CONGESTION MANAGEMENT

Queuing Techniques

There is a variety of queuing techniques some of which have being in place for a long time and have become the basis for new and more complex techniques, before we can move on to study them we have to gather an understanding of the basic queuing methods first.

The following are three queuing methods that we will be concentrating on for now:

• First-In, First-Out (FIFO) Queuing
• Fair Queuing (FQ)
• Weighted Fair Queuing (WFQ)

First-in First-out (FIFO) Queuing

First-in First-out queuing is a basic form of queuing it works on a first-come first-served basis. In this method of queuing, packets leave the router interface in the order that they arrived. High speed routers generally use this approach, as they don not have problems with traffic flow.  A set back to this approach however is that there is no preference as to which packets leave the interface first.

In reality FIFO is not queuing is more like a sort of buffering, packets arrive at the router interface, and are stored in the router memory until they are transmitted.

Fair Queuing

The problem with FIFO is that it does not allow packets ready to be transmitted to leave before packets that are still "preparing" to be transmitted.

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Fair Queuing is a methodology that allows packets that are ready to be transmitted to leave, even if they started to arrive after another packet.

Central Idea of Fair Queuing:

• Routers have multiple queues for each output line, one for each user.
  
   
• When line becomes idle, router scans the queues round robin, taking first packet on next queue.

“In this way a number of users competing for a given output line, each user gets to send one out of every number of packets.

With this scheme, user sending large packets will get more bandwidth. To avoid this "byte by byte round robin" can be simulated by estimating a finish time of each packet.

Nevertheless the problem with this scheme is that all users get the same priority. For example a file server and other servers should have more bandwidth than clients. This can be achieved by "weighted fair queuing" where some users have higher priority”. [i]

Weighted Fair Queuing (WFQ)

Weighted Fair Queuing is a queuing technique which allows guaranteed bandwidth services. The idea of WFQ is to enable different sessions to share the same link.

“WFQ is one of Cisco's premier queuing techniques. It is a flow-based queuing algorithm that does two things simultaneously: It schedules interactive traffic to the front of the queue to reduce response time, and it fairly shares the remaining bandwidth between high bandwidth flows”. [ii]