EE5412 Telecommunications Nework

[Lan_rev.doc]
Network access method or sharing techniques - random access vs deterministic access

Random access – aloha; deterministic – polling, token ring

Concentrate on random access –

Aloha – model from Hawaii U. ; model for packet radio in country area
Random transmission of packet from stations – uncorrelated
Assumptions for simple result – equal distance between every station
Slot time = packet length = propagation delay time

Under this model - cause of collision –
Packet takes time to propagate and transmit = vulnerable time
another transmission during the vulnerable time => collision

For pure aloha – vulnerable time = 2 slot time
For a successful transmission => P1 x P0 => max 18%

To improve success rate , slot aloha => vulnerable time =1 slot =>

For a successful transmission => P1 => max 36%

CSMA – sense carrier ; vulnerable time = propagation delay,
CD; CA – to improve performance after collision or to avoid collision

Characteristic of the network under heavy load – stability
On average = load < capacity

Temporary heavy load – characteristic
At light load – little collision – little retransmission
Load increase – chance of collision increase – retransmission increase
Eventually a lot of retransmission with little successful through -put. => unstable

Delay mechanism to improve stability - binary exponential backoff
IEE 802.3, Ethernet – truncated binary exponential backoff

Function of the delay mechanism –
No effect at light load
At heavy load – more traffic – more retransmission – more delay
Delay reduce effective input => maintain throughput and give the network a chance to clear the backlog without getting unstable. Overload is only transient.

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Learning Objective - Random access

1. Ethernet protocol flow diagram
2. Binary exponential backoff algorithm and its effect on performance
3. performance of aloha and slotted aloha
4. reason for collision in random access
5. brief explanation on stability