UTRAN Interfaces and Protocols

Figure 6-18 provides a generic model for the terrestrial interfaces used in
UTRAN—the Iu-CS, Iu-PS, Iur, and Iub interfaces. Each interface has two
main components—the radio network layer and the transport network
layer. The radio network layer represents the application information to be
carried—either user data or control information. This is the information
that UTRAN actually cares about. The transport network layer represents
the transport technology that the various interfaces use. In the case of
3GPP Release 1999,ATM transport is used, so the transport network layer
represents an ATM-based transport. Another transport layer could be used
instead. In such a case, the transport network layer would be different, but
the radio network layer should not be.

Looking at Figure 6-18 in the vertical direction, we see three planes—
the control plane, the user plane, and the transport network user plane.
The control plane is used by UMTS-related control signaling. It includes
the application protocol used on the interface in question. The control
plane is responsible for the establishment of the bearers that transport
user data, but the user data itself is not carried on the control plane. As
seen from control plane, the user bearers established by the application
protocol are generic bearers and are independent of the transport technology
being used. If the application protocol were to view the bearers in terms of a specific transport technology, then it would not be possible to
cleanly separate the radio network layer from the transport network
layer. In other words, the application protocol would have to be designed
to suit a particular transport technology. The signaling bearers that carry
the application signaling are established by O&M actions. These signaling
bearers are analogous, for example, to the SS7 signaling links that are
used between a BSC and a MSC in GSM.
The user plane is what carries the actual user data. This data could, for
example, be data packets being sent or received by the UE as part of a data
session. Each data stream carried in the user plane will have its own framing
structure.

The transport network control plane contains functionality that is specific
to the transport technology being used and is not visible to the radio
network layer. If standard pre-configured bearers are to be used by the user
plane and these are known to the control plane, then the transport network
control plane is not needed. Otherwise, the transport network control plane
is used. It involves the use of an Access Link Control Application Part
(ALCAP). This is a generic term that describes a protocol or set of protocols used to set up a transport bearer. The ALCAP to be used is dependent on
the user plane transport technology. 263