Congestion Management

In a subnet, when too many packets are present, its performance degrades and this situation is called as Congestion.

When the number of packets dumped in to the subnet by the hosts is within its carrying capacity, they are all delivered and the number delivered is proportional to number sent.

But, as traffic increase too far, the routers are no longer able to cope and they begin losing packets. At very high traffic, performance collapse completely and almost no packets are delivered.

The general principle of congestion control are of two types and they are : Open loop and Closed loop.

Open loop

Open loop solutions attempt to solve the problem by good design i.e., taking care of not appearing for first time. Once the system is up and running, midcourse correction are not made.

Tools for doing this type control include, deciding when to accept new traffic, deciding when to discard packets and which ones and making scheduling decisions at various point of network.

Closed loop

Closed loop solutions are based on the concept of feedback loop and this approach has three parts for congestion control and they are:

a) Monitor the system to detect when and where congestion occur.
b) Pass the information to place where action can be taken.
c) Adjust system operation to correct the problem.

Traffic Shaping

The main cause of congestion is that traffic is often bursty. If host could be made to transmit at a uniform rate, congestion would be less common.

Another open loop method to help manage congestion is forcing the packets to be transmitted at a more predictable rate. These approach is widely use in ATM network and is called “Traffic shaping”.

Thus traffic shaping is about regulating the average rate of data transmission. Here as long as the customers fulfils her part of the bargain and only send the packets according to the agreed upon contract, the carrier promises to deliver them all in a timely fashion.

The most popular shaping algorithms are (a) The leaky bucket algorithms (b) The token bucket algorithms.

Low Shedding

Low shedding is a fancy way of saying that when routers are being in undated by packets that they cannot handle, they just throw them away.

This term comes from the world of electrical power generation where it refers to the practices of utilizes intentionally blacking out certain areas to save the entire grid from collapsing on hot summary days when the demand for electricity greatly exceed the supply.

Jitter Control

The Jitter can be bonded by computing the expected transit time for each hop along the path. When a packet arrives at a router, the router checks to see how much the packet is behind or ahead of its schedule. This information is stored in the packet and updated at each hop.

If the packet is ahead of schedules, the router tries to get it out the door quickly. Thus in this method, packets that are ahead of schedule gets slowed down and packets that are behind schedule get speeded up, in both cases reducing the amount of jitter.

More Information
Pulse Code Modulations
Windows Protocol
Frequency Division Multiplexing
Data Transmission Impairments
Error Correction Methods
Error Detection Methods
Basic Transfer of Data
Simplex, Full & Half Duplex
Asynchronous Data Transmission
Electromagnetic Waves
Fibre Optic
Serial, Parallel & Synchronous Data Transmission
VSAT Communications
Satellite Communication
Transmission Lines
Data Formats
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