On 1/27/20 1:42 PM, Aled Morris via NANOG wrote:
On Mon, 27 Jan 2020 at 12:13, Rob Pickering <r...@pickering.org
<mailto:r...@pickering.org>> wrote:
Wasn't the 56/64k thing a result of CAS (bit robbed) signalling
which was a fudge AT&T did to transport signalling information
in-band on T1s by stealing the low order bit for OOB signalling (it
wasnt actually every low order bit, but meant you had to throw away
every low order bit as CPE didn't know which ones were "corrupted"
by the carrier).
Proper ISDN was always 64kbit/s clear path with separate D channels
carried OOB end to end, away from the B channel data.
There was some element of interoperability required with the
pre-existing data network architecture based on 56k channels and T1
bearers. This article has the detail:
https://en.wikipedia.org/wiki/T-carrier
/Soon after commercial success of T1 in 1962, the T1 engineering
team realized the mistake of having only one bit to serve the
increasing demand for housekeeping functions. They petitioned AT&T
management to change to 8-bit framing. This was flatly turned down
because it would make installed systems obsolete./
Compared to what was to follow, that all had to suffer the 56k channel
limitation, there can't have been that many installed systems in 1962!
Aled
I seem to also recall that you couldn't use a 56k modem unless the
far-end was digital.
