Internet Engineering Task Force Jśrg Ottensmeyer Internet Draft Siemens AG Category: Informational Date: 22 October 2002 The set of Emergency Features provided in Public Switched Telephone Networks Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 in other than as "work in progress." The list of current Internet-Drafts can be accessed a http://www.ietf.org/ietf/lid-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Copyright (c) Internet Society 2002. All rights reserved. Reproduction or translation of the complete documents, but not of extracts, including this notice, is freely permitted. Abstract This document describes the set of emergency services provided in Public Switched Telephone Networks (PSTN) and related features which are needed to provide these services. This draft is aimed as input to the discussion in the IEPREP working group to discuss further emergency features may be required on top of GETS. 1. Introduction Emergency Preparedness is an important factor for public communication networks. The IEPREP WG is currently gathering the pieces available in the Internet today into BCPs to document the required emergency services. Public Switched Telephone Networks (PSTN) have features which enable to provide several types of emergency services. And PSTN is still evolving: e.g. based on E.106 [1], a new scheme for International emergency support, methods are now being implemented in SS7 and BICC signaling to convey a calling party category on an international basis. This allows authorized users to have access to the International Telephone Service while the service is restricted due to damage, congestion and/or other faults. Internet Draft PSTN Emergency Features November 2002 This document surveys the set of emergency services and related features available in PSTN today. Hence, it is aimed as input to the discussion in the IEPREP working group. 2. Emergency Call Services available in PSTN today There are three main emergency services available. These are SOS calls, service-based calls and user-based calls. In the following, we give an informal description of these services rather than a formal definition. 2.1. Emergency Calls (911..) The "SOS call" is the classical emergency call service (e.g. 911, or 112 in Europe). It is a destination-dependent call service, i.e. calls are authorized by dialing a publicly, well-known emergency number(s), e.g. of the fire brigade, police, or ambulance, etc. SOS calls are routed to the nearest emergency center. Nearest is relative to the geographical location of the subscriber, which is held in the database of the local exchange. The selection of the region specific servicing center is done in the local exchange by mapping the dialed number to the routable number of this center. Emergency calls are available in all states of the networks. They can be placed from any subscriber line, even if the subscriber has a service restriction in the outgoing direction, the subscriber has selected an individual barring of outgoing calls, the credit limit of the subscriber is reached or the account of the subscriber is suspended. Emergency calls belong to the class of non-blockable calls. When it takes some time until the emergency caller gets connected with the emergency center, an announcement can be played to inform the caller that the emergency call is being processed and he should hold the line. Finally, emergency calls are non-chargeable calls. This means there need to be provisions to allow placing emergency calls from coinbox telephones without depositing coins. 2.2. Origin-dependent emergency calls The origin-dependent call service is used to realize emergency calls from special phones, for example calls from a police office or a fire brigade center. Authorization to use this service is granted by checking the properties of the subscriber line from which the call is originated. These properties are included in a subscriber database at the local exchange. This call service may be used during disaster recovery operations, so that these calls must be placed under all network circumstances. The call service is realized by associating a call preference with this call, which allows a preferred routing of the call throughout the network. Such calls might even preempt other calls, if the called party is currently carrying out another, non-emergency call. 2.2. User-dependent/ authenticated emergency calls The user-dependent call service is used to realize emergency calls from specially authorized users, for example calls from polices officers or fire fighters. Such call may originate form any terminal in the public communication network. Thus, users must dial a special access and authentication codes to gain access to the service. Internet Draft PSTN Emergency Features November 2002 An example for a national implementation of this call service in US is GETS (the Government Emergency Telephony Service) [2]. The international version is called International Emergency Preparedness Scheme [1]. This calls must be placed under all network conditions. This call service is realized by associating a call preference with this call. This allows a preferred routing of the call throughout the network. Such calls might even preempt other calls, if the called party is currently carrying out another, non-emergency call. 3. Features needed to realize Emergency Call Services This section lists the features required to realize the emergency call services as described before. This includes features to enable the services itself and features to fight congestion. Several features are available at all time while others are activated during exceptional network states. Under normal operation of the network, the core idea is to enable and route these calls, even if system configuration would prevent other (normal) calls (e.g. if the subscriber account is suspended due to overdue credit limit). In case of unexpected disaster situations, such as earthquakes, storms, big fires and other types of catastrophes, networks can be immensely affected or even damaged. The idea here is to fight overload and prioritize emergency calls. The first subsection covers system-based features. The second subsection describes several other (hardware) features which are used to increase availability of the emergency call services in the case of various network failures. 