ALOHA
RANDOM-ACCESS METHODS
The
random-access methods we study in this chapter have evolved from a very
interesting protocol known as ALOHA, which used a very simple procedure called
Multiple Access (MA). The method was improved with the addition of a procedure
that forces the station to sense the medium before transmitting. This was
called carrier sense multiple access (CSMA). This method later evolved into two
parallel methods: carrier sense multiple access with collision detection
(CSMA/CD), which tells the station what to do when a collision is detected, and
carrier sense multiple access with collision avoidance (CSMA/CA), which tries
to avoid the collision.
ALOHA
ALOHA,
the earliest random access method, was developed at the University of Hawaii in
early 1970. It was designed for a radio (wireless) LAN, but it can be used on
any shared medium. It is obvious that there are potential collisions in this
arrangement. The medium is shared between the stations. When a station sends
data, another station may attempt to do so at the same time. The data from the
two stations collide and become garbled.
Pure
ALOHA
The
original ALOHA protocol is called pure ALOHA. This is a simple but elegant
protocol. The idea is that each station sends a frame whenever it has a frame
to send (multiple access). However, since there is only one channel to share,
there is the possibility of collision between frames from different stations. Below
figure shows an example of frame collisions in pure ALOHA.
The
pure ALOHA protocol relies on acknowledgments from the receiver. When a station
sends a frame, it expects the receiver to send an acknowledgment. If the
acknowledgment does not arrive after a time-out period, the station assumes
that the frame (or the acknowledgment) has been destroyed and resends the
frame.
Vulnerable time
Let us
find the vulnerable time, the length of time in which there is a possibility of
collision. We assume that the stations send fixed-length frames with each frame
taking Tfr seconds to send. Below figure shows the vulnerable time for station
B.
The
vulnerable time during which a collision may occur in pure ALOHA is 2 times the
frame transmission time.
Slotted
ALOHA
Slotted
ALOHA was invented to improve the efficiency of pure ALOHA. In slotted ALOHA we
divide the time into slots of Tfr seconds and force the station to
send only at the beginning of the time slot. Below figure shows an example of
frame collisions in slotted ALOHA.
A
station is allowed to send only at the beginning of the synchronized time slot,
if a station misses this moment, it must wait until the beginning of the next
time slot. This means that the station which started at the beginning of this
slot has already finished sending its frame. Of course, there is still the
possibility of collision if two stations try to send at the beginning of the
same time slot.
Vulnerable Time for
Slotted ALOHA
The
vulnerable time is now reduced to one-half, equal to Tfr. Below
figure shows the situation.
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