What is bandwidth ? 

How it works

Bandwidth is the ability to transfer files over a given path between two devices. The data can be of any type such as xls files, voice files, a video stream , etc and the devices can be of any type such as computers , mobile phones , Play Stations , etc. but the maximum interaction permissible between them is defined as the bandwidth between them.

Fig 1.1 The ability to transfer data from yahoo.com in the above case is considered the bandwidth of the pipe to yahoo.com

Two examples of differing bandwidth ?

A very great miscommunication between most ISPs and customers is the access devices which form the A & B point of the bandwidth pipe. Most ISPs in India consider the customer premise as the A point and their PO as a B point , which in strict terms would mean a Intranet kind of scenario. We at GTTPL work slightly differently. We measure the A point as the customer premise and the B point as the peer units we have across the world. 

Fig 1.2 The thickness of the thinnest pipe in this network is 55.6 Kbps. Therefore the bandwidth between the end points would be 55.6 Kbps

Fig 1.3 The thickness of the thinnest pipe in this network is 8 Mbps. Therefore the bandwidth between the end points would be 8 Mbps

Why is the server I'm trying to download from not considered a "B" point ?

Just like any pipe from a water pipe to a bandwidth pipe the pipe as only as thick as its thinnest points. Let us take the above two examples:

In figure 1.2 the bandwidth is considered as 55.6 Kbps 

Now, if the origination part of the network (our part) of the network remains constant and the destination thickness changes (figure 1.3) , the bandwidth of the pipe changes to 8 Mbps. 

This shows that your bandwidth can go from a minimum to a maximum of subscribed bandwidth depending upon factors which are totally out of our control. 

Like in any Scientific experiment since we have no control over what or how the destination part behaves we like most International ISPs use 3rd party measuring sites to check bandwidth where the parameters which may vary are kept under standardized conditions .

The third party sites we normally use to check throughputs are : 

http://www.speakeasy.net/speedtest/ ( Dallas TX site only please - others are overcrowded and give false positives)