Many problems in complex systems, such as computer networks, can be viewed from a control
theory point of view. This approach leads to dividing all solutions into two groups: open loop
and closed loop. Open loop solutions attempt to solve the problem by good design, in essence,
to make sure it does not occur in the first place. Once the system is up and running, midcourse
corrections are not made.
Tools for doing open-loop control include deciding when to accept new traffic, deciding when to
discard packets and which ones, and making scheduling decisions at various points in the
network. All of these have in common the fact that they make decisions without regard to the
current state of the network.
In contrast, closed loop solutions are based on the concept of a feedback loop. This approach
has three parts when applied to congestion control:
1. Monitor the system to detect when and where congestion occurs.
2. Pass this information to places where action can be taken.
3. Adjust system operation to correct the problem.
The presence of congestion means that the load is (temporarily) greater than the resources (in
part of the system) can handle. Two solutions come to mind: increase the resources or
decrease the load. For example, the subnet may start using dial-up telephone lines to
temporarily increase the bandwidth between certain points. On satellite systems, increasing
transmission power often gives higher bandwidth. Splitting traffic over multiple routes instead
of always using the best one may also effectively increase the bandwidth. Finally, spare
routers that are normally used only as backups (to make the system fault tolerant) can be put
on-line to give more capacity when serious congestion appears.However, sometimes it is not possible to increase the capacity, or it has already been
increased to the limit. The only way then to beat back the congestion is to decrease the load.
Several ways exist to reduce the load, including denying service to some users, degrading
service to some or all users, and having users schedule their demands in a more predictable
way.
theory point of view. This approach leads to dividing all solutions into two groups: open loop
and closed loop. Open loop solutions attempt to solve the problem by good design, in essence,
to make sure it does not occur in the first place. Once the system is up and running, midcourse
corrections are not made.
Tools for doing open-loop control include deciding when to accept new traffic, deciding when to
discard packets and which ones, and making scheduling decisions at various points in the
network. All of these have in common the fact that they make decisions without regard to the
current state of the network.
In contrast, closed loop solutions are based on the concept of a feedback loop. This approach
has three parts when applied to congestion control:
1. Monitor the system to detect when and where congestion occurs.
2. Pass this information to places where action can be taken.
3. Adjust system operation to correct the problem.
The presence of congestion means that the load is (temporarily) greater than the resources (in
part of the system) can handle. Two solutions come to mind: increase the resources or
decrease the load. For example, the subnet may start using dial-up telephone lines to
temporarily increase the bandwidth between certain points. On satellite systems, increasing
transmission power often gives higher bandwidth. Splitting traffic over multiple routes instead
of always using the best one may also effectively increase the bandwidth. Finally, spare
routers that are normally used only as backups (to make the system fault tolerant) can be put
on-line to give more capacity when serious congestion appears.However, sometimes it is not possible to increase the capacity, or it has already been
increased to the limit. The only way then to beat back the congestion is to decrease the load.
Several ways exist to reduce the load, including denying service to some users, degrading
service to some or all users, and having users schedule their demands in a more predictable
way.
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