Internet DRAFT - draft-goldman-ieprep-comparison
draft-goldman-ieprep-comparison
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
Title
POTS, GETS,MLPP call comparison
file name draft-goldman-ieprep-comparison-00.txt
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Abstract
This document provides a high level conceptual discussion on the
significant modeling differences affecting the treatment of a POTS
(Plain Old Telephone Service) call, a GETS (Government Emergency
Telephone Service) call, and an MLPP (Multilevel Priority and
Preemption) call in the traditional voice PSTN (Public Switched
Telephone Network).
The genesis for this discussion on these services is primarily from
experience with the United States capabilities and it is appreciated
that these types of calls may be very different or non-existent in
other regions of the world. Still, it may be useful to provide this
discussion as a background, which may be useful in understanding the
legacy operations as the chartered ieprep work progresses.
It should be made clear from the onset that this document suggests no
requirements or solutions, but is merely informative about the
tradition voice network treatments.
Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
This work is being discussed on the ieprep@ietf.org mailing list.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
Table of Contents
1. Introduction 3
2. Request for Service 3
3. Signaling Desired Destination 4
4. Routing Congestion 4
5. Levels of Preference 5
6. Security Considerations 6
7. IANA Considerations 6
8. Acknowledgments 6
9. References 7
9.1. Normative References 7
9.2. Informative References 7
Author's Addresses 7
Intellectual Property Statement 7
Disclaimer of Validity 7
Copyright Statement 8
Acknowledgment 8
1. Introduction
As has been already discussed in the ieprep re-charter, essential
users of public telecommunications networks have a need for
telecommunications in crisis situations. These (voice and
data/multimedia) communications will be needed at the same time as
the public networks might be restricted due to damage, congestion, or
faults. This situation warrants mechanisms that provide secure and
manageable ways to identify authorized users, and provide priority
communications from access and call set up, through call completion.
Any network mechanisms defined should provide preferential treatment
to authorized users. While mechanisms have been defined to provide
priority service in traditional voice networks, it is crucial that
priority services continue to be provided in IP-based Next Generation
Networks.
Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
Since the charter refers to the existing mechanisms in traditional
voice networks, it thus seems useful to view at a high level the
significant conceptual differences between the treatment of a POTS
(Plain Old Telephone Service) call, a GETS (Government Emergency
Telephone Service) call, and an MLPP (Multilevel Priority and
Preemption) call in the traditional voice networks, and especially
the PSTN (Public Switched Telephone Network).
The discussion on these services is primarily from experience with
the United States capabilities and it is appreciated that these types
of call may be very different or non-existent in other regions of the
world. Still, it may be useful for the ieprep work to provide this
discussion as a background which may be useful in understanding the
traditional voice network operations.
The follow sections attempt to compare the conceptual network
treatment differences for each of the call types.
2. Request for Service
Ordinary wireline calls (or POTS calls by which they are often
referred) begin with the subscriber originating a request for service
by going off-hook. Typically this line state transition is treated by
the originating switch as a waiting system request for dial tone
and is not subject to a timeout nor is the request immediately
blocked in an overloaded system.
An originating switch in overload may shed a significant portion of
the overload traffic by temporarily limiting the number of new local
call attempts that are processed by reverting to only scanning those
lines marked as essential service (or any one of a number of similar
service names). When the switch load then drops below a threshold
because so many fewer call originations are being detected, the
switch may again start to scan all the lines. This process can of
course result in continued oscillation of which line off-hooks are
detected for call acceptance until the root cause of overload is
finally resolved. However, even in this scenario a patient caller
would eventually have the off-hook transition detected and be given a
proceed-to-send signal (dial tone) if sufficient resources are
available (ex. software call record and a DTMF (Dual tone Multi-
Frequency) receiver if the receivers are based on a pool rather than
being dedicated per line.)
GETS calls are not identified as such until dialing occurs so their
initial treatment is the same as POTS calls.
Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
MLPP lines have an intrinsic priority associated with them, but the
off-hook must still be recognized and resources allocated for the
call before dial tone can be provided.
GETS and MLPP design has not focused on resource congestion between
the caller and the switch, but rather at the originating switch and
the rest of the network.
3. Signaling Desired Destination
Once dial tone has been received by the subscriber indicating that
the switch has sufficient resources to accept a call attempt, the
subscriber enters the desired number. Historically this has been
referred to dialing, as there was a long period of time when typical
telephones had a rotary dial used by subscribers to enter the called
telephone number.
Once the called telephone number has been received, it is translated
by the switch and call routing can occur. At this point the call can
also be recognized as a GETS or MLPP call.
