Pure
Aloha Protocol
With Pure Aloha, stations are
allowed access to the channel
whenever they have data to transmit.
Because the threat of data collision
exists, each station must either
monitor its transmission on
the rebroadcast or await an
acknowledgment from the destination
station. By comparing the transmitted
packet with the received packet
or by the lack of an acknowledgement,
the transmitting station can
determine the success of the
transmitted packet. If the transmission
was unsuccessful it is resent
after a random amount of time
to reduce the probability of
re-collision.
Figure
Pure Aloha Protocol
Advantages:
· Superior to fixed
assignment when there is a large
number of bursty stations.
· Adapts to varying
number of stations.
Disadvantages:
· Theoretically proven
throughput maximum of 18.4%.
· Requires queueing
buffers for retransmission of
packets.
Comparison
Slotted Aloha
The first of the contention
based protocols we evaluate
is the Slotted Aloha protocol.
The channel bandwidth is a continuous
stream of slots whose length
is the time necessary to transmit
one packet. A station with a
packet to send will transmit
on the next available slot boundary.
In the event of a collision,
each station involved in the
collision retransmits at some
random time in order to reduce
the possibility of recollision.
Obviously the limits imposed
which govern the random retransmission
of the packet will have an effect
on the delay associated with
successful packet delivery.
If the limit is too short, the
probability of recollision is
high. If the limit is too long
the probability of recollision
lessens but there is unnecessary
delay in the retransmission.
For the Mars regional network
studied here, the resending
of the packet will occur at
some random time not greater
than the burst factor times
the propagation delay.
Another important simulation
characteristic of the Slotted
Aloha protocol is the action
which takes place on transmission
of the packet. Methods include
blocking (i.e. prohibiting packet
generation) until verification
of successful transmission occurs.
This is known as "stop-and-wait".
Another method known as "go-back-n"
allows continual transmission
of queued packets, but on the
detection of a collision, will
retransmit all packets from
the point of the collision.
This is done to preserve the
order of the packets. In this
simulation model queued packets
are continually sent and only
the packets involved in a collision
are retransmitted. This is called
"selective-repeat"
and allows out of order transmission
of packets.
Slotted Aloha Protocol
By making a small restriction
in the transmission freedom
of the individual stations,
the throughput of the Aloha
protocol can be doubled. Assuming
constant length packets, transmission
time is broken into slots equivalent
to the transmission time of
a single packet. Stations are
only allowed to transmit at
slot boundaries. When packets
collide they will overlap completely
instead of partially. This has
the effect of doubling the efficiency
of the Aloha protocol and has
come to be known as Slotted
Aloha.
Figure
11: Slotted Aloha Protocol
Advantages:
· Doubles the efficiency
of Aloha.
· Adaptable to a changing
station population.
Disadvantages:
· Theoretically proven
throughput maximum of 36.8%.
· Requires queueing buffers
for retransmission of packets.
Synchronization required.
Slotted
Aloha
Pure Aloha
Aloha
and Network Stability
Aloha
Simulation & Reservation
Aloha Protocol
Slotted
ALOHA Simulation Parameters
ALOHA
PROTOCOL IN C - LANGUAGE
