Media Access Control [Link Layer]

Media Access Control [Link Layer]

1. Random Access Protocols

   • Transmitting nodes are always transmitting at full rate of channel. R bps. When there is a collision, each node repeatedly retransmits its frame, until the frame gets through. It does not retransmit straight away, there is usually a random delay involved. 
   • Slotted ALOHA
     • Advantages
       • Node to transmit continuously at full rate, R - It is also highly decentralised, nodes act independently of each other.  - Very good for only one active node.
     • Disadvantages
       • - Lots of wasted slots, either from collision or empty
     • Maximum Efficiency
   • ALOHA
     • Unslotted, fully dencentralised protocol.
     • Maximum Efficiency
       • Only Half of ALOHA
   • Carrier Sense Multiple Access (CSMA)

2. Channel Partitioning Protocols

   • Divides channel into smaller pieces (timeslots, frequency, code)
   • TDMA
     • Divides time into time frames and divides time frame into slots. Each time slot to each node
     • Advantages: Eliminates Collision and is perfectly fair system, Transmission rate of R/N bps
     • Disadvantages: - Node is limited to an average rate of R/N bps even if it's the only one. - Node must always wait its turn in the transmission sequence, even if it's the only one
   • FDMA
     • Divits R bps into diff frequencies, each with BW R/N
   • CDMA
     • Code division multiple access. Assigns different code to each node. Each node uses unique code to encode databits

3. Taking Turns Protocols

   • When M nodes are active then each active node has throughput of R/M bps, ONLY taking turns protocol satisfy this property
   • Polling Protocol
   • Token-Passing Protocol