3.3 Basic Structure of the Core Network

Logically, CN is divided into the circuit switched domain (CS domain), the packet switched domain (PS domain) and the broadcast domain (BC domain). The CS domain equipment provides circuit service for subscribers, or provides the entities for related signaling connections. Specifically, it covers the following entities: MSC, GMSC, VLR and IWF. The PS domain provides packet data services for subscribers, including the following specific entities: SGSN and GGSN. Other equipment, for example, HLR (or HSS), AuC and EIR are shared by the CS domain and the PS domain.  
The overall structure of the WCDMA network is defined in 3GPP TS 23.002.Now, there are the following three versions:
l   R99 3GPP TS 23.002
l   R4   3GPP TS 23.002
l   R5   3GPP TS 23.002

3GPP began to formulate 3G specifications at the end of 1998 and beginning of 1999. As scheduled, the R99 version would be completed at the end of 1999, but in fact it was not completed until March, 2000. After R99, the version was no longer named by the year. At the same time, the functions of R2000 are implemented by the following two phases: R4 and R5. In principle, the R99 specification is a subset of the R4 specification set. If R99 is added with new features, it will be upgraded to R4. Similarly, the R4 specification set is a subset of the R5 specification set. If R4 is added with new features, it will be upgraded to R5.

For the above three versions, the specific equipment of the PS domain does not change, but only their protocols are upgraded and optimized. The CS domain and GSM network of the R99 version do not fundamentally change. In the R4 network, MSC as the CS domain of the CN is divided into the MSC Server and the MGW, at the same time, a SGW is added, and HLR can be replaced by HSS (not explicitly specified in the specification). In the R5 network, the end-to-end VOIP is supported and the core network adopts plentiful new function entities, which have thus changed the original call procedures. With IMS (IP Multimedia Subsystem), the network can use HSS instead of HLR.

3.3.1  Structure and Interfaces of the R99 Network

To guarantee the investment benefits of operators, the design of R99 network structure has considered fully the 2G/3G compatibility, so as to support the smooth transition of GSM/GPRS/3G. Therefore, the CS domain and the PS domain are parallel in the network. The R99 core network includes the equipment such as MSC/VLR, IWF, SGSN, GGSN, HLR/AuC and EIR. To support 3G services, the corresponding interface protocols are added to some equipment and the original interface protocols are improved.

