Composition of WCDMA Systems
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The Universal Mobile Telecommunication System (UMTS) is a 3G mobile communication system adopting WCDMA air interface. Therefore, the UMTS is usually called a WCDMA system.

In terms of functions, the network units comprise the Radio Access Network (RAN) and Core Network (CN).

The RAN accomplishes all the functions related to radio communication.

The CN handles the exchange and routing of all the calls and data connections within the UMTS with external networks. The RAN, CN, and the User Equipment (UE) together constitute the whole UMTS.

UE (User Equipment )

The UE is an equipment which can be vehicle installed or hand portable.

Through the UU interface, the UE exchanges data with network equipment and provides various CS domain and PS domain services, including common voice services, broadband voice services, mobile multimedia services, and Internet applications (such as E-mail, WWW browse, and FTP).

UTRAN (UMTS Terrestrial Radio Access Network )

The UMTS Terrestrial Radio Access Network (UTRAN) comprises Node B and Radio network Controller (RNC).

1)  Node B1.

As the base station (wireless transceiver) in the WCDMA system, the Node B is composed of the wireless transceiver and baseband processing part.

Connected with the RNC through standard Iub interface, Node B processes the Un interface physical layer protocols.

It provides the functions of spectrum spreading/despreading, modulation/demodulation, channel coding/decoding, and mutual conversion between baseband signals and radio signaling.

2)  RNC

The RNC manages various interfaces, establishes and releases connections, performs hand off and macro diversity/combination, and manages and controls radio resources.

It connects with the MSC and SGSN through lu interface. The protocol between UE and UTRAN is terminated here.

The RNC that controls Node B is called Controlling RNC (CRNC).

The CRNC performs load control and congestion control of the cells it serves, and implements admission control and code word allocation for the wireless connections to be established.

If the connection between a mobile subscriber and the UTRAN uses many RNS resources, the related RNC has two independent logical functions:

Serving RNC (SRNC). The SRNC terminates the transmission of subscriber data and the Iu connection of RANAP signaling to/from the CN.

It also terminates the radio resource controlling signaling (that is the signaling protocol between UE and UTRAN).

In addition, the SRNC performs L2 processing of the data sent to/from the radio interface and implements some basic operations related to radio resources management.

Drift RNC (DRNC) All the other RNCs except the SRNC are DRNCs. They controls the cells used by the UEs.

CN (Core Network)

The CN is in charge of the connections with other networks as well as the management and communication with UEs. The CN can be divided into CS domain and PS domain from the aspect of logic.

The CS domain equipment refers to the entities that provide circuit connection or related signaling connection for subscriber services. The specific entities in the CS domain include:

  1. Mobile switching center (MSC)
  2. Gateway mobile switching center (GMSC)
  3. Visitor location register (VLR)
  4. Interworking function (IWF).

The PS domain provides packet data services to subscribers. The specific entities in the PS domain include:

  1. Serving GPRS support node (SGSN)
  2. Gateway GPRS support node (GGSN)

Other equipment such as the home location register (HLR) or HSS, authentication center (AuC), and equipment identity register (EIR) are shared by the CS domain and PS domain.

The major functional entities are as follows:


As the functional node in the CS domain of the WCDMA core network, the MSC/VLR connects with the UTRAN through

Iu CS interface, with external networks (PSTN, ISDN, and other PLMNs) through PSTN/ISDN interface, with the HLR/AUC through C/D interface, with the MSC/VLR, GMSC or SMC through  E interface, with the SCP through CAP interface, and with the SGSN through Gs interface.

The MSC/VLR accomplishes call connection, mobility management, authentication, and encryption in the CS domain.

2)  GMSC

As the gateway node between the CS domain of WCDMA network and external networks, the GMSC is an optional entity.

It connects with the external networks (PSTN, ISDN, and other PLMNs) through PSTN/ISDN interface, with the HLR through C interface, and with the SCP through CAP interface.

The GMSC accomplishes the incoming and outgoing routing of the Visited MSC (VMSC).

3)  SGSN

As the functional node in the PS domain of WCDMA core network, the SGSN connects with the UTRAN through Iu_PS interface, with GGSN through Gn/Gp interface, with the HLR/AUC through Gr interface, with the MSC/VLR through Gs interface, with the SCP through CAP interface, with the SMC through Gd interface, with the CG through Ga interface, and with the SGSN through Gn/Gp interface.

The SGSN accomplishes the routing forward, mobility management, session management, authentication, and encryption in the PS domain.

4)  GGSN

The GGSN connects with the SGSN through Gn interface and with the external data networks (Internet /Intranet) through Gi interface.

The  GGSN provides routes to the data packets between the WCDMA network and external data networks, and encapsulates these data packets.

The major function of the GGSN is to provide the interface to the external IP packet-based network, thus the UEs can access the gateway of the external packet-based network.

To the external networks, the GGSN seems like the IP router that can be used to address all the mobile subscribers in the WCDMA network.

It exchanges routing information with external networks.

5)  HLR

The HLR connects with the VMSC/VLR or GMSC through C interface, with the SGSN through Gr interface, and with the GGSN through Gc interface.

The HLR stores subscriber subscription information, supports new services, and provides enhanced authentication.


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