There are 2 types of “links”:

• Point-to-point (direct connection)
  • e.g. PPP for dial-up access, point-to-point link between Ethernet switch and host
• Broadcast (shared wire/medium)
  • e.g. old-fashioned Ethernet, upstream HFC, 802.11 wireless LAN
    • On old-fashioned links, bits can be intercepted on in-between links ⇒ encryption is important

Links can be wired or unwired.

The environment of a multiple access protocol involves:

• Single shared broadcast channel
• 2+ simultaneous transmissions by nodes ⇒ interference is possible
  • If node receives 2+ signals at the same time, we have collision

A multiple access protocol is is a distributed algorithm that determines how nodes share a channel.

• i.e. Protocol determines when nodes can transmit
• Communication about channel sharing must use the channel itself
  • No out-of-band channel for coordination

Given a broadcast channel with rate of $R$ bits/second, an ideal multiple access protocol would have the following properties:

• When one node wants to transmit, it can send at rate $R$
• When $M$ nodes want to transmit, each can send at an average rate of $R/M$
  • i.e. broadcast link is fairly allocated
• Algorithm is fully decentralized
  • No special node needed to coordinate transmissions
  • No synchronization of clocks or slots
• Algorithm is simple