Measuring Bandwidth

Bandwidth in a network is the amount of data, that can be passed in a specified period of time over an interface.

In computer networking and computer science, the words bandwidth, network bandwidth, data bandwidth, or digital bandwidth are terms used to refer to various bit-rate measures, representing the available or consumed data communication resources expressed in bits per second or multiples of it (bit/s, kbit/s, Mbit/s, Gbit/s, etc.).

Mind your bits and bytes

 A bit is a single binary digit (a zero or a one). A kilobit (kb) is about thousand bits (actually 1024, or 2 to the power 10), and a megabit (Mb) is around a million bits (1024 x 1024, or 2 to the power 20). You're probably more familiar with bytes, which are a group of eight bits, and can represent a single character or digit. Hard disk drives, for example, are measured in megabytes (MB - very small), gigabytes (GB) or terabytes (TB - very large) - note that in abbreviations, bytes are capital B's and bits are lower case!

This table shows the maximum bandwidth (the physical layer net bitrate) of common Internet access technologies.

56 kbit/s	Modem / Dialup
1.5 Mbit/s	ADSL Lite
1.544 Mbit/s	T1/DS1
2.048 Mbit/s	E1 / E-carrier
10 Mbit/s	Ethernet
11 Mbit/s	Wireless 802.11b
44.736 Mbit/s	T3/DS3
54 Mbit/s	Wireless 802.11g
100 Mbit/s	Fast Ethernet
155 Mbit/s	OC3
600 Mbit/s	Wireless 802.11n
622 Mbit/s	OC12
1 Gbit/s	Gigabit Ethernet
2.5 Gbit/s	OC48
9.6 Gbit/s	OC192
10 Gbit/s	10 Gigabit Ethernet
100 Gbit/s	100 Gigabit Ethernet

Theory: Short Summary

The Maximum bandwidth can be calculated as follows:

 Bandwidth ≤ RWIN/RTT

where RWIN is the TCP Receive Window and RTT is the round-trip time for the path. The Max TCP Window size in the absence of TCP window scale option is 65,535 bytes.Example: Max Bandwidth = 65535 bytes / 0.220 s = 297886.36 bytes/s * 8 = 2.383 Mbit/s.

We multiply the Byte per second times 8 to get the Bit per second rate. Over a single TCP connection between those endpoints, the tested Bandwidth will be restricted to 2.376 Mbit/s even if the contracted Bandwidth is greater