What is General Packet Radio Service (GPRS)

 

What is GPRS ?

As per Wikipedia,General Packet Radio Service (GPRS) is a packet oriented mobile data standard on the 2G and 3G cellular communication network’s global system for mobile communications (GSM). GPRS was established by European Telecommunications Standards Institute (ETSI) in response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now maintained by the 3rd Generation Partnership Project (3GPP).

What is GMM ?

The management of General Packet Radio Service(GPRS) mobility in the network called GPRS Mobility Management(GMM), which  ensures the continuity of packet services when a given subscriber moves from one GPRS LA(Location area) to another. It implies that the network must know the identifier of the GPRS LA indicating where the MS is located.

The GMM functions enable the network infrastructure to keep track of subscribers’ locations within the PLMN or within another PLMN. The SGSN, which is the serving node of an MS, handles the mobility context management related to it. This context contains information such as the IMSI, the P_TMSI, the RAI, and the CI. This mobility context management is also stored at the MS side, in the SIM card.

GPRS ARCHITECTURE:

Mobile Stations: (MSs; the mobile terminals) that communicate with the network through the Base Station System (BSS)

Base Substation System:The BSS consists of the Base Transceiver Station (BTS) and the Base Station Controller (BSC).

Base Transciver Station:The BTS communicates with the MS through the radio interface Um, based on the Time Division Multiple Access (TDMA) technology.

BTS connects to the BSC through the A-bis interface.The BSC communicates with exactly one Mobile Switching Center (MSC) via the A interface.

Mobile Switching Center: MSC is a special telephone switch tailored to support mobile applications. The MSC connects the calls from the MSs to the Public Switched Telephone Network (PSTN).

Visitor Location Register: VLR is the logical database where all the current subscriber database(IMSI,MSISDN, profile ) are stored

Home Location Register: Permanent database of subscriber

GPRS evolved from GSM, where existing GSM nodes such as BSS, MSC, VLR, and HLR are upgraded.GPRS introduces two new Core Network nodes:
Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN).

Gateway GPRS Support Node:The GGSN provides connections and access to the integrated services Internet.It maintains routing information for the GPRS attached MSs to tunnel Protocol Data Units (PDUs) to the SGSN through the Gn interface .The GGSN communicates with the HLR for session management through the Gc interface.

Serving GPRS Support Node: SGSN is responsible for the delivery of packets to the MSs within its service area. The SGSN performs security, mobility management, and session management functions by communicating with the HLR through the Gr interface.The BSC of the GPRS BSS is connected to the SGSN through the Gb interface using the frame relay link.

UMTS evolved from GPRS by replacing the radio access network.

UMTS Terrestrial Radio Access Network (UTRAN): It consists of Node Bs (the UMTS term for BTS) and Radio Network Controllers (RNCs) connected by an
ATM network.

  • The RNC and the Node B serving an MS are called the Serving Radio Network Subsystem (SRNS).
  • The User Equipment (UE; the UMTS term for MS) connects with Node Bs through the radio interface Uu based on the WCDMA (Wideband Code Division Multiple Access) technology
  • In UMTS, every Node B is connected to an RNC through the Iub interface.Every RNC is connected to an SGSN through the IuPS interface, and to an MSC through the IuCS interface.
  • An RNC may connect to several RNCs through the Iur interface.

The core network consists of two service domains: the circuit-switched (CS) service domain (that is, PSTN/ISDN) and the packet switched (PS) service domain (that is, the Internet).

  • In the CS domain, an MS is identified by International Mobile Subscriber Identity (IMSI) and Temporary Mobile Subscriber Identity (TMSI).
  • In the PS domain, an MS is identified by IMSI and Packet TMSI (P-TMSI).

Three operation modes are defined for GPRS MS:
• Class A MS allows simultaneous CS and PS connections.
• Class B MS provides automatic choice of CS or PS connection, but only one at a time.
• Class C MS supports only PS connection.

Three operation modes are defined for UMTS UE:

  • PS/CS mode UE is equivalent to GPRS Class A MS.
  • PS mode UE is equivalent to GPRS Class C MS.
  • CS mode UE can attach to the CS domain only.

How GPRS Service initiated in GSM network, we can know by following procedure.

 GPRS Attach Procedure:

When an MS needs to signal its presence to the network in order to access to GPRS services, it performs an IMSI attach procedure for GPRS services. During this procedure a MM context is created between the MS and the SGSN.

