Page 180 - From GMS to LTE

166 From GSM to LTE-Advanced Pro and 5G

Radio Network Subsystem (SRNS) Relocation Request. If the core network components
agree to perform the change, the D‐RNC becomes the new S‐RNC and the resources on
the Iur interface can be released.
An SRNS relocation is also necessary if a handover needs to be performed due to
degrading radio conditions and no Iur connection is available between two RNCs. In
this case, it is not the optimization of radio network resources that triggers the procedure
but the need to maintain the radio bearer. Along with the SRNS relocation it is neces-
sary to perform a hard handover into the new cell, as a soft handover is not possible due
to the missing Iur interface.
When the first GSM networks were built at the beginning of the 1990s, many earlier‐
generation networks already covered most parts of the country. The number of users
was very small though, so it was not immediately necessary to reach the same coverage
area with GSM as well. When the first UMTS networks became operational, the situation
had changed completely. Owing to the enormous success of GSM, most people in
Europe already possessed a mobile phone. As network deployment is a lengthy and
costly process it was not possible to provide countrywide coverage for UMTS right
from the start. Therefore, it was necessary to ensure seamless integration of UMTS into
the existing GSM infrastructure. This meant that right from the beginning the design of
UMTS mobile devices had to incorporate GSM and GPRS. Thus, while a user roams in
an area covered by UMTS, both voice calls and packet data are handled by the UMTS
network. If the user roams into an area that is only covered by a 2G network, the mobile
device automatically switches over to GSM, and packet‐switched connections use the
GPRS network. In order not to interrupt ongoing voice or data calls, the UMTS stand-
ards also include procedures to hand over an active connection to a 2G network. This
handover procedure is called intersystem handover (see Figure 3.30).

Figure 3.30 3G to 2G handover.
PSTN
MSC

Iu-interface A-interface

RNC BSC

UMTS GSM
cells cells