GSM Location Update Precedure

The location update flow is as follows:

- When a MS finds that better service is provided in a certain location area, this MS notifies the MSC of the MSC/VLR which this new location area belongs to. 

- If the original location area and new location area belong to the same MSC/VLR, the MSC only changes the location information of this MS in the VLR. If the original location area and new location area belong to different MSCs/VLRs, the new MSC/VLR requests the data of this MS from the HLR. 

- The HLR returns the information needed by the new MSC/VLR and at the same time notifies the original MSC/VLR to delete the original registration information of this MS and notifies the new MSC/VLR to register this MS. 

Location Update of MS in the Same VLR

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The Signaling Process of MS Originated USSD Call

When the MS makes originate the USSD operation according to the input of the subscriber, the MS first sets up a connection to the network, and then sends a request to the MSC and waits for the response. Once the MS receives the response from the network side, the MS displays the information in the response message on its screen to the subscriber. During the waiting process, the MS may receive the USSD REQUEST or USSD NOTIFY message originated by the network. At this time, the MS must handle these messages immediately and continuously wait for the response of the original USSD operation from the MSC.

After the MSC/VLR receives the MS originated USSD operation: 

- If the Service Code in this operation indicates that this operation must be handled by the HPLMN, the MSC/VLR tries to set up the conversation with the network. If such a conversation cannot be set up, the MSC/VLR returns error information to the MS. Otherwise, the MSC/VLR forwards this operation to the network side. The MSC/VLR transfers the subsequent USSD operations transparently between the MS and network. 

- If the Service Code in this operation does not indicate that this operation must be handled by the HPLMN, the MSC/VLR decides how to handle this USSD operation according to the configuration of the USSD Control table, that is, handles this USSD operation by itself or forwards it to other network element. 

- If the MSC/VLR does not support the characters in the MS originated USSD operation, the MSC/VLR must forwards this USSD operation to the network side. If the network side does not support the characters in the USSD operation, the network side must notify the MS and release the conversation. 

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Basic Core Network Feature and Technologies

Core Network is devided into CS domain and PS domain. CS domain is based on original GSM network. PS domain is based on original GPRS network. CS domain is used to provide Circuit-switched service. Network mode can support TDM, ATM and IP. Physical entities include switching equipment (such as MSC/VLR, GMSCs), and inter-working equipment (IWF). PS domain is used to provide Packet-switched service. Network mode is IP. Physical entities include SGSN, GGSN, CG , BG etc.

Common Function of CS domain and PS

HLR: to realize mobile subscriber management and location information management ;

VLR: to deal with all kinds of data information of current mobile subscriber ;

AUC: to store authentication information of mobile subscriber ;

EIR: to store IMEI data of mobile subscriber; 

SMS: Short Message Center. 

Function entity of CS domain

MSC: to realize switching and signaling control function of circuit service. 

GMSC: to realize addressing function for different network. GMSC and MSC can be integrated together or separated independently. 

IWF: integrated together with MSC, to realize inter-working with PLMN and ISDN, PSTN , PDN (Mainly complete signaling transfer function), its functions can be defined based on specified services and network types. 

Function entity of PS domain

SGSN: to realize packet switching function, signaling control function  and route function of packet service. 

GGSN: to realize inter-working with PS domain and external data network. 

CG: to realize billing function of packet service. 

BG: to realize inter-working of two GPRS network and guarantee the security of network.

Interface between CN and RAN/BSS

A Interface: to realize mobile station management, base station management, mobility management and call processing, etc. 

Gb interface: to realize packet data transmit and mobility management.

Iu-CS interface: to realize RNS management, mobility management and call control function. 

Iu-PS interface: its basic function is similar to Iu-CS interface.

CS internal interface

B interface (MSC-VLR): A private interface between VLR and MSC. It is used for the MSC to query the current location information of a Mobile Station (MS) from the Visit Location Register (VLR), or request the VLR to update the current location information of the MS or is used for the operations of supplementary services. 

C interface (MSC-HLR): to get MSRN,  the IN service related subscriber status and location information. 

D interface (VLR-HLR): used to exchange the location information of the MS and the subscriber management information. 

E interface ( between MSC): to be used for handover process between two MSCs. 

F interface (MSC-EIR) : to be used for EIR to verify IMEI status information of mobile equipment.

G interface (between VLR): to exchange IMSI and authentication information when MS moves from one VLR to another VLR. 

PS internal interface

Gr interface (SGSN-HLR): to exchange mobile location information and manage subscriber signed information. It is similar to C interface.

Gn, Gp interface (SGSN-GGSN): GTP is adopted, used for establish tunnel and transmit data packet between GSN equipment. 

Gc interface (GGSN-HLR) : optional.

Gf interface (SGSN-EIR) .

Gi interface (GGSN – external network).

Common interfaces of CS and PS

Gs interface (MSC/VLR-SGSN): used for combined location update between MSC and SGSN) .

H interface (HLR-AUC): used for authentication and ciphering. It is a private  interface.

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GSM Location Update Flow

The procedure of location update is as follows:

• When a MS finds that better service is provided in a certain location area, this MS notifies the MSC of the MSC/VLR which this new location area belongs to.
• If the original location area and new location area belong to the same MSC/VLR, the MSC only changes the location information of this MS in the VLR. If the original location area and new location area belong to different MSCs/VLRs, the new MSC/VLR requests the data of this MS from the HLR.
• The HLR returns the information needed by the new MSC/VLR and at the same time notifies the original MSC/VLR to delete the original registration information of this MS and notifies the new MSC/VLR to register this MS.

Location Update When the MS Roams Inside the Same VLR Area

Location Update When the MS Roams In the Areas of Different VLRs  

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