There are two types of GPRS attach procedures:

Normal GPRS Attach:

In this scenario, the MS signals itself to the network by sending it its old P-TMSI identifier associated with the old RAI identifier. When the SGSN receives this information, it analyzes the RAI identifier in order to determine the associated SGSN. If there is an SGSN change, the new SGSN must contact the old SGSN from its RAI identifier in order to retrieve the MS identity. Authentication functions may be performed; they are mandatory if no MM context information related to the MS, such as IMSI, P-TMSI, CI, and RA exists anywhere in the network. Then the new SGSN informs the HLR of SGSN change, and location information in the HLR database is updated via the MAP protocol on SS7 signaling. If the HLR receives an indication from an SGSN different from the one stored in its table for a GPRS subscriber, it requests the old SGSN to remove GPRS data related to this subscriber, and then transmits this data to the new SGSN.

The new SGSN then transmits to the MS the GPRS-attach confirmation. If a new P-TMSI is allocated by the SGSN, it is acknowledged by the MS to end the GPRS-attach procedure.

Combined Attach Procedure:

In this  combined GPRS/IMSI attach procedure takes place in a cell that supports GPRS in network operation mode I. The difference with the previous scenario is that the new SGSN sends an IMSI attach request to the MSC/VLR via the Gs interface as soon as the new SGSN receives data related to the GPRS subscriber from the HLR. When the new SGSN receives the acceptance of IMSI attach from the MSC/VLR entity, it transmits the IMSI and GPRS attach confirmation. If the MS receives a new P-TMSI identifier or a new TMSI identifier, then it acknowledges it to end the combined attach procedure.

GPRS Detach:

When an MS does not need to access GPRS services anymore, an IMSI-detach procedure is initiated, either by the MS or by the SGSN. During this procedure, the MM context between the MS and the SGSN is removed.

There are two types of GPRS-detach procedures:

MS-Initiated Detach Procedure:

MS-Initiated Normal GPRS Detach When a GPRS MS wishes to be IMSI detached for GPRS services, it initiates a GPRS-detach procedure to the SGSN. The procedure is ended upon the receipt of the DETACH ACCEPT message by the MS.

MS-Initiated Combined GPRS Detach:

When an MS both IMSI and GPRS attached wishes to perform a GPRS detach in a cell that supports GPRS in network operation mode I, it initiates a combined detach procedure to the SGSN. The latter sends an explicit request to the MSC/VLR to deactivate the association between SGSN and MSC/VLR in order that circuit-switched incoming calls are no longer routed to SGSN.

Network-Initiated Detach Procedure:

Network-Initiated Normal GPRS Detach When an SGSN wishes to IMSI detach a given MS for GPRS services, it initiates a GPRS-detach procedure. The procedure is ended upon the receipt of DETACH ACCEPT message by the SGSN. The network may request the MS to perform a reattach in the case of a network failure condition.

Network-Initiated Combined GPRS Detach:

When an SGSN wishes to IMSI detach a class A or B MS for GPRS or non-GPRS services, it notifies the relevant MS of a GPRS detach. It also sends an explicit request to MSC/VLR to deactivate the association between SGSN and MSC/VLR. Circuit-switched incoming calls are no longer routed to SGSN.


An HLR may initiate a GPRS detach for operator purposes in order to remove the subscriber’s MM and PDP contexts at the SGSN. The HLR sends a CANCEL LOCATION message in order to delete the subscriber’s MM and PDP contexts from the SGSN. This latter then notifies the relevant MS of a GPRS detach. If the MS is both IMSI and GPRS attached, the SGSN sends an explicit request to the MSC/VLR to deactivate the association between the SGSN and the MSC/VLR

Paging on PCCCH:

An MS, IMSI attached for GPRS services, may be paged on PCCCH channels if they are allocated in a cell it is camped. There are two types of paging on PCCCH:

  • GPRS paging in a cell that supports GPRS in network operating modes I or III;
  • Circuit-switched paging in a cell that supports GPRS in operating mode I (presence of Gs interface).

In a cell supporting PCCCH channels and operating in network operation modes I or III, an MS both IMSI and GPRS attached in GMM STANDBY state can be paged by the network if the SGSN receives a PDU for it. When the MS receives a paging for packet-switched services, it requests of the BSS a TBF establishment in order to send any LLC frame to the SGSN. On LLC frame transmission, the MS goes to GMM READY state. Upon receipt of the LLC frame, the SGSN goes to GMM READY state. As the MS is now located at cell level, the packet transfer mode may start on downlink.

