Bandwidth by Alistair Banks

The term bandwidth can be used in many contexts to describe many things. In digital media and computing it is most commonly defined as the amount of data per second that ...

  1. a (network) connection is capable of delivering
  2. a storage device (hard drive, CD or DVD disc etc) is capable of outputting
  3. a piece of hardware is capable of delivering

You may wish to read the articles Computer counting systems (binary / bit depth explained) and Units of memory and storage (bits, bytes megabytes etc) before reading this article.

How bandwidth is measured

Be aware that bandwidth can be expressed in ...

  • Bits per second (Bits/sec, b/sec, bps). Be warned, sometimes the expression "bps" may mean bytes per second.
  • Kilobits per second (kb/sec, kbps). Network bandwidths are often expressed in kilobits per second.
  • Bytes per second (Bytes/sec, B/sec, bps). Be warned, sometimes the expression "bps" may mean bits per second.
  • Kilobytes per second (Kbytes/sec, KB/Sec, Kb/sec, KBps, Kbps)
  • Megabytes per second (Mbytes/sec, MB/sec, MBps, Mbps)

Network connection technology bandwidths

56K (V90) MODEM

Because 1 byte is 8 bits, a 56K (56 Kilobits/sec) modem connection has a maximum bandwidth of 7 Kilobytes/sec (56 divided by 8). in practice such modems rarely connect at more than 42Kbps (see Contention and other problems below).

56K modems (or more correctly V90 modems) perform at best at 56Kbps, under dream laboratory conditions, which don't ever exist in either the UK or US. Usually they connect at around 42Kbps, sometimes worse, sometimes better, never I have seen better than 52Kbps anywhere. It's all to do with line length and noise.

ISDN (single channel)

A single channel ISDN connects at 64Kbps everywhere in the world, and most places in USA, except a very few there where there is a "different" (older) ISDN which connects at 56Kbps. Most digital telecoms things were built around multiples of 64kbps, but the Americans used to drop the top bit of the byte, hence a 7/8th multiple being only 56Kbps, but the US got that fixed for the most part.

ISDN (dual channel)

Given that most residential ISDN lines are 2 channel capable you can expect a minimum bandwidth for ISDN of 128Kbps. Business lines with multiple channels (up to 30) may offer significant speed increases.

Every ISDN modem that I've ever bought is able to connect 2 channels, because a residential ISDN line typically supports 2 channels (whereas a business ISDN line may support up to 30 channels) so most people's home ISDN achieves 128Kbps - far better than V90's typical 42Kbps, and that's not even taking contention into account.

Channel bonding is another issue for ISDN, since when you connect with 2 x 64Kbps channels, they're quite separate at the receiving end, unless the equipment there can do channel bonding. Although you can dial up with more than 2 ISDN channels, I don't know of any of the 0845-style ISPs which support more than 2 channel bonding, whereas commercial ISDN ISPs will offer bonding of lots of channels. You can aggregate bandwidth over lots of media as well, mixing analog dial-up with ISDN with leased line & with wireless, if the routers at the back end and the software or router at the front end will do this.


This is a US standard high speed link and offers speeds of 30 x


This is the UK equivalent to T1 and offers 30 x 64Kbps. T1 & T3 are US standards, where E1 & E3 are what's used in the UK. T1 = 30 x 56Kbps, and E1 = 30 x 64Kbps. You hear about fractional T1 as well, and contention, so it's all pretty meaningless. I forget what multiple T3 & E3 are. The bits-per-second and contention ratios and delivery protocol and connection are what you're really interested in, e.g. my BT Net start, at 64Kbps (OK, not broadband), guaranteed less than 20:1 contention, usually 1:1, delivered as ethernet 10BT to an RJ45 socket.


Broadband has no fixed definition, so can mean anything better than dual-channel ISDN up to gigabits/sec. It can be symmetric, or not, and can mix satellite, wired, and wireless.


Contention and other problems

Modems and other network technologies suffer from several problems, preventing them reaching their theoretical maximum quoted performance ...

  • Noise is analogue interference which enters the telephone line between home and digital exchange.
  • Bandwidth Contention is the number of people who share the bandwidth at the receiving end.
  • Modem Contention ratios refer to the number of subscribers in any given time period divided by the number of modems available in that time period.

Transmission time/Line delays & Jitter

Satellite at a given bandwidth isn't as good as a land-line, since being a longer transmission path, the added line-delay makes the tcp/ip protocol go much slower. TCP/IP requires hand-shaking, so since the hand-shake can't be any quicker than twice the time taken to send a packet down the wire, then tcp/ip generally "goes slower"

Jitter is the "randomness" of the transmission time. If the line-delay is predictably slow, then you can fiddle the tcp/ip protocol settings to get best throughput, but if there's lots of jitter, and it sometimes goes faster, you want to set the settings for smaller packet sizes for the better times, which might result in slower throughput on the bad times.