How is SMS delivered?

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Every cell phone user has experienced text message service. It seems to be so easy. Just type and send a message and the recipient receives it. But actually, this simple process is completed by a set of complicated tasks that run in the background. Let us understand the process how a SMS works.

  • 1. A message sent by the sender is first delivered to the mobile operator’s SMSC where it is processed.
  • 2. The SMSC sends request to Home Location Register (HLR) and receives the routing information.
  • 3. SMSC then sends the message to Mobile Switching Centre (MSC).
  • 4. MSC collects the recipient’s information from the Visitor Location Register (VLR) and authenticates it.
  • 5. MSC then forwards the message to a mobile server.
  • 6. MSC returns the outcome of the Forward Short Operation (FSO) to the SMSC.
  • 7. SMSC reports the delivery status of the SMS back to the sender.

Now let us understand the complexities. Even when you are not using the cell phone, it is constantly talking to the cell phone tower over a pathway called the ‘Control Channel’. This is how the network keeps a track as to in which cell location your cell phone is currently located. Control Channel is also used for call set-up i.e. when someone dials your number you end up receiving the intended call. When someone calls you, the mobile tower sends a message to your phone over the Control Channel which then directs your phone to play the ringtone. It also provides you with a pair of voice frequencies so that two way chat is possible between the caller and the receiver. Similarly, the control channel also provides a pathway for text message. SMS sent to SMSC is eventually sent to the mobile tower that sends the packet of data to your phone over the control channel. The information in the text message is sent in the form of ‘0’ and ‘1’ i.e. binary codes.

Very often an SMSC is dedicated to handle the SMS traffic of one wireless network. A network operator usually manages its own SMSC(s) and locates them inside its wireless network system. However, it is possible for a network operator to use a third-party SMSC that is located outside the wireless network system.

You must know the address of the wireless network operator's SMSC in order to use SMS messaging with your mobile phone. Typically an SMSC address is an ordinary phone number in the international format. A mobile phone should have a menu option that can be used to configure the SMSC address. Normally, the SMSC address is pre-set in the SIM card by the wireless network operator, which means you do not need to make any changes to it.