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What is an SMS?
Short message service (SMS) is a globally accepted wireless service that enables the transmission of alphanumeric messages between mobile subscribers and external systems such as electronic mail, paging and voice mail systems.
What are the benefits of SMS?
SMS provides a powerful vehicle for service differentiation. The benefits of SMS to the service provider are:
- Increased call completion on wireless and wire-line networks by leveraging the notification capabilities of SMS.
- An alternative to alphanumeric paging services
- Enabling wireless data access to corporate users
- Provision of value added services as email, voicemail, and fax mail integration, reminder service, stock and currency quotes, and airline schedules
- Provision of key administrative services such as advice of charge, over the air downloading and service provisioning
What are the Elements of the SMS Architecture?
SMS Architecture

The above figure shows the basic network structure of the SMS. It consists of the following:
- Short Messaging Entity: SME is an entity, which may receive or send short messages. The SME may be located in the fixed network, a mobile station, or another service center
- Short Message Service Center: SMSC is responsible for the relaying, storing and forwarding of a short message between an SME and mobile station.
- SMS-Gateway MSC: is an MSC capable of receiving a short message from an SMSC, "interrogating a home location register (HLR) for routing information, and delivering the short message to the visited MSC of the recipient mobile station.
- Home Location Register: HLR is 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.
- Mobile Switching Center: The mobile switching center (MSC) performs the switching functions of the system and controls calls to and from other telephone and data systems
- Visitor Location Register: (VLR) is a database that contains temporary information about subscribers. This information is needed by the MSC in order to service visiting subscribers
What is the Short Message Service Center?
See SMS Architecture
What are the basic MAP operations necessary to provide end-to-end Short Message Service?
The following basic MAP operations are necessary to provide end to end short message service:
- Routing Information Request: The SMSC needs to retrieve routing information from the HLR, in order to determine the serving MSC for the addressed mobile station. This process is carried out before delivering the message.
- Point to Point Short Message Delivery: The mechanism provides a means for the SMSC to transfer a short message to the MSC, which is serving the addressed mobile station.
- Short message waiting indication: This operation is activated when a short message delivery attempt by the SMSC fails due to a temporary failure. This provides a means for the SMSC to request the HLR to add an SMSC address to the list of SMSC s to be informed when the indicated mobile station becomes accessible.
- Service Center Alert: The operation provides a means for the HLR to inform the SMSC which has previously initiated unsuccessful short message delivery attempts to a specific mobile station, that the mobile station is now recognized by the mobile network to be accessible.
What Service Elements, does the SMS comprise of?
The SMS comprises several service elements relevant to the reception and submission of short messages:
- Validity Period: The validity period indicates how long the SMSC shall guarantee the storage of the short message before delivery to the intended recipient
- Priority: Priority is the information element provided by an SME to indicate the priority message
In addition, SMS provides a time stamp reporting the time of submission of the message and an indication to the handset of whether or not there are more messages to send (GSM) or the number of additional messages to send (IS41)
How is the message status notified to the SMSC?
The SMSC is notified of the message status through MAP operations. Go to Basic Map Operations.
What Potential Applications can be envisaged, using the SMS?
Some of the potential applications of SMS technology, utilizing the Mobile Terminated are:
- Notification Services
- E-mail Interworking
- Paging Interworking
- Information Services
What are the various steps involved in a "Successful short message transfer attempt"?
The following steps are involved in a Successful short message transfer attempt: the short message is submitted from the SME to the SMSC. After completing its internal processing, the SMSC interrogates the HLR and receives the routing information for the mobile subscriber. The SMSC sends the short message to the MSC using the Forward Short message operation. The MSC retrieves the subscriber information from the VLR. This operation may include an authentication procedure. The MSC transfers the short message to the MS. The MSC returns to the SMSC the outcome of the Forward Short message operation. If requested by the SME, the SMSC returns a status report indicating delivery of the short message.
Delayed Messages or Undelivered
Messages sent to a mobile phone may be delayed for a number of reasons. The two most common are absent subscriber and mobile memory capacity exceeded.
Absent Subscriber
In the case of an absent subscriber a mobile phone may be switched off or temporarily out of coverage. If the SMSC tries to deliver a message to a mobile phone under either of these conditions then the mobile phone is noted as being absent. The SMSC will retry according to the absent subscriber retry schedule for a limited number of times just in case the mobile phone has temporarily gone out of coverage (e.g. into a tunnel). If the mobile phone becomes available (is switched on, changes cells, receives or makes a voice call, sends a short message or the mobile phones periodic location update timer expires) then the SMSC will be informed and the message waiting in the SMSC will get delivered. Also any new message arriving in the SMSC while the mobile is absent will trigger a delivery attempt and restart the retry schedule from the beginning. It should be noticed that when a mobile phone goes temporarily unavailable and becomes available again in the same cell there is no notification to the SMSC that it has become available unless the phone gets turned off then on, changes cells, receives or makes a voice call, sends a short message or the mobile phones periodic location update timer expires. Hence the need for the retry schedule for absent subscriber.
Mobile Memory Capacity Exceeded
In the case of memory capacity exceeded, the SMSC will continually retry to deliver the message with gradually lengthening intervals. The retry will continue for the validity period of the message.
Other Message Delays
Other less common errors encountered (e.g. Unidentified Subscriber) are subject to different retry schedules according to the error condition. Some messages will retry a few times and will then be purged because by retaining them at the head of the queue would result in other messages which could be delivered expiring before delivery can take place.
Duplicated Messages
Duplication of messages can occur if the sending entity fails to receive an acknowledgement that a message has been successfully received by the SMSC and the message is re-submitted

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