Subject: Re: [Ieprep] RE:MLPPoIP - (2) Preemption of low priority calls

From: [email-masked]
Date: Thu, 20 Nov 2003 18:57:02 EST

In a message dated 11/20/2003 2:42:10 PM Eastern Standard Time, 
[email-masked] writes:

> SS:  
> It seems to me that counting pkt losses per time period is easy.  The 
> question is whether it is useful.  If calls were suspended for perhaps 10 seconds
> with a message indicating that this was being done, that might be more 
> acceptable than disconnection.  Currently in the CS environment, preempted calls 
> are simply given a tone and terminated.  I would think that this would be 
> preferable. If the congestion lasted more than the 10 seconds, 
> then the call would be terminated.  It would be nice if a practical  CAC 
> were implemented but I am skeptical about that. 
> 

[MAP] Yes, counting pkt loss per time is easy. Just doesn't mean much for 
this purpose.

> Suspending a call for a short period sounds worse than the random packet 
> loss. 
> 
> SS:  I'd like to try this on some people and see which they prefer.  But it 
> seems to me that disconnecting 5% of the routine calls is preferable to 
> having 50% of the calls continue unintelligibly.  That appears to me to be a major 
> waste of bandwidth not to mention the time and frustration of the  actual 
> people involved.  
> 

I think we need to consider what scenarios need to be addressed. The example 
I gave was of 100 calls, with 95 of them low precendence and 5 high. I believe 
a fair example of what needs to be supported is more like the following 
scenario

1. A facility is carrying 100 low precedence calls, taking up the entire 
capacity, and being successfully limited to 100 calls by an effective CAC. (Let's 
ignore silence suppresssion for a moment.)
2. An emergency occurs and a whole lot of additional low precedence calls are 
attempted, but are all blocked by CAC.
3. 10 high precedence calls are attempted, 1 per second for the next 10 
seconds, and allowed by CAC which uses a higher limit for high precedence calls.
4. Each new high precedence call takes several seconds (at least 5) to get to 
the stage of media flowing.
5. With avarage holding times, the existing low precedence calls release 
about 1 every 2 seconds.
6. Presume that the CAC mechanism can respond quickly to lower the limit on 
low precedence calls to 90.

Analysis of the above shows that there is almost no significant packet loss, 
since low precedence calls release fast enough to make room for the new high 
precedence calls. Whatever loss does occur is put on the low precedence calls, 
which are already in progress, so unitelligible speech just results in someone 
repeating what they said. If unintelligible speech occured during the initial 
seconds of a high precedence call, it might result in the user hanging up to 
try again

The selective packet discard is really only needed to accomodate a less than 
instantaneous response by the CAC, and as you said, you're skeptical about 
that.

Mike Pierce

---

From: "Steve Silverman" <[email-masked]>
Date: Thu, 20 Nov 2003 21:07:21 -0500

Mike,

You have a great deal of  confidence in the CAC.  Do you have any real idea
how all of this
is going to work?   Does anyone claim to really understand the operation of
this all knowing CAC,
across a large network with congested trunks.

What I'm afraid of is the following scenario:

100 routine users filling the trunk.  Now an emergency occurs and 10 high
priority users call in (or some call out).
In the absence of a priority scheme at the packet level, 10% of each call's
packets are dropped.
All calls become unintelligible.  Everyone hopes the congestion is temporary
and holds on.  Nobody
can be understood.  Knowing that in previous congestion cases, redialing
never works, everyone holds on
since they really want to know what is going on and "their" call is
critical.  Normal call attrition rates go out the window (porthole).

I don't claim the CAC is impossible.  I hope it can be done and that would
solve a number of problems.  I just want some
strong arguments that it is possible before we invest a great deal of money
in systems that won't work very well without an unattainable technology.

Steve

  I think we need to consider what scenarios need to be addressed. The
example I gave was of 100 calls, with 95 of them low precendence and 5 high.
I believe a fair example of what needs to be supported is more like the
following scenario

  1. A facility is carrying 100 low precedence calls, taking up the entire
capacity, and being successfully limited to 100 calls by an effective CAC.
(Let's ignore silence suppresssion for a moment.)
  2. An emergency occurs and a whole lot of additional low precedence calls
are attempted, but are all blocked by CAC.
  3. 10 high precedence calls are attempted, 1 per second for the next 10
seconds, and allowed by CAC which uses a higher limit for high precedence
calls.
  4. Each new high precedence call takes several seconds (at least 5) to get
to the stage of media flowing.
  5. With avarage holding times, the existing low precedence calls release
about 1 every 2 seconds.
  6. Presume that the CAC mechanism can respond quickly to lower the limit
on low precedence calls to 90.

  Analysis of the above shows that there is almost no significant packet
loss, since low precedence calls release fast enough to make room for the
new high precedence calls. Whatever loss does occur is put on the low
precedence calls, which are already in progress, so unitelligible speech
just results in someone repeating what they said. If unintelligible speech
occured during the initial seconds of a high precedence call, it might
result in the user hanging up to try again

  The selective packet discard is really only needed to accomodate a less
than instantaneous response by the CAC, and as you said, you're skeptical
about that.

  Mike Pierce