Chapter 12Chapter 12

Computer NetworksComputer Networks

Chapter OutlineChapter Outline

IntroductionIntroduction Basic concepts in computer Basic concepts in computer

networkingnetworking• Communication linksCommunication links• Local area network (LAN)Local area network (LAN)• Wide area network (WAN)Wide area network (WAN)

Network servicesNetwork services The InternetThe Internet The World Wide WebThe World Wide Web

Basic ConceptsBasic Concepts

A A computer networkcomputer network is a set of is a set of independent computer systems independent computer systems connected by telecommunication connected by telecommunication links that together are called links that together are called interconnection network.interconnection network.

Individual computers in the network Individual computers in the network are referred to as are referred to as nodesnodes or or hostshosts. .

Communication LinksCommunication Links

Vary widely in the physical characteristicVary widely in the physical characteristics, error rate, and transmission speed.s, error rate, and transmission speed.

Switched dial-up telephone lines. Switched dial-up telephone lines. • The term “switched” means that when we The term “switched” means that when we

dial a phone number, a circuit (path) is temdial a phone number, a circuit (path) is temporarily established between called and the porarily established between called and the callee. The circuit lasts for the duration of thcallee. The circuit lasts for the duration of the call.e call.

Analog vs. DigitalAnalog vs. Digital Voice-oriented dial-up telephone network Voice-oriented dial-up telephone network

is (in part) an analog medium.is (in part) an analog medium. However, a computer produces digital However, a computer produces digital

information. information. Use modem (Use modem (modmodulator/ulator/demdemodulator) to odulator) to

convert binary information into analog and convert binary information into analog and vice versa.vice versa.

Telephone line has a rather low Telephone line has a rather low bandwidthbandwidth. Physical limits on transmission . Physical limits on transmission speed over the telephone line are being speed over the telephone line are being reached. (56000 bits per second)reached. (56000 bits per second)

Asymmetric Digital Subscriber Line (Asymmetric Digital Subscriber Line (ADSL)ADSL)

Amplitude ModulationAmplitude Modulation

Dedicated Communication LinksDedicated Communication Links

Twisted-pair copper wireTwisted-pair copper wire Coaxial cableCoaxial cable

• ISDN: 128 KbpsISDN: 128 Kbps• T1: 1.544MbpsT1: 1.544Mbps• T3: 44.736 MbpsT3: 44.736 Mbps

Fiber-optic cableFiber-optic cable• OC-3: 155.5 MbpsOC-3: 155.5 Mbps• OC-12: 622 MbpsOC-12: 622 Mbps• OC-48: 2.488 Gbps (gigabit networking) OC-48: 2.488 Gbps (gigabit networking)

Transmission TimeTransmission Time

16mb color image16mb color image

Line typeLine type speedspeed timetime

Dial-up phone lineDial-up phone line 33.6 Kbps33.6 Kbps 7.9 min7.9 min

Dedicated ISDNDedicated ISDN 128 Kbps128 Kbps 2.1 min2.1 min

T1T1 1.544 Mbps1.544 Mbps 10.4 s10.4 s

T3T3 44.737 Mbps44.737 Mbps 0.36 s0.36 s

Fiber-optic OC-3Fiber-optic OC-3 155.5 Mbps155.5 Mbps 0.1 s0.1 s

Fiber-optic OC-12Fiber-optic OC-12 622 Mbps622 Mbps 0.026 s0.026 s

Fiber-optic OC-48Fiber-optic OC-48 2.488 Gbps2.488 Gbps 0.006 s0.006 s

Wireless Data CommunicationWireless Data Communication

Use radio, microwave, infrared Use radio, microwave, infrared signals instead of wires/cables.signals instead of wires/cables.

Mobile computingMobile computing In the future,In the future,

• Fiber-optics to support high-speed error-Fiber-optics to support high-speed error-free wired connectionsfree wired connections

• Radio and microwave-based wireless Radio and microwave-based wireless links to for places that fiber does not or links to for places that fiber does not or cannot reach.cannot reach.

Local Area NetworksLocal Area Networks

A local area network connects hardware A local area network connects hardware devices such as computer, printer, and devices such as computer, printer, and mass storage devices that are all in close mass storage devices that are all in close physical proximity.physical proximity.

