The recent earthquake in Türkiye and Syria reminded everyone how important the disaster and recovery management is. Mobile network was the first part of the critical infrastructure which was involved in the emergency situation. Although it was partially damaged itself, right from the beginning it had to withstand the initial load of calls and further support coordination of AFAD responders, police, gendarmerie, soldiers, volunteers and other personnel. At the same time it had to maintain the regular calls, which were not less important. Mobile communication was helping to reunite families as well as to provide access to important information and news.
The same way everyone should recap first aid procedures time to time, operators should recap the disaster and recovery plans and emergency communication flows. I’m currently working on mobile network deployment in Mexico, where earthquakes are a common place. Therefore we take this very seriously and we know there is no excuse for us if we fail.
Let’s quickly go through the basic requirements on emergency calling in mobile networks.
As you might know from This is an emergency! there is plenty of standards for emergency communication. Moreover there are many legal requirements which differ country to country. High-level description of Emergency Communication is presented in GSMA NG.119 which we are referring to in this post.
What is Emergency Call?
Emergency Call (EC) is established by
- dialling specific emergency numbers identified by the UE
- dialling specific emergency numbers identified by the network (and not by UE)
- using UE “red button”, without the need to dial a dedicated number
EC must be supported by UE without a SIM being present, therefore subscriber can use any operator to make the call. It is always free of charge for the user. ECs are routed to the emergency services (PSAP, ECS) based on the emergency type and location of the subscriber.
There are specific ways of handling ECs per network type. We’ll focus on 4G and 5G networks, which are making use of IMS. However 3G network is still in charge in many countries.
As defined in 3GPP TS 22.003, EC is initiated by the UE using
- 3GPP TS12 (Emergency Call) for emergency calls identified by UE.
- 3GPP TS11 (Telephony) for local emergency calls (not identified by the UE)
When an emergency setup is received, the MSC generates a call to the Public Safety Answering/Access Point (PSAP). The right PSAP is chosen based on the Service Category and Location of the originator. Called Party is then set as a long number associated with the local PSAP.
In 4G, two major technologies are used for emergency call management:
- CSFB – Circuit Switched Fall Back: initially mobile operators can decide to use CSFB capability to manage the emergency call (see the previous section related to 2G/3G).
- IMS – Voice over LTE (VoLTE): Emergency call is managed by the IMS Core Network.
A CS and IMS capable UE attempting an emergency call follows TS 23.167.
With 5G Core (5GC), the only way to manage the voice service is Voice over IMS (VoIMS). CS Fall-back is not supported with 5GC. Nevertheless, the overall logic and principles to manage Voice over New Radio VoNR remain unchanged. It is recommended to use the same approach as 4G to manage the IMS emergency call. Only small updates related to 5G need to be implemented.
- Emergency calls over 5GC rely on dedicated emergency PDU session (counterpart of Emergency PDN connection in EPC).
- Functions in charge of retrieving or using trustable user location (network provided) like LRF (Location Retrieval Function) and RDF (Routing Determination Function) need to be updated to support New Radio NR Cell-ID format.
VoLTE Emergency Calling
Download of Emergency Numbers to the UE
The serving network can download additional emergency numbers to the UE in order to enable UE detection of an emergency session. These additional emergency numbers are downloaded in 4G during the Attach procedure included in Attach Accept message sent by the network 3GPP TS 24.301.
This is mainly important for roaming scenarios, where the visited MME downloads the local Emergency number list/Extended emergency number list to visited UE. Emergency numbers are linked to countries defined by Mobile Country Code (MCC) and the Emergency number list is discarded by the UE when entering in a new country. See also GSMA IR.65.
If the UE of a roaming subscriber is not made aware of the emergency number, the call will be handled via normal session establishment.
As mentioned, we distinguish two main scenarios how to setup an emergency call, depending on if the UE is able to recognize the emergency number.
- Red button usage
- Emergency Numbers
- Standard emergency numbers dialled by the user (112 and 911)
- Any emergency call number stored on a SIM/USIM
- 000, 08, 110, 999, 118 and 119 when a SIM/USIM is not present (these numbers are stored in the UE).
- Additional emergency numbers that may have been downloaded by the serving network when the SIM/USIM is present.
If the UE has identified an emergency number as defined above, the UE initiates a emergency call setup procedure, enabling high priority in case of network congestion.
The UE may or may not be normally registered in IMS network.
- The UE may perform an IMS emergency session establishment without prior emergency registration when it is already IMS registered and it is in the home network.
- Otherwise, the UE performs an IMS emergency registration.
The UE has to comply to TS 23.167 for initiating the emergency registration procedure. The emergency attach procedure described above is especially important in context of roaming. Via the emergency registration, the emergency session is managed by the visited network. In case of roaming, Local Breakout (LBO) is always used.
SIP INVITE (emergency) arrives at the Visited IMS (E-CSCF is the visited IMS network). E-CSCF utilizes the UE provided location information (P-Access-Network-Info) and/or queries the LRF (Location Retrieval Function) to retrieve the proper routing information for PSAP.
E-CSCF routes the emergency session establishment request to an appropriate PSAP taking also into account the UE emergency type (if provided, e.g. Marine Guard, Fire Brigade, Ambulance, Police).
Non UE detectable EC
MNOs also have to support “Local Emergency number” to comply with the legislation. These numbers are not made UE detectable, so the UE is not able to identify the emergency number dialed by the end user. Therefore UE does not setup an emergency call but a normal call.
The call request is sent to the P-CSCF (Home P-CSCF in case of S8 Home Routing, S8HR roaming) as per a normal session establishment procedure.
In case the P-CSCF can detect that this is a request to establish a session related to an Emergency call, the P-CSCF rejects the session initiation request with an indication that this is for an emergency session via SIP 380 Alternative Response (3GPP 24.229). When the UE receives the session rejection then the UE establishes the Emergency session to E-CSCF (Visited E-CSCF in case of roaming). The VoLTE emergency call procedure could take place as described before when the UE detects Emergency number.
Network Architecture and Sizing
Obviously it is not just about the flows. Telecom network is a mission critical system. We have to make sure, that sizing is right even for unexpected scenarios. Signaling storms during emergency situations are expected. Available resources should be sufficient to accommodate immediate spikes of traffic. The number of signaling messages during the spike can be several times higher than the regular traffic.
Moreover in case of a big disaster we have to make sure, that even with some infrastructure losses, the network is still able to absorb the load. The geo-redundant systems should have enough capacity to withstand requests coming to unavailable sites. Although it may happen, that the network is temporarily overloaded, it can never fail completely (domino effect). That’s why there are many mechanisms preventing overload situation (Rate Limit and Traffic Shaping).