Page 162 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Unit 10: Network Layer in the Internet

Closed loop solutions: make decision based on the concept of a feedback loop. The feedback Notes
loop enables the closed loop system to monitor the system to detect when and where congestion
occurs. Thereafter, it passes the information to the places where actions can be taken. This
enables to adjust system operation to correct the problem.
Monitoring of the system is dependent upon the percentage of all packets discarded for lack of
buffer space, the average queue lengths, the number of packets that time out and are retransmitted,
the average packet delay and the standard deviation of packet delay. The monitored congestion
information is provided to all places of actions when the router detects the congestion; it sends
a separate warning packet to the traffic source immediately. This done by reserving a bit or field
in each packet which is filled in each outgoing packet in case of a congested state encountered by
a router to caution the neighbors. Secondly, hosts or routers send packets periodically to explicitly
know about congestion so that the traffic around congested areas may be routed to alternate
destination paths.

The congestion may be controlled as given below:
1. Increase the bandwidth in the network. Increasing an additional line temporarily increases
the bandwidth between certain points.

2. Split traffic to follow multiple routes.
3. Increase the resources. For example, use spare routers.
4. Decrease the load by denying service to some users or degrading service to some or all
users.
5. Estimate users schedule and demands in a more predictable way.

10.2.2 Traffic Management

The traffic management facility allows maximizing available network resources and ensures
efficient use of resources that have not been explicitly allocated. Traffic management will be
mainly dependent on transmit priority and bandwidth availability. In the transmit priority,
delay-sensitive traffic is assigned a higher transmit priority. Support for bandwidth availability
deals bandwidth allocation for each VCC, connection admission control (CAC) preventing
network users from allocating more bandwidth than the network can provide, traffic policing to
ensure that a VCC, once established, does not attempt to use more bandwidth than the network
currently has available and selective cell discard dealing with momentary over subscription of
the buffer capacity of an output port.

Average Packet Delay

Suppose

Average arrival rate of packets at a router for processing = packets per second
Average processing rate of packets one at a time at the router =  packets per second
Utilization of the channel =  = /

From Queueing theory, the average delay a packet experiences at a router before being forwarded
is given by:
T = / (1/1-)

From the above, it is clear that average delay approaches infinity as utilization approaches
unity.

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