3.2 UMTS Network Structure
3.2.2 Core network
The core network is divided into two domains: the domain of packet switching and circuit switching. The circuit switched domain provides the main element Switching Center Mobile Services (MSC, Mobile Switching Center), while the packet domain covers the main elements of the Service Support Node GPRS (SGSN, Serving GPRS Support Node) Node Server and Service Support GPRS (SGSN).
The following describes these and other elements that make up the core network:
• Switching Center Mobile Services (MSC) is the central element of the Core Circuit Switched (CS-CN, Circuit Switching Core Network). The MSC of the GSM network to 3G can be used that allows updates to comply with the requirements of 3G. This element is connected to access networks of GSM and UMTS, with the Public Switched Telephone Network (PSTN, Public Switched Telephone Network), as well as with other MSC, SGSN and the various registers of the core of the network (HLR, EIR), among others.
• Visitor Location Register (VLR, Visitor Location Register): This is a temporary database that contains all information from users who are present at any given time in the location area controlled by the VLR. Overall MSC and VLR are physically combined. Among its main features are, allow authentication of the mobile and also connects to other VLR and HLR through the network signaling system.
• Local Location Register (HLR, Home Location Register) contains a permanent record of subscriber data. While the records are temporary VLR in the HLR are permanent, although they handle almost the same information.
• Equipment Identification Register (EIR, Equipment Identify Register): Stores Identities international Mobile Station Equipment (IMEI, International Mobile Equipment Identities). Usually contains three lists to indicate the status of equipment: IMEI of computers that are authorized to operate normally (white list), stolen equipment and therefore are prevented from connecting to the network (black list) and finally the gray list are registered with any equipment malfunctions occur.
• Authentication Center (AuC, Authentication Center) is associated with the HLR. Authentication keys stored subscriber and International Mobile Subscriber Identity (IMSI).
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• Server Node Service Support GPRS (SGSN) is the principal element in the packet switching network, which contains subscription information and user location.
• Support Node Gateway GPRS (GGSN, Gateway GPRS Support Node):
Makes the interface between the packet core with external data networks, such as the Internet.
The 3GPP Release 5 brings significant changes to the core architecture of the network. The next figure shows the network architecture, Release 5:
Figure 14 - 3GPP Release 5 architecture
In this new architecture incorporates a control architecture known as IMS. At the same time as bringing new elements such as Base subscriber server (HSS, Home Subscriber Server), which functions as an HLR evolved, and is also the element of connection between IMS and the packet switched domain.
For its part, this new architecture the MSC is divided into two entities, the media gateway (MGW) and a MSC server. The control logic is performed by the MSC, while the switching is done MGW. This separation allows the network to make use of more efficient routes for the transmission of high-speed data, while control messages may follow other routes.
For his part, Release 5 incorporates an all-IP network, which means that all traffic, including voice, is carried as IP packets.
The next figure shows the IMS architecture:
Figure 15 - Architecture of the IMS domain
In the domain of IMS data traffic is transported through SGSN and GGSN. For his part, HSS combines and performs the functions that make the HLR and AuC. On the other hand comes a function element called Call Session Control (CSCF Call
Session Control Function), which is the central element in IMS. There are three types of CSCF:
• Servant CSCF (S-CSCF): Provides the session control services for the computer terminal. These include the decision of routing and session establishment, maintenance and release of multimedia sessions. It also generates information charges for the billing system;
• Proxy CSCF (P-CSCF) is the first entity of IMS is contacted by the user's
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• Interrogating CSCF (I-CSCF) is the point of contact within the operator's network for all connections destined for that network subscribers or subscribers are roaming.
Finally, other elements within the domain of IMS are:
• Disengage Control Function in Gateway (BGCF, Breakout Gateway Control Function): selects the network in which the domain interoperability and circuit-switched PSTN is going to happen. If given in the same network BGCF select a MGCF responsible for such interoperability. If another network with BGCF redirects the session signaling to another BGCF in the net.
• Function Control Media Gateway (MGCF, Media Gateway Control Function) is an entity that is responsible for interoperability. Perform conversion of
protocols between the PSTN and IMS protocols call.
• Processor Media Resource Function (MRFP, Multimedia Resource Function Processor): handles the bearer channels and can handle different flows of information.
• Controller Media Resource Function (MRFC, Multimedia Resource Function Controller) controls the flow of information resources in the MRFP, a task accomplished by interpreting information that comes from application servers, S-CSCF and the MRFP.
• Media Gateway Intermediate (IM-MGW, Intermediate Media Gateway):
terminates bearer channels from a circuit network and information flows in a packet network. MGFC interacts with resource control, in addition to owning and managing other resources such as echo cancellers.
• Subscriber Locator Function (SLF Subscription Locator Function): it is only necessary when there are plenty of HSS bodies on a network.
• Application Server (AS, Application Server) offers value-added services and can reside either on the local network or in a third location.
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4 Standardization and technical requirements according to 3GPP LTE
Based on the above shown in the following chapter provides the key technical requirements and standardization proposed to move towards LTE.