Bandwidth

In computer networking and computer science, digital bandwidth or just bandwidth often refers to a bit rate measured in bits/s, for example, network throughput. The reason for the connection of data rate with the term bandwidth is that according to Hartley's law, the limit to the data rate of a physical communication link is related to its bandwidth in hertz, sometimes denoted analog bandwidth in computer networking literature. For the case of high signal-to-noise ratio, the digital bandwidth consumption (i.e. the throughput) of a bit stream or logical link in a computer network is proportional to the average spectral bandwidth of the analog signal representing the bit stream during a studied time interval. However, measuring bandwidth in bits/s is disputed due to confusion with the original definition. Less ambiguous terms are for example gross bit rate, net bit rate, throughput, goodput or channel capacity.

In website hosting, the term "bandwidth" is often used to describe the amount of data transferred to or from the website or server within a prescribed period of time. Another more specific phrase used for this meaning of bandwidth is monthly data transfer. Web hosting companies often quote a monthly bandwidth limit for a website, for example 500 gigabytes per month. If the total amount of data downloaded from the website in a particular month reaches this limit, the hosting company may shut off further public access to the site.

People are often concerned about measuring the maximum data throughput rate of a communications link or network access. A typical method of performing a measurement is to transfer a 'large' file and measure the time taken to do so. The throughput is then calculated by dividing the file size by the time to get the throughput in megabits, kilobits, or bits per second. But the results of such an exercise will result in the goodput which is less than the maximum throughput, leading to people believing that their communications link is not operating correctly. In fact, there are many overheads in transmission that mean the calculated goodput does not reflect the maximum achievable throughput. Actually, an "applivative" session has a bandwidth limitation based on TCP Window Size of Reception machine & Latency in addition to overheads.

The Maximum bandwidth can be calculated as followed:

=> Max. Throughput = TCP Window Size / Round-trip time.

The Max TCP Window size in the absence of window scaling is 65 535 bytes.

Example: Max Bandwidth "Toulouse-Dubai" = 65535 bytes / 0.220 s = 297886.36 bytes/s = 2.38 Mbit/s.

Over a single TCP connection between those endpoints, the tested Bandwidth will be restricted to 2.38 Mbit/s even if the contracted Bandwidth is greater.

Bandwidth test software is used to determine the maximum bandwidth of a network or internet connection. It is typically undertaken by attempting to download or upload the maximum amount of data in a certain period of time, or a certain amount of data in the minimum amount of time. For this reason, Bandwidth tests can delay internet transmissions through the internet connection as they are undertaken, and can cause inflated data charges.