As shown in above figure, PLMN is logically divided into core and access network.CN is further divided into CS domain, PS domain and IM Subsystem while AN is divided into BSS (if GSM) or UTRAN (known as RAN) or E-UTRAN.
Some basic functionality of these network, domain and entities are as below.
CS domain:
A "CS type of connection" is a connection for which dedicated network resources are allocated at the connection establishment and released at the connection release.
The entities specific to the CS domain are: MSC, GMSC, VLR.
PS domain:
A "PS type of connection" transports the user information using autonomous concatenation of bits called packets: each packet can be routed independently from the previous one.
CS and PS domains are overlapping, i.e. they contain some common entities. A PLMN can implement only one domain or both domains.
The entities specific to the PS domain are the GPRS specific entities, i.e. SGSN and GGSN and EPS specific entities, i.e. PDN GW, S-GW, MME, SGSN.
IMS:
The IM subsystem comprises all CN elements for provision of IP multimedia services comprising audio, video, text, chat, etc. and a combination of them delivered over the PS domain. The entities related to IMS are CSCF, MGCF, MRF, etc.
To enable communication to a mobile station the network must know where this mobile station is located. This information is stored in a function named location register.
The location register is handled by the following entities.
- The Home Location Register (HLR).
The Home Location Register (HLR) is the location register to which a mobile subscriber is assigned for record purposes such as subscriber information. For EPS, the HLR functionality is provided via HSS.
- The Visitor Location Register (VLR).
The Visitor Location Register (VLR) is the location register for Circuit Switched (CS) services, other than the HLR, used by an MSC to retrieve information for, e.g. handling of calls to or from a roaming mobile station currently located in its area.
- The Serving GPRS Support Node (SGSN).
The location register function in the SGSN stores subscription information and location information for Packet Switched (PS) services for each subscriber registered in the SGSN.
The SGSN is needed only in a PLMN which supports PS Domain with GERAN or UTRAN access.
- The Gateway GPRS Support Node (GGSN).The location register function in the GGSN stores subscriber information and routeing information (needed to tunnel packet data traffic destined for a GPRS MS to the SGSN where the MS is registered) for each subscriber for which the GGSN has at least one PDP context active.
The GGSN is needed only in a PLMN which supports GPRS with GERAN or UTRAN access.
- The Mobility Management Entity (MME).
The location register function in the MME stores subscription information and location information for Packet Switched (PS) services for each subscriber registered in the MME for EPS.
- The Packet Data Network Gateway (PDN GW).
The location register function in the PDN GW stores subscriber information and routeing information (needed to tunnel packet data traffic destined for an EPS UE to the Serving GW, where the UE is registered in the MME, in the SGSN or in the 3GPP AAA server in case of non-3GPP access) for each subscriber for which the PDN-GW has at least one PDN connection active.
fig (b) is showing PLMN supporting CS and PS services (using GPRS and EPS) and interfaces for that service support.
reference: 3gpp: TS 23.002 v9.2.0
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