Packet multiple access and the Aloha protocol Eytan Modiano Massachusetts Institute of Technology Department of Aeronautics and Astronautics Eytan Modiano
Packet multiple access and the Aloha protocol Eytan Modiano Massachusetts Institute of Technology Department of Aeronautics and Astronautics Eytan Modiano Slide 1
Packet Multiple Access SHARED UPLIN TERMINAL APPL TERMINALL TRANS NET LLC DLC MAC PHYS TERM|NA凵 TERMINAl Medium Access Control(MAC) Regulates access to channel TERMINA Logical Link Control (LLC) Eytan Modiano All other dlc functions
Packet Multiple Access TERMINAL TERMINAL TERMINAL TERMINAL TERMINAL PMA SHARED UPLINK PHYS DLC NET TRANS APPL LLC MAC • Medium Access Control (MAC) – Regulates access to channel • Logical Link Control (LLC) Eytan Modiano Slide 2 – All other DLC functions
Examples of multiple Access channels Local area networks lANs) · Satellite channels Wireless radio Characteristics of Multiple Access channel Shared transmission medium A receiver can hear multiple transmitters a transmitter can be heard by multiple receivers The major problem with multiple access is allocating the channel between the users Nodes do not know when other nodes have data to send Need to coordinate transmissions Eytan Modiano
Examples of Multiple Access Channels • Local area networks (LANs) • Satellite channels • Wireless radio • Characteristics of Multiple Access Channel – Shared Transmission Medium A receiver can hear multiple transmitters A transmitter can be heard by multiple receivers – The major problem with multiple access is allocating the channel between the users Nodes do not know when other nodes have data to send Need to coordinate transmissions Eytan Modiano Slide 3
Approaches to Multiple Access Fixed Assignment (TDMA, FDMA, CDMA Each node is allocated a fixed fraction of bandwidth Equivalent to circuit switching very inefficient for low duty factor traffic Packet multiple access Polling Reservations and Scheduling Random Access Eytan Modiano
Approaches to Multiple Access • Fixed Assignment (TDMA, FDMA, CDMA) – Each node is allocated a fixed fraction of bandwidth – Equivalent to circuit switching – very inefficient for low duty factor traffic • Packet multiple access – Polling – Reservations and Scheduling – Random Access Eytan Modiano Slide 4
Aloha Single receiver, many transmitters Receiver Transmitters E.g., Satellite system wireless Eytan Modiano
Aloha Single receiver, many transmitters Receiver ... . Transmitters E.g., Satellite system, wireless Eytan Modiano Slide 5