Ch 8 LAN Technologies and Network Topology

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Ch 8 LAN Technologies and Network Topology
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Scope

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

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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

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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)

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Network Topologies

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

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Star Topology

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

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Ring Topology

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

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Bus Topology

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

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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)

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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

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Manchester Encoding

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

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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

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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

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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 stations signal
  interferes
• Back off from interference and try again

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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

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Wireless LAN

• IEEE 802.11

AP
DHCP server
Cat 5
LAN
Cat 5
switch
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Wireless LAN

• Wireless ADSL

ADDSL router
CO
line
Cat 5 straight
Private IP
Cat 5
ADDSL router
CO
line
4-in-1 ????? (NAT, DHCP and Hub)
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Wireless LAN

• Wireless ADSL

AP
Cat 5
ADDSL router
CO
Cat 5
line
4-in-1 ????? (NAT, DHCP and Hub)
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CSMA/CA
STA1
STA2
STA3

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

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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)
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CSMA/CA

• 4-way MAC frame exchange protocol

Source
Destination
RTS
CTS
Data
ACK
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Token Passing Ring Transmission

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

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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

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Strengths of Token Ring Approach

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

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Weaknesses of Token Ring Approach

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

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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

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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)

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Illustration of FDDIFailure Recovery
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FDDI Terminology

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

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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

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The End
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Example Star Network ATM

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

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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

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Details of ATM Connection

• Full-duplex connections
• Two fibers required
• Operates at 155 Mbps or faster

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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