In a general sense, this term describes the information-carrying capacity of a given transmission medium. It is a measurement of how much information can be carried in a given time period (usually a second).  It can apply to analog telephone (POTS), Ethernet networks, digital computer system buses, radio frequency signals, and VGA video signals used to connect projectors and monitors.

Bandwidth (the width of a band of electromagnetic frequencies) originally meant how much of the electromagnetic frequency spectrum is allotted for a given transmission path.  Any digital or analog signal has a bandwidth.

More technically, bandwidth is the width, or the range of frequencies, that an electronic signal occupies. Bandwidth is literally the width of a band of frequencies: one simply subtracts the lower limit (-3 dB point) of the frequencies used from the upper limit of the frequencies used.  Bandwidth is most commonly measured in cycles per second, or hertz (Hz).

In many cases for transmission over twisted pair conductors or fiber optics it is often the same as the highest passed frequency (-3 dB point) because the low limit is assumed to be so close to zero as to have no real effect.  The higher this frequency, the wider the bandwidth and therefore the greater the capacity of a channel to carry information.

It should be remembered that a real communications path usually consists of a succession of links, each with its own bandwidth. If one of these is much slower than the rest, it is said to be a bandwidth bottleneck.

Analog Bandwidth

In analog systems, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. These frequencies are measured in the number of cycles of change per second, or hertz therefore analog bandwidth is expressed in hertz (Hz).

As an example, consider the typical telephone (POTS) where the audio signals are limited to the 300 to 3300 Hz range. A total of 3 kHz bandwidth is required to transmit this signal. If we are sufficiently clever we can manage to encode data onto this bandwidth with about 10 bps/Hz and get a modem to transmit data at 28.8 kbps.

An analog television (TV) broadcast video signal is limited by the FCC to a bandwidth of six megahertz (6 MHz) -- some 2,000 times as wide as a telephone voice signal.

Digital Bandwidth

Digital Bandwidth has over time acquired a general meaning of how much information can be carried in a given time period (usually a second) over a wired or wireless communications link. For example, a link with a broad bandwidth - that is, a broadband link - is one that may be able to carry enough information to sustain the succession of images in a video presentation (see Video Bandwidth).

In digital systems, bandwidth is expressed as bits (of data) per second (bps). Thus, a modem that works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps.

As a matter of simplicity, we often do not attempt to make a distinction between the two kinds of ways of measuring capacity and simply talk about ``bandwidth''.  However, one must remember these are two very different things: bandwidth sometimes refers to a measurement of the range of frequencies used in an analog signal and sometimes to the bits/second of digital data rate.

Different but related

Nyquist's theorem relates the bandwidth (analog bandwidth) to the data rate (digital bandwidth) by stating that given a bandwidth of W, the highest data rate is 2W. The data signal need not be encoded in binary, but if it is, then the data capacity in bits per second (bps) is twice the bandwidth in Hertz. Various manufactures use proprietary signaling protocols to increase this capacity by the transmitting more bits per data signal unit.

The bad news about signaling is that the receiver must be able to distinguish between the encoded data symbols in the presence of outside interference, usually referred to as ``noise.'' Shannon's law sets an upper limit on the bps/Hz ratio which increases logarithmically with the signal-to-noise ratio. Theoretically, one should be able to obtain between 2 and 12 bps/Hz, but current technology is blasting past these theoretical limits.

For example Bell labs recently announced there BLAST technology. “BLAST is an extraordinarily bandwidth-efficient approach to wireless communication which takes advantage of the spatial dimension by transmitting and detecting a number of independent co-channel data streams using multiple, essentially co-located, antennas.

The central paradigm behind BLAST is the exploitation, rather than the mitigation, of multipath effects in order to achieve very high spectral efficiencies (bits/sec/Hz), significantly higher than are possible when multipath is viewed as an adversary rather than an ally.

