Mobile Network Services with Linux

GSM (Global System of Mobile Communications) short messages have become tremendously popular in just a few years. Popularity of Short Message Services (SMS) was not foreseen and surprised telecommunication companies, but its popularity had an effect. For wireless phones it meant larger displays for easier reading, predictive language input software and external keyboards. For telecommunication companies this meant improvements to message centers, new services and huge revenues. Such a large set of users sending messages to each other spurred operators to bring a huge set of SMS services to the markets, such as weather forecasts, timetables, sports results, stock information, etc. Of course all events do not have to be initiated by the user. You can also get positive notification by e-mail, network alarms, news, etc., on your wireless phone.

Nowadays close to 500 GSM operators exist in 158 countries. Most of the operators provide the ability to send SMS messages. SMS services also function abroad where the GSM operators have voice roaming agreements and SMS service is available.

SMS usage is still growing strongly. In October 1999, two billion SMS messages were sent. Three billion messages were sent in December 1999. Naturally, the millennium celebrations contributed to the increase, but still the growth was tremendous. In May 2000, about eight billion SMS messages were delivered, and ten billion messages per month were sent by December 2000. The price per message is something like $.25 US per message, so it is a cheap way to communicate.

Among youngsters, ring tones, logos and picture messages have achieved great popularity. These are also based on SMS technology, and there are lots of service providers who offer this kind of service, such as http://www.jippii.fi/. It is easy to come up with plenty of applications such as e-mail notification, burglar alarm, calendar events and network alarm.

SMS messages also have their weaknesses. The length of each message is limited to 160 characters. This forces users to send more messages to transfer larger amounts of data. For example, if you want to read an e-mail from a friend, she may have to send several SMS messages before you get the whole message. On the other hand, if you want to send an e-mail from your phone, you'd better use short expressions.

The major threat for SMS services is new technology. General Packet Radio Service (GPRS) is based on GSM technology and will complement existing services. It will also facilitate new applications that haven't been available over GSM networks. Compared to GSM, it is clearly faster, and unlike SMS, it doesn't have a 160 characters per message limitation. GPRS may decrease SMS traffic, but it will take a while for the mass of users to shift their mobile phones to GPRS-compatible models.

In North America, there were only 7.4 million GSM customers in June 2000, while in Western Europe the number was as high as 215.4 million. There would be huge markets in the US if GSM became more popular, but forecasts assume there will be only 21 million customers by the end of 2004. It is possible that by then the number of short messages won't increase further, but this is all speculation. Europe, especially Finland, seems to be full of SMS service providers, but there is still room for new services and new ideas. Using SMS can still be a hit anywhere in the world.

There are multiple ways to send a message to your wireless phone: another wireless phone, gateways for modem and Internet-based connections, internet sites or wireless modem.

Wireless phone companies offer gateways to their networks through which messages can be sent. Companies usually charge quite a bit for service initiation, monthly charges and per message charges. These charges are often too high for just occasional message sending and receiving. Internet sites, which allow people to send free short messages, are usually not very reliable. Their operation is usually based on temporary free SMS message-sending provided by some telecommunications company.

Our solution is to use a wireless modem. Connect a wireless modem to your computer and use it almost like a regular modem. Wireless PCMCIA (Personal Computer Memory Card International Association) versions are widely available, and many wireless phones can be connected via serial or infrared connection to your PC. There are specific AT commands (standardized command set for modems) for sending and receiving SMS messages and managing SIM-cards from terminal programs.

Figure 1. All SMS messages are routed via a SMS center regardless of the way they are sent.

SMS messages are sent to message centers and from there they are forwarded to the receiver. GSM allows users to roam from one network to another. The receiver does not have to be reachable at all times as messages are stored temporally at a message center. This is great news for people who do not have access to a network all the time. A traveler can send a message to a spouse from an airport in Great Britain, switch off the phone before flying to South Africa and receive reply messages upon switching the phone back on at arrival time. Note that SMS messages are not guaranteed to be reliable, and message centers purge them at some point. Expiration time is usually three days, but it is known to decrease to even one hour during extremely heavy traffic.