Mobile Phone Technology

SMS (Short Messaging System) is available on all mobile phones. A user can send and receive SMS messages via an app on their phone. An SMS message is limited to 1,120 bits, which are coded as either 160 seven-bit characters or 140 eight-bit characters. Therefore, SMS is a basic messaging system.

It was a key feature of 2G phones, and the first SMS was sent in the UK by a Vodafone engineer on 3 December 1992. It read, "Happy Christmas".

The customer SMS service evolved from the system initially used by service engineers to communicate with each other. However, the MNOs soon realised that this would be a useful feature for customers and that it could generate revenue. Initially, each SMS message cost 10p. Later, SMS messages were bundled into contracts, and now unlimited messages are included in all but the most basic customer packages.

SMS messages are sent over the control channel, rather than the data network. This channel manages phones and devices on the network, and messages are sent through it. As signal strength deteriorates with distance from the antenna, data services (HTTP connections) are lost first. Then, the phone connection may become crackly before ultimately being lost, leaving the SMS system as the most reliable form of communication in areas with poor or limited connectivity.

When data is sent over HTTP, it is the client that requests the data — this is a form of pull technology. SMS uses a different technology called 'push'. SMS messages can be sent at any time and do not rely on the recipient being online. Rather than the recipient having to look for messages, these SMS messages are pushed to their phone from the SMSC (Short Messaging Service Centre). SMS messages are fundamentally different from other messaging systems.

Sending an SMS

• The sender creates the SMS message within an app on their phone.
• The app sends the message to the SMSC (Short Message Switching Centre) via the control channel. The phone number of the local network SMSC can be found within the phone's settings.
• The SMSC stores the message and then finds the recipient.
• If the recipient is on the same network as the sender, the SMSC sends the message to the recipient.
• If the recipient's phone is on a different network, the SMSC forwards the message to the other network's SMSC, which stores the message and forwards it to the recipient.
• It is possible to track SMS messages. The sender will know when the recipient has received the message.

In the era of 4G/5G, SMS is considered outdated. In most phone systems, it has been replaced by RCS messaging systems. However, RCS systems default to SMS if the RCS app has difficulty sending the message. Other popular messaging systems include WhatsApp and iMessage, but these are sent as data and will not work when there is a poor data connection, whereas SMS can still work. Many organisations still use SMS to remind customers of appointments, inform them of outages in key services, and for other business applications. These are powered by bulk SMS providers. Sadly, just as email is, SMS messages are also used to scam phone users.

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