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ALOHA PROTOCOL

Slotted Aloha
Pure Aloha
Aloha and Network Stability
Aloha Simulation & Reservation Aloha Protocol
Slotted ALOHA Simulation Parameters
ALOHA PROTOCOL IN C - LANGUAGE

ALOHA PROTOCOL:

Aloha, also called the Aloha method, refers to a simple communications scheme in which each source (transmitter) in a network sends data whenever there is a frame to send. If the frame successfully reaches the destination (receiver), the next frame is sent. If the frame fails to be received at the destination, it is sent again. This protocol was originally developed at the University of Hawaii for use with satellite communication systems in the Pacific.

In a wireless broadcast system or a half-duplex two-way link, Aloha works perfectly. But as networks become more complex, for example in an Ethernet system involving multiple sources and destinations in which data travels many paths at once, trouble occurs because data frames collide (conflict). The heavier the communications volume, the worse the collision problems become. The result is degradation of system efficiency, because when two frames collide, the data contained in both frames is lost.

To minimize the number of collisions, thereby optimizing network efficiency and increasing the number of subscribers that can use a given network, a scheme called slotted Aloha was developed. This system employs signals called beacons that are sent at precise intervals and tell each source when the channel is clear to send a frame. Further improvement can be realized by a more sophisticated protocol called Carrier Sense Multiple Access with Collision Detection (CSMA).

In 1070s, Norman Abram son and his colleagues at the University of Hawaii devised a new and elegant method to solve the channel allocation problem. Many researchers have extended their work since then. Although Abranson's work, called the Aloha System, used ground-based radio broadcasting, the basic idea is applicable to any system in which uncoordinated users are completing for the use of a single shared channel.

The two several of ALOHA are:

· PURE ALOHA
· SLOTTED ALOHA

The Aloha Protocol

simple: if you have Packet to send, "just do it"
if Packet suffers collision, will try resending later

Analyzing the Aloha Protocol

Goal: quantitative understanding of performance of Aloha protocol

fixed length Packets
Packet transmission time is unit of time
throughput: S: number Packets successfully (without collision) transmitted per unit time
- in previous example, S = 0.2 Packet/unit time
offered load: G: number Packet transmissions attempted per unit time
- note: S<G, but S depends on G
- Poisson model: probability of k Packet transmission attempts in t time units: