2.3 Brief Introduction to the Implementation of Typical 3G Services

2.1.1  CAMEL Phase 3 Intelligent Service

To introduce the intelligent network into the mobile communication system, the European Telecommunications Standards Institute (ETSI) defined CAMEL in Gsm Phase 2+ in 1997 to provide subscribers with service consistency unrelated to the specific service network. The CAMEL feature is not a supplementary service but a network feature. Even the subscriber is not in the HPLMN (Home public land mobile network), the CAMEL feature can be a means of helping network operators provide the subscriber with the specific service.
The network structure of CAMEL Phase3 is shown in Figure 2-3. Several function entities are added into the GSM network: GsmSSF (Service Switching Function), GsmSRF (Specialized Resource Function) and GsmSCF (Service Control Function). CAP Phase3 protocol interface is employed between GsmSCF and GsmSSF, and between GsmSCF and GsmSRF. While an internal protocol interface is used between MSC and GsmSRF, the others use MAP Phase3 interfaces.
The equipment designed specially for GsmSCF implementation is called the SCP, for GsmSSF implementation the SSP, and for GsmSRF implementation the IP.

CAMEL mainly embodies the separation of switching and services. The fundamental idea is as follows: The switch only implements the basic call connection function, but the control of all intelligent services is implemented by another network layer, i.e., the intelligent network. Of them, the Service Switching Function (SSF) implements the switching function, reports various events during the call to the Service Control Function (SCF) and possibly suspends the call, waiting for further instruction of SCF. The triggering points of these events are called the Detection Points (DPs). SCF implements the service logic control function. The essential of the CAMEL mechanism is a control mechanism between SCF and SSF.

2.1.2  LCS

Figure 2-4 shows the network structure of LCS implementation. Here, when MSC/SGSN supports LCS, new interfaces to various network entities are added: The Lg interface between MSC/SGSN and GMLC, the Lh interface between GMLC and HLR, and the Lc interface between GMLC and gsmSCF.

The functions of the LCS system are described as follows:
l   LCS Client
LCS Client is the source of sending location requests, and uses the location result to implement related services based on location. There are four kinds of clients according to the LCS Client functions.
1)      Value-added LCS Clients - Using LCS to support various value-added services, they may include UEs or no specific UEs.
2)      PLMN operator LCS Clients - Using LCS to enhance or support some tasks related to O&M, such as supplementary service, IN related service, bearer service and telecommunication service.
3)      Emergency services LCS Clients - Using LCS enhance the support of emergency calls from the subscribers.
4)      Lawful Interception LCS Clients - Using LCS to implement various legal requests and acceptance services.
l   GMLC (Gateway Mobile Location Center)
GMLC is a gateway device in the network connecting to the external LCS Client. After getting related location request messages through the Le interface, it is responsible for HLR addressing, and delivering the location requests to the SGSN through the Lg interface. GMLC is also responsible for delivering related location results to related LCS Clients, or convents the results into local coordinate information upon request.
MSC/SGSN/VLR mainly implements the coding/decoding of related location information, version negotiation and processing of related signaling protocol information. In addition, it provides interface functions of related signaling tracing, maintenance and management. MSC/SGSN/VLR needs to implement the main processing and control of location procedure, and user privacy protection, and provides charging information according to the processing.
l   HLR
HLR stores the subscription data related to LCS, and provides the MSC number of the located subscriber.
l   Target UE
The Target UE (also referred as MS) is a target mobile phone located. The network needs to locate the current or last location of the mobile subscriber according to the location request. Generally, the target MS is the object to be located. But for MO-LR (Mobile Originated-Location Request), the target MS is the MS that initiates the location request.
l   RNC
In 3G networks, RNC implements the specific locating testing and calculation in LCS implementation.

The external client requests the location information of a target UE/MS from GMLC (or non real-time location information request).
1)      GMLC checks the ID of the client and the requested service, and then gets UE/MS identification from the request information.
2)      GMLC delivers a message to HLR/HSS to query the address of SGSN or MSC/MSC Server. Upon receipt of the needed address, GMLC will deliver a location request to SGSN.
3)      If GMLC belongs to another PLMN, SGSN needs to check whether the LCS request is allowed. Then SGSN will check whether the request can be initiated according to the subscription information of the target UE/MS. If any item fails, SGSN will directly return a failure response. If the check is passed, the SGSN then delivers a location request to RAN.
4)      If RAN stores location information that meets the requirements of SGSN, it returns a location report to SGSN. Otherwise, RAN needs to initiate a special location processing message with the used location method. RAN returns a location information report that SGSN has estimated.
5)      SGSN returns the estimated location information and acquisition time to GMLC.
6)      GMLC returns the location information to the LCS Client. GMLC records the LCS Client CDR and the CDR of SGSN inter-network cooperation.

2.1.3  MMS Service

MMS can run in different types of networks. The terminals can be used in 2G and 3G networks. The MMS Environment (MMSE) includes all necessary service units, such as transfer, storage and notification functions. These service units can be in one network, or in different networks. Figure 2-6 illustrates the structure of the MMS system.

NMS User Agent: As the MMS functional part of the user terminal equipment, it must be able to support the MMS capability.
MMS Server: As the core part, it receives, notifies, dispatches, sends and forwards the multimedia messages. Equivalent to a control center, it dispatches different services. In one MMSE there may be multiple MMS Servers, e.g. MMS Server, E-Mail Server, SMS Server and FAX Server.
MMS Relay: Acting as a bridge between the MMS User Agent and the MMS Server, it eliminates the difference between different servers and between different networks.
MMS User DB: Composed of the MMS Subscription Database, the MMS Profile Database and the HLR, it enables users to flexibly customize services as they wish.
In terms of physical entities, the MMS Server, the MMS Relay and the MMS User DB can be integrated to form a Multimedia Messaging Service Center (MMSC). In this way, the MMSC exists as an independent entity and can be directly superimposed on the existing GPRS network.
In practice, different manufacturers may adopt different networking modes based on their own comprehension of the protocols. Next we will introduce a WAP-based networking mode in the GPRS network. In this mode, the WAP gateway is added between the MMSC and the wireless network to implement the interconnection between these two. Figure 2-7 lists the implementation flow of the multimedia messaging service.

1)      The MS activates the MMS service and sends a message to the MMSC via BTS, BSC, SGSN, GGSN and WAP Gateway in turn.
2)      The MMSC distributes the message according to the terminal type and sends a short message notification to the MS via the SMSC if the type of terminal is an MS.
3)      Upon receipt of the notification, the called accesses the MMSC via the GPRS network and the WAP Gateway, so as to distribute the MMS short message.
4)      If the subscriber does not get the message within the specified time limit, the MMSC forwards the message to the mailbox system.

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