3.2 Technology Capabilities
3.2.1 GPRS
GPRS gives a direct link between the worlds of the Internet and mobile communications.
GPRS is different from current GSM data services since it is a packet-based IP connectivity solution supporting a wide range of enterprise and consumer applications. GPRS allows users to instantly access data, as they would use their office Local Area Network (LAN). The mobile user does not have to connect to the network each time he wants to transfer data; they can stay connected all day. GPRS enables instant, always connected applications. With throughput rates of up to 48 Kb/s (utilising coding schemes 1 & 2) using four time-slot devices, users have the same effective access speeds as a modem but with the convenience of being able to connect from anywhere (Rysavy, 2002). The technology enables users to be
charged for the actual amount of data they transfer. This makes a completely new area of mobile data applications possible. GPRS is a significant development step toward 3G, which is why it is also classed as being "2.5G" technology. GPRS is now available worldwide in over 65 countries, with service from over 200 operators with a choice of over 85 handsets (Ericsson, 2002a).
GPRS is the first GSM Phase 2+ service that requires major changes in the network infrastructure, bringing packet switching into a circuit-switched transmission mode GSM network. Standard GSM uses circuit-switched connections, i.e. each time a connection is required between two points, a link between the two points is established and the resources used are reserved for the use of that single call for the complete duration of the call. Data networks, such as the Internet and LANs, use packet-switched connections. With packet switching, the data to be transferred is divided into packets, which are then sent through the network and re-assembled at the receiving end. The digitised contents (text, images, tones, software etc.) are broken down into small data packages, compressed, and coded. These data packages are then sent to the recipient in "packaged" form more or less in the "gaps" arising in voice communication (Ericsson, 2002a).
GPRS upgrades GSM data services to be more compatible with other data networks, such as LANs, WANs and the Internet. The physical radio path used by the GPRS service is based on the same principles as the current GSM system. The same frequency domain is used, thus allowing the GPRS and circuit switch traffic to use the air interface simultaneously. To support GPRS, additional network elements are added to the current GSM network. These are the Serving GPRS Support Node (SGSN), Gateway GPRS Support Node (GGSN) and GPRS backbone (Trillium, 2000). An illustration of the network architecture is shown in Figure 3.1.
FIGURE 3.1: GSM/GPRS NETWORK ARCHITECTURE
Adapted From: Third Generation (3G) Wireless White Paper. [Online]. Available at http://www. trillium.com/pages/white papers (Last accessed: March 2004).
GPRS is essentially the addition of a packet-data infrastructure to GSM. The functions of the data elements are as follows:
1. The base station controller (BSC) directs packet data to the SGSN, which is an element that authenticates and tracks the location of mobile stations.
2. The SGSN performs the types of functions for data that the mobile switching centre (MSC) performs for voice. There is one SGSN for each serving area and it is often co-located with the MSC.
3. The SGSN forwards user data to the GGSN, which is a gateway to external networks. There is typically one GGSN per external network such as the Internet.
The GGSN also manages IP addresses, assigning IP addresses dynamically to mobile stations for their data sessions (Rysavy, 2002).
In 2002, MTN had to purchase the new and expensive GPRS nodes (the SGSN and GGSN) in order to offer the new data service. There were also software upgrades required for the switching centres and the base stations. No hardware changes were needed for the base stations, any of the switching nodes or the Home Location Register (HLR).
With the GPRS standard, charging is no longer based on the length of transmission time instead the basis is the volume of data exchanged or the type of service. It means terminals can be "always on" and display arriving messages without delay. Depending on the coding, much faster data transmission rates can be achieved per timeslot with GPRS than the 9.6 Kb/s with GSM. In the GPRS standards there are four different channel coding schemes: CS-1 to CS-4. CS-1 offers the highest error correction, using half-rate convolutional coding and the lowest user data rate. CS-2 and CS-3 are punctured versions of the half rate code used in CS-
1, while CS-4 achieves the highest data rate by providing no error correction to the user data.
Table 3.2 shows the data rates for the different coding schemes (Ericsson, 2002a).
Channel Coding Scheme CS1
CS2 CS3 CS4
Single Timeslot Data Rate 9.05
13.4 15.6 21.4
Table 3.2: Data Rates for GPRS
Adapted From: Ericsson. (2002a). EDGE- Introduction of High Speed Data in GSM/GPRS Networks.
Sweden: Ericsson AB.
The rates shown above is for one channel, so a Mobile Station (MS) using all eight timeslots could achieve a theoretical data rate of 171.2 Kb/s with CS-4.
While GPRS is indeed an evolutionary technology, it has however demonstrated some limitations. According to Rysavy (2002), GPRS impacts on a network's existing cell capacity. There are only limited radio resources that can be deployed for different uses - use for one purpose precludes simultaneous use for another. For example, voice and GPRS calls both use the same network resources. The extent of the impact depends upon the number of timeslots, if any, that are reserved for the exclusive use of GPRS. In the radio link, GSM uses radio channels of 200 kHz width, divided in time into 8 time slots that repeat every 4.6 ms.
The network can have multiple radio channels (referred to as transceivers) operating in each cell sector. The network assigns different functions to each time slot, such as circuit switched
functions like voice calls or the packet broadcast control channel and packet data channels.
The network can dynamically adjust capacity between voice and data functions and can also reserve a minimum amount of resources for each service (Ericsson, 2002a).
Ericsson (2002b), have demonstrated that achieving the theoretical maximum GPRS data transmission speed would require a single user taking over all 8 timeslots without error protection. Clearly, it is unlikely that a network operator will allow all timeslots to be used by a single GPRS user. The bandwidth available to a GPRS user will, therefore, be severely limited. The reality is that mobile networks are always likely to have lower data rates than fixed networks. Relatively high mobile data speeds may not be available to individual mobile users until EDGE or UMTS are introduced.