CS412 Introduction to Computer Networking

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CS412 Introduction to Computer Networking
Telecommunication

• Medium Access Control Sublayer

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Topics

• Introduction
• Channel Allocation Problem
• Multiple Access Protocols
• CDMA

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Introduction

• Broadcast networks
• Key issue who gets to use the channel when there
  is competition
• Referred to as
• Multiaccess channels
• Random access channels
• MAC (Medium Access Control) sublayer
• LANs
• Satellite networks

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Channel Allocation Problem

• Channel Allocation
• Static
• Dynamic
• Performance factors
• Medium access delay
• Time between a frame is ready and the frame can
  be transmitted
• Throughput
• frames can be transmitted in unit time interval

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Static Channel Allocation

• FDM
• Bandwidth divided into N equal sized portions for
  N users
• Problems
• senders large
• senders continuously varies
• bursty traffic
• Discussion users gt N ? ?
• lt N ? ?
• N ? ?
• N times worse than all frames queued in one big
  queue

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Static Channel Allocation

• TDM
• Each user is statically allocated every Nth time
  slot
• Same problems as FDM
• Under what circumstances are static channel
  allocation schemes efficient?

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Dynamic Channel Allocation

• Key assumptions
• 1. Station model
• Independent
• Work is generated constantly
• One program per station
• Station is blocked once a frame has been
  generated until the frame has been successfully
  transmitted
• 2. Single channel assumption

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Dynamic Channel Allocation

• Key assumptions
• 3. Collision Assumption
• Collision
• Two frames are transmitted simultaneously,
  overlapped in time and resulting signal garbled
• Can be detected by all stations
• No other errors

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Dynamic Channel Allocation

• Key assumptions
• 4. Time either continuous or discrete (slotted)
• Continuous
• Frame transmission can begin at any instant
• No "master clock" needed
• Slotted
• Time divided into discrete intervals (slots)
• Frame transmissions begin at the start of a slot
• frames contained in a slot 0 ? ?
• 1 ? ?
• gt1 ? ?

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Dynamic Channel Allocation

• Carrier sense ("carrier" refers to electrical
  signal) either Y or N
• Yes
• A station can check channel before transmission
• If busy, station idle
• No
• Just do it"
• Can tell if transmission successful later
• LANs carrier sense
• Satellites -gt not carrier sense (why?)

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Multiple Access Protocols

• ALOHA
• Carrier sense multiple access protocols (CSMA)
• CSMA w/ collision detection (CSMA/CD)
• Collision-free protocols
• Limited-contention protocols

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ALOHA

• Applicable to any contention system
• System in which uncoordinated users are competing
  for the use of a single shared channel
• Two versions
• Pure ALOHA
• Slotted ALOHA

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Pure ALOHA

• Let users transmit whenever they have data to be
  sent
• Colliding frames are destroyed
• Sender can always find out destroyed or not
• Feedback (property of broadcasting) or ACK
• LANs immediately
• Satellites propagation delay (e.g., 270msec)
• By listening to the channel
• If frame is destroyed
• wait a random amount of time and retransmit
• (why "random"?)

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Figure 13.4 Procedure for ALOHA protocol
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Pure ALOHA
Where are the collisions?
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Pure ALOHA
Frames are assumed to have the same size (same
frame time) for analysis
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Slotted ALOHA

• Discrete time
• Agreed slot boundaries
• Synchronization needed
• Performance
• Which ALOHA has a shorter medium access delay?
• Which ALOHA has a higher throughput?

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Performance of ALOHA

• Slotted ALOHA can double the throughput of pure
  ALOHA

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Carrier Sense Multiple Access (CSMA) Protocols

• Stations can listen to the channel (i.e., sense a
  carrier in the channel)
• Types
• 1-persistent CSMA
• Nonpersistent CSMA
• p-persistent CSMA

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Figure 13.5 Collision in CSMA
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Figure 13.6 Persistence strategies
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(No Transcript)
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CSMA w/ Collision Detection (CSMA/CD)

• Can listen to the channel and detect collision
• Stop transmitting as soon as collision detected
• Widely used on LANs (e.g., Ethernet)
• Collision detection
• Analog process
• Special encoding is used

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CSMA w/ Collision Detection (CSMA/CD)

• Conceptual model
• 3 states
• Contention
• Transmission
• Idle
• Minimum time to detect collision determines time
  slot
• Depends on propagation delay of medium

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CSMA/CD Model
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13.7 CSMA/CD procedure
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Collision-Free Protocols

• Model
• N Stations 0,1, ..., (N-1)
• Question
• Which station gets the channel after a successful
  transmission?
• Protocols
• Bit-map (i.e., reservation) protocol
• Token passing protocol
• Example Token ring

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Bit-Map Protocol
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Figure 13.12 Token-passing network
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Figure 13.13 Token-passing procedure
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Performance of Contention and Collision-Free
Protocols

• Contention
• Low load gt low medium access delay )
• High load gt low channel efficiency (
• Collision-Free
• Low load gt high medium access delay (
• High load gt high channel efficiency )

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Code Division Multiple Access (CDMA)

• In FDMA, the bandwidth is divided into channels.
• In TDMA, the bandwidth is just one channel that
  is timeshared.
• In CDMA, one channel carries all transmissions
  simultaneously.

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Figures 13.14 13.15 Chip sequences and
encoding rules
The chip sequences must be orthogonal! A?B
A?C A?D B?C B?D C?D 0 A?A B?B
C?C D?D length of chip sequence
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Figure 13.16 CDMA multiplexer
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Figure 13.17 CDMA demultiplexer
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CDMA

• (a) Binary chip sequences for four stations
• (b) Bipolar chip sequences
• (c) Six examples of transmissions
• (d) Recovery of station Cs signal

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Summary of Channel Allocation Methods/Systems