Network and the Internet

國立清華大學資訊工程學系

CS1356 資訊工程導論

112/04/18

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Network

• An intricately connected system of things or people – Webster dictionary

• Computer network– A linked computer

system

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Outline

• Layered approach for network

• Link layer

• Network layer/Transport layer

• Application layer

Link

Network

Transport

Application

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Layered Approach

What and why?

Link

Network

Transport

Application

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A Story

某日，某資工系某家族決定要去竹北某餐廳家聚。他們決定先在某資電館前集合，再出發到餐廳。家族共有 12 人，某學長有一輛車，可以坐 4 人，另外還有 2 個人有摩托車，共可以載 4 人，另外 4 個人決定坐公車。某學長車上有 GPS ，可以找出路要怎麼走；有摩托車的同學上網查出地圖路線；要坐公車的同學查出要轉一次車，再走一小段路就可以到餐廳了…

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Layered Approach

• 去竹北某餐廳家聚– Decide where to go and what to do

• 4 人坐車 , 4 人騎車 , 4 人搭公車 – Decide the transportation methods

• GPS, 地圖路線 , 公車班次– Decide the routes from the source to the

destination

• 汽車 , 摩托車 , 公車 , 司機 , 馬路 , 紅綠燈 , 路標…– Make the real transportation happen

Application layer

Transport layer

Network layer

Link layer

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Network layers

• Application: Constructs message with dest address

• Transport: Provides reliable data transfer services

• Network: Handles routing through the network

• Link: Handles actual transmission of packets

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Layered Approach: Why?

• Each task in different layer can be handled more easily without considering the details of the tasks in other layers.– For example, they can decide a “best”

restaurant without considering how to go there

• Methods in different layers can be changed easily.– For example, different applications can use the

same transport protocol.

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Protocols

• A set of rules used to communicate with each other across a network. – Example 1: 先在資電館前集合，再出發到餐廳– Example 2: bus route and schedule– Example 3: traffic regulations

• Protocol is a special algorithm that enables and controls the data communications.

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(Fig. 4.14)

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Message Encapsulation

• To transmit the data, each layer appends some information to the original message.

• The corresponding layer in the receiver side needs to “decapsulate” the message.

address 1name 1address 2

name 2address 3name 3address 4name 4

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Link Layer

Link

Network

Transport

Application

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LAN, MAN, WAN

• Network can be classified by the scope– Local area network (LAN)– Metropolitan area (MAN)– Wide area network (WAN)

• Network for different scopesuses different media and has different protocols.

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Network Topology

• Popular network topologies used in LAN– Bus (Ethernet)

– Star (Wireless network with central Access Point, AP)

• Different topology uses different protocols.

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Ethernet and CSMA/CD

• CSMA/CD: protocol for Ethernet– Carrier Sense, Multiple Access with Collision

Detection

– Broadcast messages to all machines on the bus

– Resend message if detecting more than one messages sent at the same time (collision)

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WiFi and CSMA/CA

• CSMA/CA: protocol for WiFi– Carrier Sense, Multiple Access with Collision

Avoidance

• Similar to CSMA/CD– Sensing carriers before sending

• WiFi cannot detect collisions – Self signal drowns out others– Hidden terminal problem

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Collision Avoidance

• Before sending data, a node listens to the channel for a period.– If the channel is silent, it sends data.– If the channel is busy, it waits another random

period. After waiting, if the channel is silent, it sends data immediately.

• Cannot avoid collisions totally.– Before sending data, it sends a request to AP.

AP sends an acknowledgement to all machines after receive the data.

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Network Equipments for LAN

• Hub: relays signals it received

• Repeater: connects two buses

• Bridge: similar to repeater,but only forwards necessarymessages

• Switch: a bridge that connects multiple buses– All those do not change the protocol

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Network layer/Transport layer

IP, TCP, UDP, and the Internet

Link

Network

Transport

Application

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Terminology

• An internet: – a network of networks– It is different from “the Internet”.

• The Internet: a global internet that uses TCP/IP protocol suite.– TCP/IP protocol suite: IP+TCP+UDP– TCP and UDP: Transport layer protocols – IP : Network layer protocol

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Internet Service Provider (ISP)

• A company/organization that offers its customers access to the Internet.

