1974 by the first computer network architectureIBM’s SNA is a proprietary architecture

NOTE:Not SNA(storage area network)

Most network architectures developed since use a layered approach.

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Communication standard

Chapter 3 Network Architetures and Standarts

Communication standard

The rules that are established to ensure compatibility among similar communications products and services.

Communications standards are the flesh on the architectural skeleton.

They specify how a particular communications product, service,or interface will operate.

V.90 modem

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3 3 why have network architectures

Chapter 3 Network Architetures and Standarts

3.3 why have network architectures?

1.Communication system were becoming too complex.

2. wanted to be isolated from the complexities of the network.

3.Wanted to connect different types of devices to the network.

4.Distributed processing and client-server processing became practical.

5.Network needed to be managed.

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3 4 two viewpoints about standards

Chapter 3 Network Architetures and Standarts

3.4 TWO VIEWPOINTS ABOUT STANDARDS

Background:

Communications vendors realized early standards.

computer manufacturers were slower to recognize the need for standards.

Customers demanded that product was standardization.

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Computer networking

Chapter 3 Network Architetures and Standarts

One

People who have a strong networking background see the computer as a tool to enhance a network`s capability.

Tend to wait for standards and follow them closely.

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Figure 3 1 two different views of the relative roles of networks and computers

Chapter 3 Network Architetures and Standarts

Figure 3-1 Two different views of the relative roles of networks and computers.

Two

Those who have a strong computer background tend to focus on what can be done with software or specially designed circuit chips for computers.

They forge ahead of the standards-making process

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Purport

Chapter 3 Network Architetures and Standarts

Purport

While neither viewpoint is wrong ,it leads to differing philosophies.

Both are legitimate and have their place.

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3 5 stands and stands making organizations

Chapter 3 Network Architetures and Standarts

3.5 STANDS AND STANDS-MAKING ORGANIZATIONS

Because of the recognized need for common international standards, there is a great deal of cooperation among communications standards making organizations.

Data communication standards are more complicated than Voice communication standards.

Early standards were mostly electrical in nature.

The standards-setting process is slow.

Writing standards is difficult work.

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Itu the international telecommunication union

Chapter 3 Network Architetures and Standarts

ITU (The International Telecommunication Union)

1865 in the U S was established in Paris,

1947 as part of the United Nations, members from 188 countries, headquartered in Geneva, Switzerland.

The most important telecommunications standards-setting organization in the world is the ITU .

ITU-T has created two sets of standards for the electrical connection of terminals to communications networks.

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Iso international standards organization

Chapter 3 Network Architetures and Standarts

ISO:International Standards Organization

Don’t confuse the OSI with ISO and IOS,.

ISO :means the international organization for standardization.

IOS :(internet operation system) is used in CISCO system’s routers and switches. 

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Computer networking

Chapter 3 Network Architetures and Standarts

Figure of the organizations involved in setting telecommunications standards or in passing input to the standards-setting bodies.

The American National Standards Institute (ANSI) is not involved in setting communications standards.

The official TCP/IP model is now maintained by the IETF.

All communications standards are developed by one organization, the ISO.F

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3 6 standard x standard

Chapter 3 Network Architetures and Standarts

3.6V.STANDARD X.STANDARD

The ITU-T has created two sets of standards for the electrical connection of terminals to communications networks.

They are called the V. and X. standards

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Chapter 3 network architetures and standarts2

Chapter 3 Network Architetures and Standarts

V.standards :

define the connection of digital equipment to the PSTN`s analog lines

V.32, V.33, V.42 standard for the use of telephone transmission of data made a clear statement .

X.standards:

Define the connection of digital equipment to digital lines.

E.G X.25, X.400, X.500

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3 7 advantages and disadvantages of standards

Chapter 3 Network Architetures and Standarts

3.7 ADVANTAGES AND DISADVANTAGES OF STANDARDS

Adv Standards allow products from multiple vendors to be connected to one another and to communicate.

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Disadvance

Chapter 3 Network Architetures and Standarts

disadvance

The standards-setting process is slow.

Standards tend to freeze technology.

Multiple conflicting standards.

The standards-setting process can occur quickly(Writing standards is relatively easy ), when an architecture is in place.F

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3 8 communications architectures

Chapter 3 Network Architetures and Standarts

3.8 COMMUNICATIONS ARCHITECTURES

ISO and ITU-T developed OSI reference model in 1978.

OSI-RM: Open Systems Interconnection reference model.

The standards development process took so long that the osi model never came into widespread use.

