How SMS Works

Overview

SMS messages are transmitted to a subscriber mobile number via an SMSC (Short Message Service Centre). In general, a mobile operator owns an SMSC, but it is also possible for 3rd parties to operate their own SMSC that is connected to a mobile operator's network.

SMSCs are not restricted to sending SMS just to the subscribers of the mobile network the SMSC belongs to, they can send to any international mobile subscriber that the mobile network, or its backbone provider, has interworking or roaming agreements with. Therefore, a direct connection to the SMSC of each destination mobile network is not required.

The most widely used technology for transmission and reception of SMS is GSM. Other standards include CDMA, iDEN, and Satellite.

CardBoardFish connects to approximately 70 global SMSC’s, which provides unparalleled coverage of over 200 countries, and the ability to continue sending SMS without interruption even if multiple SMSC’s experience problems. Please see the article on SMS Reliability for more information.

SMS Specification

A standard GSM SMS offers the following features:

• Up to 160 characters of plain text per SMS (utilises GSM Encoding)
• Support for concatenated and long SMS (using multiple SMS parts)
• Ability to set the originator (sender ID) to up to 11 alphanumeric characters or 16 digits
• Support for setting the data coding allowing for specific character sets and data types including:
  • Unicode UCS2
  • Binary for ringtones, wap push, vCards etc
  • GSM encoding
  • Other SMSC specific character sets such as Greek ISO-8859-7
• Delivery reporting with error codes
• Validity period
• Retry Schedule

What’s in a Mobile Number

MSISDN (Mobile Subscriber Integrated Services Digital Network number) is the technical term used to describe a mobile number in international format. However, the MSISDN is not a direct mapping to a mobile handset, or even to the SIM (Subscriber Identity Module) card. An MSISDN allows an SMSC to determine the country and mobile operator that a subscriber number belongs to; and from that the SMSC can query this mobile operator’s HLR (Home Location Register) for information used to route the SMS to the handset.

Example

The international MSISDN 447973000000 is broken down as follows:

• 44 = Country Code for United Kingdom
• 7973 = A network prefix or exchange prefix for Orange UK
• 000000 = Subscriber number

An SMSC would send a “Send Routing Info” (SRI); request to the correct HLR to obtain the address of the MSC (Mobile Switch Centre) that is serving the destination SIM, and to obtain the IMSI (International Mobile Subscriber Identity) in order to address the SMS correctly.

The IMSI address is made up of 3 parts:

• MCC (Mobile Country Code)
• MNC (Mobile Network Code)
• MSIN (Mobile Subscriber Identity Number)

The HLR can also return other information such as the status of the subscriber (absent, invalid number, available), and even specific error codes like “memory full”.

SMS Delivery to Handset

After an SMSC has received the routing data from the destination network HLR, it can then send an "mt-ForwardSM" transaction to the MSC address, addressed to the IMSI. Note that the IMSI address is stored in the SIM, and it is this address that is given to the MSC when delivering an SMS, not the MSISDN.

The SMSC is notified by the MSC if the SMS termination (in acknowledgement) was successful, therefore allowing the SMSC to generate a delivery report indicating success. In case of SMS delivery failure, the SMSC is also notified so that it can take steps to retry SMS delivery. Please see article on SMS Reliability.

This method of SMS transmission is referred to as MT (Mobile Termination), which concerns the role of the SMSC in delivering an SMS to a subscriber handset.

In this case, the ESME (External Short Messaging Entity) refers to the party originating the message.

Mobile to Mobile SMS

SMS that are sent from a mobile handset are referred to as MO (Mobile Originated). This describes the process of a handset sending a Short Message Submission to its servicing MSC, and the MSC utilising the mo-ForwardSM transaction to send the SMS to the SMSC for storing, and subsequent forwarding to the destination mobile number. An acknowledgement is also sent back to the originating handset.

SS7 Network

The majority of the world’s mobile networks communicate with each other via Signalling System 7. This is a set of telephony protocols that enables the transmission of voice, data, control/session management information, and of course SMS.

The SS7 protocol has layers based on the OSI (Open Systems Interchange) model. The MAP (Mobile Application Part) layer provides the functionality required for SMS transmission. The SMSC, HLR, VLR (Visitor Location Register), and MSC communicate together over the SS7 network utilising MAP commands.