• The ISP hierarchy

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How ISPs Work

• Connection method to ISPs– Direct connection– Dial-up, DSL– Cable modem– Wireless

TANet 台灣學術網路 SeedNet 新世紀資通Hinet 中華電信 Cable modem 東森有線電視

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Internet Protocol (IP)

• A protocol used for communicating data across an internet.

• It is a connectionless protocol.– Telephone network is connection-oriented. – For the same source

and destination, different message may be routed differently.

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IP address

• How to design a global address–台灣 新竹市 光復路二段 101號– 886-3-571-5131

• IP address: the global identification for each computer connected to an internet– Pattern of 32 or 128 bits, usually represented

in the dotted decimal notation.– For example: 140.114.87.5 is for 楓橋驛站

nation-city-road-number

nation-area-phone number

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Gateway and Router

• Gateway: the “point” linked to an internet

• Router: forwards and routes messages– Forwarding: relay messages on an internet– Routing: update and maintain the forwarding

table

Destination Next hop Cost

140.114.87.5 103.121.1.2 …

140.114.79.3 … …

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Domain Name

• The mnemonic address for human– For example, bbs.cs.nthu.edu.tw is the

domain name for 楓橋驛站– Top level domains (TLD): suffixes in the

domain name: .gov, .edu, .com, .net, .org,…– Country-code TLD: .tw, .ca, .jp, .au, …– Subdomains: prefixes of an domain name,

bbs., www., …

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Domain Name System (DNS)

• Provides translation between IP addresses and domain names.– Name server– DNS lookup

Machines like binary addresses

Humans prefer domain names

DNS

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Transport Layer

TCP UDP

Handshaking

Retransmission

Flow control

Congestion control

• Two protocols• TCP (Transmission

Control Protocol)• UDP (User Datagram

Protocol)

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Transmission Control

• Handshaking: – Make reservation before go to a restaurant

• Retransmission: – Ask someone to repeat what he/she said

• Flow control: – Ask someone to talk slower

• Congestion control: – Lineup of a crowd of people

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Port Number

• TCP/UDP need to know which application should a received message be sent to– This is identified by port numbers– IP address is for the address of computer

• ExamplesPort number Application

53/TCP,UDP Domain Name System (DNS)

80/TCP,UDP Hypertext Transfer Protocol (HTTP)

513/TCP Login

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Application Layer

Client/server, peer to peer, distributed system

The Internet applications and WWW

Link

Network

Transport

Application

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Client/server Model

• Server provides service to clients– One server, many clients– Server must execute

continuously– Client initiates

communication

• Examples: – Printer server, – File server, – Web server, – Email server, – BBS server,– …

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Peer-to-peer (P2P) Model

• Two processes communicating as equals

• Examples:– Peer-to-peer file sharing:

• FastTrack, BitTorrent, …

– VoIP: Skype, …– Streaming media: TVant, …– Instant message, online

chat: MSN, google talk, …

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Distributed System

• Consists of software units that execute as processes on different computer.

• Examples:– Multitier architecture– Computer cluster– Cloud computing– Grid computing– BOINC system– …

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The Internet Applications

• Telnet, ssh

• Email

• WWW

• File transfer

• Peer2peer

• Internet phone (VoIP), instant message

• Streaming media

• …

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Electronic Mail (email)

• Email address

• Email server

• Email client– Outlook– Thunderbird– Pine

• Webmail – Yahoo mail, gmail, hotmail, …

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World Wide Web (WWW)

• A system, invented by Tim Berners-Lee (1989), of interlinked hypertexts via Internet.

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Web Browser

• A software application for retrieving, presenting, and traversing information resources on the World Wide Web.– Internet Explore– Mozilla Firefox– Apple Safari– Google Chrome

• Bookmark, RSS, download management

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HTTP and URL

Figure 4.8 A typical URL

• Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information

• Uniform Resource Locator (URL): web address

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HTML

• Hypertext Markup Language (HTML): encoded as text file

• Uses tags to communicate with browsers

• XML: eXtensible Markup Language – A language for constructing markup

languages (meta-language)

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Related Courses

• Computer network–計算機網路概論、密碼與網路安全概論、訊號與系統、作業系統

• Textbook chap 4 sec1,2,3,4

• The Internet

References