GPRS Attach Call Flow, GPRS Call Flow, PDP attach sequence, PDP context activation

GPRS Attach Call Flow, GPRS Call Flow, PDP attach sequence, PDP context activation

I will be covering sequence of events involved in a GPRS terminal attaching to a network and in PDP context activation. We can also call it as GPRS/Data location update and PDP context activation procedure.

GSM entities involved :

•GSM coverage (CELL)

• BSS (BSC)

• Core Network (SGSN, DNS server, MSC VLR)

• GGSN site (GGSN, Radius Server, DHCP server)

• GSM Database (EIR, HLR)

When GPRS enabled MS is switched ON it initiates a GPRS attach request procedure. GPRS attach request contain old TMSI ( Temporary Mobile Subscriber Identity),Mobile Network Identity(MCC+MNC),location area code ( LAC),Routing area code ( RAC) information. The SGSN (Serving GPRS Support Node) searches for TMSI in its database. No entry is found for the TMSI, so the SGSN uses the old location area information to identify the old SGSN where this terminal was being served. The old SGSN responds with the GPRS mobile's IMSI (International Mobile Subscriber Identity) to the SGSN. The SGSN asks the terminal to identify itself. The terminal responds back. The SGSN authenticates the GPRS mobile by sending a RAND value (a random value). The SIM applies secret GSM The SGSN then requests the identity of the GPRS mobile. GPRS mobile responds back with the identity. IMEI Check Request Verify that that GPRS mobile being used by the user is not a stolen one. The IMEI (International Mobile Equipment Identity) obtained from the GPRS mobile is sent to the Equipment Identification Register (EIR).The EIR clears the subscriber and responds back to the SGSN with the status.

The SGSN now informs the Home Location Register (HLR) about the new location of the GPRS mobile. The HLR informs the old SGSN that the GPRS mobile has moved to a new location. The old SGSN acknowledges back. The HLR updates the new SGSN with all the subscriber information. The HLR updates the new SGSN with all the subscriber information. The SGSN responds back to the HLR.The HLR now responds back to the SGSN's "Update Location" message. The mobile had initiated a combined attach, so the SGSN also updates the location information at the MSC-VLR that will handle the voice calls. The MSC also initiates an update at the HLR. The sequence of actions here is identical to that of the SGSN's HLR update.

PDP context activation

Step1.

The GPRS enabled MS initiates PDP context activation procedure to obtain IP for the device. PDP context request from GPRS mobile contain APN (Access Point Name) provided by the service provider as a parameter.

Step2.

The request reaches SGSN using Gb interface (interface between service BSC (cell) and SGSN node), which initiates a DNS query to find the GGSN (Global GPRS support node) corresponding to the APN specified by the mobile. The DNS (Domain name server) provides the GGSN IP address.

Step3.

SGSN routes the PDP context activation query to the GGSN IP address provided by DNS query. The transmission link or interface between SGSN node and GGSN node is known as Gn interface.

Step4.

GGSN uses RADIUS server to authenticate GPRS subscription. RADIUS server replies with successful authentication status of GPRS subscription and replies back to GGSN.

Step5.

After successful authentication of GPRS subscription, GGSN will now query DHCP (Dynamic Host Control Protocol) server to allocate dynamic IP address for the GPRS mobile.

Step6.

 The GGSN responds back to SGSN indicating completion of PDP context activation procedure.

Step7.

SGSN responds back to the GPRS mobile, this signals completion of the PDP context activation procedure. Hope this precise tutorial was helpful to you, make sure to post your comments.

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