telephone bandwidth (was Re: ASR-33 Data Format?)

From: Eric Smith <eric_at_brouhaha.com>
Date: Thu Apr 1 12:52:43 1999

Hans wrote:
> POTS is about analouge transmition, but the media isn't as clear as
> you might think. Depending on the kind of connection, and the used the
> bandwidth may reach up beyond 30 kHz, alowing rates of more than
> 100 Kbps transmission - remember, POTS starts as direct coupled

In the US, POTS service is bandlimited to roughly 300-3300 Hz. The
local loop (between you and the CO) has more bandwidth, which is why
it can (sometimes) be used for ISDN or DSL, but the switched voice
network does not.

The phone companies here have offered special leased lines with higher
bandwidth that were used for such things as links between studios and
broadcast facilities. But even though those were analog, they were (by
definition) not POTS. Nowdays those facilities have largely been replaced
by digital leased lines.

I was of the impression that POTS in the rest of the world worked
similarly, but I could easily be mistaken about that.

> But in fact, this is all history, POTS no longer exists over here
> since years.

Really? You can't use cheap analog telephones any more? I knew ISDN
was more widely deployed in some other countries than in the US, but I
had no idea that it had completely displaced POTS.

Even so, however, if you use an ISDN B channel for a voice bearer to carry
alaw or mulaw audio samples, you still can't get more than 8000 baud through
it in any particularly useful or effective manner.

The actual baud rate of the raw ISDN BRI varies. In the US it uses 2B1Q
signalling, with a rate of 80 Kbaud in each direction. Other countries
use other signalling schemes, such as 4B3T. IIRC, Germany uses a different
scheme than most of Europe. However, in most countries the telephone
company provides the ISDN NT1, so the customer equipment is connected to
the S/T interface, which is the same worldwide.

Eric
Received on Thu Apr 01 1999 - 12:52:43 BST