[Guest]

What is Bandwidth?
Why some providers give unlimited bandwidth and some sell it for money?

[HostExpert]

Generally speaking, bandwidth is directly proportional to the amount of data transmitted or received per unit time. Typically the Unit of Time is Seconds. In digital systems, bandwidth is expressed as data speed in bits, Kilobits, Megabits, or Gigabits per second.

Every Web Host will have a limited amount of bandwidth available. This amount directly corresponds to the bandwidth provided by their Internet Connections. Typically this is either 1.6Mbps or 45Mbps, but it can be higher, lower or somewhere in between.

[Mike]

The word bandwidth is also used to mean the amount of data that can be transferred through a digital connection in a given time period (in other words, the connection's bit rate). In such cases, bandwidth is usually measured in bits or bytes per second.

In the physical world, a digital signal is usually represented in an analog form for actual transmission. This can be a complex process. First the bit pattern must undergo a suitable form of channel coding, appropriate to the expected noise level of the analog channel. Then it must be transformed into an analog waveform using line coding, and modulated onto a carrier signal. The latter two processes depend upon the actual nature of the transmission medium, whether it be electrical, optical or electromagnetic.

Mathematically, the maximum digital bit rate for a given analog bandwidth and noise level is determined by the Shannon-Hartley theorem. How closely this is approximated depends to a great extent upon the choice of channel coding, which must introduce just enough redundancy to match the noise level. Too little redundancy, and expensive retransmissions will reduce the useful bitrate. Too much, and the error-correction overhead will reduce the bitrate left over for the signal. The Shannon-Hartley limit is approached closely by Reed-Solomon codes used on optical media, and even more closely by Turbo codes used in satellite communication.