GPRS 

INTRODUCTION:

 General packet radio service (GPRS) is a packet oriented mobile data service available to users of the 2G cellular communication systems, 3G systems and GSM. GPRS re-use the existing GSM infrastructure. It interworked with existing circuit-switched services. It is based on standardized open interfaces.

GPRS usage is typically charged based on volume of data transferred, contrasting with circuit switched data, which is usually billed per minute of connection time. 5 GB per month for a fixed fee or on a pay-as-you-use basis. Usage above the bundle cap is either charged per megabyte or disallowed.

GPRS is a best effort service, implying variable throughput and latency that depend on the number of other users sharing the service concurrently, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection. In 2G systems, GPRS provides data rates of 56–114 kbit/second. 2G cellular technology combined with GPRS is sometimes described as 2.5G, that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate-speed data transfer, by using unused time division multiple access (TDMA) channels in, for example, the GSM system. GPRS is integrated into GSM Release 97 and newer releases.

 GPRS provides two services: 

1.  Point-to-point (PTP) 

2.  Point-to-multipoint (PTM)

 In point-to-point packet delivery service, in which packet is transfer between two users and in point-to-multipoint (PTM) service, in which packet is delivering to multiple destinations within one service request.

In PTP versions are PTP Connection oriented Network service (PTP-CONS), which establish a logical relation in between users. Multiple packets are sent between single source and a single destination. Other version is the PTP Connectionless Network Service (PTP-CLNS), which does not require a logical link between users. Packets are sent between a single source and a single destination. Each packet is independent of its predecessor and successor.

QoS-profile can be specified by the users of the GPRS. It is maintained in the PDP context. PDP Context is nothing but which is created in each communication session. QoS-profile is used to indicate the network and radio resources required for data transmission. It has the attributes such as service precedence (high,normal,low),reliability class, delay class, peak throughput class, mean throughput class. GPRS must allocate radio resources to fulfill these user specifications. GPRS network is suffered by the following delays such as channel access delay, coding for error correction and transfer delay in the fixed part and wireless part of the network. GPRS also includes several security services namely authentication, user identity confidentiality, access control and user information confidentiality.

 Main benefits

 Resources are reserved only when needed and charged accordingly. Connection setup times are reduced. It will enable new service opportunities. It has High Speed (Data Rate 14.4 – 115 kbps). It uses the efficient radio bandwidth (Statistical Multiplexing).Circuit switching & Packet Switching can be used in parallel. It has Constant connectivity.

Characteristics of GPRS:

 1. GPRS uses packet switched resource allocation.

 2. Flexible channel allocation.

 3. Support for leading internet communication protocols.

 GPRS Terminal Classes:

 1. Class A

 It can be connected to GPRS service and GSM service (voice, SMS), using both at the same time. Such devices are known to be available today.

2. Class B

 It can be connected to GPRS service and GSM service (voice, SMS), but using only one or the other at a given time. During GSM service (voice call or SMS), GPRS service is suspended, and then resumed automatically after the GSM service (voice call or SMS) has concluded. Most GPRS mobile devices are Class B.

3. Class C

 They are connected to either GPRS service or GSM service (voice, SMS). Must be switched manually between one or the other service.

GPRS ARCHITECTURE

 In order to understand the GPRS network architecture, some fundamental GSM terminology is necessary. This section describes some of the main components of the GSM network.

GPRS Networks

GPRS architecture has two network elements, which are called as GPRS support nodes (GSN).They are, 

1.     Gateway GPRS Support Node(GGSN) 

2.     Serving GPRS Support Node (SGSN)

All GSNs are integrated into the standard GSM architecture and many interfaces (see figure 1). The network elements are gateway GPRS support node (GGSN) is provisioned by router, which supports traditional gateway functionality. It is the interworking unit between the GPRS network and external packet data networks (PDN). This node contains routing information for GPRS users. It performs address conversion and tunnels data to a user via encapsulation.

The other element is the serving GPRS support node (SGSN) which connects BSS and GGSN. It supports the MS via the G_b interface. It requests the user address from the GPRS register (GR). It keeps track of the individual MSs‘ location, is in charge for collecting billing information. It performs many security functions.

Packet Control Unit (PCU) 

The PCU separates the circuit switched and packet switched traffic from the user and sends them to the GSM and GPRS networks respectively. It also performs most of the radio resource management functions of the GPRS network. The PCU can be either located in the BTS, BSC, or some other point between the MS and the MSC. There will be at least one PCU that serves a cell in which GPRS services will be available. Frame Relay technology is being used at present to interconnect the PCU to the GPRS core.

