Messaging
Every digital, and even some analog, mobile phone systems already incorporate some form of messaging. This allows subscribers to receive and sometimes send short text messages—essentially the same as paging, but with the data appearing on a mobile phone instead of a separate pager.
In theory, messaging services should allow people to receive email through their mobile phones and to dispense with pagers altogether. But in practice, neither of these objectives has been achieved, even in markets such as Europe, where every phone can receive messages and more than 90 percent can send them. The reasons are partly technical, thanks to length limits imposed by the very low data capacity, but mostly commercial.
Most operators charge for SMS using the same model as for phone calls themselves—whoever sends the message pays for it. This allows customers to control their own costs; no matter how many messages they are sent, they won't be charged unless they decide to reply. Some operators have begun to offer email to SMS gateways, where a customer can be sent a message at an address something like [email protected], but there is no way to charge for this. It also opens a network's customers up to the possibility of spam, so many operators place a limit on the number of messages that can be sent to a single user or from a single email address.
Similarly, mobile phone operators are not geared up to deal with the type of services offered by paging companies. Most of the messages sent to a pager originate on the regular telephone network—to page someone, the user calls a human operator, who transcribes the message and transmits it. Setting up call centers that employ actual people is very expensive, and many operators also have interests in paging businesses, which they are reluctant to cannibalize.
Operators have made slightly more progress in interconnecting their own messaging networks so that customers of one operator can send messages to those of another. Unfortunately, most are more keen to form international alliances than to cooperate with their direct competitors at home. The result is that it can be easier to send a message from Paris to London than to send a message within the same city.
Digital cellphones use three types of messaging services, though two are so far only supported by GSM. These are summarized in Table 6.1, and described below.
| Service Name | Message Length | Phone system | Direction |
|---|---|---|---|
| SMS (Short Message Service) | 140–260 bytes | All digital technologies | Two-way and one-way |
| CBS (Cell Broadcast Service) | 1395 bytes | GSM only | One-way only |
| USSD (Unstructured Supplementary Services Data) | 182 bytes | GSM and UMTS | Two-way only |
SMS
The only messaging standard to have achieved widespread acceptance is the Short Message Service (SMS). It began as part of the original GSM specification, but has since spread to all the other digital systems, some of which have improved on it. Limited versions have also been standardized for the AMPS and NMT analog systems, though few operators have implemented these, as they would prefer that customers upgraded to digital.
SMS's greatest limitation is hinted at in its name: messages have to be short. GSM imposes a limit of only 160 bytes, or characters—the length of this paragraph.
The length limit is caused by the way that SMS is transmitted. It usually rides on the control channels, the same frequencies or time slots used for call setup information. This means that users can send or receive SMS messages while they are making a phone call, though they need a hands-free kit to read the screen or type on the keypad. Different systems use different types of control channels, so the precise limit depends on the system. Table 6.2 shows a full list. The exception is the analog NMT, which uses the same channels as phone calls themselves. This means that message size is limited only by the handset's memory, but prevents simultaneous talking and messaging.
A byte usually corresponds to a single alphanumeric character, but it doesn't have to. The AMPS system can use its 14 bytes to represent numbers instead, allowing up to 33 digits. This is useful for sending someone a long phone number or credit card details. At the other end of the scale, many Asian countries need more complicated character sets, such as the Japanese or Chinese alphabet. GSM, cdmaOne, and PDC can all support these, but allow only around 70 characters because each one requires two bytes.
SMS is known as a store-and-forwardservice because messages may be stored for a few seconds before being transmitted. In this respect, it is like email or the regular postal service—a sender has no guarantee that the message will be received with a certain time limit and no automatic acknowledgement when it does. This makes SMS unsuitable for interactive services.
Some implementations of SMS only allow messages to be sent from the network to the mobile phone, and are known as Terminate Only. Others allow two-way messaging, called Terminate and Originate. SMS support also needs to be built in to the phone. Every GSM phone can terminate SMS and most manufactured since 1996 can also originate it, but for other standards, many low-end phones can only terminate, or they lack SMS altogether.
| Technology | Message Length | Simultaneous Speech | Direction | |
|---|---|---|---|---|
| Digital | GSM | 160 bytes | Yes | Terminate and Originate |
| D-AMPS | 160 bytes | Yes | Terminate Only | |
| cdmaOne | 256 bytes | Yes | Terminate and Originate | |
| PDC | 160 bytes | Yes | Terminate and Originate | |
| TETRA | 260 bytes | Yes | Terminate and Originate | |
| iDEN | 140 bytes | Yes | Terminate and Originate | |
| Analog | NMT | No limit | No | Terminate and Originate |
| AMPS | 14 bytes | Yes | Terminate Only | |
CBS
If the same information needs to be sent to many different users, broadcasting is more efficient than transmitting a separate transmission to each one. This is the theory behind the Cell Broadcast Service (CBS), a variant of SMS used only in GSM. Each message is known as a page and can be only 93 bytes long, but up to 15 pages can be concatenated for a total message length of 1395 bytes—enough for several paragraphs of text or a short program.
