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seminar on introduction to MAC protocols

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  • 1. Comparison of MAC protocols in wired systems and wireless systems Mridula Sharma 19.07.2010 Comparison of MAC protocols in wired systems and wireless systems

2. Outline

  • Introduction
  • Multiple Access Protocols
  • Wired Network
  • Wireless Networks
  • Conclusion
  • References

Comparison of MAC protocols in wired systems and wireless systems 3. Introduction

  • Large class of networks is built on top ofbroadcast channels
  • Problem : if youre sharing a channel, then two stations may decide to start frame transmission at the same time
    • Frame collision , which means rubbish on the wire
  • Solution : Allocate the channel to one of the competing stations
  • Allocation can be of two types:
    • Static:FDMandTDM
    • Dynamic

Comparison of MAC protocols in wired systems and wireless systems

    • Protocol to determine who goes next on a multi-access channel belongs to the
    • Medium Access Control (MAC)sublayer
      • Especially important for LANs
      • Provides addressing and channel access control mechanisms

4. Introduction Comparison of MAC protocols in wired systems and wireless systems 5. Introduction Functions of MAC layer Comparison of MAC protocols in wired systems and wireless systems

  • On transmission, assemble data into a frame with address and error detection fields
  • On reception, disassemble frame, and perform address recognition and error detection
  • Govern access to a LAN transmission medium

6. Multiple Access Protocols

  • ALOHA
  • Carrier Sense Multiple Access (CSMA) Protocols
  • Collision Free Protocols
  • Limited Contention Protocols
  • Wavelength Division Multiple Access Protocols
  • Wireless LAN Protocols

Comparison of MAC protocols in wired systems and wireless systems 7. ALOHA

  • Radio-based communication network
    • Developed in 1970s at the University of Hawaii
    • Applicable to any system withcompeting uncoordinated users
  • Two versions
    • Pure ALOHA
    • Slotted ALOHA

Comparison of MAC protocols in wired systems and wireless systems 8. Pure ALOHA

  • Let users transmit whenever they have data to be sent !!!
  • If a collision occurs, finish your current transmission and retry later
  • Performance is maximized if all frames have the same size
  • Random waiting time required else collisions occur

Comparison of MAC protocols in wired systems and wireless systems Fig. : Frames transmitted at completely random intervals 9. Pure ALOHA- Efficiency

  • Probability that k frames are generated during a given frame time with G attempts per packet time:
  • Let P0 be the probability that frame does not suffer from collision
    • Throughput , S= G.P0
  • V ulnerable period = 2Tframe
  • Probability that a frame will not be damaged during two frame times
  • long is:

Comparison of MAC protocols in wired systems and wireless systems 10. Slotted ALOHA Comparison of MAC protocols in wired systems and wireless systems

  • Fr ame transmission can only start at fixed times
  • Since the vulnerable period is now halved,

Fig. : Throughput versus offered traffic for ALOHA systems 11. Carrier Sense Multiple Access (CSMA) Comparison of MAC protocols in wired systems and wireless systems

  • CSMAprotocols do better thanALOHA : you monitor the channel
  • before and/or during transmission
  • 1-persistent
  • Nonpersistent
  • p-Persistent
  • .

12. Comparison of MAC protocols in wired systems and wireless systems Fig. : Comparison of channel utilization versus load for various random access protocols 13. CSMA with Collision Detection (CSMA/CD) Comparison of MAC protocols in wired systems and wireless systems

  • Sense the channel, but immediately stop transmission when you detect a collision
  • Saves Channel Bandwidth and time
  • Ethernetworks like this!!!

Fig. : CSMA/CD can be in one of the three states:Contention, Transmission or Idle . 14. Collision Free Protocols Comparison of MAC protocols in wired systems and wireless systems

  • Bit-Map protocol
  • Binary Countdown Protocol

15. Limited Contention Protocols Comparison of MAC protocols in wired systems and wireless systems

  • Contention systemsare good when theres not much going on
  • Collision-free systemsare good when theres generally a lot oftraffic
  • What we really want is thecontention strategy during light loads , and
  • collision-free strategy during rush hours!!!
  • Solution: Limited Contention Protocols
  • Dynamically regulate the number of competing stationsduring a contention period.
  • If theres not much traffic, the first station will be immediately
  • allowed to transmit a frame.
  • With a lot of traffic, the strategy reduces to the bit-map protocol.

16. Limited-Contention Protocols The Adaptive Tree Walk Protocol Comparison of MAC protocols in wired systems and wireless systems 17. Wavelength Division Multiple Access Protocols Comparison of MAC protocols in wired systems and wireless systems

  • If you have a lot of bandwidth, just divide the channel into sub-channels, and dynamically allocate the sub-channels !!!
      • used in fiber optics

Fig. : Wavelength Division Multiple Access 18. Ethernet Comparison of MAC protocols in wired systems and wireless systems

  • CSMA/CDbased
    • sense the channel, wait until idle, and backoff if collision
  • Exponential Backoff
  • Near implementation ofIEEE 802.3protocol

19. 802.3 Frame Layout Comparison of MAC protocols in wired systems and wireless systems

  • Preamble : Seven times 10101010 is used to synchronize the receivers clock with that of the sender
  • Start : Just a delimiter to tell that the real info is now coming
  • Address : Generally 48-bit fields. Leftmost bit indicates ordinary or group addresses (multicast / broadcast). Second bit indicatesglobal or local address
  • Length : Ranges from 0-1500. Frames should always be at least
  • 64 bytes
  • Pad : used to fill out the frame to the minimum size
  • Checksum : Calculated over the data field. CRC-based

20. Wireless Networks Comparison of MAC protocols in wired systems and wireless systems

  • Often, there are a number ofbase stations(a.k.a. access points)connected through guided media
  • Nodes can also group together to form anad hoc network

21. Wireless LAN Requirements Comparison of MAC protocols in wired systems and wireless systems

  • Throughput
  • Number of Nodes
  • Connection to backbone LAN
  • Service Area
  • Battery Power Consumption
  • Transmission Robustness and Security
  • Collocated Network Operation
  • License-free Operation
  • Handoff/ roaming
  • Dynamic Configuration

22. Problems Comparison of MAC protocols in wired systems and wireless systems

  • There can be subtle interference:
  • Issue (a)
  • How can C be prevented from trying to transmit something to B ?
      • in that case it will ruin any receipt by B
      • hidden station problem
  • Issue (b)
  • How can we tell C that it is allowed to transmit to D, because this will not interfere with the communication from B to A ?
      • exposed station problem

23. Multiple Access with Collision Avoidance (MACA) Comparison of MAC protocols in wired systems and wireless systems

  • A first sends aRequest To Send (RTS)
  • B answers with aClear To Send (CTS)
  • C hears only RTS and can freely transmit, knowing it will notinterfere with As transmission-solves exposed station problem
  • D hears only the CTS and keeps still for otherwise it would
  • interfere with Bs reception-solves hidden station problem

24. MACA for Wireless (MACAW) Comparison of MAC protocols in wired systems and wireless systems

  • G ets rid of Ethernet like unfairness associated with binary exponential backoff algorithms !!!
  • Acknowledges the importance of link layer acknowledgements

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