It is the standard reference point by which data communications networks are described

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3 9 the iso s osi reference model

Chapter 3 Network Architetures and Standarts

3.9 THE ISO`S OSI REFERENCE MODEL

3.9.1 Objective

Provide a basis for interconnecting dissimilar systems for the purpose of information exchange.

Define communications rules.

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Chapter 3 network architetures and standarts3

Chapter 3 Network Architetures and Standarts

OSI and most network architecture use a layered approach.

A layer in the OSI RM represents a component of the total process of communicating.

Each one layer requests services from the layer below it.

N layer Provide services to N+1 layer

The same network, any two-system must have the same level.

The actual physical communication completed  only in the bottom layer.

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Why layering

Chapter 3 Network Architetures and Standarts

Why layering?

1) Layering forces the modularization of function .

Dealing with complex systems.

The output of laying definition process is standards.

eases maintenance, updating of system.

One layer can be changed without affecting other layers

Different implementations can be substituted relatively

Transparently。

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Layer

前置处理机

中央主机

前置处理机CCP

中央主机

前置处理机

中央主机

Chapter 3 Network Architetures and Standarts

Layer

Layer requires reasonably sophisticated software and hardware at each end of the connection.

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Computer networking

The philosopher-translator-Secretary architecture.

Chapter 3 Network Architetures and Standarts

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Chapter 3 network architetures and standarts4

CN scientists

Chapter 3 Network Architetures and Standarts

Example:Scientific exchange layered

Scientific opinion

JP scientists

protocol

English

JP Translate

CN Translate

protocol

Letters

CNTelegrafisten

JPTelegrafisten

protocol

Telegraph code

Telex machine

Telex machine

protocol

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Network architecture layered principle

Chapter 3 Network Architetures and Standarts

Network Architecture Layered principle

Basic concept  :

Entity:Can send or receive any information of hardware / software process

Peer layer:Two different systems the same level 。

Interface:Interaction between the two neighboring layer

SERVE:for the upper classes a capacity to provide services through the interface

Protocol:Communication both in communication must comply with the rules.

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Layered network architecture

SYSB

SYSA

SYSB

SYSA

entity

Peer layer

Pn+1

N+1

N+1

server

Pn

N

N

peer entity

Pn-1

N-1

N-1

P3

3

3

P2

Physical communication line

2

2

P1

1

1

Physical communication line

Chapter 3 Network Architetures and Standarts

Layered network Architecture

interfact

The way that the interaction between the layers actually takes place is that each one requests services from the layer above it.

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Services

Chapter 3 Network Architetures and Standarts

Services

The way that the interaction between the layers actually takes place is that each one requests services from the layer above it.

Each layer provides services to the next higher layer

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Service primitives

Chapter 3 Network Architetures and Standarts

Service Primitives

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Relations among service primitives

Chapter 3 Network Architetures and Standarts

Relations among Service Primitives

服务用户

(N+1)

服务提供者

(N)

服务提供者

(N)

服务用户

(N+1)

系统A

系统B

Request

PDU

Indication

需证实

Response

Confirm

无需证实

Request

Indication

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Computer networking

Chapter 3 Network Architetures and Standarts

电话系统工作示例

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Chapter 3 network architetures and standarts5

Chapter 3 Network Architetures and Standarts

Services, interface protocols and functions

Protocols can provide a service for upper layer .

Protocols is “level”,

But Services is “Vertical ”,

Service is adopted by the lower layer of interface.

Functions is the internal activities 。

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Services to protocols relationship

Services to Protocols Relationship

The relationship between a service and a protocol.

Chapter 3 Network Architetures and Standarts

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Layers of the iso s osi reference model

Chapter 3 Network Architetures and Standarts

Layers of the ISO’s OSI reference model.

A layer in the OSI model represents a compon-

ent of the total process of communicating. T

The layers of the OSI model on each node communicate with each other.

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Computer networking

Chapter 3 Network Architetures and Standarts

Actual communication passes down through the layers on the sending node and then up through the layers on the receiving node.

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Computer networking

Chapter 3 Network Architetures and Standarts

X.25

The first three layers of the OSI model have been standardized, and together are called the X.25 standard for data transmission.

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Gosip

Chapter 3 Network Architetures and Standarts

GOSIP

government open system Interconnetion Protocol.

a subset of the OSI model that vendors must support if they want to sell network services or equipment to the U.S. government.

GOSIP is neither a new architecture nor a New standard.It is a more precise specification of what parts of the OSI model the government will use.

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3 9 2the function of each layer

Chapter 3 Network Architetures and Standarts

3.9.2The Function Of Each Layer

LAYER 1 THE PHYSICAL LINK LAYER

It specifies the electrical characteristics between the communications line and the terminal or computer .

