Preserving Emergency Call During Failure to Transfer

Abstract

A system and method in a user equipment (UE) for transferring an emergency session from a first network to a second network. The method includes transmitting a first registration request message to the second network; receiving a registration reject message from the second network; detecting the emergency session between the UE and the first network is ongoing; transmitting a second registration request message comprising an emergency registration request to the second network; and receiving a registration accept message for the emergency session.

What is claimed is: 
     
         1 . A method for transferring an ongoing emergency session from a first network to a second network, the method comprising:
 receiving, by a network component in the second network from a user equipment (UE), a first attach request message, the first attach request message comprising a PDN CONNECTIVITY REQUEST message with request type set to “handover”;   transmitting, from the network component to the UE, an attach reject message;   receiving, by the network component from the UE responsive to the attach reject message, a second attach request message, the second attach request message comprising a PDN CONNECTIVITY REQUEST message with request type set to “handover of emergency bearer services” for the ongoing emergency session; and   transmitting, by the network component to the UE, an attach accept message for transferring the ongoing emergency session from the first network to the second network.   
     
     
         2 . The method of  claim 1 , wherein the second attach request message is an emergency attach request message. 
     
     
         3 . The method of  claim 1 , wherein the attach reject message comprises an evolved packet system (EPS) mobility management (EMM) cause code. 
     
     
         4 . The method of  claim 3 , wherein the EMM cause code is #19 and indicates EPS Session Management (ESM) failure. 
     
     
         5 . The method of  claim 1 , wherein the first network is a fifth generation (5G) network and wherein the second network is a fourth generation (4G) network. 
     
     
         6 . The method of  claim 1 , wherein the first network is part of a fifth generation system (5GS) and wherein the second network is part of an EPS. 
     
     
         7 . The method of  claim 2 , wherein the emergency attach request message comprises an attach type set to “EPS emergency attach”. 
     
     
         8 . The method of  claim 1 , wherein the second attach request message comprises an attach type not set to “EPS emergency attach”. 
     
     
         9 . A network component in a first network comprising:
 a processor; and   a memory storing instructions which, when executed by the processor, cause the network component to:
 receive, by the network component from a user equipment (UE), a first attach request message, the first attach request message comprising a PDN CONNECTIVITY REQUEST message with request type set to “handover”; 
 transmit, from the network component to the UE, an attach reject message; 
 receive, by the network component from the UE responsive to the attach reject message, a second attach request message, the second attach request message comprising a PDN CONNECTIVITY REQUEST message with request type set to “handover of emergency bearer services” for the ongoing emergency session; and 
 transmit, by the network component to the UE, an attach accept message for transferring the ongoing emergency session from a second network to the first network 
   
     
     
         10 . The network component of  claim 9 , wherein the second attach request message is an emergency attach request message. 
     
     
         11 . The network component of  claim 9 , wherein the attach reject message comprises an evolved packet system (EPS) mobility management (EMM) cause code. 
     
     
         12 . The network component of  claim 11 , wherein the EMM cause code is #19 and indicates EPS Session Management (ESM) failure. 
     
     
         13 . The network component of  claim 9 , wherein the second network is a fifth generation (5G) network and wherein the first network is a fourth generation (4G) network. 
     
     
         14 . The network component of  claim 9 , wherein the second network is part of a fifth generation system (5GS) and wherein the first network is part of an EPS. 
     
     
         15 . The network component of  claim 10 , wherein the emergency attach request message comprises an attach type set to “EPS emergency attach”. 
     
     
         16 . The network component of  claim 9 , wherein the second attach request message comprises an attach type not set to “EPS emergency attach”. 
     
     
         17 . A non-transitory computer readable storage medium comprising instructions which, when executed by a processor, cause the processor to:
 receive, by a network component in a second network from a user equipment (UE), a first attach request message, the first attach request message comprising a PDN CONNECTIVITY REQUEST message with request type set to “handover”;   transmit, from the network component to the UE, an attach reject message;   receive, by the network component from the UE responsive to the attach reject message, a second attach request message, the second attach request message comprising a PDN CONNECTIVITY REQUEST message with request type set to “handover of emergency bearer services” for the ongoing emergency session; and   transmit, by the network component to the UE, an attach accept message for transferring the ongoing emergency session from the first network to the second network.   
     
     
         18 . The non-transitory computer readable storage medium of  claim 17 , wherein the second attach request message is an emergency attach request message. 
     
     
         19 . The non-transitory computer readable storage medium of  claim 17 , wherein the attach reject message comprises an evolved packet system (EPS) mobility management (EMM) cause code. 
     
     
         20 . The non-transitory computer readable storage medium of  claim 19 , wherein the EMM cause code is #19 and indicates EPS Session Management (ESM) failure.