Using our laboratory testbed, the BLAST team recently demonstrated what we believe to be unprecedented wireless spectral efficiencies, ranging from 20 - 40 bps/Hz. By comparison, the efficiencies achieved using traditional wireless modulation techniques range from around 1 - 5 bps/Hz (mobile cellular) to around 10 - 12 bps/Hz (point-to-point fixed microwave systems). In the 30 kHz bandwidth utilized by our research testbed, the raw spectral efficiencies realized thus far in the lab correspond to payload data rates ranging from roughly 0.5 Mb/s to 1 Mb/s. By contrast, the data rate achievable in this bandwidth using typical traditional methods is only about 50 kb”

Examples of Digital Bandwidth:

For modem users, bandwidth is usually limited to 56 Kbit/sec, but depending on various factors such as network congestion, it may fluctuate well between 1Kbit/s and 50 Kbit/s. For higher speed connections such as cable modem or DSL, bandwidth may easily go beyond 1 Mbit (1024 Kbit/sec).  See the examples below for other examples of common digital transmission protocols and there respective bandwidths:

T1, DS-1 - 1.544Mbps
E1, DS-1 - 2.048Mbps
T2, DS-2 - 6.312Mbps
E2 - 8.448Mbps
E3 - 34.368Mbps
T3 or DS3 - 44.736Mbps
OC-1, STS1 - 51.840Mbps
Fast Ethernet - 100.00 Mbps
OC-3, STS3 - 155.520Mbps
OC-3c - 155.520Mbps
OC-12, STS12 - 622.080Mbps
OC-48 - 2.488Gbps
OC-96 - 4.976Gbps
OC-192 - 10Gbps
OC-255 - 13.21Gbps

Memory Bandwidth

Generally, bandwidth refers to data-carrying capacity and is expressed in cycles per second or Hertz (Hz). In the case of RAM, bandwidth is a function of its rated speed and the size of its data path.

A memory bandwidth statistic describes the amount of traffic, measured in megabytes per second, that a computer system can move from one level of memory to another.

Memory Bandwidth is usually expressed in Bytes per Second.

Peak Memory Bandwidth = (Memory Bus Width) x (Data Rate) x Number of Channels

where Data Rate = (Memory Bus Speed x Operations/Clock Cycle)

In order to calculate the bandwidth in Megabytes per seconds, the bus speed must me divided by 8 to change bits to bytes.

DDR333's bandwidth is 64 bits x 333MHz / 8, which equals about 2664MB/s or aproximaly 2700MB/s, hence the reason that it is called PC2700.

In the case of RDRAM the formula is almost the same with the exception that RAMBUS is only 16 bits. PC800's Bandwidth therefore would be calculated as 16 bits x 800MHz / 8, which equals 1600MB/s.  We must now take into consideration that RDRAM supports dual channel transfers; this doubles the effective bandwidth to 3200MB/s.

Peak Bandwidth Summary

 PC100 SDRAM 800 MB/s PC133 SDRAM 1.1 GB/s Single-Channel PC800 RIMM 1.6 GB/s Dual-Channel PC800 RIMM 3.2 GB/s Single-Channel PC1066 RIMM 2.1 GB/s Dual-Channel PC1066 RIMM 4.2 GB/s

Video Bandwidth

Video bandwidth refers to a monitor's ability to refresh the screen. High bandwidths allow more information to be painted across the display in a given amount of time, which translates into support for higher resolutions and higher refresh rates. Lower bandwidths result in flickering, ringing artifacts, and ghosting.

To calculate the bandwidth of a monitor (measured in megahertz, or MHz), multiply the horizontal resolution by the vertical resolution, and then multiply the product of the two figures by the refresh rate. For example, 1024x 728 x 75 = 56 MHz.

Incorrect Usage

You may hear bandwidth incorrectly used as the amount of time it takes a Web page to fully load or as the amount of traffic on a Web site (this is also incorrect, but widely used). HTML programmers often refer to larger graphics as "bandwidth hogs" (because they take up so much room and download so slowly).