3.1 System-based features o "Digit translation on the dialed number" means that the dialed number may be replaced by a new code and call preferences are added or altered. This is applied to convert a uniform emergency call number (e.g. 911) to the region specific servicing center number. o "Subscriber emergency override" means that it is possible, to override subscriber service restrictions for specific dialed numbers (e.g. emergency numbers). Thus subscribers with outgoing call restrictions can still place emergency calls. o "Disable Calling Line Identification Restriction". Originating subscribers may hide the presentation of the calling line identification on the called party's terminal. For emergency calls, it is possible to define categories for terminating subscribers (e.g. the emergency centers) to get the ability to override the presentation restriction and have the calling party's identity presented. o "Preference category during catastrophe" means that the local exchanges will impose service restrictions when a catastrophe state is activated. Then, call admission is based on the following criteria: o the destination of the call. Call are classified as "regular calls" and "emergency calls" (e.g. 911 calls). Internet Draft PSTN Emergency Features November 2002 o the source of the call. All subscriber lines are administratively assigned a preference category (which are held within the local exchange). There are several possible levels. Normal subscribers get preference category 0 while e.g. the "red phone of the president" gets the highest category. o the identity of the caller. By means of a specific service, the identity of the caller is verified and if eligible, the preference category of the subscriber line is increased (for the call to be placed). o the active level of service restriction in the local exchange. The administration can activate different levels of service restrictions in the local exchange; these are for example: o Level 0: Normal Operation of the Local Exchange o Level 1: Only subscribers with preference category 1 or 2 are permitted to originate calls. Subscribers without preference can place emergency calls. The terminating traffic is not restricted. o Level 2: Only subscribers with preference category 2 are permitted to originate calls. Emergency calls are accepted only if the calling subscriber has a preference category greater than zero. The terminating traffic is not restricted. o "Priority Routing" means preferential treatment to calls originating from certain subscribers and their dial preferences in order to receive special path selection. o "Automatic Repeat Attempt" means that in case of congestion, repeated set-up attempts can be made on the last-choice route for priority subscribers or emergency traffic. The feature requires that the signaling system transmits the calling category information. o "Last trunk reserved for special traffic" means that the last idle trunk within a trunk group can be reserved for special traffic, e.g. origin-dependent (priority calling category) or destination-dependent (e.g. emergency) traffic. o "Authentication and authorization" means that a caller is authenticated (e.g. via an account number and a PIN) and is then authorized to receive a special service (e.g. a higher preference category). This is typically done in special service offices reached through a dialed access code (e.g. for GETS). o "Hard-to-Reach Control (HTRC)" is a kind of congestion control which fights overloads resulting from catastrophes. In these cases, communication from outside with subscribers inside the catastrophe area is almost impossible. The number of call attempts to reach the affected area increase rapidly and the network is severely overloaded. Many call attempts fail because the called destination is congested. Additionally, calls to other areas (not affected by the catastrophe) cannot be successfully routed to their destinations due to the propagated congestion; hence the telecommunication companies are not able to provide their service effectively. Therefore, the networks run a Hard-to-Reach Control (HTRC). HTRC is based on the following functions: Internet Draft PSTN Emergency Features November 2002 - Automatic determination of HTR destinations - Automatic reduction of HTR traffic through network management controls (e.g. by means of trunk reservation) - Administration of destinations to be automatically detected, to be excluded from HTR determination, to be excluded from HTR reduction or even to be marked as HTR in order to benefit from HTR traffic reduction through network management controls. 3.2 Hardware-based features There are several features to increase the availability of emergency call services in case of network or other hardware failures. Those are for example: o Provision of backup lines for emergency calls If a remote part of the local exchange is in stand alone service (which means, all PCM-links from the remote part to the local exchange are out of service), calls to emergency numbers can be routed to subscriber ports, which are defined as emergency call ports. These emergency call ports can be connected via suitable transmission equipment to an emergency center. o Power Supply of User Terminals The interface S0 (between customer premise equipment and the local exchange) for analog and ISDN lines include power supply of the terminal equipment in normal and emergency operation. This means that appropriate terminals can be operated even without external power supply. o Power Supply for Network Elements Typically, regional regulators demand 48 hours of operation without external power supply. Therefore, reliable battery backup and/or fuel based generators are provided for exchanges, transmission equipment and other essential elements. 4. Denial of Service Protection PSTN provides denial of service protection through statistical and other means. Thereby individual channels are supervised. There are two major features. o "noisy port supervision" provides supervision of the subscriber line by comparing the actual usage pattern against the dialing characteristics of well-behaved subscribers. This feature detects attacks or malfunctions if the subscriber requests more than a certain number of incomplete, short, or faulty calls on that line in a given period of time. o "killer trunk supervision" works the same way as noisy port supervision, it just compares against the dialing characteristics on trunk lines. 5. PSTN Emergency Call Services and Features In this section we give a brief listing which features are needed to provide which emergency services. The aim is to make clear that not all features are needed for each service. Internet Draft PSTN Emergency Features November 2002 Service "Emergency "origin-dependent "user-dependent/ call" emergency call service" authenticated call emergency service" Feature - digit translation on x x x dialed number/ address replacement - subscriber emergency x override - preference category x x x during catastrophe - Disable calling line x identification restriction - priority x x - automatic repeat x x attempt - last trunk reserved for x x x special traffic - authentication and x authorization Table 1: Mapping of Emergency Features and Services 6. Conclusions PSTN systems offer features to aid all kind of catastrophe and emergency state. This ranges from prioritization of personal emergency calls under normal network operation up to blocking of other than emergency calls during catastrophe state. Additionally, parts of the network are protected from congestion by automatic detection of hard-to-reach catastrophe areas. Thus, originating calls from different areas can still reach emergency centers within this catastrophe area. This paper summarizes a set of services that may find their way into the requirement sheets of the IEPREP working group. Of course, not all of these services may be applicable in the Internet, but further discussion is needed to select an appropriate subset of the above described features. Security Considerations This draft is on security. Specific requirements to other security functions may be derived if further considered. IANA Considerations Service and feature name spaces may be needed if further considered. References [1] "International Emergency Preparedness Scheme", ITU-T Recommendation E.106, March 2000 [2] "Framework for Supporting IEPS in IP Telephony", Internet Draft , June 2002 Author's Address Jśrg Ottensmeyer Siemens AG M’nchen, Germany email: joerg.ottensmeyer@siemens.com Internet Draft PSTN Emergency Features November 2002