Subject: RE: [Ieprep] Preemption

From: "Gunn, Janet NE" <[email-masked]>
Date: Mon, 1 Apr 2002 14:07:25 -0500

As I understand it, it is a bit more complicated than that.

In order to establish a mobile call, first the handset must acquire a
control channel.  Once it has a control channel, it can request a traffic
channel (voice or data if supported).

WIthin GSM, eMLPP (extended MultiLevel Precedence and Pre-emption) has 7
levels.  Of these, the two highest are reserved for network internal use,
leaving 5 levels available for subsribers.  These 5 levels affect the
assignment of voice (and maybe data, I am not sure) traffic channels, and
can be passed to the land-line PSTN as MLPP parameters.

But if you can't get a control channel, you can't even request a traffic
channel, and cannot be added to the queue.  In a very high level sense,
getting a control channel is analogous to getting dial tone in the landline
PSTN.

To address this, GSM has 15 levels of access class for control channel
control. When the network operator  detects congestion of the control
channels, it can "turn off" all mobile sets with a given access class.
These sets cannot even request a control channel until that access class is
turned back on.

Ordinary GSM mobile sets are typically "randommly" assigned to access
classes 0 to 9.  This is often just the last digit of the serial number.
Under congestion, the network operator will typically deny one or more
access classes in turn for a short time (say 5 and 8), then re-activate the
them and deny another combination (say 3 and 7), and so on.  This is (again
on a high level) analogous to "call gapping" or other Network Management
Controls in the landline PSTN.

In order to exempt certain mobile handsets from this access control, they
can be assigned to access classes 12, 13 or 14 (classes 11 and 15 are
reserved for network internal use).  Such special mobile sets would tpically
have access class (say)13 IN ADDITION TO an access class 0 - 9.

Note that there is no inherent "ordering" of the access classes.  From a
technical perspective the network operator could block  access classes in
any order or combination they chose.

This is probably more than you wanted to know, but I wanted to explain where
the 15 levels came from.

Janet

-----Original Message-----
From: Alistair Munro [mailto:[email-masked]]
Sent: Monday, April 01, 2002 1:40 PM
To: Ken Carlberg
Cc: [email-masked]
Subject: Re: [Ieprep] Preemption

Hi Ken, (and Stu also),

> > I wonder if anyone on the operational side of PSTN, ISDN, GSM has any
> > comment on the preemption issue? It would certainly help my
> > understanding of practice.
>
> from my experience, the preemption that has been deployed is almost
> exclusive to private networks, and I'll have to pass on comments.
>
> my understanding is that the UK, under the guidance of Oftel, has
> deployed multiple priority indicators for their cellular/mobile
> telephony service, but i don't believe preemption is part of that
> service.  i'll look into that.

This is roughly in line with what I have heard. I believe that for GSM (in
the UK, don't know about elsewhere) that there are multiple levels (15
possibly) that can be assigned and stored on the SIM. Engineering staff may
occupy the top-levels, emergency about half way down (there may be multiple
emergency priorities too), and the common herd (paying customers) lurk
around in the nether regions.

I also understand that there is no preemption for GSM in the UK. I can
check further but Fred's comments reminded me that I may be drifting
off-topic. If we can specify a priority and preemption scheme that works
for the IP domain then the best we can do in this area should be consistent
with the behaviour at the gateways.

I wonder how we might test some of these things...

Regards,

Alistair

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From: Alistair Munro <[email-masked]>
Date: Mon, 01 Apr 2002 20:36:09 +0100

Hi Janet,

> ....
> This is probably more than you wanted to know, but I wanted to explain
> where the 15 levels came from.

Aargh, thanks for the corrections and clarifications :-).

Sorry if I caused some confusion - I was not thinking about the issue of
getting the control channel but about what happened from the first
switch inwards. I imagine that all the access class issues can be mapped
somehow into the SIP domain, and that the issues of
prioritising/preempting calls would be similar whether we were using a
SIP switch or a PSTN one.

For GSM systems (or TETRA, or many of the other digital cellular systems
around), as far acquiring the control channel is concerned, is this
orthogonal to the treatment of the call subsequently?  It is a
competitive process on a shared medium (layer 2) up to a point, which I
believe is a sub-IP issue that could be raised in a similar way for an
802.11 network with 802.1d/1p/1q, but, once established its progress is
certain - which would not apply to an IP platform. Would an IP platform
perform any better or worse than a GSM one? I think yes, if only because
a TDM system will run out of slots. What about GPRS?

Regards,

Alistair