Cabling

Interconnect methods (topology / devices)

Data encoding (bits to waves)

Electrical properties

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Typical equipment

Chapter 3 Network Architetures and Standarts

Typical equipment

What are the Physical Layer components on my computer?

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Computer networking

Chapter 3 Network Architetures and Standarts

Physical Layer

This is the only layer at which actual bits pass.

The only layer that is implemented strictly in hardware.

The only layer that actually communicates across the transmission line to the other node.

Layer1 only knows about a strem of bits.

Data transmission unit is bits. 

Transmission media does not belong to the physical layer. 

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Computer networking

Chapter 3 Network Architetures and Standarts

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Layer2 the data link layer

Chapter 3 Network Architetures and Standarts

LAYER2 THE DATA LINK LAYER

Function

Framing(成帧)

Line discipline

Flow control

Error control

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Data link layer

Chapter 3 Network Architetures and Standarts

Data link layer

It is responsible for establishing a link between two points on a network and ensuring that data are successfully transferred between the two points .

Divide the data into smaller units for transmission.(FRAME)

Detecting transmission errors and correcting errors.

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Chapter 3 network architetures and standarts6

Chapter 3 Network Architetures and Standarts

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Chapter 3 network architetures and standarts7

Chapter 3 Network Architetures and Standarts

Network Interface Card Bridge

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Computer networking

Chapter 3 Network Architetures and Standarts

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Layer 3 the network layer

Chapter 3 Network Architetures and Standarts

LAYER 3 THE NETWORK LAYER

It is responsible for routing a message all the way through the network from the transmitter to the receiver is the network layer.

Transmission units is package.

Point-to-point

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Layer4 the transport layer

Task

两端点间可靠的透明数据传输

(应用进程间的逻辑通信)

network

end-to-end

Chapter 3 Network Architetures and Standarts

LAYER4 THE TRANSPORT LAYER

Identifies actual address .but don’t know route.

Determine if the entire message was received correctly.

is responsible for end-to-end (source to destination) packet delivery

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Chapter 3 network architetures and standarts8

Chapter 3 Network Architetures and Standarts

Also calculates a check sum for the entire message.which is recalculated by layer4 at

The receiving end and compared.

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Layer 5 the seeion layer

Chapter 3 Network Architetures and Standarts

LAYER 5 THE SEEION LAYER

It is responsible for establishing the communications rules between specific machines or applications.

provides the mechanism for opening, closing and managing a session between end-user application processes.

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Computer networking

Chapter 3 Network Architetures and Standarts

Session Layer Dialog

Pacing the rate of transmission so data are not sent faster than the receiver can handle it.

Certain accounting function.

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Layer 6 the presentation layer

Chapter 3 Network Architetures and Standarts

LAYER 6 THE PRESENTATION LAYER

Provides common data formatting between communicating devices.

Components make sure the receiving station can read the transferred data

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Computer networking

Chapter 3 Network Architetures and Standarts

Concept of Encryption and Decryption

Direct and Indirect Methods of Translation

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Computer networking

Chapter 3 Network Architetures and Standarts

Data Compression Methods

Run-Length Encoding

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Layer 7 the application layer

Chapter 3 Network Architetures and Standarts

LAYER 7 THE APPLICATION LAYER

It is “where” the user works at a terminal with computer software.

It provides a set of interfaces for applications to obtain access to networked services as well as access to the kinds of network services that support applications directly.

OSI- FTAM,VT,MHS,DS,CMIP

TCP/IP- FTP,SMTP,TELNET,DNS,SNMP

Although the notion of an application process is common to both, their approaches to constructing application entities is different.

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Five common application of osi

Chapter 3 Network Architetures and Standarts

Five common application of OSI

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Typical equipment1

Chapter 3 Network Architetures and Standarts

Typical equipment

Gateway proxy firewall

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Computer networking

Chapter 3 Network Architetures and Standarts

Summary of Layer Functions

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Computer networking

Chapter 3 Network Architetures and Standarts

Chapter 3 Network Architetures and Standarts

OSI模型是协助网络检修人员识别网络问题一种理想的框架

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3 10 tcp ip architecture

Chapter 3 Network Architetures and Standarts

3.10 TCP/IP ARCHITECTURE

TCP/IP :

Transmission Control Protocol/Internet Protocol

The official TCP/IP model is now maintained by the IETF(ITU-T).

Because progress on setting standards for the OSI model was slow, DARPA developed a set of protocols called TCP/IP.

TCP/IP has become the de facto standard protocol for internets.

