How does SMS work?

What is SMS?

SMS stands for short messaging service. It's a way to send text over wireless phone systems. The text appears on your handheld's screen, or in the case of a cellular phone, on the phone's display. SMS messages are text messages of up to 160 characters.
No phone call is required to send or receive a text message. In fact, it is possible to send and receive messages on Handspring phones while on a voice call. SMS has been around since 1991, has gained huge usage in Europe and Asia, but has come into widespread use in North America recently. SMS is a universal text messaging system but has come to be associated with its prime adopter, the GSM/GPRS wireless networks.

How SMS works
SMS works on a store-and-forward basis. Instead of being sent directly to the recipient, SMS messages travel through several important nodes before reaching the recipient (skip this part if you find it boring; there's more good stuff below).

• The SMS message is submitted to your wireless service provider's SMS Center.
• After the message is processed internally, the SMS Center sends a request to the Home
• Location Register (HLR) and receives the routing information for the recipient.
• The SMS Center sends the message to the Mobile Switching Center (MSC).
• The MSC collects the recipient's information from the Visitor Location Register (VLR) and, sometimes, proceeds with an authentication operation.
• The MSC forwards the message to a Mobile Server.
• If the recipient has a Treo or VisorPhone, the SMS is placed on the SIM card until the handheld picks up the message and transfers it to the internal memory on the unit.
• This is how Treos and VisorPhones can store hundreds of SMS messages, unlike ordinary GSM mobile phones that store only a few SMS messages.
• Treo or VisorPhone owners may encounter the error message "Your SIM contains at least one SMS message. Do you want to move the message to your Visor?" The MSC returns the outcome of the Forward Short operation to the SMS Center.
• The SMS Center reports delivery status of the short message back to the sender.

What are the components of an SMS message?

• The actual text of the SMS message isn't the only thing that's being transmitted. Here are the elements of a complete SMS transaction:
• Header: identifies the type of message.
• Service Center Timestamp
• Originator Address: the phone number of the sender
• Protocol Identifier
• Data Coding Scheme
• User Data Length: tells how long the message is
• User Data: the message itself (140 bytes: 160 7-bit characters, or 140 8-bit characters)

You won't see any of these components except the User Data (the message itself).
When you send an SMS to an email address, the message is still sent to the SMS Email center, but then it is relayed to the email server of the recipient. The recipient's email address is actually embedded in the User Data portion of the message, transparently on a Treo, or manually on a VisorPhone