Iu-PS Interface

The protocol architecture for the Iu-PS interface
is shown in Figure 6-20.We first notice that no transport network control
protocol is involved. It is not needed because of the protocol that is used in
the user plane. Specifically, in the user plane, we find that the GPRS Tunneling
Protocol (GTP) tunnel extends to the RNC. This is different than
standard GPRS where the tunnel ends at the SGSN and a special Gb interface
is used from SGSN to BSC. The fact that the tunnel extends to the
RNC means that only a tunnel identifier and IP addresses for each end are required for establishment of the bearer. These are included in the application
messages used for establishment of the bearer, which means that no
intermediate ALCAP is needed.

As mentioned, the user plane uses the GTP (GTP-U indicates a GTP user
plane). This protocol uses the User Datagram Protocol (UDP) over IP.AAL5
over ATM is used as the transport. For a packet data transfer, the identification
of individual user packets is supported within the GTP-U protocol.
Consequently, it is not necessary to structure these user packets according
to ATM cell boundaries. This means that multiple user packets can be multiplexed
on a given ATM cell, thereby reducing ATM overhead.
In the control plane, we again find RANAP at the application layer.We
have a choice of signaling bearer, however. One option is to use the standard
ATM SS7 stack, as described previously, for the Iu-CS interface. Another
option is to use SCCP over IP-based SS7 transport over ATM. For IP-based SS7 transport, we use the MTP3 User Adaptation (M3UA) protocol over the
Stream Control Transmission Protocol (SCTP). Both of these protocols are
described in Chapter 8, “Voice over IP Technology.”