Shared Communication Channels - Computer Network - Lecture Slides, Slides of Computer Networks

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COMPUTER

NETWORKS

Lecture-

Review Lecture 6

 CRC Hardware Components

 Frame Format and Error Detection

 LAN technology and Network Topology (Chapter 8)

 Direct Point-to-Point Communication

Shared Communication Channels

 Each LAN consists of a single shared medium

 The computers take turns using the medium

 Sharing a single medium over long distances is inefficient, due to the long delays

Growth of LAN Technologies

 LAN technologies reduce cost by reducing number of connections

 But ... attached computers compete for use of shared connection

 Local communication almost exclusively LAN

 Long distance almost exclusively point-to- point Docsity.com

Locality Of Reference Principle

 Principle of Locality of Reference helps predict computer communication patterns:

• Spatial (or physical) locality of reference
  • Computers are likely to communicate with other computers that are located nearby
• Temporal locality of reference
  • Computers are likely to communicate with the same computers repeatedly

Locality Of Reference Principle

 Thus

• LANs are effective because of spatial locality of reference, and
• Temporal locality of reference may give insight into which computers should be on a LAN

Star Topology

 All computers are attached to a central point

 Center of star is sometimes called a “Hub”

 Logical versus Physical Star

Star Topology in Practice

 Previous diagram is idealized

Ring Topology

Bus Topology

 All computers are attached to a single long cable

 Any computer can send data to any other computer

 Coordination required to decide which computer uses the line at what time

Reason for Multiple Topologies

 Each topology has advantages and disadvantages

• In a Ring, it is easy to coordinate access, however entire network is disabled if a cable cut occurs
• In a Star, only one computer is affected, when a cable cut occurs
• In a Bus, needs fewer wires than a star, however entire network is disabled when a cable cut occurs

Example Bus Network: Ethernet

 Widely used LAN technology

• Invented at Xerox PARC (Palo Alto Research Center) in 1970s
• Defined in a standard by Xerox, Intel and Digital Equipment Corporation - DIX standard
• Standard now managed by IEEE (802.3) defines formats, voltages, cable lengths, ...

Ethernet Speeds

 Originally 3Mbps

 Current standard is 10Mbps

 Fast Ethernet operates at 100Mbps

 Giga-Bit Ethernet

Encoding used in Ethernet

 Manchester Encoding: Uses signal changes to encode data

• A change from positive voltage to 0 encodes a 0 bit
• A change from 0 voltage to positive voltage encodes a 1 bit