Page 163 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 149

The RNC is connected to the core network via the Iu interface. As shown in Figure 3.1,
UMTS continues to use independent circuit‐switched and packet‐switched networks
for the following services:
For voice and video telephony services, the circuit‐switched core network, already
known from GSM, continues to be used. The MSC therefore remains the bridge between
the core and access network. Owing to the new functionalities offered by UMTS like,
for example, video telephony, a number of adaptations were necessary on the interface
that connects the MSC to the radio network. While in GSM, the Transcoding and Rate
Adaptation Unit (TRAU) was logically part of the radio network, it was decided to put
this functionality into the core network for UMTS. This was done because even in
GSM, the TRAU is physically located near the MSC to save transmission resources, as
described in Section 1.5. In the Release 4 BICN network architecture, the UMTS TRAU
is part of the MGW. The interface between the MSC/TRAU and RNC has been named
Iu(cs), which indicates that this interface connects the radio network to the circuit‐
switched part of the core network. The Iu(cs) interface therefore corresponds to the
GSM A‐interface and reuses many functionalities on the higher layers for MM and CC.
The BSSMAP protocol, which is used on the GSM A‐interface, has been enhanced
and modified for UMTS and renamed Radio Access Network Application Part
(RANAP). In the standards, RANAP is described in 3GPP TS 25.413 [7] and forms the
basis for MM, CC and the SM. Furthermore, RANAP is used by the MSC and SGSN for
requesting the establishment and clearing of radio bearers (RABs) by the RNC.
In practice, the same MSC can be used with both the UTRAN (via the Iu(cs) inter-
face) and the GSM radio network (via the A‐interface). With GSM and the A‐interface,
the MSC can only handle 12.2 kbit/s circuit‐switched connections for voice calls and 9.6
or 14.4 kbit/s channels for data calls. With UMTS and the Iu(cs) interface, the MSC is
also able to establish 64 kbit/s circuit‐switched connections to the RNC, which equals
the speed of an ISDN B‐channel. This functionality was mainly used for circuit‐switched
video telephony but has since been replaced with IP‐based video telephony by network
operators or Internet‐based companies. The Iu(cs) interface is, like all other interfaces
in the UTRAN, based on ATM or IP and can thus use a number of different transmis-
sion technologies.
All packet‐switched services, which in most cases require a connection to the Internet,
are routed to and from the core network via the Iu(ps) interface. The functionality of
this interface corresponds to the GSM/GPRS Gb interface, which was described in
Chapter 2. SGSNs usually support both the Gb and Iu(ps) interface in a single node,
which allows the use of only a single SGSN in a region to connect both types of radio
network to the packet‐switched core network.
Similar to the Iu(cs) interface, the higher‐layer GSM/GPRS signaling protocols were
reused for UMTS and only slightly enhanced for the new capabilities of the radio net-
work. For the lower layers, however, ATM or IP are used instead of the old Frame Relay
protocol.
The handling of user data has changed significantly for the SGSN with UMTS. In the
GSM/GPRS system, the SGSN is responsible for processing incoming GTP packets
from the GGSN and converting them into a BSSGP frame for transmission to the cor-
rect PCU and vice versa. In UMTS, this is no longer necessary as the SGSN can forward
the GTP packets arriving from the GGSN directly to the RNC via an IP connection and
can send GTP packets it receives from the RNC to the GGSN. The UMTS SGSN is thus