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How a Mobile Phone works > SMS

Short Message Service (SMS) messages are 160 character text messages, sent using a SDCCH (slow speed data channel). This means that they can be sent at the same time as a fax or data call, but also means that they can be a little slow because of the limited bandwidth available to carry them.

SMS delivery is a store-and-forward system, where the message is sent to a Short Message Service Centre (SMSC), which then forwards them on to the destination mobile.

How quickly?
Generally, a SMS arrives within two or three seconds, but if there is a lot of traffic, or if there is another problem, there can be considerable delays. If the mobile is not available, the SMSC retries frequently at first, but soon falls back to only trying every few hours, and then to once or twice a day, so if your phone dips out of coverage for a while, incoming SMS can stack up in the system.

When a lot are queued, some networks disable your SMS mailbox to prevent the queue getting ever-longer.

when the handset receives the SMS, it tells the network, and optionally, senders can choose to be told when the message has been delivered. This sometimes increases the cost of sending the SMS, depending on the network's tariff.

In bad coverage, the destination mobile may not be able to acknowledge receipt, so the network may keep sending copies!

Where to?
In theory, you can send SMS messages to any GSM phone, wherever it is in the world, but some networks, particularly ones in the USA, are not really geared up to handle SMS reliably. Because of problems caused by bulk SMS "spammers", some networks refuse SMS from certain other networks.

Because of the popularity (and profitability) of text messaging, some fixed line phones are being produced with the ability to send and receive SMS text messages. As a result, you may be able to send SMS to fixed line phones as well as mobiles.

Where from?
You can usually use the standard home SMSC to send SMS wherever you are, even when you are roaming, although you may pay a hefty premium on the cost of sending them from some foreign networks.

What type?
Many mobile handsets allow you to change the SMS message type, allowing you to choose between sending SMS as Voice, Text, Fax, X.400, Paging, Email or ERMES. Don't be fooled: you can only send as text in the GSM networks currently in use. You can send email and fax by SMS, but this isn't the way to do it. See the User Tech section to find out how.

You can also set the expiry period of messages you send, so that if they're not delivered, they are dropped after a certain number of hours or days. You can't increase this past your mobile network's maximum, but you can reduce it if appropriate.

There are several types of special sms that can be sent: some networks are able to amend sim settings by SMS, and you can send custom ring tones, display graphics by SMS. How to do this is dealt with in the User Tech section, but the underlying mechanism is the same, although it requires the correct facilities in the SMSC. Some WAP implementations use SMS as a transport medium. See the WAP page for details of this.

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