Cisco Cisco ASR 5000
ISR with Circuit Switched Fallback
▀ ISR with CSFB - Feature Description
▄ SGSN Administration Guide, StarOS Release 18
ISR with CSFB - Feature Description
Idle-mode Signaling Reduction (ISR) feature allows the UE to move between LTE and 2G/3G without performing
Tracking Area (TA) or Routing Area (RA) updates once it has been activated. A pre-requisite for ISR activation is that
the UE, SGSN, MME, Serving GW and HSS all support ISR. At the first attach to the network, ISR is not activated. ISR
can only be activated when the UE has first been registered in an RA on 2G/3G and then registers in a TA or vice versa.
Tracking Area (TA) or Routing Area (RA) updates once it has been activated. A pre-requisite for ISR activation is that
the UE, SGSN, MME, Serving GW and HSS all support ISR. At the first attach to the network, ISR is not activated. ISR
can only be activated when the UE has first been registered in an RA on 2G/3G and then registers in a TA or vice versa.
If the UE first registers on GERAN/UTRAN and then moves into an LTE cell, the UE initiates a TA update procedure.
In the TA update procedure, the SGSN, MME and Serving GW communicate their capabilities to support ISR, and if all
the nodes support ISR, the MME indicates to the UE that ISR is activated in the TAU accept message.
In the TA update procedure, the SGSN, MME and Serving GW communicate their capabilities to support ISR, and if all
the nodes support ISR, the MME indicates to the UE that ISR is activated in the TAU accept message.
Circuit-Switched Fallback (CSFB) is an alternative solution to using IMS and SRVCC to provide voice services to
users of LTE. The IMS is not part of the solution, and voice calls are never served over LTE. Instead, the CSFB relies
on a temporary inter-system that switches between LTE and a system where circuit-switched voice calls can be served.
users of LTE. The IMS is not part of the solution, and voice calls are never served over LTE. Instead, the CSFB relies
on a temporary inter-system that switches between LTE and a system where circuit-switched voice calls can be served.
The ISR feature must be enabled for the CSFB feature to work, the ISR feature is a license controlled feature.
The LTE terminals 'register' in the circuit switched domain when powered and attaching to LTE. This is handled
through an interaction between the MME and the MSC-Server in the circuit-switched network domain over the SGs
interface.
through an interaction between the MME and the MSC-Server in the circuit-switched network domain over the SGs
interface.
Consider the following scenarios:
Voice calls initiated by the mobile user: If the user makes a voice call, the terminal switches from a LTE system
to a system with circuit-switched voice support. Depending on where the UE latches on after completion of the
voice call:
voice call:
The packet-based services that are active on the end-user device at this time are handed over and
continue to run in a system with circuit-switched voice support but with lower data speeds.
OR
The packet-based services that are active on the end-user device at this time are suspended until the
voice call is terminated and the terminal switches back to LTE again and the packet services are
resumed.
resumed.
Voice calls received by the mobile user: If there is an incoming voice call to an end-user that is currently
attached to the LTE system, the MSC-Server requests a paging in the LTE system for the specific user. This is
done through the SGs interface between the MSC Server and the MME. The terminal receives the page, and
temporarily switches from the LTE system to the system with circuit-switched voice support, where the voice
call is received. Once the voice call is terminated, the terminal switches back to the LTE system.
done through the SGs interface between the MSC Server and the MME. The terminal receives the page, and
temporarily switches from the LTE system to the system with circuit-switched voice support, where the voice
call is received. Once the voice call is terminated, the terminal switches back to the LTE system.