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Tcp ip and the internet

Chapter 3 Network Architetures and Standarts

TCP/IP and the Internet

The acronym of the U.S. government's version of the open system interconnection protocol is Internet.

In 1969

ARPA funded and created the “ARPAnet” network

In 1975

Convert from experimental to operational network

TCP/IP begun to be developed

In 1983

The TCP/IP is adopted as Military Standards

ARPnet MILNET + ARPnet = Internet

In 1985

The NSF created the NSFnet to connect to Internet

NSF = National Science Foundation

In 1990

ARPA passed out of existence, and in 1995, the NSFnet became t

he primary Internet backbone network

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Tcp ip 4 layer or 5 layer

Application

Layer

Application

Layer

Transport

Layer

Transport

Layer

Internet

Layer

Internet

Layer

Network

Interface

Network Interface

Physical

Chapter 3 Network Architetures and Standarts

TCP/IP 4-layer or 5-layer

CISCO and IBM use the four-layer model.

We use the five-layer model .

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A comparison of the layers of the osi reference model and the implementation of tcp ip

Chapter 3 Network Architetures and Standarts

A comparison of the layers of the OSI reference model and the implementation of TCP/IP.

Network Interface

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Protocols

Chapter 3 Network Architetures and Standarts

PROTOCOLS

Each layer has several protocols.

A layer define a data communication function that may be performed by certain protocols.

A protocol provides a service suitable to the function of that layer.

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Computer networking

Chapter 3 Network Architetures and Standarts

TCP/IP and the OSI Model

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Application layer

Chapter 3 Network Architetures and Standarts

Application layer

Provide services that can be used by other applications.

Incorporate the functions of top 3 OSI layers

Contains the programming required to support the user`s application.

Different module are required for each application:

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Transport layer

Chapter 3 Network Architetures and Standarts

Transport layer

Application layer directly run over the transport layer, corresponding to OSI transport layer.

Two kinds of services: TCP & UDP.

TCP—Transmission Control Protocol,

reliable connect-oriented transfer of a byte stream.

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Udp user datagram protocol

Chapter 3 Network Architetures and Standarts

UDP—(User Datagram Protocol)

best-effort connectionless transfer of individual messages.

The advantage of a connectionless protocol is that there is no setup time at the beginning of the communication.

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Chapter 3 network architetures and standarts9

Application

Application

Router/Gateway

Machine A

Machine B

Transport

Transport

Internet

Internet

Internet

Network Interface

Network Interface

Network Interface

Network 1

Network 2

Chapter 3 Network Architetures and Standarts

Internet layer

The internet layer of TCP/IP routes data between networks when necessary using the IP protocol.

  • Transfer of information across networks through gateways/routers

  • Global unique IP address and IP packets

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Figure 2.11


Chapter 3 network architetures and standarts10

Application

Application

Machine B

Machine A

Router/Gateway

Transport

Transport

IP

packet

IP

packet

IP

packet

Internet

Internet

Internet

Network Interface

Network Interface

Network InterfaceS

Packet

of network1

Packet

of network2

Packet

of network2

Packet

of network1

Network 1

Network 2

Chapter 3 Network Architetures and Standarts

Network interface layer

Handles the connection between the end system and the network to which it is attached.

IP

packet

  • Corresponding to part of OSI network layer and data link layer

  • Different network interfaces: X.25, ATM, frame relay, Ethernet, etc

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Figure 2.11


Some protocols in tcp ip suite

Chapter 3 Network Architetures and Standarts

Some Protocols in TCP/IP Suite

App.

Transport

Internet

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A critique of the tcp ip reference model

A Critique of the TCP/IP Reference Model

Problems:Service, interface, and protocol not distinguished.

Not a general model.does not require strict layering.

Application

Layer

Application

ayer

Transport

Layer

Transport

Layer

Internet

Layer

Internet

Layer

Network

Interface

Network

Interface

(a)

(b)

TCP/IP network architecture

Chapter 3 Network Architetures and Standarts

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3 11 architecture of the internet

3.11 Architecture of the Internet

Chapter 3 Network Architetures and Standarts

Guide: Informal architectural,it was not formalized until later.

The Internet is a network of networks.

The architects of the Internet don't insist on documenting every detail of Internet operation before implementation work began.

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Chapter 3 network architetures and standarts11

Chapter 3 Network Architetures and Standarts

The four initial requirements confronting the Internet’s architects were :

Multiplexing Survivability service generality

supporting diverse network technologies

The primary initial requirement that the architects of the Internet addressed were diverse network technologies.

