Feature Description
The MME's emergency bearer services are provided to support IMS emergency sessions. Emergency bearer services are functionalities provided by the serving network when the network is configured to support emergency services.
Emergency bearer services are provided to normal attached UEs, depending on local regulation, to UEs that are in limited service state. Receiving emergency services in limited service state does not require a subscription. Depending on local regulation and an operator's policy, the MME may allow or reject an emergency attach request for UEs in limited service state.
In release 19.2, the Emergency Bearer Service feature provides a functionality to disable Emergency Bearer Service at a TAI management database level.
Important |
This feature is license controlled. Please consult your Cisco Account Representative for information about the specific license. This license was not enforced in earlier releases. |
Feature Capabilities
The Emergency Bearer Support is equipped with the following capabilities:
UE capabilities
For Emergency Bearer Services in EPS, the UE supports the following functionalities:
- IMS Voice Calls.
- ATTACH requests with IMEI as the mobile identity if SIM is not detected in the network.
- PDN Connectivity procedures with request type set to "EMERGENCY".
MME Capabilities
The MME can accept an Attach Request for an Emergency Bearer Service or a PDN Connectivity Request to an emergency PDN if the network capabilities are configured to support Emergency Bearer Services.
As of 19.2, the MME now also supports:
- Emergency Bearer Service profile configuration. The profile
should include the following to complete the profile configuration:
- APN name
- PGW FQDN or IP Addresses
- QoS parameters to setup a session
- Configuration to disable emergency services at TAI management object level to reject emergency calls for a configured list of TAIs
Call Admission Control
In this release, all emergency calls are allowed until the configuration limit is reached. Deletion of existing calls to admit emergency attaches is not in scope of this release.
Attach for Emergency Bearers
Emergency Bearer Support supports the following ATTACH behaviors:
- Valid UEs only: Only normal UEs that have a valid subscription, authenticated and authorized for PS service in the attached location, are allowed. The normal Authentication and Attach procedures will be executed. The HSS subscription of the UE should allow the UE to be attached to EPS in its current TAI and in the current CSG (if applicable). The emergency attach procedure is not any different from the normal ATTACH procedure in this case.
- Authenticated UEs: These UEs must have a valid IMSI. These UEs are authenticated and may be in limited service state due to being in a location that is restricted from service. The Authentication procedure should complete successfully. The Update Location procedure to the HSS failing, or any further validation of HSS provided subscription data does not affect the processing of the ATTACH request successfully.
- IMSI: These UEs must have an IMSI. If authentication fails, the UE is granted access and the unauthenticated IMSI is retained in the network for the records purposes.
- All UEs: Along with authenticated UEs, this includes UEs with an IMSI that can not be authenticated and UEs with only an IMEI. In this case, an emergency attach request with the IMEI is accepted by the network.
- ISR is deactivated for an emergency attached UE.
As of release 19.2
- MME rejects the emergency attach procedure if emergency services are disabled for a TAI from which attach request is initiated.
Important |
When authentication fails, the MME queries the UE for IMEI, and the received IMEI is used as the key for the UE in the network. The IMSI is used for recording purposes only. If IMEI is used as the key for identifying the UE in the network, there will be no backup database context associated with the call. |
PDN Connectivity for Emergency Bearer Service
A UE that is already attached to the network for EPS services requests for Emergency Bearer Service using a PDN connectivity request. The request-type in PDN Connectivity request is set to "emergency", and no APN information is supplied by the UE.
The MME does not consider HSS provided information to setup a connection, rather uses the locally configured PGW and APN information to setup the PDN connection. The UE is not allowed to request bearer allocations from this PDN, the requests are rejected.
As of release 19.2, the MME rejects emergency PDN activation if emergency services are disabled for a TAI.
Important |
The setup for PDN connection and associated bearers should not be affected by the policy configuration on the MME. |
Tracking Area Update Procedure
MME supports the following in the TAU procedure:
- Skip Authentication procedure for a UE that only has PDNs for Emergency Bearer Services.
- If the UE is
restricted on the new TAI, and the UE has PDN connection for Emergency Bearer
services, the MME:
- Deactivate all non-Emergency service PDN using with signaling to the UE if the UE is in ECM-CONNECTED.
- Deactivate all non-emergency service PDN locally, and sending the EPS Bearer Context status IE in the TAU accept message if the UE is ECM-IDLE.
- The MME shall also indicate to the UE that ISR is turned off
- If
re-authentication fails the MME,
- Deactivate all non-Emergency service PDN using with signaling to the UE if the UE is in ECM-CONNECTED
- Deactivate all non-emergency service PDN locally, and sending the EPS Bearer Context status IE in the TAU accept message if the UE is ECM-IDLE.
