How SMS Works

Here’s a summary of my understanding of 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.

  1. The SMS message is submitted to your wireless service provider’s SMS Center.
  2. 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.
  3. The SMS Center sends the message to the Mobile Switching Center (MSC).
  4. The MSC collects the recipient’s information from the Visitor Location Register (VLR) and, sometimes, proceeds with an authentication operation.
  5. The MSC forwards the message to a Mobile Server
  6. The MSC returns the outcome of the Forward Short operation to the SMS Center.
  7. The SMS Center reports delivery status of the short message back to the sender.

Remember for serving a search warrant on undelivered SMS, they are stored at the originator’s SMSC. Here is a graphic

SMS Summary

In relation to the great debate about interception of communications, here is an US interpretation:

  • In order for the SMS to fall under wiretap (Title III in US, RIPA for UK) the SMS would need to be recieved in real time.
  • Since the SMS is in fact stored at the provider level it is not in real time
  • Therefore, considering the second point, the SMS is a stored electronic communication and subject to the Electronic Comunications Privacy Act (ECPA) and can be obtained via probable cause and a search warrantHope this is helpful to the Community.




  1. Fred

    if the sender of the SMS is in the US and the receiver in Europe, will the SMSC send the SMS to another SMSC based in europe so that the receiver could receive the SMS or the receiver will receive the SMS directly from the SMSC of the sender ?

  2. Mo Leung

    May be. The international SMS delivery have two approach:
    1) the SS7 signaling between the Home operators and the receiver’s operators
    2) by some international SMS hubbing provider.

    If it is using the first approach, the SMS will not send the SMS to the SMSC based in Europe but if the home operator is using the second approach, the SMS message will send to the SMS hubbing provider then the SMSC of the receiver’s operator.

  3. Kunle

    I have always wondered if SMS is sent and delivered to the recipient solely by the Originator’s SMS Center or does the recipient SMS Center gets involved at all. You may be surprised that many articles and text book do not specify this point. They just allude to the ‘SMS Center’?! Yes but whose’s SMS Center?!

    Thanks very much Mike, I can now lay this question to rest in my mind. Thanks a lots