4. The Medium Access Sub Layer
Broadcast channels [or multi-access channels] are a category of networks and the key issue is how to determine who gets to use the channel when there is competition for it. The protocols which define these factors belong to a sub layer of data link layer called the MAC(medium access control)sub layer.
ALOHA: Norman Abramson devised a new and elegant method to solve the channel allocation problem called the “ALOHA” system which used ground-based Radio broad casting. Two categories are present in this ALOHA system. They are: a. Requires global time synchronization [SLOTTED ALOHA] b. Doesn’t require global time synchronization. [PURE ALOHA]
(a). SLOTTED ALOHA: Roberts published a method for doubling the capacity of an ALOHA system by dividing the time up into discrete intervals, with each interval corresponding to a single frame. Time synchronization was achieved by having a special station that emits a pip at the start of each interval, like a clock. In this system, a computer is not permitted to send whenever a carriage return is typed. Instead, it is required to wait for the beginning of next slot. Since the vulnerability period is now halved, the probability of no other traffic during the some slot is e-g leads to……………. S = Ge-g
Throughput Vs offered traffic graph:
From the graph, SLOTTED ALOHA peaks at G=1 with a throughput of about 0.368.
Operating at higher values of G……
• Reduces the no of empties.
• Increases the no of collisions exponentially.
(b).PURE ALOHA: In an ALOHA system, users transmit whenever they have data to be sent in fixed length frames. when collision occur, the sender gets the information due to the feedback property of broadcasting. If the frame was destroyed, the sender just waits a random amount of time and sends it again.
Frame generation in an ALOHA system:
If the first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be re-transmitted later.
Let the mean frame (new) generated by different number of users per frame time be ‘s’(frames without collisions). The value of S can be therefore, either 0 or 1.
Let the mean frames (new + retransmitted)generated by different no of users per frame time without collisions be ‘G’. The value of G is obviously greater than of equal to S (2)
At low load i.e., S=0……
There will be few collisions….so, few retransmissions are required. So, (3)
At high load i.e., S=1…….
There will be many collisions …….so, few re-transmissions are required. So, (4)
If P0 is the probability that a frame doesn’t suffer from any collision,
If Pr[k] is the probability that ‘k’ frames are generated during a given frame time, then
The probability of ‘0’ frames is Pr  = G0 e-G
P0 = e-G ( 7)
The mean no. of frames generated in an interval of 2 frame...
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