Page 290 - From GMS to LTE

276 From GSM to LTE-Advanced Pro and 5G

On the packet‐switched side of the GSM or UMTS network, the mobile device per-
forms a routing area update. The request includes information on the previously used
MME/S‐GW, which, from the point of view of the GSM/UMTS SGSN, is another SGSN.
On the basis of this information, the SGSN derives the IP address of the previous SGSN
(MME) and requests the subscriber’s current context over the Gn interface, as shown in
Figure 4.23. From the SGSN’s point of view, this is a standard inter‐SGSN routing area
update procedure, which is also used to retrieve the context of a subscriber who has
performed a cell reselection from a GSM or UMTS cell connected to a different SGSN.
With the information provided in the MME, the SGSN then authenticates the sub-
scriber and requests a user data tunnel modification from the PDN‐GW, which acts as
a GGSN over the Gn interface. The procedure is finished with an update of the sub-
scriber’s location in the HLR and confirmation of the routing area update to the mobile
device. The device then goes back to the idle state. The details of this procedure are
described in 3GPP TS 23.060 [22].
When an LTE‐capable mobile device roams through a GSM, UMTS or CDMA net-
work, it should return to an available LTE network as soon as it roams into an LTE‐cov-
ered area. Again, System Information messages are used to inform the mobile device of
nearby LTE cells. In GSM, this information is broadcast in SIB 2‐quarter messages and
in UMTS in SIB 19. Again, usage priorities can be used to steer the network selection,
in addition to taking signal strength into account, so that LTE cells with a lower signal
level can be preferred to cells of other radio technologies.
Once the mobile device performs a cell reselection to an LTE cell, it performs a track-
ing area update procedure with the MME. In the Tracking Area Update message, the
mobile device includes information about the last used location and the routing area so
that the MME can determine the previously used SGSN. It will then contact the SGSN
over the Gn interface to request the subscriber’s context to preserve the session. The
2G/3G SGSN will again see this as a standard inter‐SGSN routing area update. Once the
subscriber has been authenticated the MME contacts a Serving‐GW and the PDN‐GW
that has so far acted as an SGSN to redirect the core network user data tunnel. Finally,
the MME also contacts the HSS to inform it of the new location of the subscriber.
Afterward, the tracking area update is confirmed to the mobile device and it can return
to RRC‐idle state.

4.9.2 RRC Connection Release with Redirect between LTE and GSM/UMTS
While the mobile device is in the LTE RRC connected state, the network is responsible
for mobility management. The network needs to coordinate times during which the
mobile device is not available to receive data in the downlink direction because of meas-
urements on other channels and on other frequency bands. Network control of the
mobility management process in the connected state is also important so that no data is
lost when the mobile is handed over to a different cell.
Depending on the capabilities of the mobile device and the network, the eNode‐B can
instruct the mobile device to start a search for neighboring LTE, UMTS and GSM cells
on other channels and frequency bands once the mobile device reports deteriorating
signal quality. The eNode‐B then sends the mobile device a list of frequencies and bands
in which to search for other cells. Furthermore, a transmission gap pattern in which the
measurements are to be performed is given. The pattern can be adapted to the number