GSM networks use a technique called circuit switching to establish connections between mobile phones and the network. This involves creating a dedicated path for data to travel through.
The GSM network uses a combination of frequency division multiple access (FDMA) and time division multiple access (TDMA) to allow multiple phones to share the same frequency band. This is essential for supporting a large number of users.
Each mobile phone is assigned a unique temporary mobile subscriber identity (TMSI) to identify it on the network. This TMSI is used to authenticate and authorize the phone's access to the network.
The GSM network uses a system of cells, each with its own base station, to provide coverage to a specific geographic area.
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What Is
Short Message Service (SMS) is a store-and-forward system that allows users to send and receive short messages between mobile phones.
The system is based on the concept of a message center, which acts as a central hub for all SMS messages. This message center is responsible for storing and forwarding messages between mobile phones.
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Each SMS message is made up of three parts: the header, the user data, and the tail. The header contains information about the message, such as the sender's phone number and the message type. The user data is the actual message content, while the tail contains error correction information.
The GSM network uses a technique called concatenation to send long messages. This involves breaking down the message into smaller chunks, called segments, and sending each segment as a separate SMS message.
The maximum length of an SMS message is 160 characters, including spaces and punctuation. This is determined by the length of the user data field in the SMS message format.
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Message Transfer
A Short Message (SM) is sent from a handset to a Short Message Service Center (SMSC) via the air interface, accompanied by the destination address and SMSC address stored on the SIM card.
The SMSC address is crucial in determining where the message will be forwarded.
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The Mobile Switching Center (MSC) or Serving GPRS Support Node (SGSN) invokes the MAP service package MAP_MO_FORWARD_SHORT_MESSAGE to send the text to the Interworking MSC of the Service Centre.
This service sends the mo-ForwardSM MAP operation to the SMSC identified in the SM Submission from the handset, embedded within a Transaction Capabilities Application Part (TCAP) message.
The Interworking MSC of the SMSC receives the MAP mo-ForwardSM message and passes the SMS-PP Application Protocol Data Unit (APDU) containing the text message to the actual Service Centre (SC) for storing and subsequent delivery.
The message submission status is then forwarded to the subscriber's handset over the air interface.
The SMS delivery process involves two main phases: Mobile Originated (MO) and Mobile Terminated (MT).
Here's a breakdown of the SMS Mobile Originated and Mobile Terminated Call Flow:
- The SMS-SUBMIT is the MO FSM message which is between origin MSC and YateSMSC.
- The message received by SMSC is acknowledged to the origin subscriber by SMS-SUBMIT REPORT.
- YateSMSC gets the required information from HLR regarding destination MSC/VLR to route the SMS.
- The message is delivered to the respective destination MSC/VLR, which in turn delivers the SMS to the subscriber.
- Once the SMS is delivered, the UE gets an indication message as “message delivered” or “message successfully delivered”.
Delivery and Failure
The HLR sets a flag against the destination account, indicating that it is unavailable for short message delivery, and stores the SMSC's address in the Message Waiting Data (MWD) list for the destination party.
Valid flags are Mobile Not Reachable Flag (MNRF), Memory Capacity Exceeded Flag (MCEF), and Mobile Not Reachable for GPRS (MNRG).
The HLR will now start responding to SRI-for-SM requests with a failure, indicating the failure reason, and will automatically add the requesting SMSC's address to the MWD list for the destination party.
However, if the SRI-for-SM message has priority flag set, the HLR will reply with VLR address if available.
The HLR may be informed of a subscriber becoming available for Short Message delivery in several ways:
- Where the subscriber has been detached from the network, a reattach will trigger a Location Update to the HLR.
- Where the subscriber has been out of coverage, but not fully detached from the network, on coming back into coverage it will respond to page requests from the Visitor Location Register (VLR).
- Where the MS has had its memory full, and the subscriber deletes some texts, a Ready-for-SM (memory available) message is sent from the VMSC/VLR to the HLR.
Upon receipt of an indication that the destination party is now ready to receive short messages, the HLR sends an AlertSC MAP message to each of the SMSCs registered in the MWD list for the subscriber, causing the SMSC to start the Short Message delivery process again, from the beginning.
The retry schedule interval will depend on the original failure cause - transient network failures will result in short retry schedule, whereas out of coverage will typically result in a longer schedule.
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Service Operations
Service operations for GSM's Short Message Service (SMS) involve managing the flow of messages between the mobile network and the Short Message Service Center (SMSC).
The SMSC acts as a central hub, storing and forwarding messages between the mobile network and the SMS center.
The SMSC uses a database to store messages temporarily, allowing for efficient handling of large volumes of messages.
This temporary storage enables the SMSC to handle network congestion and ensure timely delivery of messages.
Messages are typically stored in the SMSC for a short period, usually a few minutes, before being forwarded to their destination.
The SMSC also performs error checking and correction to ensure the integrity of messages being sent.
This helps to prevent errors and ensures that messages are delivered correctly to the recipient's mobile phone.
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1.2.2 Infrastructure
SMS infrastructure is designed for efficient transmission of messages, allowing for no delay in call setup.
SMS messages can be sent using the signaling channels of the underlying GSTN infrastructure, which enables fast and reliable delivery.
Related reading: SMS Home Routing
SMS clients can run on computers not directly connected to a GSTN network supporting SMS, making it possible to access SMS services from a variety of devices.
SMS messages sent with the GSTN SMS service must be sent as class 1 SMS messages, if the client is able to specify the message class.
