What is GPRS ?
GPRS (General Packet Radio Service) is a step between GSM and 3G cellular
networks. GPRS offers faster data
transmission via a GSM network within a range 9.6Kbits to 115Kbits. This new technology makes it possible for users to make
telephone calls and transmit data at the same time. (For example, if you have a
mobile phone using GPRS, you will be able to simultaneously make calls and
receive e-mail massages.) The main benefits of GPRS are that it reserves radio
resources only when there is data to send and it reduces reliance on traditional
circuit-switched network elements.
With GPRS, an IP data transmission protocol, which is characteristic of
computer networks, is being introduced to GSM. IP is a data transmission
protocol which is used in Internet, the largest computer network in the world
Main features of GPRS
Before introduction of GPRS, the radio capacity was used for calls and data
transmission within the GSM network in a rather inefficient way. For data
transmission the entire channel was occupied and was thus insufficiently used.
With the GPRS technology, the channel is used more efficiently owing to the
possibility of more than one user sharing the same channel. GPRS telephones user
several channels for data transfer thus facilitating greater transfer speeds.
The GPRS infrastructure and mobile phones support a data transmission speed
of up to 13.4Kbits per channel.
GPRS signaling and data traffic do not travel through the GSM
network. The GSM network is only used for table look up, in the Location
Register (HLR and VLR) data bases, to obtain GPRS user profile data.
Owing to the fact that more than one channel is used for downlink, the GPRS
mobile phones make possible greater data transmission speeds. There are several
types of phones with regard to the number of channels they use for data
Type 2+1 – two downlink channels and one uplink data transmission
Type 3+1 – three downlink channels and one uplink data transmission
Type 4+1 – four downlink channels and one uplink data transmission
The GPRS mobile phones can be classified into the following three classes in
terms of the possibility of simultaneous calls (via GSM) and data transmission
Class A – Simultaneous calls (via GSM) and data transmission (via GPRS)
Class B – Automatic switching between the GSM and the GPRS mode is
possible according to telephone settings.
Class C – Hand operated switching between the GSM and the GPRS mode
Data Transmission Speeds
The supported data transmission speed per channel is 13.4Kbits. Depending
on the type of phone, the following data transmission speeds are theoretically
Type 2+1: Receive 26.8Kbits and send 13.4Kbits.
Type 3+1: Receive 40.2Kbits and send 13.4Kbits.
Type 4+1: Receive 53.6Kbits and send 13.4Kbits.
In the core network, the existing MSCs are based upon
circuit-switched technology, and they cannot handle the GPRS style packet
traffic. Thus two new components, called GPRS Support Nodes, are added:
The SGSN can be viewed as a "packet-switched MSC;" it
delivers packets to mobile stations (MSs) within its service area. SGSNs send
queries to home location registers (HLRs) to obtain profile data of GPRS
subscribers. SGSNs detect new GPRS MSs in a given service area, process
registration of new mobile subscribers, and keep a record of their location
inside a given area. Therefore, the SGSN performs mobility management functions
such as mobile subscriber attach/detach and location management. The SGSN is
connected to the base-station subsystem via a Frame Relay connection to the PCU
in the BSC.
GGSNs are used as interfaces to external IP networks such as the
public Internet, other mobile service providers' GPRS services, or enterprise
intranets. GGSNs maintain routing information that is necessary to tunnel the
protocol data units (PDUs) to the SGSNs that service particular MSs. Other
functions include network and subscriber screening and address mapping. One (or
more) GGSNs may be provided to support multiple SGSNs. More detailed technical
descriptions of the SGSN and GGSN are provided in a later section.
GPRS security functionality is equivalent to the existing
GSM security. The SGSN performs authentication and cipher setting procedures
based on the same algorithms, keys, and criteria as in existing GSM. GPRS
uses a ciphering algorithm optimised for packet data transmission.
Network Protocols Used
There are several protocols used in the network equipment. These
protocols operate in both the data and signalling planes. The following is a
brief description of each protocol layer:
Sub-Network Dependent Convergence Protocol (SNDCP):
the protocol that maps a network-level protocol, such as IP or X.25, to the
underlying logical link control. SNDCP also provides other functions such as
compression, segmentation and multiplexing of network-layer messages to a
single virtual connection.
Logical Link Control (LLC): a data link layer
protocol for GPRS which functions similar to Link Access Protocol - D (LAPD).
This layer assures the reliable transfer of user data across a wireless
Base Station System GPRS Protocol (BSSGP): BSSGP
processes routing and quality of service (QoS) information for the BSS.
BSSGP uses the Frame Relay Q.922 core protocol as its transport mechanism.
GPRS Tunnel Protocol (GTP): protocol that tunnels the
protocol data units through the IP backbone by adding routing information.
GTP operates on top of TCP/UDP over IP.
GPRS Mobility Management (GMM): protocol that
operates in the signalling plane of GPRS and handles mobility issues such as
roaming, authentication, and selection of encryption algorithms.
Network Service: protocol that manages the
convergence sub-layer that operates between BSSGP and the Frame Relay Q.922
Core by mapping BSSGP's service requests to the appropriate Frame Relay
BSSAP+: protocol that manages paging for voice and
data connections and optimizes paging for mobile subscribers. BSSAP+ is also
responsible for location and routing updates as well as mobile station