Page 304 - From GMS to LTE

290 From GSM to LTE-Advanced Pro and 5G

however, SMS remains a non‐IP service as the message over the air interface is not
embedded in an IP packet but in an RRC signaling message. As a consequence, no IP‐based
higher‐layer application is required to send and receive SMS messages.
To send and receive SMS messages while in the LTE network, a mobile device has
to inform the MME during the attach procedure of its SMS capabilities. This is done
by setting a flag in the attach message that the MME should also register the mobile
device with the circuit‐switched GSM or UMTS core network. This is also referred
to as a ‘combined attach SMS only’ in the specification documents and is mainly used
today for non‐voice LTE devices such as tablets that can send and receive SMS
messages.
To deliver SMS messages over the SGs interface, the MME registers itself with the
HLR for the delivery of SMS messages during the attach procedure. When a subscriber
sends an SMS message to another subscriber who is currently located in the LTE net-
work, the message is first delivered to the SMS service center. The SMS center then
queries the HLR to get the address of the network node to which the SMS should be
forwarded for delivery over the radio network. In this case, it will receive the address of
the MME and then forward the SMS message to an SGs‐capable MSC. From there, it is
routed over the IP‐based SGs interface to the MME.
If the subscriber is in RRC connected state at the time the SMS is delivered to the
MME, it can be forwarded immediately to the mobile device in an NAS signaling
message. If the mobile device is currently in RRC idle state the MME first needs to page
the device in the last‐reported tracking area. Once the mobile device responds and
a signaling connection to the MME has been reestablished, the SMS is forwarded and
the device returns to RRC idle state.

4.13.2 CS‐Fallback for Voice Calls
In addition to SMS messages, the SGs interface can be used to deliver Paging mes-
sages that inform the mobile device of an incoming call. The call itself, however, is
not delivered over the LTE interface and the mobile device has to fall back to a GSM
or UMTS network where a circuit‐switched connection is then established for the
call. This method of delivering voice calls is therefore referred to as CS (circuit‐
switched) fallback and is executed as follows. Further details can be found in 3GPP
TS 23.272 [26].

The Preparation Phase
When the GSM/UMTS/LTE‐capable device first connects to the EPS (that is, to LTE),
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it indicates to the network that it wants to perform a ‘combined attach’ in the same
way as for the SMS over SGs functionality described above. In practice, this means
that it requests the network to also register its presence in the 2G/3G circuit‐switched
network.
Registration of the mobile in the 2G/3G network is performed on behalf of the mobile
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device by the MME. From the MSC point of view, the MME acts as an SGSN. To the
MSC, the mobile device seems to be attached to the 2G/3G network via an SGSN by
performance of a combined circuit‐switched/packet‐switched location update.
For registration in the network, the MME has to inform the MSC of the 2G/3G
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Location Area Identity (LAI) in which the mobile device is currently ‘theoretically’