Cisco Cisco ASR 5000
ISR with Circuit Switched Fallback
▀ Call Flows
▄ SGSN Administration Guide, StarOS Release 18
Step Description
5.
On receiving the paging message, the UE performs a service request procedure which establishes the RRC connection and
sends a Service Request to the MME. The Service Request message includes a special CS Fall-back indicator that informs
the MME that the CS fallback is required.
sends a Service Request to the MME. The Service Request message includes a special CS Fall-back indicator that informs
the MME that the CS fallback is required.
6.
This triggers the MME to activate the bearer context in the eNodeB with an indication to perform fallback to GERAN or
UTRAN.
UTRAN.
7.
The eNodeB selects a suitable target cell, by triggering the UE to send measurements on the neighbour cells, and initiates a
handover or cell change procedure. The selection between handover or cell change procedure is based on the target cell
capabilities and is configured in the eNodeB.
handover or cell change procedure. The selection between handover or cell change procedure is based on the target cell
capabilities and is configured in the eNodeB.
8.
After a handover or cell change procedure, the UE detects the new cell and establishes a radio connection and sends a page
response to the MSC, through the target RAN.
response to the MSC, through the target RAN.
9.
When the page response arrives at the MSC, a normal mobile terminated call setup continues and CS call is activated
towards the UE.
towards the UE.
The CS fallback is primarily supports voice calls but it also supports other CS services. In the case of SMS services the
UE need not switch to other radio interfaces. The UE can remain on LTE and still send and receive SMSes. The SMS
messages are tunnelled between the UE and the MSC through the MME NAS signalling and the SGs interface.
UE need not switch to other radio interfaces. The UE can remain on LTE and still send and receive SMSes. The SMS
messages are tunnelled between the UE and the MSC through the MME NAS signalling and the SGs interface.
When ISR is activated the UE is simultaneously registered at both SGSN and MME. So any paging for CS services
occurs at both the SGSN and the MME. In a network if ISR is activated for an UE and CSFB is used in the network, the
SGSN has to support additional call flows.
occurs at both the SGSN and the MME. In a network if ISR is activated for an UE and CSFB is used in the network, the
SGSN has to support additional call flows.