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1.2.2.1 Centers
SMS messages are stored by an entity called SMS Center (SMSC) for delivery within GSTN networks. The SMSC stores the message and sends it to the recipient when they connect to the network.
The number of a cooperative SMSC must be known to the SMS sender, usually configured in the SMS client and specific for the network operator to which the sender has subscribed. In some cases, the SMSC number may need to be specified for message delivery to succeed.
The SMSC is responsible for storing and forwarding SMS messages, making it a crucial component of the SMS infrastructure. In most situations, the SMSC number is part of the sending SMS client's configuration.
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Here are some key points to note about SMS Centers:
Content and Forms
SMS messages have a maximum length of 160 characters, or 140 octets, and can be sent to and from SMS clients.
The GSM character set is used for SMS messages, but other character sets like UCS-2 16-bit characters can also be supported, resulting in 70 character messages.
SMS messages can contain various content types, including small bitmaps and simple formats for musical notes, but these formats are proprietary and not considered in this memo.
SMS messages are limited in length, and the first versions of the SMS specification did not specify any standardized methods for concatenating SMS messages.
To deal with this situation, SMS clients composing messages SHOULD use the standard concatenation method based on the header in the TP-User Data field as specified in the SMS specification.
SMS clients MAY opt to use the backwards-compatible (text-based) concatenation method defined in the SMS specification when sending a message to an SMS recipient whose support for concatenated messages is unknown.
SMS messages can be used as the transport mechanism for HTML forms, providing a way to fill in forms offline and send the data without making a TCP connection to the server.
The sender's telephone number may be included in the SMS message, thus providing a weak form of authentication.
SMS message service options like deferred delivery and delivery notification requests are not within the scope of this document.
URI Scheme and Applicability
The "sms" URI Scheme is comparable to "mailto" in that it can be used with a comma-separated list of recipient phone numbers. This allows for sending SMS messages to multiple recipients at once.
The notation for phone numbers is taken from RFC 3966, which provides a corrected syntax of the telephone number. This format was chosen for a specific reason, but the details are not specified.
SMS messages sent as a result of this URI MUST be sent as class 1 SMS messages, if the user agent is able to specify the message class.
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The URI Scheme
The URI Scheme is a way to send information to a specific recipient. It's comparable to the "mailto" URI, which can be used with a comma-separated list of email addresses.
The "sms" URI Scheme is a specific type of URI Scheme that provides information for sending an SMS message to certain recipient(s). It's based on the notation for phone numbers from RFC 3966.
A phone number in an "sms" URI MUST be written in international form with a country code, unless the number only works from inside a certain geographical area or a network. This means it should start with a "+" character.
Hyphens, dots, and parentheses in phone numbers are used only to improve readability and MUST NOT convey any other meaning.
Here's a summary of the steps for processing an "sms" URI:
- The phone number of the first recipient is extracted and MUST be written in international form with a country code.
- The body of the message is extracted, if present.
- The user agent SHOULD provide some means for message composition, either by implementing it itself or by accessing a service providing it.
- After message composition, a user agent SHOULD try to send the message first using the default delivery method employed by that user agent.
- If the URI consists of a comma-separated list of recipients, all of them are processed in this manner, and exactly the same message is sent to all of the listed recipients.
2.1 Applicability
When specifying the class of an SMS message, it's essential to consider the capabilities of the user agent.
If the user agent can specify the message class, the SMS message MUST be sent as a class 1 SMS message.
SMS messages sent as a result of a URI MUST adhere to specific guidelines for their class.
Class 1 SMS messages are the default choice when the user agent is able to specify the message class.
This ensures that the message is sent with the most basic features and limitations.
The user agent's capability to specify the message class is a crucial factor in determining the class of the SMS message.
Processing and Security
In GSM, processing refers to the handling of short message service (SMS) messages between mobile stations and the base station system.
The base station system consists of a base transceiver station (BTS), a base station controller (BSC), and a mobile switching center (MSC).
The BTS is responsible for transmitting and receiving radio signals to and from mobile stations.
The BSC manages multiple BTSs and controls the radio resources.
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The MSC connects the base station system to the public switched telephone network (PSTN) and provides call control and switching functions.
The MSC also stores the location of mobile stations and updates this information in real-time.
The home location register (HLR) is a database that stores information about each mobile station, including its location and subscription details.
The visitor location register (VLR) is a database that stores information about mobile stations that are currently roaming.
Security in GSM is ensured through the use of encryption and authentication.
The authentication center (AuC) is a database that stores the secret key and other authentication information for each mobile station.
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Explore Related Subjects
Lin's research on GSM Point-to-Point Short Message Service is a great starting point for exploring related subjects. He published his findings in the International Journal of Wireless Information Networks in 1997.
One of the key aspects of GSM Short Message Service is its technical realization, which involves understanding how it works. Lin's work provides a comprehensive overview of this process.
On a similar theme: Service Termination Point
The technical realization of GSM Short Message Service relies on the use of a point-to-point communication system. This system enables users to send and receive short messages over a network.
Lin's research highlights the importance of understanding the underlying technology behind GSM Short Message Service. By grasping this concept, developers can create more efficient and reliable messaging systems.
Curious to learn more? Check out: Service Control Point
Frequently Asked Questions
What is GSM messaging?
GSM messaging is a standard for mobile communication that optimizes text encoding for SMS messages. It allows for efficient text transmission by representing common characters and symbols in a single character.
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