This information is provided to assist the IETF SIPPING Working Group in developing requirements for Emergency Call Centres using the Session Initiation Protocol.

1. Overview of the UK Service.
The Emergency Calling Service enables members of the general public to initiate emergency calls, using the fixed calling numbers of 112 and 999 (alternative numbers, with equal treatment), to an emergency service or to other agencies that are treated as emergency services. These calls would not normally require payment by the caller or the emergency service at the time of the call and consist of two main stages.

Calls to either of the emergency numbers receive special treatment at all switches and are priority routed to an emergency operator (answering) service, which may be geographically located anywhere. The emergency operator automatically receives location information on the call origination to enable some level of detail (dependant on the type of the originating network - mobile or fixed) as to the geographic location of the caller. This information may be verified by human interaction at the emergency answering service who also establish whether the call is likely to be  genuine  and the service required. 

If it is established that this is likely to be a genuine emergency call, the second stage takes place - the caller and emergency operator are connected to the appropriate emergency service centre perceived to be local to the caller - police (who also despatch cave & mountain rescue services), fire, ambulance or coastguard. If more than one service is requested, they are connected sequentially in the order requested. Geographic location information is available automatically at the start of the call to the emergency service centre to assist in the prompt despatch of emergency assistance.

The emergency operator remains on standby in the call during the initial part of the interaction between the caller and the emergency services to check that caller is responding and to provide further assistance if required. Call termination is only achieved once the last party in the call (caller, emergency operator and emergency service centre operator) has concluded.

2. Call Handling UK Service
This section provides information on the special call handling granted by the signalling and switch data settings in the current UK PSTN/ISDN switched circuit network.
- Fixed line calls to 112 are automatically rejected, in BT's network, if additional digits are detected 4 seconds after the digit "2" has been dialled. Although this introduces a small delay on delivering the call to the emergency operator, it prevents erroneous calls (caused by spurious '1s' and '2s' from line or equipment faults, or by repeated tapping of handset switches on lines with loop-disconnect signalling). Currently this feature auto-rejects 80% of calls to 112 in the BT network. Of the remaining 20% most are still erroneous calls that have passed the network filter (only 112 tapped-out) with only about 1% considered genuine.
- All 999 calls and those 112s that pass through the network filter are marked with a protection bit.
- Initially priority is granted by trunk reservation. Any route which has the potential to carry an emergency call is considered congested to ordinary calls when all but the last 2 to 6 trunks are busy (dependent on the size of routes from the switch). Only emergency calls can use these last trunks in cases of congestion.
- Automatic alternative routing and Automatic Re-routing is applied to all emergency calls to improve successful call delivery under congestion conditions.
- Emergency calls are exempted from call gapping and exchange overload restrictions, based on the protection bit.
- If all of these advantages fail to deliver the call to an emergency operator, continuous retry over all alternative routes is applied from the originating switch.
- Emergency calls are placed in a separate, priority queue at the Call Centre switches of the emergency operator (answering) service - which may also handle other non-emergency calls (such as operator assistance, etc.).
- Once an emergency call reaches the emergency operator and is answered, it is subject to last party clear.
- The emergency operator initiates the connection of the appropriate emergency service centre using the public PSTN (but there is no dial tone for the second stage of the call).
- The full CLI of the original caller is passed to the emergency service centre in either the signalling messages that set-up the call or verbally (to enable a return call to be made if necessary).
- The protection bit is also applied to the call segment that delivers the emergency call from the emergency operator service to the selected emergency service call centre (e.g. telecommunications network operator to police call centre). Alternative routing for this part of the call is under control of the operator who has alternative numbers for the required emergency service or its back-up.
- The emergency operator remains active in the call (effectively as a third party) after successful contact with the emergency service centre has been established
- All emergency calls are recorded as part of the call record and audit trail.

