Page 128 - From GMS to LTE

114 From GSM to LTE-Advanced Pro and 5G

The changes in the circuit core network to support UMTS Release 99 were therefore
mainly software enhancements to support the new Iu(cs) interface between the Mobile
Switching Center (MSC) and the UTRAN. While the Iu(cs) interface is quite similar to
the GSM A‐interface on the upper layers, the lower layers were completely redesigned
and were based on ATM. Furthermore, the Home Location Register (HLR) and
Authentication Center (AuC) software were enhanced to support the new UMTS
features.
The GPRS packet core network, which connects users to the Internet or a company
intranet, was adopted from GSM with only minor changes. No major changes were
necessary for the packet core because GPRS was a relatively new technology at the time
of the Release 99 specification, and was already ideally suited to a high‐speed packet‐
oriented access network. Changes mostly impact the interface between the SGSN and
the radio access network, which is called the Iu(ps) interface. The biggest difference
from its GSM/GPRS counterpart, the Gb interface, is the use of ATM instead of Frame
Relay on lower layers of the protocol stack. In addition, the SGSN software was modi-
fied to tunnel GTP user data packets transparently to and from the RNC instead of
analyzing the contents of the packets and reorganizing them onto a new protocol stack
as was previously done in GSM/GPRS.
As no major changes were necessary in the core network it was possible to connect
the UMTS radio access network (UTRAN) to a GSM and GPRS core network. The
MSCs and SGSNs only required a software update and new interface cards to support
the Iu(cs) and Iu(ps) interfaces. Figure 3.1 shows the network elements of a combined
GSM and UMTS network.

UTRAN R99 core network

RNC
PSTN
Node-B
MSC GMSC
UE RNC

Node-B
HLR IN

GSM BSS
SGSN GGSN Internet
TRAU

BSC
BTS Server
PCU

Data and signaling Signaling
Figure 3.1 Common GSM/UMTS network: Release 99.