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How SMS works
What is SMS?

Definition: 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 (International Engineering Consortium, 2002) Guaranteed delivery: if a wireless recipient is switched off, out of range or if there is a network outage, the SMS message will be stored in the network and delivered when the recipient announces its presence, or when the outage is rectified. No guarantees existed with previous such services eg. Alphanumeric paging. This is the basis of the store-and-forward concept. Send or receive during voice or data calls: SMS messaging makes use of a separate channel, normally used for transfer of control messaging to transfer its packets. Being out-of-band, this means voice and data calls will not be interrupted by SMS transfer. Furthermore, the low-bandwidth requirements of transmitting short alphanumeric strings allows messaging worldwide with very low latency. This of course depends upon network operator agreements.

History of SMS
Analogue: the world-wide standard in the late 1980s and early 1990s had no capability for text messaging GSM: European networks began development of a digital standard (GSM) in 1991. Phase 2 of the standard, release in 1993, defined data bearing services over GSM – SMS was a part of this standard. Vodacom (South Africa) became the first company in the world to implement fax and data services on its network, later that year. CDMA/TDMA: The American networks decided to take an alternative route, using first TDMA and later, the superior CDMA which integrated text messaging into its standard. TDMA later gained this capability through Motorola’s iDEN development. Interoperability: When the buildout of personal communication service (PCN) networks was complete in 1998, SMS was fully deployed. All that remained was the agreement between network operators to allow this. April 2000: Australian networks agree to allow interoperability between SMS users.

The SMSC is a combination of hardware and software responsible for the relaying and storing and forwarding of a short message between an SME and a mobile device.

In this way, if one’s phone is switched off or the storage for SMS is full, the SMSC will retain any messages until the user can receive them.

Of course, the SMSC must be highly reliable, with redundant backups for its primary functionality in case of failure. It must be able to serve a large number of subscribers, and thus be able to support a high throughput of messages. It should also be scalable, to allow expansion for greater capacity or new developments in messaging, such as EMS or MMS in future.

Manufacturers and networks are increasingly incorporating other forms of access directly to the SMSC, through modem dialup, X.25 and even the Internet so that corporate customers can leverage the power of SMS without needing mobile stations such as mobile phones. This variant on the implementation of a Short Message Service Centre places one such unit at every base station, interacting through, in this example, leased lines. An alternative would be microwave links between the wireless stations.

This distributed architecture as the advantage of better redundancy in case of a single failure; all other SMSCs will remain operational throughout the network. However, load distribution will most likely be uneven through the network and so some units will be used under capacity and potentially others overloaded. This alternative implementation for the Short Message Service Centre has base stations networked through leased lines, with one centralised location for Short Message Service Centres. This implementation has a cluster of such NT servers, visible to the network as a single entity.

This form has the advantage of being better suited to load-sharing as demand in different areas and times varies in operation. A disadvantage is the requirement for higher bandwidth links between this SMSC cluster and the rest of the network; that is, the data distribution will centre upon one part of the network.

SMSC Implementation
Some of the potential applications of SMS technology, utilizing both MT-SM and MO-SM where appropriate, include the following:

Notification Services - Notification services are currently the most widely deployed SMS services. Examples of notification services using SMS include the following: Voice/fax message notification, which indicates that voice or fax mail messages are present in a voice mailbox E-mail notification, which indicates that e-mail messages are present in an e-mail mailbox Reminder/calendar services, which enable reminders for meetings and scheduled appointment E-mail Interworking - Existing e-mail services can be easily integrated with SMS to provide e-mail to short messaging and mobile e-mail and message escalation. Paging Interworking - Paging services integrated with SMS allow digital wireless subscribers to be accessible via existing paging interfaces, as well as escalation of messages. Information Services - A wide variety of information services can be provided by the SMS, including weather reports, traffic information, entertainment information (e.g., cinema, theatre, concerts), financial information (e.g., stock quotes, exchange rates, banking, brokerage services), and directory assistance. SMS can support both push (MT) and pull (MO) approaches to allow not only delivery under specific conditions but also delivery on demand, as a response to a request. WAP Integration - SMS can deliver notifications for new WAP messages to wireless subscribers but can also be used as the transport mechanism for WAP messages. These messages can contain diverse information from sources that include databases, the World Wide Web, e-mail servers, etc.

Mobile Data Services The SMSC can also be used to provide short wireless data. The wireless data may be in interactive services where voice calls are involved.

Some examples of this type of service include fleet dispatch, inventory management, itinerary confirmation, sales order processing, asset tracking, automatic vehicle location, and customer contact management. Other examples may be interactive gaming, instant messaging, mobile chat, query services, mobile banking, etc. Linking the SMSC to the network are two gateways, the SMS-gateway mobile switching centre (SMS-GMSC) and SMS interworking mobile switching centre. These are typically integrated into the SMSC.

Essentially, data in mobile networks is routed through a series of mobile switching centres (MSCs). If a SMSC needs to send a message to a mobile recipient, the SMS-GMSC interface will interrogate a Home Location Register (HLR) for the appropriate routing information and pass the message with this. If, on the other hand, the recipient is within the network, the SMS-IWMSC simply routes the message to the appropriate SMSC.

Application
Originally SMS was used for: - notification purposes: particularly voice mail (eg. Optus) - 2-way messaging: intended to supersede alphanumeric paging, the capability was introduced simply to allow general-purpose messaging between wireless entities. Newer implementations for SMS include: - E-mail: a number of services now exist that allow you to check a POP3 mail account on your handset, or employ translation of e-mail into SMS form for receipt on a handset - Fax: allows of transfer through handset to a notebook, or translation into actual SMS messages for viewing on the handset. - Interactive banking: services such as the Vodafone/Commonwealth Bank mobileBank service allow account balances, funds transfers and other transactions to be completed via SMS messaging on a user’s handset. - Information services: for a fee, a number of service providers will now send regular updates on share prices, news headlines, the weather and even goings on at the Big Brother house straight to your handset via SMS. Web integration: - a number of forms exist, including the ability to receive web content through SMS, the sending/receiving of actual WAP datagrams through SMS, as well as emerging cross-platform approaches such Internet instant messaging services being combined with SMS sending capabilities – allowing ‘SMS chat’.