GPRS PAGGING:

GPRS pagging is a process when MS or handset receives a pagging request on Pagging control channel on GMM standby state.This pagging request send by SGSN when SGSN receives a PDU packet and it sent to MS via BSS.

The main parameters included in this request are:

■ IMSI

■ P-TMSI

■ Routing area

■ Channel needed (indicating GPRS paging)

■ Negotiated QoS

■ DRX parameters

The BSS checks for the routing area in the paging request message and pages the MS in each cell belonging to RA. The MS responds with any LLC frame, e.g., RR or info frame, other than NULL LLC. The BSS, on receiving the LLC frame, adds cell global identity (CGI) and sends the LLC frame to the SGSN. The SGSN considers the LLC frame a successful paging response.

Authentication Procedure:

The authentication procedure allows the network to identify and authenticate the user to use GPRS service.

During an authentication procedure, a new SGSN needs to retrieve the triplet (Kc, signed RESult [SRES], random) from the HLR/AUC entity via the MAP protocol using the SS7 network.

When the SGSN has retrieved this triplet, it authenticates the MS by sending the random number in the AUTHENTICATION AND CIPHERING request message. Upon receipt of this number, the MS will calculate the SRES number and the ciphering key Kc. Next the MS forwards to the network the SRES number.

The network compares the SRES number calculated by the MS with the one sent by the entity HLR/AUC. If the two SRES numbers are identical, the SGSN considers that the outcome of the authentication of the GPRS subscriber is positive.

Location Procedures:

A location procedure is always initiated by the MS. Under normal circumstances, a location change occurs when the MS decides to camp on a new cell for better radio conditions.

If an MS in GMM READY state camps in a new cell within its current RA, it needs to perform a cell update procedure in order to receive directly downlink PDUs from the network without being paged. If the MS camps in a new cell belonging to a new RA, it needs to perform an RA update procedure in order to update MM context information between the MS and the SGSN.

RA Update Procedure

An RA update procedure is performed when a GPRS MS has detected a new RA. This procedure is always initiated by the MS. There are four types of RA update procedures:

Normal RA Update:

This process also called Intra-SGSN HA Update. During an RA update procedure, the MS signals itself to the SGSN by sending its old P-TMSI signature associated with the RAI identifier from its old RA. The SGSN has the necessary information about the MS if the SGSN also handles the old RA.

In the case of an intra-SGSN change, the SGSN validates the presence of the MS in the new RA by returning to it a ROUTING AREA UPDATE ACCEPT message. If the SGSN allocates a new P-TMSI identifier, it is acknowledged by the MS. This procedure is called intra-SGSN RA update since the SGSN does not need to contact an old SGSN, GGSN, and HLR.

Periodic RA Update:

When the SGSN detects that the old RA sent by the MS is handled by another SGSN, the SGSN has no information about this MS. In this case, the SGSN needs to contact the old SGSN, the GGSN, and the HLR in order to retrieve information and update the routing information. This procedure is called an inter-SGSN RA update procedure.

Thus the new SGSN is able to contact the old SGSN from the RAI identifier in order to retrieve MM and PDP context information related to the MS identified in the old SGSN by its old P-TMSI. If the old signature does not match the one saved in the old SGSN, the new SGSN performs an MS authentication procedure. If the old SGSN has saved in its buffer some packets addressed to the MS, it forwards the packets toward the new SGSN.

When the new SGSN has retrieved MM and PDP context information, it updates the data related to the new SGSN in the GGSN. The new SGSN then updates location information in the HLR database via the MAP protocol using the SS7 network. If the HLR receives an indication from an SGSN different from the one saved in its table for a GPRS subscriber, it requests the old SGSN to remove GPRS data related to this subscriber. It then transmits this data to the new SGSN.

When the new SGSN receives an RA update confirmation in the HLR database, it transmits the RA update confirmation to the MS with its new P-TMSI identifier and the receive N-PDU number. This message contains the acknowledgment of N-PDUs successfully transferred by the MS before the start of the update procedure. The RA update procedure ends as soon as the MS acknowledges its new P-TMSI identifier.

Combined RA and IMSI attach scenario generates the same message exchange between the MS, SGSN, GGSN, MSC/VLR, and HLR entities as the combined RA and LA update scenario

 

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