Business Library

After the previous discussion of what TCP/IP is all about, and why you need to know something about it, this article returns to look at Ethernet LANs from a new angle:

How can the performance of a particular network be predicted ?

This article provides a simple calculation tool, written in Java, which demonstrates how the performance of a simple Ethernet Local Area Network can be predicted and examines some of the factors which impact performance. You should see below a panel containing a set of input fields on the left and a results area on the right. (If you do not see it, your browser might not support Java or you have disabled it - in which case this article will be hard to follow!)

Acknowledgements to Mike Tanner, whose book Practical Queueing Analysis - available from the Local Links Book Shop - provided the basis for the formulae used in the program.

If you leave the initial settings as they are and press the Calculate button, you will obtain the following results (or similar -depending on what type of machine you are using):

Link Util : 6.4%
Max Util : 98.97%
Ts : 640.0 microsecs
Tq : 684.0 microsecs
Tt (Tq+Ts) : 1324.0 microsecs

If you are with me so far, let's look at what we have to tell the program and what it is telling us. If you remember from the earlier discussion of how an Ethernet LAN works, transmission performance is impacted by:

• Time to sense if the media is busy.
• Time waiting for the media to become free.
• Time to send the message - which is related to how big the message is, how fast the media is, and to a lesser extent how long the media is.
• If there is a collision then also the time to abort the transmission and retransmit.

Input Parameters

• Message Rate (packets per second) - this specifies the average rate at which traffic is presented to the LAN by the attached devices. It is assumed by the program that the traffic arrives randomly - but over a period of time at this average rate.
• Average Message Length (Bytes) - is an estimate of the size of each message crossing the LAN on average - where a byte, sometimes also called an octet, is 8 binary digits.
• Message Length Distribution - which defines how the message sizes vary between each other. Constant means that the messages are always exactly the average size defined above, or exponential means the sizes occur randomly - but the average size over a period of time is as defined above. Normally for analysis work, exponential is used.
• Cable Length (kilometres) - the total length of cabling making up the network.

Results

• Link Util - the utilisation of the LAN media as a percentage compared to its rated speed of 10 million bits per second. So 6.4% in the example means 640,000 bits per second is being carried.
• Max Util - perhaps more interesting is that for each mix of traffic there is a finite limit that the Ethernet LAN can carry. In this case, a maximum utilisation of 98.7% is predicted.
• Ts - the average time taken to pass a message of the specified size across the LAN, 640 microseconds in the example (where a microsecond is a millionth of a second).
• Tq - the average time waiting (queuing) for the media to become free - including time to recover from collisions, 684 microseconds in the example.
• Tt - the total time, on average, to pass a message including both queuing and transmitting, 1,324 microseconds in the example (1.324 milliseconds)

You might be surprised about the small times being considered here! But remember that a communication between two devices typically requires many packets to be transferred between them - and for each packet this delay will be applied. Try increasing the message rate to 1,000 per second, press Calculate and you will see that the queuing delay increases considerably. As an extreme example, set message rate to 10051, and message size to 116 - and you will see Tt is now predicted to be 1762322 microseconds - 1.76 seconds! This shows that an Ethernet LAN will behave very badly when the utilisation is very high compared to its maximum throughput as a consequence of waiting for the media to become free and the increased likelihood of collisions.

Feel free to experiment with different settings - but if the calculation produces the message UTILISATION EXCEEDS MAXIMUM and reports just Link Util and Max Util, this means that you are asking the LAN to carry more traffic can it possibly can. Please e-mail networker@vinntec.co.uk if you wish to provide any feedback on this article or the program it contains.

Business Implications?

• An Ethernet LAN will transfer large amounts of information between systems quickly and efficiently provided the loading is kept at a reasonable level.
• If an Ethernet LAN is operated at a high average utilisation, performance will suffer.
• Techniques exist for predicting the performance of a LAN before you install it. The simple example shown here is the tip of the iceberg!