How to manage network performance


SAGE-AU is a guest blogger.

SAGE-AU is a not-for-profit professional organisation that promotes the development of the system administration profession.

Before Ethernet, IP and converged services buying telecommunications services was an extremely complex business of choosing the right communications technology from a bewildering array of offerings including ISDN, X21, leased lines and Frame Relay.

Now it all looks simple - everything operates over a single set of protocols and commodity hardware. The hardware story is only half the tale.

The key problem with converged services is managing network performance. Instead of having several different channels tuned to the traffic they were carrying, a converged network is effectively a single channel carrying many different types of traffic.

The main difficulty with managing network traffic is that it is very hard to know what effect mixing two streams of data together will have. The extreme situations of adding even a tiny amount of the wrong type of traffic can cripple network performance and conversely adding large amounts of traffic to a network can result in almost no change in the overall performance occur in real life. The naive intuition of simply adding up bandwidths can result in both wildly under-specified or over-specified networks.

Streams of cars merging onto a freeway are a useful metaphor for network traffic. Three examples will help illustrate why it is difficult to know what will happen when merging network streams:

- heavy traffic entering an almost empty freeway or light traffic merging with a lightly loaded freeway behaves just as one would expect; everything just adds.

- if the stream of cars on the on ramp fits perfectly between the cars on the freeway; the traffic on the freeway is almost unaffected by the arriving cars.

- two streams of traffic try to merge where the cars arrive at the merge at just the same time; everything slows down, the order of cars becomes unpredictable and traffic starts to build up on the on ramp.

In practice, quite detailed knowledge of the traffic being added is required to have an accurate picture of what the resulting network performance will be.

Once network performance degrades due to unfavourable mixing there are a few things that can be done to resolve the problem:

- Split the traffic back into separate streams using either separate physical or virtual networks

- QoS (Quality of Service) can be enabled to allow some traffic to have higher priority that other traffic. Applying QoS always reduces the theoretical available capacity of the network, but can ensure that the resulting network runs acceptably. The drawback with QoS is that if it is only supported in part of the network then the problem may have only been postponed.

We are definitely better off for having eliminated the hardware tangle that existed prior to converged services; but it has far from eliminated all the problems that come from running a complex network.

By Maurice Castro

Maurice Castro is a member of SAGE-AU a not-for-profit IT Operations and System Administrator profession organisation, SAGE-AU. For more information go to

Tags: telecommunications services, leased lines, converged services, frame relay, IP, Ethernet, X21, ISDN

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