1) Logical channels

Once resources are allocated to GPRS, at least one channel will serve as the master channel to carry all necessary signalling and control information for the operation of the GPRS. All other channels will serve as slave and are only used to carry user and signalling information. If no master channel exists, all the GPRS users will use the GSM common control channel (CCCH) and inform the network to allocate GPRS resources. A physical channel dedicated to GPRS is called a packet data channel (PDCH). It is mapped into one of the physical channels allocated to GPRS. A PDCH can either be used as a packet common control channel (PCCCH), a packet broadcast control channel (PBCCH), or a packet traffic channel (PTCH).

All the types of logical channels are given in Table 1:

Broadcast channels

• Packet Broadcast Central CHannel (PBCCH): This is a downlink channel that is used to broadcast information to mobiles and informs them of incoming calls etc. It is very similar in operation to the BCCH used for GSM. In fact, the BCCH is still required in the initial stage to provide a time slot number for the PBCCH. The PBCCH broadcasts general information such as power control parameters, access methods and operational modes, network parameters, etc, required to set up calls.

Common control channels:

• Packet Paging CHannel (PPCH): This is a downlink only channel and is used to alert the mobile about an incoming call and to make it ready to receive data. It is used for control signalling prior to the call set up. Once the call is in progress a dedicated channel referred to as the PACCH takes over.
• Packet Access Grant CHannel (PAGCH): This is also a downlink channel and it sends information telling the mobile which traffic channel has been assigned to it. It occurs after the PPCH has informed the mobile that there is an incoming call.
• Packet Notification CHannel (PNCH): This is another downlink only channel that is used to alert mobiles that there is broadcast traffic intended for a large number of mobiles. It is typically used in what is termed point-to-point multicasting.
• Packet Random Access CHannel (PRACH): This is an uplink channel that enables the mobile to initiate a burst of data in the uplink. There are two types of PRACH burst, one is an 8 bit standard burst, and a second one using an 11 bit burst has added data to allow for priority setting. Both types of burst allow for timing advance setting.

Dedicated control channels:

• Packet Associated Control CHannel (PACCH): This channel is present in both uplink and downlink directions and it is used for control signalling while a call is in progress. It takes over from the PPCH once the call is set up and it carries information such as channel assignments, power control messages and acknowledgements of received data.
• Packet Timing Advance Common Control CHannel (PTCCH): This channel, which is present in both the uplink and downlink directions is used to adjust the timing advance. This is required to ensure that messages arrive at the correct time at the Base Station regardless of the distance of the mobile from the Base Station. As timing is critical in a TDMA system and signals take a small but finite time to travel this aspect is very important if long guard bands are not to be left.

Data traffic channel:

• Packet Data Traffic CHannel (PDTCH): This channel is used to send the traffic and it is present in both the uplink and downlink directions. Up to eight PDTCHs can be allocated to a mobile to provide high speed data. This channel is not much different from GSM traffic channels. The T.S and frame definitions remain the same. Since GPRS is mostly used as overlay of GSM, every physical channel is also of 200 KHz. Modulation and all other air specifications except switching techniques remain the same as that of GSM.

Definition of a multiframe is little different as compared to GSM multiframe which is given as under:

1.1) Multiframe of a PDCH

A multiframe structure of a PDCH is composed of 52 TDMA frames. With each frame duration being of 4.615 ms, the duration of a multiframe is calculated to be 240 ms. A group of 4 consecutive frames is called as a Block. As shown above, the frames marked with ‘T’ have been reserved for PTCCH ( timing synchronization information). Frames marked with ’X’ are idle frames reserved for future usage.

2) Data Packet Routing in the GPRS Network

In this section data packet routing for the mobile originated and mobile terminated data call scenarios is presented. In mobile originated data routing, the mobile gets an IP packet from an application and requests a channel reservation. The mobile transmits data in the reserved time slots. The packet switched public data network (PSPDN) PDU is encapsulated into a Sub-Network Dependent Convergence Protocol (SNDCP) unit that is sent via LLC ( logical link control ) protocol over the air interface to the SGSN currently serving the mobile.

