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.
Комментариев нет:
Отправить комментарий