Re: Signal Quality differences in CT1/PRI?
Karl Denninger (karl@Mcs.Net)
Thu, 28 Aug 1997 10:41:22 -0500
On Thu, Aug 28, 1997 at 10:29:59AM -0500, Derric Scott wrote:
>
> Hello:
>
> > 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
We get 31200 symmetrical connections with regularity over CT1 lines, and
some 33.6kbps connections (usually not symmetrical however).
We also get data rates between 44 and 50kbps regularly on K56Flex over
the same circuits. I've seen 52kbps (the maximum for the present
implementation) on some occasions on CT1 circuits, but that doesn't happen
often.
I've put modems into our PRI units (which are all intended for ISDN only)
and haven't seen any appreciable difference in performance or call quality
in either analog or K56Flex modes.
The important thing is to be on the trunk side of the switch; if you insist
on wink start trunks, you'll automatically be there (since the line side
can't support wink start). If you have FXS-immediate start trunks (foreign
exchange style service) you're almost always on the line side, which is
HIGHLY undesirable.
--
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