In R99, the function entities of the CS domain include MSC, VLR and others. According to the different connection modes, an operator can set MSC as GMSC, SM-GMSC or SM-IWMSC. To implement internetworking, IWF (usually working with MSC) is configured in the system.
Besides the above function entities, the specific function entities of the PS domain include SGSN and GGSN to provide packet data services for subscribers. HLR, AuC and EIR are the common devices shared by the CS domain and the PS domain.
The main function entities of R99 include:
1)      Mobile Switching Center (MSC)
MSC is specific to the CS domain to connect the radio systems (including BSS and RNS) and the fixed network. It implements all functions of CS calls, for example, controlling call proceeding, managing the communication services of MS within this network or other networks (PSTN/ISDN/PSPDN and other mobile networks), and providing charging information.
2)      Visitor Location Register (VLR)
VLR is also specific to the CS domain. It stores the information of the registered subscribers that enter the control area, so as to provide necessary data of call connection of mobile subscribers. When an MS roams to a new VLR area, this VLR will initiate the location registration procedure to HLR and get the necessary subscriber data; however, when the MS leaves this control area, the subscriber data should be deleted. Therefore, VLR can be regarded as a dynamic database.
A VLR can manage several MSCs, but usually in implementation, MSC and VLR are combined.
3)      Home Location Register (HLR)
HLR is a device shared by the CS domain and the PS domain, responsible for managing the database system of mobile subscribers. PLMN can contain one or more HLRs, and the detailed configuration mode is determined by subscriber quantity, system capacity and network structure. All mobile subscriber data of the home location area, for example, identity flags, location information and subscribed services, are stored in the HLR.
When a subscriber roams, HLR receives the new location information and requests the previous VLR to delete all data of the subscriber. When a subscriber is called, HLR will provide the routing information.
4)      Authentication Center (AuC)
AuC is a device shared by the CS domain and the PS domain to store the entity of subscriber authentication algorithm and ciphering key. AuC sends authentication and ciphering data to VLR, MSC and SGSN through HLR, to ensure the legality and security of communication. Each AuC associates with the corresponding HLR, and it communicates with others only through this HLR. Usually, AuC and HLR are combined in the same physical entity.
5)      Equipment Identification Register (EIR)
EIR is a device shared by the CS domain and the PS domain. It stores the International Mobile Equipment Identity (IMEI) of mobile devices used by the system. The mobile devices are divided into three levels: white, gray and black, and they are stored in the corresponding lists respectively. Nowadays, the EIR is not yet used in China.
A minimized EIR can only contain the smallest “white list” (the device is in the white level).
6)      Gateway MSC (GMSC)
SMSC is the specific device of CS domain. GMSC, As the interface between the system and other public communication networks, has the function of querying location information. When an MS is called, if the network cannot be sure to which HLR the subscriber belongs, it should query the HLR through GMSC, and then forwards the call to the current registered MSC of the MS.
Specifically, operators can decide which MSCs (e.g. part of MSCs or all MSCs) can be taken as GMSCs,.
7)      Serving GPRS Support Node (SGSN)
SGSN is a piece of equipment specific to the PS domain, and it provides the connection between CN and BSS/RNS of the radio access system. In CN, SGSN and GGSN/GMSC/HLR/EIR/SCP have the corresponding interfaces. SGSN implements the mobility management and session management of packet data services. It manages the mobile and communication services of each MS within the mobile network, and provides the charging information.
8)      Gateway GPRS Supporting Node (GGSN)
GGSN is also a piece of equipment specific to the PS domain. As the interface between the mobile communication system and other public data networks, it has the function of querying location information. When an MS is called, first the data is transmitted to GGSN, then GGSN queries the current location information from HLR and forwards the call to the current registered SGSN. GGSN also provides the charging interface.
The interface protocols of the R99 core network are shown in Table 3-1.
Table 3-1 Interface names and meanings of the R99 core network
Interface name
Connection entity
Signaling and protocol
A
MSC-BSC
BSSAP
Iu-CS
MSC-RNS
RANAP
B
MSC-VLR

C
MSC-HLR
MAP
D
VLR-HLR
MAP
E
MSC-MSC
MAP
F
MSC-EIR
MAP
G
VLR-VLR
MAP
Gs
MSC-SGSN
BSSAP+
H
HLR-AuC


MSC-PSTN/ISDN/PSPDN
TUP/ISUP
Ga
GSN-CG
GTP'
Gb
SGSN-BSC
BSSGP
Gc
GGSN-HLR
MAP
Gd
SGSN-SMS-GMSC/IWMSC
MAP
Ge
SGSN-SCP
CAP
Gf
SGSN-EIR
MAP
Gi
GGSN-PDN
TCP/IP
Gp
GSN-GSN (Inter PLMN)
GTP
Gn
GSN-GSN (Intra PLMN)
GTP
Gr
SGSN-HLR
MAP
Iu-PS
SGSN-RNC
RANAP

3.3.2  Structure and Interface of the R4 Network

Figure 3-6 shows the basic network structure of the PLMN in R4 version. All function entities in the figure can be regarded as independent physical devices. The standards of Nb, Mc and Nc interfaces are included in 23.205 and 29 series of technical specifications.
In actual application, some functions may be combined into the same physical entity, such as MSC/VLR and HLR/AuC, to make some interfaces become internal interfaces.


