Basics

What is GPRS?

GPRS (General Packet Radio Service) is a service within the GSM network, just like the two most popular services SMS and voice connections. GPRS is used for transmitting data in the GSM network in from of packets. The connection to the remote station is not reserved and left open during the entire connect time, but is occupied only at the time of actual data transmission.

8 time slots and 4 coding schemes (CS) with different data transmission rates are available CS1 with 9,05 kbit/s, CS2 with 13,4 kbit/s, CS3 with 15,6 kbit/s, CS4 with 21,4 kbit/s.

This results a theoretical total data rate of 171,2 kbit/s when bundling all eight time slots with coding scheme 4. The effort for the error correction will be reduced with this and longer data blocks are transmitted. The adjustment of the transmission speed is made dynamically depending on the HF transmission characteristics by switching between the coding schemes. Average data rates of 58, which are bases on four time slots, can be expected in practice.

Fakten zu GPRS

• GPRS:General Packet Radio Service
• Dienst im GSM-Mobilfunknetz
• Verfügbarkeit fast flächendeckend
• Theoretisch 171,2 kbit/s
• Praktisch mittlere Datenraten von 58 kbit/s

What do you need for using GPRS?

• An application with a cable-connected GPRS modem
• An available GSM/GPRS network
• A SIM card with an activated GPRS service
• A remote station with access to Internet or the GPRS network

With the GPRS network, several channels are occupied at the same time for data transmission whenever data needs to be transmitted. Whenever a GPRS device is not receiving or sending data the channels are available for other services and other GPRS subscribers. By way of this dynamic allotment of resources the utilization of the existing radio infrastructure is clearly optimised. A drawback of the system is that the available bandwidth may drop when many users try to access the GPRS network at the same time. In addition, the data is also compressed by means of special coding processes (CS-1...CS-4) so as to ensure a further data rate enhancement.

The dynamic allotment of resources permits a method of billing which clears the way for many potential applications in industrial data communication. The billing is not based on connect time, but rather on the data volume transmitted. This enables applications with a permanent connection, i.e. a dedicated-line operation,... "always online". Caution is demanded, however, for monthly transmission volumes beyond 50 MB. In such cases GPRS may very quickly lose its cost advantage. The usual transmission volume in industrial communication, however, clearly remains below this critical volume, and is therefore ideal for using GPRS.

Whereas in a control station many field devices used to be read out in parallel by means of conventional technology via modem racks, GPRS affords considerable simplifications. Only one Internet access for the control station is required, modem racks are dispensable.
Also connection establishment is considerably faster using GPRS rather than using conventional modem technology. Field devices are often required to allow the fastest possible readout. Here GPRS has a clear advantage over other technologies.

Transmission speed

The maximum transmission speed in the GPRS network is theoretically 171,2 kBit/s. The devices available on the market offer a maximum transmission speed of 85,6 kBit/s. Practice shows that rather values around 50 kBit/s are achieved, which corresponds with an average transmission speed of an analogue 56k modem.

Important parameters for the choice of a contract

For choosing a provider and a GPRS tarif, the following data should be carefully compared:

• Basic charge per month with GPRS
• Included volume (1 MB, 5 MB, ...)
• Block size (1 kByte, 10kByte, ...)
• Billing period (e.g. 24 h)

Any prospective subscriber will pay attention to the basic charge and the included monthly volume in choosing a provider. But just as important is the GPRS tarif unit. Since usually no connect times are billed in the GPRS network the billing is based on the amount of bytes transmitted. The billing unit used is the block size unit. In current contracts, block sizes of one kilobyte are most frequent, whereas previously block sizes of 100 kilobytes were normal. That means for the customer that always a complete block will be billed per dial-in into the GPRS network, even if the data transmitted is actually less. With a block size of 100 kByte, this may easily result in a high bill.

Where to be cautious?

Of course there are cases of applications in which GPRS is not the ideal choice for transmitting data.

• In cases of a permanent transmission of huge amounts of data
• a need for a permanently available wide band
• protocols with a critical timing

Where is the use of GPRS of particular interest?

Here a short summary:

• Wherever there is a need for a dedicated line
• Where a rented line or a landline would be too costly because the location is somewhere "out in the nowhere"
• Where high costs for overseas connections are incurred
• Where a number of devices have to be read out in succession/simultaneously
• Where there is often a need for transmission of small data volumes.