Despite these clear technical advantages, CBS has not been widely deployed. The main reason is that it offers operators no way to charge for the services; by definition, a broadcast message can be picked up by anyone within a cell. In theory, there should be a way to overcome this using GSM's encryption facilities, but no operator has managed it yet. A message has to be intended for just one user, or for everybody within a cell.
USSD
GSM networks have access to a third messaging technology, Unstructured Supplementary Services Data (USSD). Like SMS, it uses the control channel and can operate while a phone is in use. Messages are slightly longer, with a maximum of 182 bytes compared to the 160 bytes of SMS under GSM.
The main advantage of USSD is that it is connection-oriented. This means that the network establishes a connection with the phone before sending any data, rather like a phone conversation. The sender knows when a message has been received, and if necessary, a response can be sent very rapidly over the same connection. This makes it ideal for interactive applications such as Web browsing.
USSD is designed to be accessed by programs running within a phone rather than directly from a menu-driven user interface. However, people willing to enter long strings of code numbers can sometimes use it to access network features otherwise unavailable from their particular phone. To distinguish them from ordinary phone numbers, these strings usually begin with the * and # keys.
Though GSM is the only existing system that supports USSD, it has also been written into the specification for UMTS, the European third-generation standard. The other two messaging standards have not, and so the GSM Association expects their use to start declining from about 2005.
Applications of Messaging
Though messaging is not really email or paging, it is starting to find applications of its own. The most popular are:
Voice Mail Notification. Almost all mobile operators provide their customers with a voice mail facility, which is used while the phone is switched off or is out of range. Once a customer reestablishes a connection with the network, she receives an SMS telling her about the voice mail. Most phones intercept these messages and illuminate an icon so that the customer does not have to read and delete "You have voicemail" several times a day.
Reprogramming. Most phones can have some of their software updated remotely, using SMS or CBS messaging. Customers can also download new ring-tones.
Advertising. Though operators rarely tolerate blatant spam from outsiders, some like to use CBS to tell their customers about new features or special offers. Most also send SMS reminders to people whose bills are overdue.
Broadcast Information Services. Many operators are beginning to offer simple information services using CBS. These are free to the customer and give basic news headlines as they happen. Unfortunately, there is usually no way to choose between types of headlines—everything is broadcast to everyone. If customers want news, they have to take the whole package of finance, sports, and weather with it.
Specialized Information Services. SMS can be used to notify people of particular events, such as stock prices reaching a certain level or plane flights being delayed. Operators usually charge extra for these types of services, but customers are often prepared to pay.
Person-to-Person Messaging. This is the fastest growing area of messaging and requires a phone with SMS Originate capability. It is particularly popular among primary-school children, who usually don't have much money and know that sending text costs less than making a voice call. They also have the time to type in messages on a phone's awkward keypad, often while in class. Nokia says that SMS has replaced passing notes around class in Finnish schools, and estimates that teenagers send an average of more than 100 messages each month. Because of input difficulties and space limits, most are written in a code that resembles early telegrams, for example, "c u 2nite."
The SIM Toolkit
At the heart of every GSM phone is a SIM (Subscriber Identity Module), a tiny smart card that stores each customer's identity and other information, such as their phone book. The theory was that cards could be swapped around between phones operating on different frequencies, and that phones themselves could be upgraded by replacing the SIM rather than the whole phone. The former worked, though was rendered unnecessary by the arrival of dual-band and triple-band phones. The latter didn't, as improvements in battery and electronic technology usually render the rest of the phone obsolete long before the card.
Some people like to keep two or more SIM cards so that they can use their phones to access different networks without paying roaming charges. A few phones are even specially designed to hold two or more cards for this purpose, with information such as the user's personal directory stored on the phone itself.
The SIM Application Toolkit is a standard that gives the SIM access to the full range of phone features, including messaging services. The operator can send messages to the SIM to reprogram it, adding specific features or even applications. These might range from a simple program allowing the user to switch between business and personal lines to a more complicated one offering mobile banking. When the program runs, it can then send messages as the user enters or requests data. A mobile banking application might request the user's balance using USSD, then send an instruction to transfer money via SMS.
The big disadvantage of the SIM Toolkit is that many phones and operators do not support it. Though first introduced early in 1998, it has suffered from a much slower take-up than most wireless technologies. It is perceived as too complex and has been overshadowed by technologies that promise—but usually fail to deliver—mobile Internet surfing.