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.
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!)
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
