What is Bandwidth?

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What is Bandwidth?

Discussion of the data transmission capacity of various hardware is a staple of the CU-SeeMe mailing list. The following is are in the range of typical messages:

We're using ISDN here (64k) and using the PC version, so no sound yet (any idea when?). A couple of days ago I was getting 17 fps, but yesterday it was pretty poor, down to about 2 or 3 (from the oslo reflectors, a bit higher from NASA TV). So it all depends on what is happening on the Internet that day. But performance could be pretty high. Should be better than dial-up, in theory :-)

I found out today this whole network thing is based on how much bandwidth is available and how much is being used. LIke my access provider told me today that he has a T1 line, which I think he said was 1.5mbps or something like that, but each person that logs on uses a certain amount of that total. The more people on, the less available, the slower the connection becomes. Ain't that interesting?

I'm not sure if this is _the_ answer to the disconnect about ISDN, Kilostream, etc.; but I believe Europe uses a different and incompatible standard called Sonet, whereas the U.S. has the older ISDN. For general background info, ISDN is generally a combination of 2 (64kbps) B channels and 1 (16K) D channel. Sonet uses a different set of bandwidth characteristics. One of the primary drivers behind ATM (aside from speed) is that it is able to bridge the comms barrier between U.S. networks and Sonet.

Having so many people be aware of a small part of the telecommunications puzzle is tantalizing and frustrating at the same time. The remainder of this page is my attempt to combine the wise words of Richard Cogger, Richard Jones, Byron Thomason, Christopher Davis, and a cast of thousands.

SONET is Synchronous Optical NETwork, and is the basic standard, pretty much worldwide I think, but for sure in the US, for use of longhaul fiber. It specifies the framing, timing, position of mgmt info, etc. SONET links carry multiplexed collections of fixed bandwidth datastreams in multiples of 51.xxx Mbits/sec. These are designated OC-n, where OC stands for Optical Carrier and n is the multple of 51.xxx. Some popular versions are:

  • OC-3 155 Mbps Most early ATM going in at this rate
  • OC-12 622 Mbps Not used much by carrier industry
  • OC-24 1.2 Gbps Half of OC-48, probably 8 OC-3's
  • OC-48 2.4 Gbps Common speed on longhaul fiber plant

    This SONET infrastructure is replacing the older T3 (45Mbps), T1 (1.544Mbps) and so on. An OC-3 can be 3 OC-1's with the equivalent of a T3 in each or it can be a variant, OC-3c, where the c means concatenated or contiuous or somesuch, which would be used to carry ATM cells. The old T3 would carry exactly so many T1's, each of which would carry exactly 24 64Kbps channels, used for voice. (Or the T3 might not be broken down but used as one pipe, between routers, for example. When an OC-1 carries a subdivided T3, the difference in rates (45Mb, 51.xxxMb) is the SONET overhead that allows software management of SONET piping.

    Think of SONET as fat pipes with smaller pipes branching off (fibers and Muxes). The big deal for the carrier industry about SONET is that they can reconfigure bandwidth remotely to manage their networks a lot better than with the old infrastructure.

    ISDN really doesn't have a lot to do with SONET, except that the 64K B channels that ISDN provides over copper distribution can be packed, as circuit switched channels, into OC-n facilities, OR they can be chopped into ATM cells which are cell-switched and then carried on SONET pipes. They can also be carried, circuit switched and Time Division Multiplexed, thru the usual digital network of T3's, T1,s, etc.

    Europe has a differet legacy system, doing E1's at 2.xxxMbps for 32 64Kbps channels and so on. But everyone is going to SONET and maybe ATM everywhere.

    Result: Old stuff is compatible (to a point) at 64Kbps but incompatible at higher speeds. New stuff is compatible at SONET OC-n speeds and at mux points when used to carry ATM, but not when used to carry T3 compatible streams, etc.

    Have you found errors nontrivial or marginal, factual, analytical and illogical, arithmetical, temporal, or even typographical? Please let me know; drop me email. Thanks!

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