GPRS interfaces 

Um between an MS and the GPRS fixed network part. The Um is the access interface the MS uses to access the GPRS network. The radio interface to the BTS is the same interface used by the existing GSM network with some GPRS specific changes.

Gb between a SGSN and a BSS. The Gb interface carries the GPRS traffic and signaling between the GSM radio network (BSS) and the GPRS network. Frame Relay based network services is used for this interface.

Gn between two GSNs within the same PLMN. The Gn provides a data and signalling interface in the Intra -PLMN backbone. The GPRS Tunnelling Protocol (GTP) is used in the Gn (and in the Gp) interface over the IP based backbone network.

Mobile Station 

A GSM subscriber needs a terminal called Mobile Station (MS). It is used to

connect to the network using the radio interface U_m. In idle mode an MS is not reachable and all contexts will be deleted. In the standby state there is only movement across routing areas which is updated to the SGSN. Before sending any data over the GPRS network, an MS must attach to it, following the procedures of the mobility management. The attachment procedure includes assigning a temporal identifier, called a temporary logical link identity (TLLI), and a ciphering key sequence number (CKSN) for data encryption.

GPRS BSS 

Base Station Subsystem (BSS) which performs radio-related functions. BSS contains Base Transceiver Stations (BTS) and Base Station Controllers (BSC).

 BTS which provides new GPRS channel coding schemes through Channel Codec Unit (CCU). The BTS handles the radio interface to the MS. It consists of radio equipment (transceivers and antennas) required to service each cell in the network.

 BSC forwards the Circuit-switched calls to MSC and the Packet-switched data to SGSN. The BSC provides the control functions and physical links between the MSC and the BTS. A number of BSCs are served by one MSC while several BTSs can be controlled by one BSC.

The Network Switching Subsystem 

The NSS is responsible for call control, service control and subscriber mobility manage

a) Mobile Switching center (MSC) 

MSC is in charge for telephony switching functions of the network. It also performs authentication to verify the user‘s identity. It ensures the confidentiality of the calls. The Authentication Center (AuC) provides the necessary parameters to the MSC to perform the authentication procedure. The AuC is shown as a separate logical entity but is generally integrated with the HLR. The Equipment Identity Register (EIR) is on the other hand a database that contains information about the identity of the mobile equipment. It prevents calls from unauthorized or stolen MSs.

b)Home Location register (HLR) 

HLR is a database used to store and manage permanent data of subscribers. HLR is used to map an MS to one or more GGSNs. It is used to update the SGSN of the MS. It is also used to store the fixed IP address and QoS profile for a transmission path.

c) Visitor location register (VLR) 

VLR is a database used to store temporary information about the subscribers. It is needed by the MSC in order to service visiting subscribers. The MSC and VLR are commonly integrated into one single physical node and the term MSC/VLR is used instead. When a subscriber enters a new MSC area, a copy of all the ne) cessary information is downloaded from the HLR into the VLR. The VLR keeps this information so that calls of the subscriber can be processed without having to interrogate the HLR (which can be in another PLMN) each time. The temporary information is cleared when the mobile station roams out of the service area.

d)Equipment identity register (EIR) 

EIR is also a database that encloses information about the identity of the mobile equipment. It prevents calls from unauthorized or stolen MSs.

GPRS Mobility Management States 

a)Idle State

 A MS in the idle state is not traceable and can only receive PTM-M transmissions such as general broadcast events destined to a specific geographical area. The MS needs to perform the attach procedure in order to connect to the GPRS network and become reachable.

b)Ready State

Data is sent or received in this state. The MS informs the SGSN when it changes cells. The MS may explicitly request (or can be forced by the network) to detach in which case it moves to Idle. A timer monitors the Ready state and upon its expiry, the MS is put on Standby. The timer insures that resources are not wasted by an inactive MS.

c)Standby State

 A connected MS which is inactive is put in the Standby state. Moving back to Ready can be triggered by sending data or signalling information from the MS to the SGSN. Upon arrival of data destined to the MS, the SGSN pages the latter and a response to the page moves the MS back to the Ready state. The MS may wish (or can be forced by the network) to terminate the connection by requesting to detach in which case it returns to Idle. A timer is used by the SGSN to monitor the tracking of the MS, and when it expires, the MS is detached and is considered unreachable

GPRS PROTOCOL ARCHITECTURE

A GPRS network introduces many new protocols designed to convey user data in a reliable and secure way. The protocol architecture is implemented for the transmission and signaling planes in GPRS. Transmission plane protocols are used for the transmission of user data and control functions. Signaling plane protocols are used to convey signaling information that controls and supports the transmission plane functions. (See figure 2).

