Ch 8 LAN Technologies and Network Topology

Ch 8 LAN Technologies and Network Topology

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

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

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

LANs • Many LAN technologies exist • Designed for sharing (needs medium access control, MAC) • IEEE 802.3, 802.4, 802.5, 802.11 • 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)

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

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

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

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

Example Bus Network: Ethernet • Most popular LANs • IEEE standard 802.3 • Several generations • Same frame format • Different data rates (10/100/1000 Mbps) • Different wiring schemes (e.g., 10Base2, 10BaseT)

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

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

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

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

CSMA/CD Paradigm • 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

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

Wireless LAN • IEEE 802.11 AP DHCP server Cat 5 LAN Cat 5 switch

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

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

CSMA/CA • Limited range (hidden terminal problem) • Not all stations receive all transmissions • Cannot use CSMA/CD • E.g., STA2 can detect the collision STA1 STA2 STA3

CSMA/CA • Purpose: inform all stations in range of X or Y before transmission • Known as Collision Avoidance (CA) RTS CTS STA2 STA3 STA1 Area cleared by RTS (Request To Send) Area cleared by CTS (Clear To Send)

CSMA/CA • 4-way MAC frame exchange protocol Source Destination RTS CTS Data ACK

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

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

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

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

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

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)

Illustration of FDDIFailure Recovery

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

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

The End

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

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

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

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

Ch 8 LAN Technologies and Network Topology