Re: Ready for ISDN

Christopher Davis (ckd@loiosh.kei.com)
Thu, 9 Feb 1995 19:05:06 -0500


[warning: more than you probably ever wanted to know about digital
telecommunications follows]

MS> == Michael Sattler <msattler@jungle.com>

MS> The low-bandwidth side of American network usage is as follows:

MS> 14.4 - 28.8 kpbs modems

I'd even include 9.6; it's still "usable" for many purposes.

MS> 56.0 - 64.0 kpbs 1 ISDN b-channel
MS> 112.0 - 128.0 kbps 2 "bound" ISDN b-channels

MS> The reasons the ISDN numbers show a range is due to the need for a 16
MS> kbps control channel, called a d-channel. When you have things set
MS> up to do a direct dial (over Centrex, as I have here in SF) then you
MS> don't need the d-channel, and you get the full b.

No, that's not why there are 56kbps and 64kbps ISDN connections. The real
reason has nothing to do with the D channel and everything to do with
backward compatibility.

The old traditional way of putting a voice call onto a digital trunk, in
North America, is to use an 8000Hz 7-bit sample (56000bps) and put it on a
64000bps channel, using the 8th bit to signal "on/off hook" or "in use".

Many interoffice trunks (fewer and fewer) still use the eighth bit for
signalling, so ISDN data calls over those trunks must be rate-adapted to
56kbps. Newer trunks (those equipped with Signalling System 7, an
out-of-band method to transmit channel status), and calls not going over
interoffice trunks (i.e. within the same switch) are able to use the full
64kbps.

An ISDN basic rate (BRI) line is 2B+D, two 56 or 64kbps B channels and a
16kbps D channel used for call signalling and the like.

MS> I forget the exact numbers for T-* flavors (as I'm in a cafe and
MS> don't have any manuals with me), but a partial T-1 bought us 128
MS> kbps. Someone?

A full T1 is 24 64kbps channels. Some configurations may be 24 56kbps
channels for the same backward compatibility reasons as given for ISDN
above. Fractional T1s can be any number of subchannels less than 24.
Common configurations are 128kbps, 256kbps, 512kbps, and 768kbps (half a
64kbps-channelized T1).

ISDN primary rate (PRI) lines are T1s configured in a 23B+D layout, with
the D channel at 64kbps rather than the BRI's 16kbps. (Additional T1s can
sometimes be added on while still using the first T1's D channel, so those
would be 24B PRI connections).