> I've seen mentioned here before reference to the differences in signal
> quality between CT1 and PRI, regarding the fact that PRI is 8-bit
> data values whereas CT1 is 7-bit.
>
> My first question is: doesn't a lot of the voice network use CT1-type
> (i.e. 7-bit) transmission?
Depends on where your are and your telco, really. Around here it seems to
practically all be 8 bit within the BellSouth network (guess we do get
*something* for the high BellSouth (Alabama) rates :). For good support
of 64k ISDN it is easiest for the telco to have all their trunks 8 bit.
A while back on the list a guy in Chicago (I think) said that it was
mostly 7 bit there.
> My second question is: what differences does that one bit make in modem
> connection quality? I'd like to hear about real-world examples.
Well, one bit out of only eight is a pretty good amount in digital signal
processing terms. That will mean that the channel will have a measurable
amount of more noise - which WILL affect the modems to some degree.
(Anyone got their DSP book handy to show how many DB one bit in 8 is?
Mine's not here.)
> I ask because I've been getting pretty lousy connection rates to my
> PortMaster (CT1, 26400 right now but as low as 21K or 19.2K), and in
> testing from the same end-user modem to a PortMaster on PRI, get
> consistent 28.8 connects.
>
> I understand that there can be a difference, but that big?
Hmm.. 26400 I can believe - the 21 and 19.2 are too low - it's probably
something else. Others have posted fairly consistent, similar connects
between CT1 and PRI, at least at 28.8 speeds.
One real world example we've seen is that with our analog modems, any
customer who is behind a SLC can never get above 26.4. (The SLCs
use CT1 to get back to the CO).
Later.
Derric
-- Derric Scott Scott Network Services, Inc. P. O. Box 361353 derric@scott.net (205)987-5889 Birmingham, AL 35236