3. Geographic Location Information

The geographic location information is used extensively in the Emergency Calling Service. It enables delivery to the correct emergency service centre, rapid deployment of any response to correct incident location and is also a tool in the validation of the call. The essential information used is defined below.
- The service is specific to the UK and is only accessible from terminals attached (subscribed) to networks licensed in the UK.
- The emergency operator centre receives the full CLI (not the CLIP supplementary service) and it is used to automatically access a database giving geographical location for fixed lines. The database entries are provided by all network operators (automatically) and can identify the building address originating a fixed line call.
- Calls from mobile terminals carry information on the individual cell that the call originates from.
- The emergency operator uses the address or cell information to deliver the call to the local emergency services centre identified as local to the caller for the required emergency service .
- The fixed line address and cell coverage area is generally sufficient to identify which emergency service vehicle to send. Caller verification by voice that the fixed line address is also incident location, and additional caller details to narrow-down incident location within cell coverage area, is sufficient to direct vehicle to incident location. 
- Currently a growing one third of all emergency services centres in the UK are able to access location information automatically at the start of the call delivered by the emergency operator service. This location information is provided (within 2 seconds of call arrival) through a separate IP data link to the extended voice channel, which does however deliver the caller's CLI then used to rapidly retrieve location over the data link. 

4. Call Validation 
- The emergency operator performs initial validation of the call using standard questions, ensuring that there is someone at the originating terminal and they do want to contact the emergency services.
- This call validation by the emergency operator filters out around 40%  of fixed lines calls and 75% of mobile calls. The reasons for rejection vary greatly - genuine mis-dialling, young children playing with the phone, accidental pressing of keys from mobiles in pockets/handbags, etc. The emergency operator does not reject the call based on the nature of the incident.
- The emergency service centre uses the approximate location delivered automatically to help eliminate hoax calls to see if incident described is likely to be at location claimed.

5. Requirements for the Service using Packet-switched Technology

The following requirements are based on the information provided above and also the Tiphon document (TS 101-878 v.1.1.1 [2]).

- Emergency calls from end-users must provide a voice link and be recognised by the use of a nationally specified called user identity (i.e. service code) or by sending special indications.
- It should be noted that to prevent both accidental and malicious misuse, there is no requirement  for exceptional handling by-passing of validation of non-registered users.  For the same reasons careful consideration needs to be given to whether users registered on one SIP server can make emergency calls on another SIP server if that is the only one available. 
- Emergency calls must receive priority treatment once discerned by the service code or through explicit signalling. Priority treatment must be applied to both the signalling and the user stream, if appropriate.
- Emergency calls must not be subject to restrictive network management practices such as call gapping and overload restrictions.
- Emergency calls from other network domains must be recognised by the service code or through explicit signalling.
- Emergency calls must be subject to normal call recording, charging and accounting, etc. by serving entities in the call path (either the signalling or the user stream), if appropriate, even though billing and settlement may not result in any charge.
- Originating location must be provided to geographically determine:
	-	the correct national or regional operator centre to route the call to;
	-	the emergency service centre appropriate to the nature and location of the emergency;
	-	the location to accurately despatch emergency assistance.
- Emergency calls must be automatically routed accurately to the location of the appropriate emergency service, recognising that emergency services are provided on a local basis but are often reached through a non-geographic intermediate call-handling centre.
- The geographic location where the emergency call is originating from must be automatically determined as accurately as possible to ensure that emergency services can be directed to the precise location where the emergency has occurred.
- Accurate geographic location information must be automatically available at the emergency operator centre and the local emergency service centre.
- An indication of which type of emergency service is required should optionally be provided by an automatic means, but to do so is not an essential feature of an emergency calling service capability.
- Emergency calls shall be routed in accordance with the nationally agreed plan for this service capability.
- Emergency calls shall be completed in preference to any normal calls that may compete for resources.
- Emergency calls shall be completed as soon as possible if all resources are busy.
- Emergency calls shall be exempted from any restrictive management routing controls.
- Emergency calls must be migrated to multi-party calls under the control of the emergency operator.
- Emergency calls shall be subject to "last party clear" for successful termination.
- The emergency service must be able to place a call back to the calling party in the event that they need to make a subsequent call to the caller after the initial emergency call has terminated.
- Emergency calls shall be restored with priority over normal calls as an exceptional behaviour of fault restoration.

Additional Requirements:
- The transmission path from originator to operator must be maintained throughout the call, including during extended periods of apparent silence, for extended questioning/checking,.
- All available pertinent information about the origination of the emergency call must be automatically available to the emergency operator and the emergency service centre.
- An indication of an answered emergency call shall be displayed on the originating user equipment if feasible.
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6. References
[1] http://www.oftel.gov.uk/publications/ind_guidelines/emer1002.htm
[2] ETSI Tiphon TS 101-878 v.1.1.1 (section 6.7) - available from 
http://docbox.etsi.org/TIPHON/TIPHON/07-drafts/wg1/_Published/