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.
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.
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.
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.
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:
- 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.
- 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.