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Short Message Services (SMS) : 

Short Message Services (SMS)

What is SMS? : 

What is SMS? The Short Message Service (SMS) is the ability to send and receive text messages to and from mobile telephones. The text can comprise of words, numbers, an alphanumeric combination or a binary format for the transmission of simple graphical and audio information. SMS was incorporated into the Global System for Mobiles (GSM) digital mobile phone standard. The first short message was sent in December 1992 from a Personal Computer to a mobile phone on the Vodafone GSM network in UK. From then on, SMS became an accidental success that took nearly everyone in the mobile industry by surprise. Few people predicted that this difficult to use service would take off.

Network Elements and Architecture : 

Network Elements and Architecture

Slide 4: 

ESME - External Short Messaging Entity: A device that may receive or send short messages. Receiving and sending short messages can be done by voice mail, web, e-mail and other mechanisms. SMSC - SMS Center: A combination of hardware and software responsible for the relaying and forwarding of a short message between an SME and a mobile device. An SMSC acts as a store-and-forward system for short messages. The SMSC must have high reliability, subscriber capacity, and message throughput. In addition, the system should be easily scalable to accommodate growing demands for SMS in the network.

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HLR (Home Location Register): A database used for permanent storage and management of subscriptions and service profiles. Upon interrogation by the SMSC, the HLR provides the routing information for the indicated subscriber. Also, if the destination station was not available when the message delivery was attempted, the HLR informs the SMSC that the station is now recognized by the mobile network to be accessible, and thus the message can be delivered. VLR (Visitor Location Register): A database that contains temporary information about subscribers homed in one HLR who are roaming into another HLR. This information is needed by the MSC to service visiting subscribers.

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STP - Signal Transfer Point: A network element that enables communication with the wireless network elements such as the HLR and the MSC. MSC - Mobile Switching Center: Performs the switching functions of the system and controls calls to and from other telephone and data systems. The MSC will deliver the short message to the specific mobile subscriber through the proper base station. Air Interface: The air interface is defined in each one of the different wireless technologies. These standards specify how the voice or data signals are transferred from the MSC to the handset and back, as well as the utilization of transmission frequencies, considering the available bandwidth and the system’s capacity constraints.

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The Mobile Device: The mobile device is the wireless terminal capable of receiving and originating short messages. In the past, these devices used to be digital cellular phones, but more recently the application of SMS has been extended to other terminals such as handheld computers and personal digital assistants (PDAs).

Characteristics of SMS : 

Characteristics of SMS The network elements are designed to provide guaranteed delivery of text messages to the destination. Temporary failures due to unavailable receiving stations are identified, and the short message is stored in the SMSC until the destination device becomes available. An active mobile handset is able to receive or submit a short message at any time, regardless of whether a voice or data call is in progress. SMS supports several input mechanisms that allow interconnection with different message sources and destinations (e.g., voice mail, web, e-mail). SMS is characterized by out-of-band packet delivery and low-bandwidth message transfer, which results in a highly efficient means for transmitting short bursts of data.

Short Message Data Structure : 

Short Message Data Structure A short message is formally known as a Protocol Data Unit (PDU) and comprises two parts - the header information and the short message text itself (known as the user data). This short message is sent to the SMS Center that looks at the details on the header and tries to send the message to the recipient using the type of postal service the sender specified.

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The header contains the following parameters: SMSC Address The address of the SMS Center to which the short message is to be sent. Destination Address The Destination Address field denotes the final recipient of the short message. This parameter is usually specified by the sender. Originating Address The address of the sender of the short message. Usually automatically appended to the short message itself so that the recipient can identify the sender.

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Status Report Request This parameter allows the short message sender to request confirmation that the short message has been delivered to its intended recipient. Service Center Timestamp In addition to the short message text itself and the Originating Address, the time and date that the SMS Center received the short message are usually also automatically appended to outbound short messages from the SMS Center.

