Handling inbound calls while dialed-up

[bscott at ntisys.com]

On Wed, 8 Jan 2003, at 10:36am, mod+gnhlug at std.com wrote:
> Can somebody supply (or point to) an explanation of the mechanics that
> make it possible for a dialed-up peecee to receive notification of inbound
> calls?

  There are at least two methods that I know of.

  One method involves using the modem and call waiting.  The modem
recognizes the call waiting tones, and signals the host computer.  The host
computer can offer the user a choice to switch to the voice call, putting
the modem link "on hold".  Most ISPs don't give you very long before they
hang-up on you, though.

  The other method uses call-forward-on-busy.  While your modem is on the
line, calls are forwarded to some central call center somewhere.  At the
same time, you run special software that connects to the same call center
using IP.  When a call comes in, you have the option of taking the call via
some kind of voice-over-IP mechanism.

  Both methods require a "value-added service" from the phone company:  
Either call waiting, or call forwarding.  These are typically a few dollars
per month.

  As far as the bandwidth of a voice call goes: If you have a solid V.92
connect, and you're willing to take some loss of voice quality due to
compression, you can hold a conversation and still have a little bandwidth
left over.

  Remember, a voice telephone call is, nominally, 64 kilobits per second,
without any kind of compression or packet switching.  On top of that, it is
(or was) common to "rob" the high bit for signaling, leaving you with only
56 kilobits.  Take a 56 kilobit stream, use half-duplex, compress it, and
only transmit if audio is actually present, and you can get it down to
something like 20 or 25 kilobits.  Less, if you don't mind it sounding like
crap.

-- 
Ben Scott <bscott at ntisys.com>
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