Introduction to Computer Networks

– A Selective Overview

國立政治大學資訊科學系

劉吉軒 副教授

Computer Communication

Transfer a file of data Access information from a database Exchange of electronic mail Financial transaction Share the use of peripheral devices Access a remote supercomputer Industrial control/Business activity

Basic Communication Functions

Data Communication Networks

Simple computer-to-computer Involved only two computers Two alternatives

Point-to-point wire link Public carrier facilities

Public switched telephone network (PSTN) Modem link

Two Computers Only

More Than Two Computers

LAN – local area networks All computers are located in a single office

or building or site Enable all computers to communicate with

one another at different times Involve different techniques and

equipments or devices

LAN-based Systems

Wider Area

Computers are located in different sites Use public carrier facilities Wide Area Network (WAN) Enterprise-wide private network

Lease dedicated transmission lines (circuits) from the public carriers

Enterprise-wide Private Network

More General Solutions

Public switched data network (PSDN) most public carriers now provide this, in addi

tion to providing PSTN such networks, like PSTN, are now intercon

nected internationally transmit data rather than voice

Alternatively, convert existing PSTNs to enable data to be transmitted without modems ISDNs (Integrated Services Digital Networks)

Public Carrier Data Networks

Internet Embrace multiple networks, LAN-WAN-

LAN

Broadband Multiple-service Networks

Traditional networks primarily support data-only services

New applications Desktop video telephony Videoconferencing, etc.

Require high transmission bit rates ATM (Asynchronous Transfer Mode)

Broadband Multiple-service Networks

Categories of Networks

Local Area Networks (LANs) Metropolitan Area Networks (MANs) Wide Area Networks (WANs) Size, ownership, the distance it covers, ph

ysical architecture

LAN

Usually privately owned and links the devices in a single office, building, or campus

Size is limited to a few kilometers Designed to allow resources to be

shared between PCs or workstations Hardware – printer, hard disk, Software – application programs Data

LAN

LAN

A given LAN will use only one type of transmission medium

The most common LAN topologies are bus, ring, and star

Traditionally, LAN data rates – 4 to 16 Mbps

Today – can reach 100 Mbps

MAN

Designed to extend over an entire city May be a single network such as a cable television

network May be a means of connecting a number of LANs

into a larger network May be wholly owned and operated by a private

company May be a service provided by a public company Many telephone companies provide a popular

MAN service called Switched Multi-megabit Data Services (SMDS)

MAN

WAN

Provides long-distance transmission over large geographical areas that may comprise a country, a continent, or even the whole world

May utilize public, leased, or private communication devices, usually in combination

Enterprise network – wholly owned and used by a single company

WAN

Internetworks

Connecting two or more networks devices – routers, gateways

Internet – a specific worldwide network

TCP/IP

A set of protocols, or a protocol suite, that defines how all transmissions are exchanged across the Internet

Named after its two most popular protocols TCP - Transmission control protocol IP - Internetworking protocol

In active use for almost 20 years Demonstrated its effectiveness on a

worldwide scale

History

APARNET (1969) A packet-switching network of computers

linked by point-to-point leased lines Research result of a project funded by ARPA

The conventions developed to specify how individual computers could communicate across that network became TCP/IP

TCP/IP and the Internet Developed together, each shaping the

growth of the other An internet under TCP/IP operates like

a single network connecting many computers of any size and type

TCP/IP and OSI TCP was developed before OSI

Encapsulation

Message – user datagram – datagram – frame – bits (signals)

The movement of datagram across the Internet is the responsibility of the TCP/IP protocol

Transmitted as signals along the transmission media

Network Layer

More accurately, the internetwork layer IP + ARP, RARP, ICMP, IGMP (supporting pro

tocols) Internetwork Protocol (IP)

Transmission mechanism across networks Datagrams are transported independently Unreliable, connectionless datagram protocol Best-effort delivery service (no error checking

or tracking)

IP

Possible reasons of transmission failures Noise (bit errors), congestion, routing loop,

disable links Example of best effort – postal delivery IP provides bare-bones transmission

functions that free the user to add only those facilities necessary for a given application and thereby allows for maximum efficiency

