Fast GSM,
Datalink over 9600

9.10.1998

Teppo Koskinen
Department of Computer Science and Engineering
Helsinki University of Technology
koskinen@cc.hut.fi

Abstract

GSM is a widely used digital mobile phone standard developed in Europe. Today it is used in more than 40 countries and just in Finland it has more than 1.5 million subscribers.

1 Introduction

GSM data services are available in almost every GSM network and the usage is growing rapidly. Current service quality is enough for transferring small files or reading the email, but new services that need to transfer audio or video will need a greater data transfer capacity. To answer that need High Speed Circuit Switched Data (HSCSD) and General Packet Radio Service are under development.

2 Current GSM data transfer technologies

Currently GSM networks use two different data transfer technologies: CSD and SMS. Those are completely different technologies. Neither of them is fast enough for video applications but they are useful for some purposes. SMS is used to deliver textual information between phones and CSD is used with laptop computers to access Internet or LANs.

2.1 Short Message Service

There are two different kinds of SMS specified in GSM standards -Point to Point (SMS/PP) and Cell Broadcast (SMSCB). From the normal GSM phone user's point of view, the SMS is delivering short text messages from a GSM phone to another. This is the SMS Point to Point. Cell Broadcast functionality permits a number of unacknowledged general messages to be broadcast to all receivers within a particular region. Such messages could be used to deliver some local information like weather, traffic or advertising.

With SMS the maximum length of eight bit data message is 140 bytes. With text messages the maximum length is 160 characters. That is achieved by packing 7 bit GSM characters into a 8 bit data with SMS specific packing which.

The SMS PP is a connectionless protocol. That means that the delays are usually longer than with connection oriented protocols. The time needed to deliver one message between a phone and SMS service centre is typically from 3 to 5 seconds. As sending from a phone to another is actually concatenation of two deliveries the delay is cumulative and thus from 6 to 10 seconds. [4]

Conclusion is that SMS can be used to deliver text information, but not much else. One strength of SMS is that it uses the signaling channel to deliver messages. It means that phones can receive and send short messages even during calls.

2.2 Circuit Switched Data

CSD is the basic solution for data transfer in GSM network. Circuit switched means that that the data connection is always created just as voice call. Usually the connection establishment delay is around 15 seconds. The data transfer rate after connection establishment is 9600 kbit/s.

3 Future GSM data transfer technologies

To overcome the current problems with GSM limited data transfer capacity many new technologies are under development. Two of the most important ones that are coming to general use in near future are HSCSD and GPRS. [1]

2.1 High Speed Circuit Switched Data

HSCSD is a new service that can offer more than 9.6 kbit/s data transfer capacity to phones. In HSCSD phone can use more than one channel. Theoretically the maximum amount of channels is 8. But that may not be allowed in the beginning to avoid overloading the network. When HSCSD is established the minimum and maximum number of channels required by the mobile phone are registered. That also effectively specifes the data transfer rate. During the call the data transfer rate will not drop under the needed minimum and it wount rise above the upper limit.[3]

Currently about 100 billion USD is used to build GSM infrastructure. It means that the network operators are not willing to use new technologies that would require expensive changes in the network. HSCSD has been specified so that only minimum amount of changes are needed. It means that while the actual number of timeslots used in the air interface can change during the call, the channels needed between BSC and MSC must always be reserved for the maximum capacity.

Current phones are not HSCSD capable.

2.2 General Packet Radio Service

The standardization of the GSM GPRS is in progress at the ETSI. GPRS is a new GSM service that provides actual packet radio access for mobile GSM users. The main benefit of GPRS is that it reserves radio resources only when needed. The need for packet radio is based on the high burstiness of data applications, like Internet browsing using World Wide Web.

The GPRS provides a bearer service from the boundary of a data network to a GPRS MS. The users of the bearer service are the public network layer software packages (such as IP, OSI CLNP, and X.25). Also, GPRS-specific applications will use the GPRS service. The physical radio interface consists of a flexible number of TDMA time slots (from 1 to 8) and thus provides a raw data rate of almost 200 kbit/s.

The problem with the GPRS is that it requires more changes in the network than HSCSD.

# References

[1]	Hämäläinen J., Design of GSM High Speed Data Services, Väitöskirja, Tampere University of Technology, Tampere 1996 [referenced 9.10.1998] < http://www.forum.nokia.com/nf/magazine/papers/drtech.html >
[2]	Hämäläinen J. & Kari H., Packet Radio Service for the GSM Network, 1996 [referenced 9.10.1998] < http://www.forum.nokia.com/nf/magazine/papers/gprsbook.html >
[3]	Rostas P., Uudet Datapalvelut GSM verkossa, HSCSD:n ja GPRS:n tarkistelu, Seminaari esitelmä, Teknillinen korkeakoulu, Sähkötekniikan osasto, Espoo 1997 [referenced 9.10.1998] < http://keskus.hut.fi/opetus/s38116/1997/esitelmat/42690u/ >
[4]	Short Message Service of GSM Networks, Nokia Forum Magazine, 1996 [referenced 9.10.1998] < http://www.forum.nokia.com/nf/magazine/papers/gsmsms.html >

# Further Information

Wireless Application Protocol Architecture Specification, WAP forum, 30.4.1998

Information about Internet style browsing protocol developed for cellular networks