For mobile terminated data routing (see Figure 5), we have two cases:

1. Routing to the home GPRS network, and
2. Routing to a visited GPRS network.

In the first case, a user sends a data packet to a mobile. The packet goes through the local area network (LAN) via a router out on the GPRS context for the mobile. If the mobile is in a GPRS idle state, the packet is rejected. If the mobile is in standby or active mode, the GGSN routes the packet in an encapsulated format to SGSN. In the second case, the home GPRS network sends the data packet over the inter-operator backbone network to the visiting GPRS network. The visiting GPRS network routes the packet to the appropriate SGSN.

3) Security in GPRS

The General Packet Radio Service (GPRS) allows packet data to be sent and received across a mobile network (GSM). GPRS can be considered an extension to the GSM network to provide 3G services. GPRS has been designed to allow users to connect to the Internet, and as such is an essential first step toward 3G networks for all mobile operations. In GPRS, TMSI is replaced by P-TMSI and P-TMSI signature as alternative identities. The HLR GPRS register maps between internet protocol (IP) addresses and IMSI.

GPRS security functionality is equivalent to the existing GSM security. Authentication and encryption setting procedures are based on the same algorithms, keys, and criteria as in GSM systems which are A3, A5 and A8.

GPRS provides identity confidentiality to make it difficult to identify the user. This is achieved by using a temporary identity where possible. As in GSM, the device is authenticated by a challenge response mechanism. This only verifies that the smart card within the device contains the correct key.

GPRS does not provide end-to-end security so there is a point where the data is vulnerable to eavesdropping or attack. If this point can be protected, e.g., in a physically secure location, this is not a problem. However, if end-to-end security is required, there are other standards that can be used over GPRS; such as the wireless application protocol (WAP) and Internet protocol security (IPSec). In GPRS, authentication is performed by serving GPRS support node (SGSN) instead of VLR. The encryption is not limited to radio part, but it is up to SGSN. An IP address is assigned after authentication and ciphering algorithm negotiation.

4) Benefits of GPRS

GPRS technology brings a number of benefits for users and network operators alike over the basic GSM system. It was widely deployed to provide a realistic data capability via cellular telecommunications technology.

GPRS technology offered some significant benefits when it was launched:

• Speed: One of the most important benefits of GPRS technology is that it offers a much higher data rate than was possible with GSM. Rates up to 115.2 kbps are possible, although the maximum data rates realistically achievable under most conditions will be in the range 15 - 40 kbps.
• Packet switched operation: Unlike GSM which was used circuit switched techniques, GPRS technology uses packet switching in line with the Internet. This makes far more efficient use of the available capacity, and it allows greater commonality with Internet techniques.
• Always on connectivity: A further advantage of GPRS is that it offers an "Always On" capability. When using circuit switched techniques, charges are based on the time a circuit is used, i.e. how long the call is. For packet switched technology charges are for the amount of data carried as this is what uses the services provider's capacity. Accordingly, always on connectivity is possible.
• More applications: The packet switched technology including the always on connectivity combined with the higher data rates opens up many more possibilities for new applications. One of the chief growth areas that arose from GPRS was the Blackberry form of mobile or PDA. This provided for remote email applications along with web browsing, etc.
• CAPEX and OPEX: The Capital expenditure (CAPEX) and operational expenditure (OPEX) are two major concerns for operators. As GPRS was an upgrade to existing GSM networks (often implemented as a software upgrade achieved remotely), the capital expenditure for introducing GPRS technology was not as high as deploying a complete new network. Additionally OPEX was not greatly affected as the basic base-station infrastructure remained basically the same. It was mainly new core network elements that were required.

The GSM and GPRS elements of the system operated separately. The GSM technology still carried the voice calls, while GPRS technology was used for the data. As a result voice and data can be sent and received simultaneously. Some people refer to the system as GSM GPRS.

In order to further develop the capability of GPRS, further advances were made and another system known as EDGE or Enhanced GPRS, EGPRS was developed.