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Home > Protocol > Aloha Protocol
 
ALOHA PROTOCOL
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  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:
Prob[k trans in t] = ((Gt)**k )(e**(-Gt))/(k!)
  • capacity of multiple access protocol: maximum value of S over all values of G
Analyzing Aloha (cont)
 
focus on a given attempted packet transmission
S = rate attempted Packet trans * prob[trans successful]
= G*prob[no other Packet's overlap with attempted trans]
= G*prob[0 other attempted trans in 2 time units]
= Ge*(-2G)
Aloha throughput
Note: maximum thorughput is 18% of physical channel capacity
you buy 1 Mb link, thoughput will never be more than 180Kb!
 
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  Aloha Protocol