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Validity Period Each short message submitted to the SMS Center is assigned a Validity Period, which sets the maximum time that the short message is retained in the SMS Center. Failure to successfully deliver the short message within the short message lifetime causes it to be marked for purge, with no further delivery attempts made. Setting specific short message Validity Periods is important for many SMS-based applications. Some SMS Centers can inform software applications as soon as a short message lifetime expires, allowing alternative, secondary action to be taken. `All short messages have a message lifetime - those short messages that do not have a specific value when submitted are automatically assigned the default Validity Period for that mobile network. For example, some mobile network operators set a maximum short message lifetime of 72 hours (Vodafone, UK) or 48 hours (Vodafone D2, Germany), after which any short messages that haven’t been delivered are deleted.

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Date Coding Scheme The Data Coding Scheme (DCS) parameter is used for several purposes, including the following: Indicate the form in which the short message text (user data) is encoded, be it the GSM 7-bit default alphabet, 16-bit text or binary. Specify short message classes, which tell the mobile phone how to deal with the short message. For example, the Data Coding Scheme flag is used to indicate whether to store the short message in the SimCard or memory, send it directly to the display or to Terminal Equipment attached to the mobile phone. Allow a receiving Short Message Entity to display an icon associated with a short message, such as an email or voice mail icon. Indicate that a short message is compressed. The Data Coding Scheme values are described in GSM 03.38. How or if they are supported depends on the specific mobile phone. Network support of the DCS parameter also varies- application developers and users should check with their network operator to see which if any uses of the DCS are allowed.

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Protocol Identifier Another flag that is used for a wide variety of purposes is the Protocol Identifier (PID). The PID determines how the short message should be handled by the receiving entity or the SMS Center. Uses of the Protocol Identifier include: Routing short messages to the correct outbound interface. This is useful when several interfaces share the same numbering plan (e.g. PSTN fax and voice). Use of the Protocol Identifier indicates to the SMS Center where to send the short message to maximize the likelihood that it is successfully delivered to its intended Destination Address. Routing by Protocol Identifier is, for example, used in the provision of SMS to Fax services, through which a mobile phone user can send a short message to a fax machine. The SMS Center recognizes that the Protocol Identifier indicates an SMS to Fax message and routes the short message to the module within the SMS Center that incorporates fax outdial, or an SMS to Fax platform resident outside the SMS Center itself . Indicating that a mobile phone receiving a short message should check to see if a short message of the same type is currently stored and if so replace it with the new one.

Slide 15: 

Reply Path The Reply Path allows a user to indicate to the receiver that a reply to the short message is requested. When the recipient chooses to reply to a short message, the SMSC Address from which the short message came is used instead of the SMSC Address stored on the SimCard. Additionally, the Originating Address from which the short message came is automatically used as the Destination Address. This feature was incorporated to indicate to the SMS Center that the initial sending entity should be charged for the reply rather than the replying entity. The advantage is that someone sending a message can receive a reply even if the recipient of the short message has not got an SMS Center number programmed into his or her SimCard. Many mobile phones allow a recipient to reply to a short message irrespective of the setting of the Reply Path parameter in the received short message. In such cases, the SMS Center and Originating Address translation described above is also applied.

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Message Reference An identifier (1- 255) which is incremented with each short message sent.   Message Length Indicates the length of the short message. Reject Duplicates The Reject Duplicates parameter allows a sender to indicate to the SMS Center that a short message with the same Message Reference as one already stored in the SMS Center for the same Destination Address should be discarded and replaced by the new one.

Slide 17: 

User Date Header Indicator The User Data Header Indicator allows a sender to indicate that the short message text itself (the user data) is in a special format of the types defined in GSM 03:40 such as SMS concatenation. SMS Commands Some mobile phones allow the sender to send specific instructions to the SMS Center to carry out operations on previously submitted short messages. For example, different command types allow the user to inquire about the status of a short message or delete a short message that is waiting to be delivered.   Message Type Indicator The Message Type Indicator parameter indicates whether a short message is for sending, receiving, is a status report (confirmation of delivery), or a specific command to the SMS Center such as an enquiry on a short message. A user does not normally have control over this parameter from the mobile phone keypad.

