ALOHAnet Ravi.H.B
Protocol Set
of rules that govern data communication. A protocol defines what is communicated, how it is communicated and when it is communicated.
MAC Protocols Medium
Access Control. Many nodes share a common channel. Goals
Fair among users High efficiency Low delay Fault tolerant
ALOHAnet Norman
Abramson – University of Hawaii in
1970. Used for satellite communication in the pacific. Basic “transmit at will” protocols. ALOHAnet uses shared medium for transmission.
ALOHA Protocol OSI
layer 2 protocol Used for LAN networks Broadcast topology Contention type
ALOHA Two
rules
If you have data to send, send the data. If the message collides with another transmission, try re-transmitting later.
Successful
receipt of the frame at the satellite means no collision or overlap. ALOHA depends on the ability of a node to detect or learn that a collision has occurred.
ALOHA
Any terminal is allowed to transmit without considering whether channel is idle or busy If packet is received correctly, the listener transmits an ACK. If no ACK is received, re-transmission of data
ALOHA - Types Pure
ALOHA Slotted ALOHA
Pure ALOHA Any
node with a packet of data to transmit can transmit onto the shared channel. Collision will occur in 2 scenarios
Another node is already transmitting when this node starts. Another node starts to transmit while this node is transmitting
Slotted ALOHA Enhancement
of Pure ALOHA. A node can send only at the beginning of a new timeslot. Thus, collisions are reduced.
Throughput of Pure ALOHA In
Pure ALOHA, the vulnerable period for a node’s transmission starts T seconds before it initiates its frame transmission and ends T seconds after it completes its frame. Max. Throughput is 18.4% It occurs at G=0.5
Throughput of Pure ALOHA 0.2 0.18 0.16
Throughput (S)
0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0
1 2 3 AverageNum beroffram esperunittim e(G)
4
5
Throughput of Slotted ALOHA In
Slotted ALOHA, the vulnerable period of a node’s transmission is only the T seconds prior to the start of its frame. Max. Throughput is 36.8% It occurs at G=1
Throughput of Slotted ALOHA 0.4 0.35
Throughput (S)
0.3 0.25 0.2 0.15 0.1 0.05 0 0
1 2 3 AverageNum beroffram esperunittim e(G)
4
5
Thank You