This document discusses the characteristics and functioning of contention-based computer networks. Contention networks rely on stations listening to the transmission medium before transmitting to avoid collisions. They work best under light-to-medium traffic loads and are limited by network length and propagation delays. Refinements like CSMA, CSMA/CD, and CSMA/CA were developed to improve efficiency by detecting and avoiding collisions. Contention networks provide easy expansion and resilience to station failures. However, they have high overhead and access delays that increase significantly under heavy traffic loads.

2. • Smooth functioning of a contention-based
network is dependent upon high availability
of the transmission media and a low
collisions rate.

3. The network is characterised by the
following :
• Message length
• Traffic volume
• Network length constraints
• Performance
• Overhead
• Access Delay
• Station Failures
• Expansion

4.  Message length :
 Messages are divided into short packets in
order to reduce the amount of data that must
be rebroadcast after collisions.Normally, the
original message is also fairly short.

5.  Traffic volume :
 Contention protocols are designed for
networks with low traffic volume,that is one
with few time.
Low traffic volume implies a limited number
of attached workstations.

6.  Network length constraints :
The longer the network,the greater the chance of
collision.
Contention networks are limited by the time
needed for a signal to travel the length of the
transmission media and have an
acknowledgement returned (that is propagation
delay).

7.  Performance :
Contention networks are most effective
under light to medium load.
Performance under those conditions is
excellent.
Under heavy load, a contention networks
tends to be unstable,with service rapidly
degrading.

8.  Overhead :
Contention networks have high overhead
because of collisions and the need to
acknowledge the successful receipt of
messages.

9.  Access Delay :
Delay on the network is generally moderate
to long depending on traffic.
Delay under heavy load can be significantly
higher than load alone would seem to
dictate.

10.  Station Failures :
 Because operation of the network is not
dependent on the presence or absence of
any one workstation,failure of workstation
inconveniences only its users.
Rarely does failure of a single station disrupt
service on the whole network.

11.  Expansion :
 Addition of new workstations is relatively
easy because to be included in the
network,the workstation simply must
recognise its own unique address.
Expansion may be achieved with minimal
disruption of the network.

12. • Refinements on contention procedures are
used by many of the current microcomputer
local networks.
• These refinements are :
Carrier Sense Multiple Access(CSMA)
 Carrier Sense Multiple Access with Collision
Detection(CSMA/CD)
 Carrier Sense Multiple Access with Collision
Avoidance(CSMA/CA)

13. • STEP 1 :
Station A
Listening
Station B
Quiet

14. • STEP 2 :
Station A
Sending
Station B
Listening
Message A

15. • Each will be discussed in separate sections.
• For all practical purposes, charcteristics of
these refinements are identical to the
charcteristics of simple contention.

16. References
• Information Technology Today
By S. Jaiswal
• https://en.wikipedia.org/wiki/Workstati
on