1 Ch 8 LAN Technologies and Network Topology

2 Scope  Describes the concepts underlying local network technologies  Describes basic network topology  Examines examples of popular local network technologies

3 Classification Terminology  Network technologies classified into three broad categories Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN)  LAN and WAN most widely deployed

4 Scientific Justification for LANs A computer is more likely to communicate with computers that are nearby than with computers that are distant A computer is more likely to communicate with the same set of computers repeatedly  Known as the locality principles

5 LANs  Many LAN technologies exist Designed for sharing (needs medium access control, MAC) IEEE 802.3, 802.4, 802.5,  Key features of a LAN High throughput Relatively low cost Limited to short distance Often rely on shared media rather than direct connections (or said point-to-point connections)

6 Network Topologies  Specifies general “shape” of a network Star Ring Bus  Each topology has advantages and disadvantages

7 Star Topology  Central point of network known as hub  Each computer has separate connection to hub

8 Ring Topology  To be connected in a closed loop  Connections go directly from one computer to another  No central facility

9 Bus Topology  Shared medium forms main interconnect  Broadcasting oriented  Only one computer sends a signal at any time

10 Example Bus Network: Ethernet  Most popular LANs  IEEE standard  Several generations Same frame format Different data rates (10/100/1000 Mbps) Different wiring schemes (e.g., 10Base2, 10BaseT)

11 Manchester Encoding  Hardware can detect a change in voltage easily than a fixed value  Use rising and falling edges to encode data 1 and 0  One slot for a bit  Voltage change occur exactly half-way through a slot

12 Manchester Encoding  A preamble is used to have the receiver know when each time slot begins  The preamble consists of 64 alternating 1’s and 0’s sent before the frame

13 Sharing on an Ethernet  Signal propagates across entire cable (terminator located at both ends)  All stations receive transmission (only the dest. can accept the frame)  Only one station transmits at any time  CSMA/CD media access scheme

14 CSMA/CD Paradigm  Multiple Access ( MA ) Multiple computers attach to shared media Each uses same access algorithm  Carrier Sense ( CS ) Wait until medium idle Begin to transmit frame

15  Two simultaneous transmissions Interfere with one another Called collision  CSMA plus Collision Detection (CD) Listen to medium during transmission Detect whether another station’s signal interferes Back off from interference and try again CSMA/CD Paradigm

16 Backoff After Collision  When collision occurs Wait random time t, 0 ≤ t ≤ d (tome slot) Use CSMA and try again  Double range for each successive collision  Called exponential backoff

17 Wireless LAN  IEEE switch AP LAN DHCP server Cat 5

18 Wireless LAN  Wireless ADSL line ADDSL router Cat 5 straight line ADDSL router Private IP 4-in-1 頻寬分享器 (NAT, DHCP and Hub) Cat 5 CO

19 Wireless LAN  Wireless ADSL line ADDSL router Cat 5 AP Cat 5 4-in-1 頻寬分享器 (NAT, DHCP and Hub) CO

20  Limited range ( hidden terminal problem ) Not all stations receive all transmissions Cannot use CSMA/CD E.g., STA 2 can detect the collision CSMA/CA STA 1 STA 2 STA 3

21 CSMA/CA  Purpose: inform all stations in range of X or Y before transmission  Known as Collision Avoidance (CA) STA 1 STA 2 STA 3 RTS CTS Area cleared by RTS (Request To Send) Area cleared by CTS (Clear To Send)

22 SourceDestination ACK Data CTS RTS CSMA/CA  4-way MAC frame exchange protocol

23 Token Passing Ring Transmission  Station waits for token before sending  Signal travels around entire ring  Sender receives its own transmission

24 Token Passing  Token Special, reserved message Small bit pattern differs from normal data frames  Station Waits for the token to arrive Transmits one packet around ring Transmits token around ring  When no station has data to send Token circulates continuously  Guarantees fair access

25 Strengths of Token Ring Approach  Easy detection of broken ring interference (by the sender) hardware failures (passing mode)  No collision

26 Weaknesses of Token Ring Approach  Broken wire disables entire ring  Point-to-point wiring Awkward in office environment Difficult to add / move stations

27 Token Passing Ring Technologies  LocalTalk Operated at 10 Mbps (CSMA/CA)  IBM Token Ring Originally operated at 4 Mbps Later version operated at 16 Mbps  FDDI ( Fiber Distributed Data Interconnect ) Operated at 100 Mbps

28 FDDI Failure Recovery  Uses two fiber rings  Automatic failure recovery Dual-attached Counter rotating (data travels in the reverse direction across the second ring) Self healing (the process of reconfiguring to avoid failure)

29 Illustration of FDDI Failure Recovery

30 FDDI Terminology  FDDI Uses optical fibers High reliability Immune to interference  CDDI FDDI over copper Same frame format Same data rate Less noise immunity

31 FDDI Hub Technology  Part of FDDI standard  Stations attach to hub  Same frame format and data rate as FDDI  Called star-shaped ring  Advantages Wiring Reliability

32 The End

33 Example Star Network: ATM  Asynchronous Transfer Mode (ATM)  Designed by telephone companies  Intended to accommodate Voice Video Data

34  Building block known as ATM switch  Each station connects to switch (star topology)  Switches can be interconnected  Only propagate data to the communicating pair Example Star Network: ATM

35 Details of ATM Connection  Full-duplex connections  Two fibers required  Operates at 155 Mbps or faster

36 ATM Characteristics  High data rates (e.g. 155 Mbps)  Fixed size packets Called cells Important for voice  Cell size is 53 octets 48 octets of data 5 octets of header