Datagram Packets in the IP layer

Addressing

IP address – an address that identifies the connection of a host to its network An addressing convention at the Internet le

vel Physical address (contained on NICs) ident

ifies individual devices within a local network

Four bytes (32 bits) – class type, netid, hostid

IP Address Classes cover the needs of different types of

organizations

IP addresses Notation

IP address = Internet address

Class Ranges of IP Addresses

Nodes with More Than One Address

An Internet address defines the node’s connection to its network

More than one internet address Any device connected to more than one

network (any router) Any device with more than one connection to

the network

A Sample Internet

Client-Server Paradigm

One of the benefits of networks is the ability to distribute processing responsibilities

Domain Name System (DNS) Identification systems

Designed for machines and software – bit patterns and numbers

People are the actual users – prefer names DNS identifies each host on the Internet

with a unique name Given the domain name, a program can

obtain the IP address associated with it by engaging the services of a name server in a client-server session

DNS

A hierarchical system made up of a number of labels separated by dots Unique name Decentralized and easy to change From left to right – from lower to higher

Three different domains – generic (organization), country, reverse

DNS

Telnet A client-server process for remote login

Allow a user at one site to gain access to a computer at another site

Telnet Security – login and password identifiers

File Transfer Protocols Trivial File Transfer Protocol (TFTP)

Allows a local host to obtain files from a remote host but does not provide security or reliability

Simpler to use and smaller to store

FTP

Standard mechanism for copying a file from one host to another Use TCP – reliable and secure Use Telnet – require login id and password

Electronic Mail: SMTP

Simple Mail Transfer Protocol A system for sending messages or files to other

computer users based on mailbox addresses rather than a direct host-to-host exchange

Echo the postal system Addresses are used to identify both the recipient and

sender of a message Messages that cannot be delivered within a specified

amount of time are returned to the sender Received mail is stored in the mailbox until the

recipient removes or discards it

Electronic Mail

Spooling mechanism Allows a user to send mail even if a

network is currently disconnected or the receiving machine is not operational

When a message is sent, a copy of is placed in a storage facility called a spool

The background client (MTA) process looks for new messages and not-yet-sent old messages and attempt delivery

Electronic Mail Addresses Mailbox identifying name + domain name

Protocols other than TCP/IP – xxx%yyy@zzzz

Aliases Forward messages to more than one

recipient at a time by including several mailing addresses

Electronic mailing list Frequent messages are sent to the same

group of people Mail alias expansion – allows one name

(called an alias) to refer to an entirely different name or even to multiple recipients

Electronic Mail Mechanism

HyperText Transfer Protocol (HTTP)

A file-retrieving application program that can access distributed and linked documents on the World Wide Web

Hypertext – linked textual documents Hypermedia – linked documents

containing images, graphics, and sounds

HTTP Commands

World Wide Web A repository of information linked together

WWW Information is stored in a set of documents Use the concept of hypertext

WWW

A hypertext available on the Web is called a page The main page for an organization or an

individual is known as homepage Undistributed

Information about one specific subject consists of one or more Web pages on the same server

Distributed On different servers

Uniform Resource Locator When a client needs to access a Web

page, it needs an address URL is a standard for specifying any

kind of information on the Internet

URL Method – the protocol used to retrieve the

document Gopher, FTP, HTTP, news, TELNET

Host – the computer at which the information is located The name of the computer can be an alias Web pages are usually stored in computers

given alias names that usually begin with www Path – pathname of the file

Browser Architecture Access, interpret, and display Web

documents Controller, client programs, interpreters

Types of Web Documents

Based on the time when the contents of the documents are determined

Static documents fixed after creation, stored in a server The same content, independent of user ac

cess (identity, location, time) HTML (HyperText Markup Language)

A markup language for writing Web pages Let the browser format the Web pages (dis

play)

HTML

Dynamic Documents

Created by a Web server whenever a browser requests the document An application program is called upon to

create the dynamic document The server returns the output of the program

as a response to the browser that requested the document

The content of the document may depend on time, identity and location of requested host

Accessing a Dynamic Documents

CGI

Common gateway interface A standard for building dynamic Web

documents Defines the type of language to be used

in preparing the documents Most of the CGI programs have been

created using shell scripts in UNIX The script is run at the server side

Active Documents Created at the

client site by a program retrieved from the server site and run at the client site

Animated graphics

Active Documents

The interpreter at the client site is responsible for running the program

Java – a technology and a language Programming language, runtime

environment, class library Use the term applet to define an active

document program – distinguish active programs from regular program

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大、速度快

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