Random Access

• Idea: Consider the following rules for regulating access to the channel:
  • Transmission must still occur in time-slots.
  • However, any computer that has a new message to transmit may do so in the next time-slot.
  • If two or more stations transmit in the same time-slot, a collision occurs, i.e., neither message can be received correctly.
  • To resolve a collision, the stations involved in the collision retransmit their messages a random number of time slots later.
• This set of rules describes a famous random multiple-access protocol, called the ALOHA protocol.
• It was first proposed by researchers at the University of Hawaii in the late 1960.
• It was shown that with this protocol, a maximum of $\frac{1}{e}\approx 36\%$ of time-slots carry successful messages, i.e., are neither empty nor collisions.
• Hence, it the the ``raw'' bit rate of the broadcast channel is 10 MB/s then the ``useful'' bit rate with this channel is approximately 3.6 Mb/s.
• Stations that do not have messages to transmit do not prevent stations with messages to transmit.
• Furthermore, especially when there are few stations with messages to transmit, collisions occur rarely and there is virtually no delay until a transmission can commence.
• The ALOHA protocol forms the conceptual basis for the support of bursty traffic in LANs.