From: wcs@anchor.ho.att.com (bill.stewart@pleasantonca.ncr.com +1-510-484-6204)
Date: Sun, 13 Mar 94 19:46:50 PST
To: cypherpunks@toad.com
Subject: re: voice encryption
Message-ID: <9403140346.AA22368@anchor.ho.att.com>
MIME-Version: 1.0
Content-Type: text/plain


> [IBM ISDN board with TI DSP, COM1 interface, 64kbps]

The big problem with ISDN is still getting connectivity to the
people you want to talk to and getting service at reasonable prices
(prices vary widely depending on your phone monopoly's interactions
with the regulatory bureaucracy, and/or your distance from
long-distance service providers if you want PRI connections.)

Do you know if the TI DSP is user-programmable, or if it's just
performing most of the ISDN functions?

The difficulty with COM1 interfaces, when you're using DOS,
is that DOS basically gives you a single character per interrupt,
which means a system doing 8000 char/second on COM1 is taking a 
*lot* of interrupts.  There are generally other interfaces that
let you handle more characters at once, but they're not transparent
to software that's expecting simple COM1, and they're often
vendor-specific.  (The FOSSIL drivers for 16550 UARTs are addressing
the same problem; maybe somebody's got FOSSIL support for ISDN?)
On the other hand, if you're doing software to support voice over ISDN,
it's probably worth doing some board-specific hacking, especially if
you're going to use the on-board DSP anyway.

Bill