Revision History

The ePDG and P-GW support emergency call establishment over untrusted WiFi for the P-GW as per 3GPP Release 13. 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 and, depending on local regulation, to UEs that are in limited service state. Receiving emergency services in a limited service state does not require a subscription.

Authentication Authorization Requests (AAA) to Diameter now carry the new Emergency-Indication AVP for Untrusted WiFi emergency calls. Diameter requests related to PDN connections for emergency services have the highest priority. Depending on regional/national requirements and network operator policy, these Diameter requests are the last to be throttled, in the event that the 3GPP AAA Server has to apply traffic reduction.

Supported Functionality

3GPP Release 13 Emergency Call Support on the ePDG and P-GW includes the following functionality:

• Emergency call establishment over untrusted Wi-Fi for the P-GW. The P-GW includes the new Emergency-Indication AVP over the AAA S6b interface only during Emergency PDN connection establishment.

• Lawful Intercept is supported for Emergency PDNs over the S2b interface.

• Various Create Session Request message IEs have been modified to support all four different behaviors of emergency bearer establishment.

• Intra- and Inter-chassis recovery are supported for emergency call over the S2b interface.

• Network initiated dedicated bearer creation is supported for emergency calls over the S2b interface.

• The maximum APN restriction is ignored for emergency APN.

• Multiple PDNs are supported for emergency calls over the S2b interface.

• Context replacement for emergency calls over the S2b interface without IMSI with same IMEI is supported.

• P-GW emergency related statistics and bulkstats are available.

• Graceful shutdown of S2b emergency calls is supported.

Previous Behavior: Emergency calls were not supported for the S2b interface. Also, handoff between the s2b interface and LTE was not supported for emergency calls.

New Behavior: Emergency calls are now supported on the S2b interface and handover is also supported for emergency calls from the S2b interface to LTE and vice-versa for "authenticated imsi" only.

The ePDG sends a Create Session Request (CSReq) message to the P-GW. The P-GW deduces the emergency related policies to apply from the Access Point Name (APN) received in the CSReq message. For emergency attached User Equipment (UE), if the International Mobile Station Identifier (IMSI) cannot be authenticated or the UE has not provided it, then the International Mobile Equipment Identifier (IMEI) is used as UE identifier.

The P-GW sends the Emergency-Indication AVP over the s6b interface so that the 3GPP AAA server only applies specific policies for emergency services. For an unauthenticated UE, the 3GPP AAA server does not update the Home Subscriber Server (HSS) with the identity of the P-GW. For an authenticated UE, this indication is sent together with the "PDN GW currently in use for emergency services" message, which comprises the PDN GW address and the indication that the PDN connection is for emergency services to the HSS, which stores it as part of the UE context for emergency services.

Support is available for all four different behaviors of emergency bearer establishment:

• Valid UEs only.

• Only UEs that are authenticated are allowed.

• IMSI required, authentication optional.

• All UEs are allowed.

This section describes the new Attribute Value Pair (AVP) and modified Information Elements that support the feature.

Emergency-Indication AVP

A new Emergency-Indication AVP is defined in the Authentication and Authorization Request to signal a request to establish a PDN connection for emergency services.

Information Elements

This section describes other important elements in a Create Session Request that have been modified to work properly with the feature.

Emergency Handover Support

When a subscriber makes an emergency call over WLAN, user equipment (UE) adds sos to the NAI to indicate that this is an emergency attach to ePDG. ePDG informs P-GW about this emergency attach in create session request. When the caller moves away from WLAN into LTE coverage or vice versa, the call gets handed over without being dropped.

P-GW supports following emergency call handovers:

• Handover of Emergency Calls from LTE to Wi-Fi(S2b) : Handovers of emergency calls from LTE to Wi-Fi (S2b) for authenticated UE is supported. While the UE moves from LTE to untrusted Wi-Fi, LTE triggers an Authentication Authorization Request (AAR) to the S6b server with the AVP Emergency-Indication sent in that Authentication and Authorization Request (AAR). Also, an STR is sent when a Wi-Fi (S2b) call is cleared.:

  1. The ePDG sends Create Session Request to the P-GW.

  2. If the UE requested P-CSCF in the IKE Config request, P-CSCF is requested.

  3. Downlink packets are sent on LTE access. The ePDG includes the IP address that is received within the IKE message from the UE in the PAA (PDN Address Allocation) in the GTPv2 Create Session Request.

  4. The P-GW sends AAR to the 3GPP AAA to authorize the APN for the subscriber and to update P-GW address on the HSS for the APN.

  5. The P-GW sends an indication of IP-CAN modification to the PCRF with Credit Control Request (CCR).

  6. 3GPP AAA sends AAA to the P-GW.

  7. The Policy and Charging Rules Function (PCRF) acknowledges IP-CAN Session Modification with a Credit Control Answer (CCA).

  8. The P-GW identifies the S5 session and reallocates the requested IP address session and responds back to the ePDG with a Create Session Response message.

  9. ePDG sends Create Bearer Response message.

  10. P-GW sends the Delete Bearer Request message.

  11. The S-GW sends Delete Bearer Response message to the P-GW.

• Handover of Emergency Calls from Wi-Fi(S2b) to LTE : Handover of Emergency Calls from Wi-Fi (s2b) to LTE for authenticated UE is supported. Since an emergency call in LTE does not have S6b interface authorization enabled, handover of emergency calls from untrusted Wi-Fi to LTE triggers a Session Termination Request (STR) to the S6b server:

  1. The MME selects the P-GW from the MME Emergency Configuration Data and sends a Create Session Request.

  2. The S-GW sends a Create Session Request.

  3. P-GW sends an indication of IP-CAN modification to the PCRF with Credit Control Request (CCR), if Gx authentication is enabled or P-GW applies the local policy and does not query PCRF if local policy is configured. P-GW sends Session Termination Request to S6b server and P-GW provides IPv6 Prefix and/or IPv4 address in PAA.

