Page 305 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 291

located. Since this is only a theoretical value, it has to be derived from the Tracking
Area Identity (TAI), which is the corresponding identifier in LTE. In practice, this
creates a dependency between the TAI and the LAI, that is, the location areas that
describe a group of base stations in 2G/3G, and LTE must be configured in a
geographically similar way for the fallback to work later on.

The Execution Phase: Mobile‐Terminated Call
When a circuit‐switched call for a subscriber arrives at the MSC, it signals the incom-
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ing call via the SGs interface to the MME, which is, in its eyes, a 2G or 3G SGSN.
From here, the notification is forwarded to the mobile device. From the MSC point of
view, this is a legacy procedure that already exists.
If the mobile is in RRC idle state when the voice call is signaled, the MME has to page
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the mobile device to reestablish radio contact. Once contact has been reestablished,
it forwards the information about the call.
If the mobile is in RRC connected state, the MME can forward the request imme-
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diately. If the mobile wants to receive the call, it signals to the MME that it would
like to be transferred to the 2G or 3G network in which it can receive the call. The
MME then informs the eNode‐B that the mobile has to be transferred to the 2G/3G
network.
The standard contains two options as to how to proceed at this point. The first option
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is to release the connection and redirect the device to the 2G or 3G network. The
second option that is also used in practice today is to pre‐establish a channel in a 3G
cell for the device and then to perform a handover procedure. This way there is only
a very short interruption of an ongoing packet data transfer.
In case of a release with redirect the eNode‐B usually includes information about
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a 3G target frequency or a list of GSM channels to speed up the device’s acquisi-
tion of a target cell. In case a handover procedure is performed the eNode‐B
configures inter‐RAT measurements and waits for a measurement report from
the mobile device. If a suitable cell has been found the MME is contacted, which
then forwards the request to prepare a channel in the target cell to the UMTS
network. Once the 3G channel is established the MME informs the eNode‐B
which in turn sends a ‘MobilityFromEUTRAN’ command that contains all infor-
mation necessary for the mobile device to find the target cell and the prepared
channel.
Once the mobile device is in the 2G or 3G cell, it answers to the initial paging via the
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legacy cell. There are two variants of the procedure in the case where a release with
redirect was used depending on how the core network is set up. While introducing
LTE, many network operators chose to introduce an additional MSC to the network
with an SGs interface but without connectivity to the radio network. The advantage
of this approach is that existing MSCs do not have to be updated for the launch of
LTE. In this case, the location area of the target 2G or 3G cell is different from the one
in which the mobile device is registered. As a consequence, the mobile device needs
to perform a location area update that triggers a forwarding of the call from the core
network from the SGs MSC to the MSC that controls the target cell. The disadvan-
tage of this approach is that the procedure increases the call setup time by around 2.5
seconds. The call establishment time of a call between two mobile devices thus
increases from around 5 seconds if both devices are located in a 3G network to 10