3.2 Basic Structure of UTRAN

UTRAN includes one or several Radio Network Subsystems (RNSs). A RNS is composed of one RNC and one or several Node Bs. The Iu interface is used between RNC and CN while the Iub interface is adopted between RNC and Node B. Within UTRAN, RNCs connect with one another through the Iur interface. The Iur interface can connect RNCs via the direct physical connections among them or connect them through the transport network. RNC is used to allocate and control the radio resources of the connected or related Node B. However, Node B serves to convert the data flows between the Iub interface and the Uu interface, and at the same time, it also participates in part of radio resource management.

3.2.1  System Interfaces

UTRAN has the following main interfaces:

1. Cu interface

The Cu interface is the electrical interface between the USIM card and ME, and it adopts the standard interface.

2. Uu interface

The Uu interface is the radio interface of WCDMA. UE accesses the fixed network of the UMTS system through the Uu interface, so we can say the Uu interface is the most important open interface in the UMTS system.

3. Iur interface

The Iur interface is the interface connecting RNCs. It is specific to the UMTS system for mobility management of UEs in RAN. For example, when different RNCs perform soft handover, all UE data are transmitted from the working RNC to the candidate RNC through the open standard Iur interface.

4. Iub interface

The Iub interface is an open standard interface connecting Node B and RNC. It allows RNC to connect to NodeB from another equipment manufacturer.  

5. Iu interface

The Iu interface is the interface between UTRAN and CN. Similar to the A interface and the Gb interface in the GSM system, it is also an open standard interface. It allows different vendor’s UTRAN and CN to connect together, and can be divided into the Iu-CS interface and the Iu-PS interface.

3.2.2  Basic Protocol Structure of UTRAN Interfaces

The protocol structure of UTRAN interfaces is designed according to a universal protocol model. The principle of design is that logically the layer and the plane should be independent. If necessary, you can modify a part of the protocol structure without modifying other parts, as shown in Figure 3-4.

Horizontally, the protocol structure contains the radio network layer and the transport network layer. All protocols related to UTRAN are contained in the radio network layer. The transport network layer is the standard transmission technique adopted by UTRAN, and it has nothing to do with the specific functions of UTRAN.
Vertically, it contains the control plane and the user plane.
The control plane contains application protocols (RANAP in the Iu interface, RNSAP in the Iur interface and NBAP in the Iub interface) and signaling bearers to transmit these application protocols. Application protocols are used to build the bearers to UEs (For example, radio access bearer in the Iu interface, radio links in the Iur and Iub interfaces). These signaling bearers of these application protocols can be the same as or can differ from those of the Access Link Control Application Protocol (ALCAP), and they are established through O&M.
The user plane contains data flows and data bearers to carry these data flows. All information (such as voice and data) received or sent by UEs is transmitted through the user plane. The transport network control plane is located between the control plane and the user plane, and it is just in the transport layer, so it does not contain any information about the radio network control plane. It contains ALCAP and the signaling bearer required by ALCAP. ALCAP establishes the transport bearer for the user plane. By adopting the transport network control plane, the application protocol implementation of the radio network plane can be independent from the technique selected for the data bearer of the user plane.
In the transport network, the transport bearer of the data plane in the user plane is built in such a way: Application protocols in the control plane conduct signaling processing first, which triggers the establishment of data bearer in the data plane through ALCAP. However, not all types of data bearers should be established through ALCAP. Without signaling processing of ALCAP, the transport network control plane is not needed, so the pre-configured data bearer should be used instead. The signaling bearer of ALCAP can be the same as or can differ from that of the application protocol. Usually, the ALCAP signaling bearer is established through O&M operations.
The data bearer of the user plane and the signaling bearer of the application protocol both belong to the user plane of the transport network. In real-time operations, the data bearer of the transport network user plane is controlled directly by the transport network control plane. However, the control operation required for establishing the signaling bearer of the application protocol belongs to O&M operations.
In conclusion, UTRAN obeys the following principles:
1)      The signaling plane is separated from the data plane.
2)      UTRAN/CN functions are separate from the transport layer, that is, the radio network layer does not depend on the specific transmission technique.
3)      Macro diversity (FDD Only) is processed totally by UTRAN.
4)      The mobility management of RRC connections is processed totally by UTRAN.

3.2.3  Functions Implemented by UTRAN

1)      Functions related to overall system access control
l   Admission control
l   Congestion control
l   system information broadcast
2)      Functions related to security and confidentiality
l   Encryption/decryption of radio channels
l   Protection of message integrity
3)      Functions related to mobility
l   Handover
l   SRNS relocation
4)      Functions related to radio resource management and control
l   Radio resource configuration and operation
l   Radio environment survey
l   Macro diversity control (FDD)
l   Connection and release of radio bearers (RB control)
l   Allocation and cancellation of radio bearers
l   Dynamic channel allocation (TDD)
l   Radio protocol function
l   RF power control
l   RF power setting
5)      Timing advance setting (TDD)
6)      Radio channel coding
7)      Radio channel decoding
8)      Channel coding control
9)      Initial (random) access detection and processing
10)    CN distribution of NAS messages

3.2.4  RNC (Radio Network Controller)

RNC is used to control the radio resources of UTRAN. Usually, it connects with the CS domain (MSC), the PS domain (SGSN) and the broadcast domain (BC, not indicated in the figure) through the Iu interface. The RRC protocol between UE and UTRAN terminates here. Logically, RNC corresponds to the Base Station Controller (BSC) in the GSM network.
The RNC used to control Node B is called the Controlling Radio Network Controller (CRNC) of Node B, which is responsible for managing the radio resources of the cell under its control.
If more than one RNS is used to connect a UE with UTRAN, all the relevant RNSs can be divided into the following types:
l   Serving RNS (SRNS): Managing the radio connection between UE and UTRAN, It corresponds to the termination point of this UE’s Iu interface (Uu interface). All basic radio resource management is implemented by SRNC in SRNS, for example, mapping the radio access bearer parameters to the parameters of transport channels, cell handover and open loop power control. A UE connected with UTRAN should have one and only one SRNC.
l   Drift RNS (DRNS): Any RNS except the SRNS used by UE is called the DRNS. Its corresponding RNC is DRNC. A subscriber can have none, one or several DRNSs.
Usually, the actual RNC contains all functions of CRNC, SRNC and DRNC.

3.2.5  Node B

Node B is the base station (i.e. radio transceiver) of the WCDMA system, and it interconnects with RNC through the standard Iub interface to process the physical layer protocols of the Uu interface. Its main functions include: Spreading/de-spreading, modulation/demodulation, channel coding/decoding, and conversion between baseband signals and RF signals. Meanwhile, it implements such radio resource management functions as inner loop power control. Logically, it corresponds to the Base Transceiver Station (BTS) in the GSM network. 

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