Design Guidelines

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Designer

  • What entities

Naming addressing routing

security

Concerned with

Modularized

Resources are allocated

Chapter 3 Network Architetures and Standarts

Designer

It wasn`t until 1991 that the Word Wide Web was introducee and Internet began to move from being an academic and military tool to being the tool it now os for research,sales,and communication.

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3 12 manufacturers architectures

Chapter 3 Network Architetures and Standarts

3.12 MANUFACTURERS`ARCHITECTURES

Before OSI RM

IBM SNA

DEC DNA

Burroughs BNS

It is a proprietary architecture.

Manufacturers that developed network architectures were primarily interested in developing proprietary architectures on which to base their future products.

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Ibm sna

Chapter 3 Network Architetures and Standarts

IBM SNA

SNA DNA ARCHITECTURES

Because of customer and market pressures,the situation changed and computer manufacturers became fully supportive of the international communications standards.

IBM SNA

DEC DNA

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Other architecture

Chapter 3 Network Architetures and Standarts

OTHER ARCHITECTURE

The ATM Reference Model

Novell NetWare RM

Windows NT RM

A Proposed Reference Model

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Computer networking

The ATM reference model.

Chapter 3 Network Architetures and Standarts

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Novell netware rm

Chapter 3 Network Architetures and Standarts

NOVELL NETWARE RM

IPX/SPX (internetwork packet exchange /sequences packet exchange)

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Computer networking

Chapter 3 Network Architetures and Standarts

Windows NT RM

Windows NT 网络体系与模型

NetBEUI NWLink

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A proposed reference model

Chapter 3 Network Architetures and Standarts

A Proposed Reference Model 

Andrew S.Tanenbaum:

  • 5 layers of RM;

  • No presentation layer session;

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3 13 a caveat

Chapter 3 Network Architetures and Standarts

3-13 A CAVEAT

It is important to understand that writing the specifications for communications architectures and the related standards is difficult work.

As actual communications systems have been implemented over the years, most have not strictly followed the OSI model.

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3 14 advs and disadvs of layered architectures

Chapter 3 Network Architetures and Standarts

3-14 ADVS AND DISADVS OF LAYERED ARCHITECTURES

1.Layering forces the modularization of function.

2. Good flexibility

3. Easy to achieve and maintain

4. Scalability

5.Standardization 

A protocol stack is software that implements the layers of a protocol

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Osi vs tcp ip reference models

Chapter 3 Network Architetures and Standarts

OSI vs. TCP/IP Reference Models

OSI introduced concept of services, interface, protocols These were force-fitted to TCP later .

In OSI, reference model was done before protocols. In TCP, protocols were done before the model

OSI: Standardize first, build later

TCP: Build first, standardize later

As actual communications systems have

been implemented over the years, most have notstrictly followed the OSI model.

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Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

1. The first three layers of the OSI model have been standardized, and together are called the(  ).

A

A、 X.25 standard for data transmission

B、 V. standard

×

C、 consolidated transmission standard

×

×

D、 Consolidated Transmission Protocol

Redo

Next

Answer


Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

2. The ITU-T has created two sets of standards for the electrical connection of terminals to communications networks. They are called (  ).

D

×

A、 the ISO and OSI standards

B、 the TCP and IP standards

×

C、 the PC and modem standards

×

D、 the V. and X. standards

Redo

Next

Answer


Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

3. IBM’s SNA (  ).

C

×

A、 was never widely implemented

×

B、 is a data link protocol

C、is a proprietary architecture

×

D、 was an advanced mainframe computer for its time

Redo

Next

Answer


Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

4. A network architecture is (  ).

C

×

A、 the plan for the Internet’s evolution

×

B、 a description of how a network is to be implemented

C、 a set of principles used as the basis for the design and implementation of a communications network

×

D、 None of the above

Redo

Next

Answer


Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

D

5. The Interne (  ).

×

A、 is implemented using the TCP/IP protocol

×

B、 has the attribute of service generality

×

C、 allows for diverse network technologies

D、 All of the above.

Redo

Next

Answer


Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

  • A set of principles used as the basis for the design and implementation of a communications network is called a

  • ________________

network architecture

Answer

2.The_____________ reference model is the standard reference point by which data communications networks are described and measured.

OSI

Answer


Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

3.Two telephone callers may be connected but they will not be able to ___________ and exchange information if they speak different languages.

communicate

Answer

4.The________ _________ layer specifies the electrical characteristics between the communications line and the terminal or computer system.

Physical link

Answer


Computer networking

Chapter 3 Network Architetures and Standarts

课后习题

5.When data are received, the OSI presentation layer is responsible for ___________the data to meet the needs of the application layer.

formatting

Answer


Computer networking

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