- If a UE attached only for EMERGENCY SERVICES, the MME shall set the mobile reachability timer to the configured T3412 value, and locally detach the UE if mobile reachability timer expires.
As of release 19.2, MME also supports:
- If a TAU for a UE
in ECM-IDLE state is received after an emergency attach procedure, which
arrives from an area whose emergency services are disabled, MME provides the
following functions:
- Rejection of TAU in case of single emergency PDN.
- De-activation of all emergency PDNs in case of multiple PDNs.
Note
The above functions are applicable for TAU arriving in idle mode
Inbound relocation Procedures
- Handling inbound relocations with no IMSI and security context present in the incoming MM context.
- If the UE is not valid in new location, or if local policy forbids setup of all bearers in the context, ensure that bearers set up for emergency services are not torn down.
- As of release 19.2, S1 and X2 handovers occurring after an emergency attach from an area where emergency services are disabled, is allowed to continue in connected mode.
MME Emergency Configuration Data
MME is supported with the following configuration data:
- Emergency Access Point Name (em APN): A label according to DNS naming conventions describing the access point used for Emergency PDN connection (wild card not allowed).
- Emergency QoS profile: The bearer level QoS parameter values for Emergency APN's default bearer (QCI and ARP). The ARP is an ARP value reserved for emergency bearers.
- Emergency APN-AMBR: The Maximum Aggregated uplink and downlink MBR values to be shared across all Non-GBR bearers, which are established for the Emergency APN, as decided by the PDN GW.
- Emergency PDN GW identity: The statically configured identity of the PDN GW used for emergency APN. The PDN GW identity may be either an FQDN or an IP address. It has be possible to support multiple PDN GW identity to support PGW redundancy.
- Disable emergency services: In release 19.2, MME provides CLI control to disable emergency-services at TAI management object level.
Information Storage
Currently, MME-APP stores UE contexts in lists indexed by IMSI, GUTI or PTMSI. To support emergency IMS bearers for UE without IMSI, MME supports indexing the list of active call lines by IMEI too.
Interdependences
The Emergency Bearer Service feature affects the related features described in this section, during Attach/TAU processing.
Regional Zone Code Restriction
- Regional Zone Code restriction for a call in progress, and the TAI is restricted
- UE has emergency PDN connections, and emergency connections are allowed in restricted zone codes
Load Rebalancing
The MME does not impact UEs that are connected for Emergency Bearer Services during load rebalancing procedures (3GPP TS 23.401- 9.6.0 - 4.3.2.7).
SRVCC
If any of the bearers setup for emergency services have a QCI value of 1, such bearers is moved to CS domain on SRVCC activity. There is no conflict between SRVCC and Emergency Bearer Services. The Sv interface accepts messages without an IMSI, and unauthenticated UEs is supported over the Sv interface.
CSFB
The attach type IE is used for signaling either a "combined" or "emergency" attach. A UE that is "combined" attached might send a PDN connectivity request for emergency bearer services. After setup of such a bearer, if CSFB is requested, CSFB procedure will proceed with no interaction. Because the UE has been authenticated in the network, there is successful transfer of the UE context to a Gn/Gp SGSN. The SMS functionality of a UE is unaffected by a PDN Connectivity to a emergency PDN.
Gn/Gp Interface
Since the current version of Gn/Gp interface supported on the MME does not support handover of unauthenticated UE MM contexts to SGSN, Context Requests for an unauthenticated UE context from a Gn/Gp SGSN will be rejected by the MME.
Operator Policy
The interdependency of the Operator Policy that applies to the Emergency Bearer Service are as follows:
- Maximum PDN or Bearers reached
- Current TAI not supported
- Authentication required by policy fails
- Equipment identification through policy fails.
The specifications are only for calls which have both emergency and non-emergency PDNs. In any of the above policy restriction, the emergency PDN stays established, regardless of what validation level is required for emergency attach.
Interface
S11
The following changes are implemented on the S11 interface to support Emergency Service:
- IMSI is made optional in the Create Session Request.
- An indication flag is added to indicate if the IMSI is available but unauthenticated.
NAS
- New header type added to NAS parser to specify if the message header type is "integrity protected" or "integrity protected and ciphered"
- New Attach type.
- Emergency service support indicator for Attach/TAU accept.
S3/S10
The following interface changes apply to Context Response and Forward Relocation Request messages:
- Optional IMSI.
- IMSI Validation flag.
- Security Parameters if available.
S6A
The changes to the S6A interface includes the following:
- Optional HSS handle in UE_CONTEXT.
- Authentication Information Request not mandatory for all call flows.
- Update Location Requests not sent for Emergency Bearer Services Attach, if the configuration does not require it.
- Cancel Location Request will not clear a call in the MME if the associated IMSI has a PDN connection for Emergency Bearer Services.