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6.19.5 Iu interface

The structure of the Iu interface is split into two parts since the Iu connects to the circuit
and packet domains of the core network. The Iu-CS, as shown in Figure 6.105, connects
to the 3G MSC and is much the same as the Iur interface. Again, the transport and
radio network control plane are built on top of an ATM and broadband SS7 signalling
stack, with the RANAP protocol sitting on the SCCP layer to provide connection-oriented
and connectionless services. As before, user data is transported using AAL2 and thus
the AAL2 signalling stack is required for transport network control, i.e. establishment
of user plane AAL2 connections. The user data is placed in the Iu frame protocol, as
described previously.
To place this in context with the radio network protocol stack, consider the following
diagrams, which illustrate the complete connection for control and data. Figure 6.106


Radio Transport User
Network Network Plane
Control Plane Control Plane
Radio
Network
Layer
RANAP User Data




Q.2630.1

SCCP Q.2150.1

MTP3b MTP3b

SSCF-NNI SSCF-NNI
Transport
Network
SSCOP SSCOP
Layer

AAL5 AAL5 AAL2


ATM


Physical Layer




Figure 6.105 Iu-CS protocol stack
380 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM




UE BTS RNC 3G MSC

MM/CM L3 Signalling e.g. Location Updating

RRC (Direct Transfer) RANAP (Direct Transfer)



RRC TBs
RLC FP SCCP/MTB3b
MAC AAL2 SSCOP/AAL5
ATM ATM
WCDMA E1/STM-1 STM-1


Figure 6.106 Signalling protocol stack


shows the signalling control protocol stack, which is established for the transfer of L3
signalling messages between the UE and core network, in this case, the MSC (however,
the stack is the same towards the SGSN). The signalling connection is made up of
two parts, the RRC connection and the RANAP connection. Between the UE and the
RNC, although this is signalling, as far as the UTRAN is concerned, this is viewed as
user data since it is coming from the mobile, and thus is transported over the standard
AAL2 bearer.
For the transport of real-time voice data between the UE and CS-CN, the stack overview
is as shown in Figure 6.107. Between the UE and RNC, the call is transported using the
RLC transparent mode, and then to the MSC using the Iu user plane protocol in support
mode, over AAL2/ATM.
For the connection to the SGSN, the Iu-PS interface is used. The structure of this is
somewhat simpler than the other interfaces, with the absence of any transport network
signalling (Figure 6.108). This is because the user data is transported over a GTP tunnel,
using an IP over ATM connection. As will be seen in the next section, all the relevant
information for this tunnel is sent in RANAP procedures.
Once again, considering an overview of this connection from the UE to the SGSN,
Figure 6.109 shows the stacks. Since this is IP traf¬c to the packet core, the packets
¬rst have their headers compressed using the PDCP protocol, and then, in this example,
use the RLC acknowledged mode to provide reliable delivery across the radio network.
Subsequently, they are transported in a GTP tunnel and by IP over ATM towards the
packet core.
6.19 UMTS TERRESTRIAL RADIO ACCESS NETWORK (UTRAN) 381




UE BTS RNC 3G MSC

Voice Call Data


TBs
RLC-TM FP Iu-UP FP
MAC AAL2 AAL2
ATM ATM
WCDMA E1/STM-1 STM-1


Figure 6.107 Voice call overview


6.19.6 Radio access network application part (RANAP)
The RANAP protocol provides the radio network signalling between the core network
and the radio access network across the Iu interface. RANAP covers procedures for both
the circuit and packet domains of the core network. The general services offered by
RANAP are general control services, noti¬cation services and dedicated control services.
The transport layer is expected to provide connectionless and connection-oriented services
to support RANAP procedures, which it does through the broadband SS7 stack. RANAP
expects the underlying signalling transport to provide in-sequence delivery of messages,
and maintain a connection per active UE.
The key RANAP functions are as follows:

