Network usage measurement at a glance!
Posted: July 6th, 2008 | Author: TnT Admin | Filed under: Concepts | Tags: network | No Comments »Let’s understand a little bit of network measurements: two factors determine how long it takes to send a packet or frame across a single link. The amount of time it takes to put the signal onto the cable is known as the transmission time or transmission delay. This will depend on the transmission rate (or interface speed) and the size of the frame. The amount of time it takes for the signal to travel across the cable is known as the propagations time or propagations delay. Propagation time is determined by the type of media used and the distance involved.
Once we move to multi-hop paths, a third consideration enters the picture – the delay introduced from processing packets at intermediate devices such as routers and switches. This is usually called the queuing delay since, for the most part, it arises from the time packets spend in queues within the device. The total delay in delivering a packet is the sum of these three delays. Transmission and propagation delays are usually quite predictable and stable. Queuing delays, however, can introduce considerably variability.
The term bandwidth is typically used to describe the capacity of a link.
Throughput is a measure of the amount of data that can be sent over a link in a given amount of time. Throughput estimates, typically obtained through measurements based on the bulk transfer of data, are usually expressed in bits per second or packets or second. Throughput is frequently used as an estimate of the bandwidth of a network, but bandwidth and throughput are really two different things. Throughput measurement may be affected by considerable overhead that is not included in bandwidth measurements. Consequently, throughput is a more realistic estimator of the actual performance you will see.
Throughput is generally an end-to-end measurement. When dealing with multi-hop paths, however, the bandwidths may vary from link to link. The bottleneck bandwidth is the bandwidth if the slowest link on a path, i.e., the link with the lowest bandwidth.
(Source: Network Troubleshooting Tools” by Joseph D. Sloan)
Taking the above in a real-live environment (depending on the role you play in), bandwidth is fixed in a load test and usually controlled by the network guys through allocation. Therefore, there isn’t much to be done if you are a load tester. On the other hand, throughput is the amount of load that is transmitted over the network and we suggest to use it as the measurement for network usage. After the load test, you can advise the network guys that there is a predictable amount of data (expressed in throughput) being transferred before the system is to go-live. This will allow them to have better planning on the bandwidth allocation and expectation of slowness if a high level of data is being transferred when go-live.
Leave a Reply