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MAC Protocols

  • For a shared medium with multiple nodes

Random Access Protocols

  • Randomly sends the data from any node
  • Eg. ALOHA (Slotted/Pure), CSMA, CSMA CD (Detection)/CA (Avoidance)

Channelisation

  • Use of multiplexing to send the data
  • FDMA, TDMA

Pure Aloha

  • Random Access Protocol
  • Collision is possible (maybe another station is also transmitting data)
  • Acknowledgement is there, re transmission is possible
  • Vulnerable time = 2 * Transmission time
  • Efficiency = G * e ^ -2g, G -> No of stations
  • Max efficiency = 18.4%
Slotted Aloha
  • Divided into slots of transmission time
  • Only transmission at the begin of the stot
  • Vulnerable time - Transmission time
  • Efficiency = 36.8 %

Carrier Sense Multiple Access Protocol

1) 1 - Persistent - Continuously sensing the channel, once free it sends the data (collision) 2) 0 - Persistent - If the channel is busy, it backs off for some time (loss of bandwidth) 3) P-Persistent - Continuously sensing the channel, starts the probability of p (hybrid)

  • Sensing the carrier for any transmission in the medium in the meantime
  • Checks only the point of contact only, not the entire channel

Carrier Sense Multiple Access Protocol / CD

  • Used in Ethernet
  • How to check if the frame has been collided?
  • Senses collided frame while the station is transmitting the frame.
  • No acknowledge system
  • Transmission Time > Propagation Delay (Max - TT >= 2 * Transmission Time)
  • Efficiency = 1 + /1 + 6.44a, a = PD/TT

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