General Packet Radio Service(GPRS)

In the mid-1980s voice calls were the most important service for fixed and wireless networks. This is the reason why GSM was initially designed and optimized for voice transmission. Since the mid-1990s, however, the importance of the Internet has been constantly increasing. GPRS, the General Packet Radio Service, enhances the GSM standard to transport data in an efficient manner and thus allows wireless devices to access the Internet.

Circuit Switched Data Transmission over GSM

The GSM network was initially designed as a circuit-switched network. All resources for a voice or data session are set up at the beginning of the call and are reserved for the user until the end of the call. The dedicated resources assure a constant bandwidth and end-to-end delay time. This has a number of advantages for the subscriber:

• Data that is sent does not need to contain any signaling information such as information about the destination. Every bit simply passes through the established channel to the receiver. Once the connection is established no overhead, e.g. addressing information, is necessary to send and receive the information.
• As the circuit-switched channel has a constant bandwidth the sender does not have to worry about a permanent or temporary bottleneck in the communication path. This is especially important for a voice call. As the data rate is constant, any bottleneck in the communication path would lead to a disruption of the voice call.
• Furthermore, circuit-switched connections have a constant delay time. This is the time between sending a bit and receiving it at the other end. The greater the distance between the sender and receiver the longer the delay time. This makes a circuit-switched connection ideal for voice applications as they are extremely sensitive to a variable delay time. If a constant delay time cannot be guaranteed, a buffer at the receiving end is necessary. This adds additional unwanted delay especially for applications like voice calls.

While circuit-switched data transmission is ideally suited for voice transmissions, there are a number of grave disadvantages for data transmission with variable bandwidth usage. Web browsing is a typical application with variable or ‘bursty’ bandwidth usage. For sending a request to a web server and receiving the web page, as much bandwidth as possible is desired to receive the web page as quickly as possible. As the bandwidth of a circuit switched channel is constant there is no possibility of increasing the data transmission speed while the page is being downloaded. After the page has been received no data is exchanged while the subscriber reads page. The bandwidth requirement during this time is zero. The resources are simply unused during this time and are thus wasted.

Packet Switched Data Transmission over GPRS

For bursty data applications it would be far better to request resources to send and receive data and release them again after the transmission. This can be done by collecting the data in packets before it is sent over the network. This method of sending data is called ‘packet switching’. As there is no longer a logical end-to-end connection, every packet has to contain a header. The header for example contains information about the sender (source address) and the receiver (destination address) of the packet. This information is used in the network to route the packets through the different network elements. In the Internet for example the source and destination addresses are the IP addresses of the sender and receiver.

To be able to send packet-switched data over existing GSM networks, the General Packet Radio Service (GPRS) was conceived as a packet-switched addition to the circuit-switched GSM network. It should be noted that IP packets can be sent over a circuit-switched GSM data connection as well. However, until they reach the Internet service provider they are transmitted in a circuit-switched channel. GPRS on the other hand is an end-to-end packet switched network and IP packets are sent packet switched from end to end.


The packet-switched nature of GPRS also offers a number of other advantages for bursty applications over GSM circuit-switched data transmission:

• GPRS is usually charged by volume and not by time. For subscribers this has the advantage that they pay for downloading a web page but not for the time reading it, as would be the case with a circuit-switched connection. For the operator of a wireless network it has the advantage that the scarce resources on the air interface are not wasted by ‘idle’ data calls because they can be used for other subscribers.
• GPRS dramatically reduces the call set-up time. Similar to a fixed-line analog modem, a GSM circuit-switched data call takes about 20 seconds to establish a connection with the Internet service provider. GPRS accomplishes the same in less than 5 seconds.
• As the subscriber does not pay for the time when no data is transferred, the call does not have to be disconnected to save costs. This is called ‘always-on’ and enables applications like email programs to poll for incoming emails in certain intervals or allows messaging clients like Yahoo or MSN messenger to wait for incoming messages.
• When the subscriber is moving, by train for example, it happens quite frequently that the mobile has bad network coverage or even loses the network completely for some time. When this happens, circuit-switched connections are disconnected and have to be manually re-established once network coverage is available again.GPRSconnections on the other hand are not dropped as the logical GPRS connection is independent of the physical connection to the network. After regaining coverage the interrupted data transfer simply resumes.