Lectures 13&14 Packet Multiple Access: The Aloha protocol Eytan Modiano Massachusetts Institute of Technolog
Lectures 13 & 14 Packet Multiple Access: The Aloha protocol Eytan Modiano Massachusetts Institute of Technology Eytan Modiano Slide 1
Multiple Access Shared Transmission Medium a receiver can hear multiple transmitters a transmitter can be heard by multiple receivers the major problem with multi-access is allocating the channel between the users the nodes do not know when the other nodes have data to send Need to coordinate transmissions
Multiple Access • Shared Transmission Medium – a receiver can hear multiple transmitters – a transmitter can be heard by multiple receivers • the major problem with multi-access is allocating the channel between the users; the nodes do not know when the other nodes have data to send – Need to coordinate transmissions Eytan Modiano Slide 2
Examples of multiple Access channels Local area networks(LANs) Traditional Ethernet Recent trend to non -multi-access lANs satellite channels Multi-drop telephone Wireless radio Medium Access Control (MAC) MAC NET Regulates access to channel DLC LLC Logical Link Control (LLC) All other dlc functions PHY
Examples of Multiple Access Channels • Local area networks (LANs) – Traditional Ethernet – Recent trend to non-multi-access LANs • satellite channels • Multi-drop telephone • Wireless radio • Medium Access Control ( MAC) – Regulates access to channel • Logical Link Control (LLC) – All othe r DLC f unctions NET DLC PHY MAC LLC 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 Contention systems Polling Reservations and Scheduling Random access
Approaches to Multiple Access • Fixed Assignment (TDMA, FDMA, CDMA) – each node is allocated a fixed f raction of bandwidth – Equivalent to circuit switching – very inefficient for low duty factor traffic • Contention systems – Polling – Res ervations and Scheduling – Random Access Eytan Modiano Slide 4
Aloha Single receiver, many transmitters Receiver Transmitters E. g, Satellite system wireless
Aloha Single receiver, many transmitters Receiver ... . Transmitters E.g., Satellite system, wireless Eytan Modiano Slide 5