• RAB management: establishment, modi¬cation and release of radio access bearers.
• Serving RNC relocation: shifting of resources and functionality between RNCs.
• Iu connection release: release of all resources related to one Iu connection.
• Iu load control: adjusting of Iu loading and overload handling.
• User paging: offers the core network user paging service.
• Transport of NAS information: the transfer of NAS information, such as layer 3 sig-
nalling, between a user and the core network. An example would be the transfer of
location/routing area update messages.
• Security mode control: transfer of security keys for ciphering and integrity protection
to the radio access network.
• General error reporting: the reporting of general error situations.
382 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM



Radio Transport
User
Network Network
Plane
Control Plane Control Plane

Radio
Network
Layer
RANAP User Data




SCCP

MTP3b GTP-U

SSCF-NNI UDP
Transport
Network
SSCOP IP
Layer

AAL5 AAL5


ATM


Physical Layer




Figure 6.108 Iu-PS protocol stack


Table 6.34 highlights some of the principal RANAP procedures. For a complete de¬-
nition of RANAP, the reader is referred to TS 25.413. This section will address one or
two of the common ones in further detail.


6.19.6.1 Transfer of NAS messages

Two RANAP commands are provided for the transfer of NAS signalling messages. For
both of these, the contents are not interpreted, but merely passed transparently across the
Iu interface. The initial UE message is used to pass initial NAS messages from the UE
to the CN. It is used when no signalling connection exists between the user and core
network. An example of NAS messages that use this would be a location update request
or a CM service request. Since this is an initial message, it not only transfers the NAS
6.19 UMTS TERRESTRIAL RADIO ACCESS NETWORK (UTRAN) 383




UE BTS RNC SGSN

Packet Data


PDCP TBs GTP-U
RLC-UM/AM FP IP
MAC AAL2 AAL5
ATM ATM
WCDMA E1/STM-1 STM-1


Figure 6.109 Packet connection overview


Table 6.34 RANAP procedures
Message name UTRAN procedure
Initial UE message NAS signalling connection establishment
Direct transfer Uplink direct transfer
(of NAS messages) Downlink direct transfer
Radio access bearer Radio access bearer establishment
assignment Radio access bearer release
Radio access bearer modi¬cation
Iu release RRC connection release
UTRAN to GSM/BSS handover
Paging Paging for a UE in RRC idle mode
Paging for a UE in RRC connected mode
Relocation Hard handover with switching in the CN
SRNS relocation
UTRAN to GSM/BSS handover
GSM/BSS to UTRAN handover
Security Security mode procedures


message, but also contains some extra information to support this ¬rst connection. The
additional information is shown in Table 6.35.
Subsequent NAS messages between a UE and CN domain, in either direction, are
transported over the direct transfer message. This is much simpler and just transfers the
NAS message. However, in the downlink direction, the CN will specify the SAPI used
for this message (either SAPI 0 or SAPI 3), indicating a priority level. This will tell the
RNC which signalling bearer to use for RRC transport.
384 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Table 6.35 Initial UE message contents
Initial UE message
Core network domain indicator (CS or PS domain)
Location area ID (LAI)
Routing area (RAC) (conditional on PS domain)
Service area ID (SAI)
NAS message
Iu signalling connection identi¬er (a unique identi¬er allocated by the RNC)
Global RNC-ID


A service area identi¬er (SAI) is a subdivision of a location area. The SAI is de¬ned
as follows:
SAI = LAI + SAC (service area code)
where an LAI = PLMN ID + LAC.

6.19.6.2 RAB establishment, modi¬cation and release
When a user wishes to establish a call or connect to the network, either self-initiated or
as a response to a page, a RAB must be set up between the RNC and the CN to transport
that traf¬c. The RAB assignment request is used by the core network to establish a radio
access bearer. The procedure may request one or more RABs to be established, so there
may be a number of RAB assignment responses (Figure 6.110). The RAB assignment
request is the message that initiates the establishment of a radio link and associated radio
bearers on the Iub/Uu interfaces. The request contains all the necessary QoS parameters
for the RNC to determine the requisite resource allocation for the radio link, and the
requirements on the radio link to meet the QoS.
The assignment request will contain the elements listed in Table 6.36.
Note that for a connection to the PS core, the RAB assignment request contains both

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