In R4 version, the function entities (SGSN and GGSN) of the PS domain remain unchanged and so do the external interfaces. The function entities of the CS domain still contain the following devices: MSC, VLR, HLR, AuC, EIR and so on, and the interrelation between them does not change yet. To meet the development requirement of all-IP networks, the entities of the CS domain in R4 version have the following changes:
1.     When necessary, the MSC can be divided into two different entities: MSC Server (for processing signaling only) and Circuit Switched Media Gateway (CS-MGW for processing subscriber data) to implement the MSC function together. The corresponding GMSC is also divided into GMSC Server and CS-MGW.
1)      MSC Server
The MSC Server is composed of the call control module and the mobility control module to process calls of the CS domain. It terminates and converts the user-network signaling into the network-network signaling. It can also contain VLR to process service data of mobile subscribers and the related CAMEL data.
The MSC Server can control part of call status about the connection control module of the CS-MGW media channel through interfaces
2)      Circuit Switched Media Gateway (CS-MGW)
CS-MGW is the transmission termination point of PSTN/PLMN, and it connects with CN and UTRAN through the Iu interface. It may be the termination point of either the bearer channel from the CS network or the media flow from the packet network (such as RTP flow in IP networks). On the Iu interface, CS-MGW can support media conversion, bearer control and payload processing (for example, multi-media digital signal codec, echo cancellation and conference bridge), and also can support different Iu options of CS services (based on AAL2/ATM or RTP/UDP/IP).
CS-MGW:
l   Connecting with MSC server and GMSC server to control resources
l   Owning and using such resources as echo canceller
l   Can be equipped with a multimedia digital signal codec
CS-MGW may have necessary resources to support UMTS/GSM transport media. Furthermore, it can require the H.248 tailor to support multimedia digital signal codec and framing protocol.
The bearer control module and payload processing capability of CS-MGW can be used to support such mobility functions as SRNS reallocation/handover and location services. Nowadays, it is expected that the H.248 standard mechanism can be used to support these functions.
3)      GMSC Server
The GMSC Server is composed of the call control module and the mobility control module of GMSC.
HLR can be updated to become a Home Subscriber Server (HSS). For details, please refer to the R5 network introduction.
In R4, a new entity has been added: Signaling Gateway (SGW)
There are two kinds of singalling transport methods: TDM based tradictional SS7 and IP based siganlling transport network SIGTRAN.
A siganlling gateway is needed to convert the transport layer protocol (i.e. between Sigtran SCTP/IP and SS7 MTP) when two equipments communicate using different signalling transport method. The SGW does not interpret the application layer (e.g. MAP, CAP, BICC, ISUP) messages but may have to interpret the underlying SCCP or SCTP layer to ensure proper routing of the signaling.   
In R4 networks, some interface protocols are also added, as shown in Table 3-2.
Table 3-2 Names and meanings of external interfaces in R4 CN
Interface name
Connection entity
Signaling and protocol
A
MSC-BSC
BSSAP
Iu-CS
MSC-RNS
RANAP
B
MSC-VLR

C
MSC-HLR
MAP
D
VLR-HLR
MAP
E
MSC-MSC
MAP
F
MSC-EIR
MAP
G
VLR-VLR
MAP
Gs
MSC-SGSN
BSSAP+
H
HLR-AuC


MSC-PSTN/ISDN/PSPDN
TUP/ISUP
Ga
SGSN-CG
GTP'
Gb
SGSN-BSC
BSSGP
Gc
GGSN-HLR
MAP
Gd
SGSN-SM-GMSC/IWMSC
MAP
Ge
SGSN-SCP
CAP
Gf
SGSN-EIR
MAP
Gi
GGSN-PDN
TCP/IP
Gp
GSN-GSN (Inter PLMN)
GTP
Gn
GSN-GSN (Intra PLMN)
GTP
Gr
SGSN-HLR
MAP
Iu-PS
SGSN-RNC
RANAP
Mc
(G) MSC Server-CS-MGW
H.248
Nc
MSC Server-GMSC Server
BICC/ ISUP/TUP
Nb
CS-MGW-CS-MGW

Mh
HSS-R-SGW

3.3.3  Structure and Interface of the R5 Network


The structure and interface format of R5 version networks is basically the same as that of R4 networks. However, there are still some differences: In R5, when PLMN includes the IM subsystem, HLR will be replaced by HSS. Furthermore, the A and Iu-CS interfaces are supported simultaneously between BSS, CS-MSC/MSC-Server, and the Gb and Iu-PS interfaces are supported between BSC and SGSN.
For simplicity, R5 interface protocols will not be described here.

1. Home Subscriber Server (HSS)

With the IM subsystem, the network should use HSS instead of HLR.
HSS is the main database of mobile subscribers in the network and stores the related service information to support the call/session processing by the network entities. For example, through authentication, authorization, name/address resolution and location dependency, HSS supports the call control server to implement smoothly such procedures as roaming/routing. 
Similar to HLR, HSS should maintain and manage such subscriber-related information as subscriber identity, address, security, location and subscription services. Based on this information, HSS can support the CC/SM entity of different control systems (CS domain control system, PS domain control system and IM control system). 

