General Packet Radio Service was the evolution of 2G GSM to provide packet switched data at rates up to a maximum of 115.2 Kbps. GPRS was something of a revolution because all previous mobile phone systems had used circuit switched channels. Also, previous cell phone systems including GSM had focused on voice communication, but the need for mobile data was starting to come about and GPRS was one of the first to address this in a real way.

As a result, GPRS was developed to enable data to be handled and it also provided a stepping stone on the path to 3G. The name of this technology can be justified as follows:

General Packet Radio Service

• General: Means it is not restricted to GSM use and can be used for any other system (GPRS is an overlay technology that can be implemented over any other existing technology such as GSM, DECT, 3rd Generation systems to enable packetized data communication.)
• Packet Radio : It enables packet mode communication over air.
• Service, not System : GPRS uses existing BSS (partially also NSS) and add special nodes to the existing infrastructure.

1) Concept of Packet Switching

• Using the traditional approach, a circuit (a frequency channel) was switched ( allocated) permanently to a particular user. This is known as a circuit switched mode. In case of the bursty nature of data transfer, there are periods when it will not be carrying data and is being unused by the subscriber. Thus, there is efficient utilization of frequency channel is not seen
• To utilize the spectrum efficiently, the overall channel capacity can be shared between several users. To achieve this, the data is split into packets and headers are inserted into the packets to provide the destination address, source address and other important information. The frequency channels are not dedicated to the data users. As and when, the demand from the user arises, frequency channel is allotted only till the time there are packets to be transmitted. And then the frequency channel is free to serve some other user. Packets from several sources can thus be transmitted over the same link. As it is unlikely that the data burst for different users will occur all at the same time, by sharing the overall resource in this fashion, the channel, or combined channels can be used far more efficiently. This approach is known as packet switching and it is at the core of many cellular data systems such as GPRS.
• The key element of GPRS technology was that it used packet switched data rather than circuit switched data, and this technique made much more efficient use of the available capacity. This is because most data transfer occurs in what is often termed a "bursty" fashion. The transfer occurs in short peaks, followed by breaks when there is little or no activity.
• GPRS provided a packet data capability for the 2G cellular systems, enabling the evolution of GSM to provide a data capability.
• A packet data network architecture is overlayed or added to the existing GSM architecture to provide the data capability. The existing GSM network architecture is used to carry the circuit switched voice calls as well as the network access etc.
• GPRS uses a concept of statistical multiplexing.

2) Statistical multiplexing

• In real time, a user might require to transmit/receive a very small amount of data or huge chunks of data.
• Statistical multiplexing (as illustrated in Figure 2) is either allotting different instants of a time slots to multiple users or allotting multiple time slots to a single user. All this depends on the amount of data required by a user.
• The MAC layer is central to this and there are three MAC modes that are used to control the transmissions. These are named fixed allocation, dynamic allocation, and extended dynamic allocation.
• The fixed allocation mode is required when a mobile requires a data to be sent at a consistent data rate. To achieve this, a set of Packet data channels (PDCHs) are allocated for a given amount of time. When this mode is used there is no requirement to monitor for availability, and the user can send and receive data freely. This mode is used for applications such as video conferencing.
• When using the dynamic allocation mode, the network allocates time slots as they are required. A user is allowed to transmit in the uplink when it sees an identifier flag known as the Uplink Status Flag (USF) that matches it's own. The mobile then transmits it's data in the allocated slot. This is required because up to eight mobiles can have potential access to a slot, but obviously only one can transmit at any given time. We require statistical multiplexing in this mode of allocation.
• A further form of allocation known as extended dynamic allocation is also available. Use of this mode allows much higher data rates to be achieved because it enables users to transmit in more than one slot. When the USF indicates that a mobile can use this mode, it can transmit in the number allowed, thereby increasing the rate at which it can send data.
• Statistical multiplexing ensures that slots will not be wasted (whereas TDM can waste slots). The transmission capacity of the link will be shared by only those users who have packets.

3) Architecture of GPRS

GPRS can be implemented in the existing GSM systems as illustrated in Figure 3. Changes are required in an existing GSM network to introduce GPRS. The Base Station Subsystem (BSS) consists of a Base Station Controller (BSC) and Packet Control Unit (PCU). The PCU supports all GPRS protocols for communication over the air interface. Its function is to set up, supervise, and disconnect packet switched calls. The packet control unit supports cell change, radio resource configuration, and channel assignment. The base station transceiver (BTS) is a relay station without protocol functions. It performs modulation and demodulation. To allow the GPRS network to provide the packet data capability additional network entities are required to be added to the overall architecture - the serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN).

SGSN: It is functionally connected with BSC, physically can be at MSC or BSC site. There are one SGSN for few BSCs or one (or few) per every BSC .One SGSN can support BSCs of several MSC sites. The Main functions of SGSN are,

• Authenticates GPRS mobiles
• Handles mobile’s registration in GPRS network
• Handles mobile’s mobility management
• Relays MO and MT data traffic
• TCP/IP header compression, data compression using V.42bis algorithm
• Error control between MS- SGSN (ARQ)
• Collect charging information of air interface usage.

GGSN: The GGSN acts as a logical interface to external packet data networks. Within the GPRS networks, protocol data units (PDUs) are encapsulated at the originating GGSN and decapsulated at the destination GGSN. In between the GSNs, IP is used as the backbone to transfer PDUs. This whole process of routing data packets through N number of nodes from source GGSN to destination GGSN is referred to as tunnelling in GPRS. The path taken by a packet is called as a tunnel. Special algorithms are used to add, modify, delete tunnels and requires a separate study. The GGSN also maintains routing information used to tunnel the PDUs to the SGSN that is currently serving the mobile station (MS).

All GPRS user related data required by the SGSN to perform the routing and data transfer functionality is stored within the HLR. In GPRS, a user may have multiple data sessions in operation at one time. These sessions are called packet data protocol (PDP) contexts. The number of PDP contexts that are open for a user is only limited by the user’s subscription and any operational constraints of the network. The GGSN provides the gateway to the external IP network, handling security and accounting functions as well as the dynamic allocation of IP addresses. The GGSN contains routing information for the attached GPRS users. The routing information is used to tunnel PDUs to the mobile’s current point of attachment, SGSN.

In addition, there are some more elements in GPRS architecture which have not been shown in the Figure 3. They are as follows:

BG (Border Gateway) (Not defined within GPRS)

• Routes packets from SGSN/GGSN of one operator to a SGSN/GGSN of another operator
• Provides protection against intruders from external networks

DNS (Domain Name Server)

• Source and destination addresses of packets from MS to BTS is in the form of ggsn1.oper1.fi (oper1 is the name of the operator). When the packet transits outside its own network, it’s address should be in the IPv4 format.
• DNS translates addresses from ggsn1.oper1.fi -format to 123.45.67.89 format (i.e. as used in Internet)

Charging Gateway

• Collects charging information from SGSNs and GGSNs

PTM-SC (Point to Multipoint -Service Center)

• PTM Multicast (PTM-M): Downlink broadcast; no ciphering
• PTM Group call (PTM-G): Closed or open groups; Down/up -link; ciphered