Re[6]: Ready for ISDN

Richard Cogger (
Thu, 9 Feb 1995 15:00:46 -0500

> 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.
This is somewhat confused. I don't know anything about "kilostream"
services, but:

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 Mbits/sec. These are designated OC-n, where OC stands for
Optical Carrier and n is the multple of 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 (I think)
32 64Kbps channels and so on. But (I think) 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.

Hope I'm not spreading any disinformation. Corrections welcome.

Cheers, -Dick