Slide 18: 

Some of these parameters are added by the mobile network entities and some are accessible by the originator of the short message. A default value for all of these parameters except the Destination Address is usually built into the application software, the SMS Center and mobile phone. Different combinations of these Short Message Data Structure parameters are formed depending on what SMS-related action is being carried out. For example, the Validity Period parameter is only set when submitting short messages. Some of the Short Message Data Structure parameters are optional and some are mandatory. The presence and order of the parameters for different types of short message transactions are defined within GSM 03:40. The user data plus the other parameters is collectively known as the Protocol Data Unit (PDU) or SMS-TPDU (SMS-Transport Protocol Data Unit). There are several different types of PDU, each of which contains different parameter combinations. For example, the Submit PDU is the format sent by the originating Short Message Entity to send a short message. The Deliver PDU is the format presented to the receiving message. The Command PDU is the format sent by the originating SME to request an action on previously submitted but not yet delivered short messages. The Status Report PDU indicates the status of a previously submitted short message to the originating SME (for example, confirming successful delivery to the Destination Address).

The SMS Center (SMSC) : 

The SMS Center (SMSC) SMS Center Criteria Because SMS is a store and forward service, every single short message of any type passes through an SMS Center. As such, the selection of an SMS Center vendor is critical to success of the mobile network operator SMS-based services. The key selection criteria for an SMS Center vendor are:0

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1. Design Because the design of the SMS Center was not closely defined in SMS standards, they do vary significantly. Some SMS Centers take an integrated approach and incorporate services other than SMS into the SMS Center. Others take an interface approach with their SMS Center designed as an SMS engine devoted to processing inbound and outbound short messages. Some early adopters of SMS amongst the first commercial GSM mobile network operators saw the integrated SMS Center approach as an opportunity to get multiple mobile network services up and running quickly by buying an integrated platform offering several built-in services such as voice mail. Other SMS Center vendors took the long view and realized that the integrated approach could mean that the SMS Center gets busy doing lots of other things, slowing down message delivery times. The integrated approach also reduces the network operator’s freedom to subsequently develop tailored services for its customers without having to go back to the SMS Center provider.

Slide 21: 

2. Scalability It is important that as SMS usage grows, the SMS Center can be easily and cost effectively (i.e. quickly and cheaply) expanded so that overall service reliability does not fall as usage increases. Naturally, because of the hardware duplication, platform maintenance, internal message routing and so on, it costs more to add the same SMS capacity by deploying a new SMS Center than to upgrade and expand an existing one. 3. Performance SMS Center performance is usually measured by several different variables. SMS Centers are usually dimensioned in terms of short messages throughput per second and SMS Center transit delays. Throughput denotes message handling capacity: the ability to input, process and output short messages. Transit delays denote the time it takes for the short message to be processed within the SMS Center.

Slide 22: 

4. Availability And Resilience An SMS Center is fully available if it is in a position to receive and send all short messages. Availability is defined in terms of Mean Time Between Failure (MTBF) and Mean Time to Recovery (MTTR). High availability is associated with high values of MTBF and low values of MTTR.An SMS Center is fully resilient if it is able to withstand partial or complete hardware failure without the loss of stored short messages. 5. Connectivity Ideally, the SMS Center will allow a network operator to offer its customers a variety of different inbound connectivity options and a variety of different types of recipient. This is shown is the diagram below.

Future Strategy - SMSC and IP : 

Future Strategy - SMSC and IP In the first and second generations of mobile communications, every SMS Center vendor had their own architecture and Application Programming Interfaces that had to be implemented to develop applications and services on that platform. Then along came the Internet and the Internet Protocol (IP). Directory stores, messages stores and other components of an architecture can be taken from different vendors and integrated using common standards such as IP and LDAP. This is the IP SMSC- an SMS Center that is based on IP philosophies and principles.SMS is an extremely popular service, with a single network handling several hundred million SMS messages per month. This traffic can result in the SS7 signaling network, that the SMS messages are transmitted over, becoming congested.  As such, next generation SMS Center vendors are working on high speed SS7 and SS7 over IP solutions and replacement of internal SS7 signaling networks with IP links. SMS messages are then broken out at the nearest switch or base station and terminated in the usual way.

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