4. Routing Congestion
The following paragraphs discuss the aspects of Call Admission
Control that apply to the three types of calls. The descriptive terms
of "blocking", "waiting", and "preempting" are introduced.
A blocking model means that if the resource is immediately available
the setup can proceed, but if the resource is not immediately
available the call attempt is immediately abandoned. A waiting model
means that the system will wait for a period of time for a resource
to become available prior to abandoning the attempt. A preempting
model means that if a resource is not immediately free, it may be
preempted from a call with less priority to allow the higher priority
call setup to proceed.
If a POTS call encounters congestion on the primary route for the
call the switch may try some alternate routes. If that is not
successful, a blocking model used and the caller is given indication
that the attempt has failed and the call is released. It should be
appreciated by the reader that congestion can be encountered not only
at the originating switch, but at any subsequent switch as the call
progresses to the intended destination and the blocking treatment
model would be applied.
Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
If a GETS call encounters congestion routing the call, a waiting
model is used rather than the blocking model described above and for
a period of time the switch will wait until a trunk becomes
available. This behavior can be repeated in subsequent switches as
the call progresses through the network. (While not germane at this
level of discussion, the switch may attempt more routes by including
less desirable routes than would be the case with a POTS call.
If an MLPP call encounters congestion routing the call, resources
being used by calls in the same domain with lower priority then the
call being attempted may be preempted for the completion of the call.
It should be stressed here that there are both private networks and
public networks, and that MLPP is often relegated to a private
network. (Thus, the potential of preempting a call in the public
network may be eliminated.)
Thus we can see that POTS calls follow a blocking model, GETS calls
follow a waiting model, and MLPP calls follow a preemptive model.
5. Levels of Preference
As stated earlier, telephone wireline lines may be ordinary or marked
for essential service. The determination used to qualify a particular
line as essential service is not germane to this discussion.
The traditional voice network Signaling System Number 7 (SS7)
protocol in the United States can in call priority information in the
signaling that is used for call setup. SS7 specifies that the Initial
Address Message (IAM) for calls to the public emergency service (9-
1-1 in the United States), MLPP, or High Probability of Completion
(HPC, which is the Standard used for the GETS service) should be set
to an Message Transport Part (MTP) Link Level Congestion level of 1
rather than the 0 assigned to ordinary calls. This preference may be
help where there is congestion on the SS7 signaling link between two
nodes.
GETS calls have one level of priority. (In the form that they are
identified as GETS calls in the SS7 Calling Party's Category (CPC.)
MLPP calls have multiple levels of priority as was discussed above.
The number of levels and the determination used to qualify a
particular line or user at a given level is not germane to this
discussion. It is germane that there a multiple levels in order of
priority.
It is not the intent in this ID to discuss the merits of the various
mechanisms but rather to merely describe the fundamental differences
between the POTS blocking model, the GETS waiting model, and the MLPP
preempting model.
Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
It should also be appreciated by the reader that these mechanisms
were designed as voice based services with a Time Division
Multiplexing (TDM) network in mind, and only address the
establishment of the call path since once a path is established the
Quality of Service of the path is not an issue since a fixed
bandwidth is allocated for the time slot.
6. Security Considerations
This document does not introduce any new security considerations
beyond those that are already well known in the SIP community and
documented in [2] (Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
"SIP: Session Initiation Protocol," June 2002.).
7. IANA Considerations
This draft does not require any IANA considerations.
8. Acknowledgments
The author acknowledges the gracious help provided by Igor Faynberg
and Kimberly King.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2](Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler,
"SIP: Session Initiation Protocol," June 2002.). [RFC2234]
Crocker, D. and Overell, P.(Editors), "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, Internet Mail
Consortium and Demon Internet Ltd., November 1997.
9.2. Informative References
[3] Framework for Supporting Emergency Telecommunications Service (ETS)
in IP Telephony (Work in progress)
[4]Emergency Telecommunications Services (ETS) Requirements for a Single
Administrative Domain (Work in progress)
[5]A Framework for Supporting Emergency Telecommunications Services
(ETS) Within a Single Administrative Domain (Work in
progress)
Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
Author's Addresses
Stuart Goldman
Lucent Technologies
5531 E. Kelton Ln
Scottsdale, AZ, 85254
Phone: +1 623 582 7136
Email: sgoldman@lucent.com
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
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Internet-Draft draft-goldman-ieprep-comparison-00
IEPREP S.goldman
Internet Draft Lucent Technologies
Exprires April 8, 2006 October 13,2005
POTS, GETS,MLPP call comparison
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