     Note	If the MME indicates Piggyback support, then, the P-GW piggybacks the Create Bearer Request message to the Create Session Response message.

  4. The MME sends a Modify Bearer Request message to the S-GW.

  5. The S-GW processes each message independently. The S-GW forwards the Create Bearer Response to the P-GW(without piggybacking).

• Emergency PDN Handover with HO=0: Handovers from LTE to Wi-Fi is supported:

  1. The ePDG sends Create Session Request to the P-GW. P-CSCF is requested if the UE requested P-CSCF in the IKE Config request.

     Note	Downlink packets are dropped at the P-GW while the session is being handed over to WLAN.

  2. P-GW checks for an LTE session.

  3. If there is an LTE session, the P-GW sends AAR to the 3GPP AAA to authorize the APN for the subscriber and to update P-GW address on the HSS for the APN.

  4. 3GPP AAA sends AAA to the P-GW.

  5. The P-GW sends an indication of IP-CAN modification to the PCRF with Credit Control Request (CCR).

  6. The PCRF acknowledges of IP-CAN Session Modification with a Credit Control Answer (CCA) message. This message includes the Policy and Charging rules. The P-GW enforces and triggers for events that must be reported by the P-GW.

  7. If Online flag is enabled and if P-GW does not have quota for the WLAN rating group, or if Online Charging Server (OCS) has not sent a 4011 for the WLAN rating group previously, then the P-GW sends a CCR-u to the OCS reporting the usage.

  8. The P-GW identifies the S5 session and reallocates the requested IP address session and responds back to the ePDG with a Create Session Response message.

  9. After the P-GW sends the Create Session Response, the P-GW sends an interim Accounting Request (ACR) to the OFCS.

  10. The OFCS responds with an ACA to the P-GW.

  11. P-GW sends the Delete Bearer Request to the S-GW.

  12. The S-GW sends Delete Bearer Response to the P-GW.

Emergency calls over the S2b interface should not be rejected due to a failure from the S6b server. To ensure this, failure handling must be configured in the APN which is used for emergency calls .

Handling is configured in the aaa group so that emergency calls continue regardless of failures as indicated by the result code.

To configure AAA failure handling for S2b emergency calls:

configure 
   context ingress_context_name 
      aaa group default 
         diameter authentication failure-handling authorization-request result-code 3000 to 5999 action continue 
           diameter authentication failure-handling authorization-request request-timeout action continue 
               end 

Note the following assumptions:

• If an IP‑CAN Session Modification Request triggered by the PCRF removes all PCC rules with a QCI other than the default bearer QCI and the QCI used for IMS signaling, then the PCEF starts a configurable emergency inactivity timer. When the configured period of time expires, the P-GW initiates an IP‑CAN Session Termination Request for the IP‑CAN session serving the IMS Emergency session

• If the Gx/S6b interface returns a Virtual APN, which is not configured as an emergency APN, then the call is rejected with the cause code "APN_DENIED_NO_SUBSCRIPTION

configure 
   failure-handling-template  gx_template   
     msg-type any failure-type diabase-error action continue local-fallback 
         end 

Following example shows failure handling template configuration for Gx failure (PCRF return ErrorCode):

configure 
   failure-handling-template  gx_template   
        msg-type credit-control-initial failure-type diameter result-code 3000 to 5999 action continue local-fallback 
            msg-type credit-control-update failure-type diameter result-code 3000 to 5999 action continue local-fallback 
                end 

Following example shows failure handling template configuration for Gx delayed response:

configure 
   failure-handling-template  gx_template   
      msg-type credit-control-initial failure-type resp-timeout action continue 
         msg-type credit-control-update failure-type resp-timeout action continue 
            end 

To configure local policy for Gx failure (PCRF down or PCRF return ErrorCode):

configure 
          local-policy-service  service_name   
            ruledef  ruledef_name 
               condition priority  priority  { variable { eq | ge | gt | le | lt | match | ne | nomatch } regex | string_value | int_value | set } 
                end 

configure 
   local-policy-service  service_name   
      actiondef   actiondef_name 
         action priority  priorityaction_name arguments  
            end 

configure 
   local-policy-service  service_name   
      eventbase   eventbase_name 
         rule priority  priority   [ event list_of_events ] ruledef ruledef_name actiondef actiondef_name [ continue ]  
          end 

configure 
   context contect_name   
      ims-auth-service   service_name 
         [ no ] policy-control   
            associate failure-handling-template gx_template 
               associate local-policy-service  service_name 
                  end 

Configuring APN to attach emergency PDN on LTE

For emergency PDN handover with S6b Gx, configure APN mode to attach emergency PDN on LTE.

configure 

   context  context_name

      apn apn_name 

         emergency-apn  

         end 

Enabling S6b Authorization

Following is the sample configuration to enable S6b authorization:

configure 

   context  context_name

      pgw-service  service_name 

         apn apn_name 

            authorize-with-hss  [ egtp[gn-gp-enabled] [ s2b [gn-gp-enabled] [ s5-s8 [gn-gp-enabled | gn-gp-enabled]] [ report-ipv6-addr ]| lma [ s6b-aaa-group aaa-group-name  | report-ipv6-addr ] | report-ipv6-addr ] [ default | no ] authorize-with-hss  

             end 

The following sections describe commands available to monitor the feature