Transmission protocols in the Um interface

 a) Physical layer

 The physical layer can be divided into the Radio Frequency (RF) layer and the Physical Link layer.

 The Radio Frequency (RF) is the normal GSM physical radio layer. Among other things the RF layer specifies the carrier frequency characteristics and GSM radio channel structures. It uses the radio modulation scheme for the data. The GSM RF physical layer is used for GPRS with the possibility for future modifications.

 The Physical Link layer supports multiple MSs sharing a single physical channel and provides communication between the MSs and the network. Network controlled handovers are not used in the GPRS service. Instead, routing area updates and cell updates are used.

b)Medium Access Control (MAC)

 MAC protocol handles the channel allocation and the multiplexing. The RLC and the MAC together form the OSI Layer 2 protocol for the U_m interface. The

 radio interface at U_m need GPRS which does not require changes compared to GSM.

c) Radio Link Control (RLC)

 RLC protocol establishes a reliable radio link to the upper layers. It also works either in acknowledged or unacknowledged modes.

Logical Link Control (LLC)

 LLC layer establishes a secure and reliable logical link between the MS and the SGSN for upper layer protocols. It works either in acknowledged or unacknowledged modes. The data confidentiality is ensured by using ciphering functions.

 Subnetwork dependent convergence protocol (SNDCP)

 SNDCP is used to transfer data packets between SGSN and MS. It is used to provide multiplexing of several connections of network layer onto one logical connection of underlying LLC layer. It provides functions that help to improve channel efficiency. This is achieved by means of compression techniques. Data Link layer is divided into LLC layer and RLC/MAC Layer.

 Transmission protocols in the Gb interface a)Physical Layer Protocol

 Several physical layer configurations and protocols are possible at the Gb interface and the physical resources are allocated by Operation & Maintenance (O&M) procedures. Normally a G703/704 2Mbit/s connection is provided.

b) Network Services layer

 The Gb interface Network Services layer is based on Frame Relay. Frame Relay virtual circuits are established between the SGSN and BSS. LLC PDUs from many users are statistically multiplexed onto these virtual circuits. These virtual circuits may traverse a network of Frame Relay switching nodes, or may just be provided on a point to point link between the BSC and the SGSN.

Base station subsystem GPRS protocol (BSSGP)

 BSSGP is used to deliver routing and QoS-related information between the BSS and SGSN. It is to enable two physically distinct nodes, the SGSN and BSS. It is to operate node management control functions. There is a one-to-one relationship between the BSSGP protocol in the SGSN and in the BSS. If one SGSN handles multiple BSSs, the SGSN has to have one BSSGP protocol device for each BSS. BSSGP does not perform error correction and works on top of a frame relay (FR) network.

Transmission protocols in the Gn interface

 a)Layer 1 and Layer 2

 The L1 and the L2 protocols are vendor dependent OSI layer 1 and 2 protocols. It carries the IP datagrams for the GPRS backbone network between the SGSN and the GGSN.

 b) Internet Protocol (IP)

 The Internet Protocol (IP) datagram in the Gn interface is only used in the GPRS backbone network. The GPRS backbone (core) network and the GPRS subscribers use different IP addresses. This makes the GPRS backbone IP network invisible to the subscribers and vice versa. The GPRS backbone network carries the subscriber IP or X.25 traffic in a secure GPRS tunnel.

c) TCP or UDP

 TCP or UDP are used to carry the GPRS Tunnelling Protocol (GTP) PDUs across the GPRS backbone network. TCP is used for user X.25 data and UDP is used for user IP data and signalling in the Gn interface.

d) GPRS tunneling protocol (GTP)

 GTP is the basis for tunnel signaling. It uses two transport protocols such as reliable TCP and non-reliable UDP. The GPRS Tunnelling Protocol (GTP) allows multi-protocol packets to be tunnelled through the GPRS backbone between GPRS Support Nodes (GSNs).

 SIGNIFICANCE: 

This unit is important for cellular telephone technique and GSM networks. and also include security related techniques and recent technology in mobile computing