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