Chapter 12 Computer Networks

Chapter Outline Introduction Introduction Basic concepts in computer networking Basic concepts in computer networking Communication linksCommunication links Local area network (LAN)Local area network (LAN) Wide area network (WAN)Wide area network (WAN) Network services Network services The Internet The Internet The World Wide Web The World Wide Web

Basic Concepts A computer network is a set of independent computer systems connected by telecommunication links that together are called interconnection network. A computer network is a set of independent computer systems connected by telecommunication links that together are called interconnection network. Individual computers in the network are referred to as nodes or hosts. Individual computers in the network are referred to as nodes or hosts.

Communication Links Vary widely in the physical characteristics, error rate, and transmission speed. Vary widely in the physical characteristics, error rate, and transmission speed. Switched dial-up telephone lines. Switched dial-up telephone lines. The term “ switched ” means that when we dial a phone number, a circuit (path) is temporarily established between called and the callee. The circuit lasts for the duration of the call.The term “ switched ” means that when we dial a phone number, a circuit (path) is temporarily established between called and the callee. The circuit lasts for the duration of the call.

Analog vs. Digital Voice-oriented dial-up telephone network is (in part) an analog medium. Voice-oriented dial-up telephone network is (in part) an analog medium. However, a computer produces digital information. However, a computer produces digital information. Use modem (modulator/demodulator) to convert binary information into analog and vice versa. Use modem (modulator/demodulator) to convert binary information into analog and vice versa. Telephone line has a rather low bandwidth. Physical limits on transmission speed over the telephone line are being reached. (56000 bits per second) Telephone line has a rather low bandwidth. Physical limits on transmission speed over the telephone line are being reached. (56000 bits per second) Asymmetric Digital Subscriber Line ( ADSL) Asymmetric Digital Subscriber Line ( ADSL)

Amplitude Modulation

Dedicated Communication Links Twisted-pair copper wire Twisted-pair copper wire Coaxial cable Coaxial cable ISDN: 128 KbpsISDN: 128 Kbps T1: 1.544MbpsT1: 1.544Mbps T3: MbpsT3: Mbps Fiber-optic cable Fiber-optic cable OC-3: MbpsOC-3: Mbps OC-12: 622 MbpsOC-12: 622 Mbps OC-48: Gbps (gigabit networking)OC-48: Gbps (gigabit networking)

Transmission Time 16mb color image 16mb color image Line type speedtime Dial-up phone line 33.6 Kbps 7.9 min Dedicated ISDN 128 Kbps 2.1 min T Mbps 10.4 s T Mbps 0.36 s Fiber-optic OC Mbps 0.1 s Fiber-optic OC Mbps s Fiber-optic OC Gbps s

Wireless Data Communication Use radio, microwave, infrared signals instead of wires/cables. Use radio, microwave, infrared signals instead of wires/cables. Mobile computing Mobile computing In the future, In the future, Fiber-optics to support high-speed error-free wired connectionsFiber-optics to support high-speed error-free wired connections Radio and microwave-based wireless links to for places that fiber does not or cannot reach.Radio and microwave-based wireless links to for places that fiber does not or cannot reach.

Local Area Networks A local area network connects hardware devices such as computer, printer, and mass storage devices that are all in close physical proximity. A local area network connects hardware devices such as computer, printer, and mass storage devices that are all in close physical proximity. Owner of the computers is also owners of the means of communications. Owner of the computers is also owners of the means of communications. A private network. A private network. Most widely used LAN: Ethernet. Most widely used LAN: Ethernet.

Ethernet Developed in the mid-1970s by computer scientists at Xerox PARC. Developed in the mid-1970s by computer scientists at Xerox PARC. Originally designed to operate at 10 Mbps using coaxial cables. Originally designed to operate at 10 Mbps 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 + bridgeCable, single wire + transceiver or multiple cable + bridge HubHub

Constructing Ethernet

Communication Protocol The rules describing how messages are transmitted between network nodes are called communication protocols. The rules describing how messages are transmitted between network nodes are called communication protocols. Protocols are “ algorithm of communication ”. They describe the step-by-step procedures required to guarantee an orderly exchange of information across a network. Protocols are “ algorithm of communication ”. They describe the step-by-step procedures required to guarantee an orderly exchange of information across a network.

Ethernet Protocol Use contention-based transmission technique. Use contention-based transmission technique. Users compete for access to a single shared communication line. Users compete for access to a single shared communication line. When a node wants to send a message, it first listens to the line to see if it is currently in use.When a node wants to send a message, it first listens to 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 identifies the destination.Address field attached to the front of the message that identifies the destination. Every node reads the address filed, if not its own, discards remainder of the message. If it is, accepts and store it.Every node reads the address filed, if not its own, discards remainder of the message. If it is, accepts and store it. What to do in a collision? What to do in a collision? Control of Ethernet network is distributed. More robust. Control of Ethernet network is distributed. More robust.

Wide Area Network A wide area network (WAN) connects devices that may be across town, across the country, or across the ocean. A wide area network (WAN) connects devices that may be across town, across the country, or across the ocean. Cross public property, must purchase services from external providers. Cross public property, must purchase services from external providers. Point-to-point links. Point-to-point links.

WAN Protocol Most WANs use a store-and-forward, packet switching communication protocol. Most WANs use a store-and-forward, packet switching communication protocol. Unit of transmission in WAN is a packet: an information block with fixed maximum size. Unit of transmission in WAN is a packet: an information block with fixed maximum size. A message is “ chopped ” into N separate packets and sent independently through the network. A message is “ chopped ” into N separate packets and sent independently through the network. At the destination node, the packets are reassembled to restore the original message. At the destination node, the packets are reassembled to restore the original message.

Packet Switching

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

Routing Algorithm Shortest path problem: O(N 2 ) Shortest path problem: O(N 2 ) Highly complex because of: Highly complex because of: Massive amount of dataMassive amount of data Topological changes: routing table must be re-computed often.Topological changes: routing table must be re-computed often. Network failuresNetwork failures

Router A router transmits message between two distinct networks that uses different protocols. A router transmits message between two distinct networks that uses different protocols. LAN/WAN connection. LAN/WAN connection.

Network Services Resource sharing: Resource sharing: Physical resources: printer or disks.  printer serverPhysical resources: printer or disks.  printer server Logical resources: data file or software.  file serverLogical resources: data file or software.  file server Client-server computing Client-server computing Information sharing: electronic data interchange Information sharing: electronic data interchange Information utility Information utility

Network Services (cont’d) Groupware: support group efforts in producing shared documents. Groupware: support group efforts in producing shared documents. Bulletin board Bulletin board News groups News groups Electronic commerce Electronic commerce

Network Security Authentication problem Authentication problem Encryption/decryption problem Encryption/decryption problem Fault-tolerance problem Fault-tolerance problem