In message <20061116070138(_dot_)D12393(_at_)openss7(_dot_)org>
"Brian F. G. Bidulock" writes:
Janet,
On Thu, 16 Nov 2006, Janet P Gunn wrote:
In the circuit switched world, a circuit is either up or down, and
"preemption" means taking the circuit down.
Well, no. I think that I have mentioned here before in a previous
discussion on the topic of preemption that local telephone switches
often congest on the basis of line-side call attempts. This often
happens during natural disasters than invoke a collective behaviour
and result in mass calling attempts (often to 911). The result on
the switch is delayed dial tone (sometimes minutes depending on the
persistence of other callers). It has always been technically possible
in the design of telephone exchanges (based on principles applied to
military switching systems) to preempt or further degrade service to
some lines to allow other lines a higher probability of completing a
call is a shorter interval of time. Some local switches had a line
class option that would permit this. Most jurisdictions, (and carrier
lawyers), deemed that, in the public telephony, the importance of a
telephone call cannot be determined by the line that it originates on,
as it surely can in the case of military telephony. In public telephony
it is deemed only that the importance of a call can be determined by
its destination (e.g. 911) rather than its origin. I think that the
same principles apply to public telephone calls regardless of whether
they traverse the so antiquited circuit switched network, or the new
fangled Internet. I think most regulatory bodies responsible for
both agree with that too. I don't think that whether idle capacity
is incremental in unit circuits or continous bandwidth really has
much to do with it.
--brian
.... except that everything is different wrt PSTN and VOIP.
For example, not getting a dial tone (your example above) is due to
not enough cards in the local switch to convert DTMF to call attempts.
SIP doesn't use DTMF and so that bottleneck doesn't exist for a SIP
originated call.
A well designed VOIP based 911 could eliminate many technical
bottlenecks leaving only the number of available 911 dispatchers as
the bottleneck and do a lot to make their job eaier.
There may be a lack of forward thinking in ITU since all VOIP ideas
there tend to mirror PSTN capability rather than allow PSTN to
gradually become an antiquated subset of VOIP capability.
VOIP has yet to reach critical mass. Deployment is quite significant.
Its still mostly a gateway to PSTN with small but growing pure VOIP
connections. In a sense PSTN is already almost a subset capability
since full service features and voice mail for free and extras like
free or very cheap fax to email gateway come bundled with VOIP
services but not PSTN, plus abilities like managing your voice mail
queue using your computer screen and keyboard rather than voice
prompts and DTMF pressing plus ability to archive voice mail on a PC.
Of course, IEPREP is just about priority treatment for emergency calls
and not extra VOIP features above and beyond the PSTN.
Curtis
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