Page 306 - From GMS to LTE

292 From GSM to LTE-Advanced Pro and 5G

seconds if both are located in the LTE network and have to perform a fallback first. As
this has a significant impact on user experience, most network operators therefore
chose to upgrade all MSCs in their network over time with SGs capabilities. This
means that the location area the mobile device is registered in while in LTE is the
same as that of the target cell. With this setup, no location update procedure is
required and the CS‐fallback procedure takes only around half a second longer than
a conventional 3G call setup.

The Execution Phase: Mobile‐Originated Call
This procedure is similar to the mobile‐terminated example above. The difference is
that no paging is sent by the network, unlike in the case of an incoming call, and there
is no paging response to the MSC after the device is in the legacy cell.

SMS and Call‐Independent Supplementary Services (CISS)
For receiving SMS text messages, the mobile device can remain in the LTE network as
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the text message is forwarded by the MSC to the MME via the SGs interface and from
there via RRC signaling over the LTE radio network to the mobile device. Sending text
messages works in a similar way and hence there is also no need to fall back to a legacy
network.
For call‐independent supplementary services (CISS) such as changing call forwarding
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configuration, checking prepaid balance via USSD messaging, and so on, a fallback to
the legacy network is required.
While only the support of the SGs interface has to be added to the circuit‐switched
core network, the solution is relatively simple to implement. However, there are a num-
ber of drawbacks for which most network operators have put mitigations in place today
to reduce their impact. These include:
The fallback to a GSM or UMTS network takes several seconds if a location update
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procedure is necessary, which need to be added to an already increased call setup
time compared to fixed‐line networks. This has a negative impact on the overall user
experience compared to fixed‐line networks and mobile voice calls established in
GSM or UMTS networks. This time can be reduced significantly by upgrading all
MSCs to support the SGs interface and to ensure that LTE Tracking Areas and GSM/
UMTS Location Areas overlap as closely as possible.
If a GSM network is used for the voice call, no packet‐switched services can be used
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during the conversation as most GSM networks do not support the dual transfer
mode (DTM) functionality for simultaneous voice and data transmission. In addition,
even if DTM is supported, datarates will be very low compared to those in the LTE
network. To counter this effect, network operators can configure their networks to
prefer a release with redirect to 3G rather than to 2G if both technologies are available
at the location where the redirect is to be performed.
After the voice call, the mobile device has to return to the LTE network, which again
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consumes many seconds, during which time no data transfers can take place. A miti-
gation against this is not to send a standard 3G Radio Bearer Release message but to
include information that the mobile device shall reselect to a given LTE channel.
While not widely used in practice so far, another option is to perform a handover
from 3G back to LTE once the voice call has terminated.