Owner of the computers is also owners of Owner of the computers is also owners of the means of communications.the means of communications.

A private network.A private network. Most widely used LAN: Ethernet.Most widely used LAN: Ethernet.

EthernetEthernet

Developed in the mid-1970s by computer Developed in the mid-1970s by computer scientists at Xerox PARC.scientists at Xerox PARC.

Originally designed to operate at 10 Mbps Originally designed to operate at 10 Mbps using coaxial cables.using coaxial cables.

Fast Ethernet: 100 Mbps.Fast Ethernet: 100 Mbps. Two ways to construct an Ethernet LAN:Two ways to construct an Ethernet LAN:

• Cable, single wire + transceiver or multiple Cable, single wire + transceiver or multiple cable + bridgecable + bridge

• HubHub

Constructing EthernetConstructing Ethernet

Communication ProtocolCommunication Protocol

The rules describing how messages The rules describing how messages are transmitted between network are transmitted between network nodes are called communication nodes are called communication protocols.protocols.

Protocols are “algorithm of Protocols are “algorithm of communication”. They describe the communication”. They describe the step-by-step procedures required to step-by-step procedures required to guarantee an orderly exchange of guarantee an orderly exchange of information across a network.information across a network.

Ethernet ProtocolEthernet Protocol Use contention-based transmission technique.Use contention-based transmission technique. Users compete for access to a single shared Users compete for access to a single shared

communication line. communication line. • When a node wants to send a message, it first listens to When a node wants to send a message, it first listens to

the line to see if it is currently in use.the line to see if it is currently in use.• Broadcast message if line is not busy.Broadcast message if line is not busy.• Address field attached to the front of the message that Address field attached to the front of the message that

identifies the destination.identifies the destination.• Every node reads the address filed, if not its own, Every node reads the address filed, if not its own,

discards remainder of the message. If it is, accepts and discards remainder of the message. If it is, accepts and store it.store it.

What to do in a collision?What to do in a collision? Control of Ethernet network is distributed. More Control of Ethernet network is distributed. More

robust. robust.

Wide Area NetworkWide Area Network A wide area network (WAN) connects devices that A wide area network (WAN) connects devices that

may be across town, across the country, or across may be across town, across the country, or across the ocean.the ocean.

Cross public property, must purchase services Cross public property, must purchase services from external providers.from external providers.

Point-to-point links.Point-to-point links.

WAN ProtocolWAN Protocol

Most WANs use a store-and-forward, packet sMost WANs use a store-and-forward, packet switching communication protocol.witching communication protocol.

Unit of transmission in WAN is a packet: an infUnit of transmission in WAN is a packet: an information block with fixed maximum size.ormation block with fixed maximum size.

A message is “chopped” into N separate pacA message is “chopped” into N separate packets and sent independently through the netwkets and sent independently through the network.ork.

At the destination node, the packets are reassAt the destination node, the packets are reassembled to restore the original message. embled to restore the original message.

Packet SwitchingPacket Switching

RoutingRouting

Selecting the specific path to use for Selecting the specific path to use for relaying data.relaying data.

Routing AlgorithmRouting Algorithm

Shortest path problem: O(NShortest path problem: O(N22)) Highly complex because of:Highly complex because of:

• Massive amount of dataMassive amount of data• Topological changes: routing table must Topological changes: routing table must

be re-computed often.be re-computed often.• Network failuresNetwork failures

RouterRouter

A router transmits message between A router transmits message between two distinct networks that uses two distinct networks that uses different protocols.different protocols.

LAN/WAN connection.LAN/WAN connection.

Network ServicesNetwork Services

Resource sharing:Resource sharing:• Physical resources: printer or disks. Physical resources: printer or disks.

printer serverprinter server• Logical resources: data file or software. Logical resources: data file or software.

file server file server Client-server computingClient-server computing Information sharing: electronic data Information sharing: electronic data

interchangeinterchange Information utilityInformation utility

Network Services (cont’d)Network Services (cont’d)

Groupware: support group efforts in Groupware: support group efforts in producing shared documents.producing shared documents.

E-mailE-mail Bulletin boardBulletin board News groupsNews groups Electronic commerceElectronic commerce

Network SecurityNetwork Security

Authentication problemAuthentication problem Encryption/decryption problemEncryption/decryption problem Fault-tolerance problemFault-tolerance problem