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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 8, 2006. Comments are solicited and should be addressed to the working group's mailing list at Ieprep@ietf.org and/or the author(s). Copyright Notice Copyright (c) The Internet Society (2005). 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. 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