Medium Access Control in Communication Networks: Infrastructure vs Distributed Approaches , Study notes of Organizational Communication

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Distributed

Medium Access Control

Medium Access Control

• Problem:
  • Single shared communications resource
    • RF spectrum, electrical cable, etc
  • Multiple users
  • How do you decide who communicates when?
• Two approaches:
  • Infrastructure
    • Central controller handles all packet scheduling
  • Distributed
    • Distributed algorithm for determining who communicates when

Multiple Access

• Infrastructure:
  • Control access to uplink
• Distributed:
  • Control access to shared medium
  • Distributed networks are half-duplex
  • No central controller -> users = network

Early MAC Example

• In the 1970s, University of Hawaii constructed a packet radio network to interconnect Hawaiian islands
• In 1972 it was connected to ARPANET
• Basic MAC approach:
  • If you have data to send, send it
  • If the message you send interferes with someone else, try again

ALOHA Performance

• Per-user rate as a function of users
• Total rate converges to 0.

ALOHA Performance

• Poor performance due to statistical

independence of time slots

• If collision occurs
  • Wait before retransmitting
    • How long?
  • Probability of retransmitting in next slot decreases

exponentially

• Exponential backoff

Hidden Terminal Problem

• In RF, range of shared medium is limited
• Result: Hidden Terminal Problem
• Scenario:
  • User A starts transmitting to user B
  • User C listens, hears idle
  • User C starts transmitting to user B
  • Packet collision

User A User B User C

Exposed Terminal Problem

• Nodes may determine medium is busy when it

is in fact idle