HSS can integrate different kinds of information to enhance the service support of application and service domain from the core network, as well as shielding different types of network structures from the upper layer. It supports the following functions: Subscriber control function requested by the IM subsystem, related HLR function subset requested by the PS domain and HLR function of the CS domain (in case that subscribers are allowed to access the CS domain, or roam to the traditional networks)

2. Call Status Control Function (CSCF)

CSCF has the following forms: Proxy CSCF (P-CSCF), Serving CSCF(S-CSCF) or Interrogating CSCF (I-CSCF):
P-CSCF: Serves as the first access point of UE in the IM sub-system.
S-CSCF: Processes the session status in the network.
I-CSCF: Mainly process the route related SIP call request, just like the GMSC function in CS domain.
The Policy Decision Function (PDF) is a logical entity of the P-CSCF. If the PDF is implemented in a separate physical node, the interface between the PDF and the P-CSCF is not standardised.

3. Media Gateway Control Function (MGCF)

l   Controlling part of calling status of the connection control module in the IMS-MGW media channel
l   Communicating with CSCF
l   Selecting CSCF according to the call routing numbers from traditional networks
l   Converting the call control protocols between ISUP and the IM subsystem
l   Receiving out-band information and forwarding it to CSCF/IMS-MGW

4. IP Multimedia Subsystem-Media Gateway (IMS-MGW)

A IMS-MGW may terminate bearer channels from a switched circuit network and media streams from a packet network (e.g., RTP streams in an IP network). The IMS-MGW may support media conversion, bearer control and payload processing (e.g. codec, echo canceller, conference bridge),.
IMS-MGW implements the following functions:
l   Interacts with the MGCF for resource control.
l   Owns and handles resources such as echo cancellers etc.
l   May need to have codecs.
The IMS-MGW will be provisioned with the necessary resources for supporting UMTS/GSM transport media. Further tailoring (i.e. packages) of the H.248 may be required to support additional codecs and framing protocols, etc.

5. Multimedia Resource Function Controller (MRFC)

The MRFC:
l   Controls the media stream resources in the MRFP.
l   Interprets information coming from an AS and S-CSCF (e.g session identifier) and control MRFP accordingly.
l   Generates CDRs .

6. Multimedia Resource Function Processor (MRFP)

The MRFP:
l   Controls bearers on the Mb reference point.
l   Provides resources to be controlled by the MRFC.
l   Mixes incoming media streams (e.g. for multiple parties).
l   Sources media streams (for multimedia announcements).
l   Processes media streams (e.g. audio transcoding, media analysis).

7. Subscription Locator Function (SLF)

The SLF:
l   Is queried by the I-CSCF during the Registration and Session Setup to get the name of the HSS containing the required subscriber specific data. Furthermore the SLF is also queried by the S-CSCF during the Registration.
l   Is accessed via the Dx interface
The SLF is not required in a single HSS environment. An example for a single HSS environment is a server farm architecture.

8. Breakout Gateway Control Function (BGCF)

The Breakout Gateway control function (BGCF) selects the network in which PSTN/CS Domain breakout is to occur. If the BGCF determines that the breakout is to occur in the same network in which the BGCF is located within, then the BGCF shall select a MGCF which will be responsible for the interworking with the PSTN/CS Domain. If the break out is in another network, the BGCF will forward this session signalling to another BGCF in the selected network.
The functions performed by the BGCF are:
l   Receives request from S-CSCF to select appropriate PSTN/CS Domain break out point for the session
l   Select the network in which the interworking with the PSTN/CS Domain is to occur. If the interworking is in another network, then the BGCF will forward the SIP signalling to the BGCF of that network. If the interworking is in another network and network hiding is required by the operator, the BGCF will forward the SIP signaling via an I-CSCF(THIG) toward the BGCF of the other network.
l   Select the MGCF in the network in which the interworking with PSTN/CS Domain is to occur and forward the SIP signalling to that MGCF. This may not apply if the interworking is a different network.
l   Generation of CDRs.

The BGCF may make use of information received from other protocols, or may make use of administrative information, when making the choice of which network the interworking shall occur.

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