An SMS gateway is a telecommunications network facility for sending or receiving Short Message Service (SMS) transmissions to or from a telecommunications network that supports SMS. Most messages are eventually routed into the mobile phone networks. Many SMS gateways support media conversion from email and other formats.
Some SMS gateway providers can be classified as aggregators or SS7 providers. The aggregator model is based on multiple agreements with mobile carriers to exchange two-way SMS traffic into and out of the operator's short message service center (SMSC), also known as local termination model. Aggregators lack direct access into the SS7 protocol, which is the protocol where the SMS messages are exchanged. These providers have no visibility and control over the message delivery, being unable to offer delivery guarantees. SMS messages are delivered in the operator's SMSC, but not the subscriber's handset.
Another type of SMS gateway provider is based on SS7 connectivity to route SMS messages, also known as international termination model. The advantage of this model is the ability to route data directly through SS7, which gives the provider total control and visibility of the complete path during the SMS routing. This means SMS messages can be sent directly to and from recipients without having to go through the SMSC of other mobile operators. Therefore, it is possible to avoid delays and message losses, offering full delivery guarantees of messages and optimized routing.
Several operators have true fixed-wire SMS services. These are based on extensions to the ETSI GSM SMS standards and allow fixed-fixed, fixed-mobile and mobile-fixed messaging. These use frequency-shift keying to transfer the message between the terminal and the SMSC. Terminals are usually DECT-based, but wired handsets and wired text-only (no voice) devices exist. Messages are received by the terminal recognising that the CLI is that of the SMSC and going off-hook silently to receive the message.
Direct-to-mobile gateway appliance
A direct-to-mobile gateway is a device which has built-in wireless GSM connectivity. It allows SMS text messages to be sent and/or received by email, from web pages or from other software applications by acquiring a Subscriber Identity Module (SIM card) . Direct-to-mobile gateways are different from SMS aggregators because they are installed on an organization's own network and connect to a local mobile network. The connection to the mobile network is made by acquiring a Sim card from the mobile operator and installing it in the gateway. Typically, direct-to-mobile gateway appliances are used for low 100s or 1,000s texts per month. More model appliances now offer the capability of send up to 100,000 messages each day. Several vendors that have historically provide GSM Gateway equipment for voice also have an SMS capability. Some are more primitive than others. The more sophisticated devices are engineered with SIM management to regulate the number of SMS messages per SIM; ODBC to connect to a database and HTTP interfaces to interactive with third party applications.
Regulation: SMS Gateway equipment is regulated via the Mobile Telegraphy Act in the UK and can be used legally by and company to send sms to their own customers or prospects.
A direct-to-SMSC gateway is a device which allows SMS text messages to be sent and received by email, from web pages or from other software applications. The gateway connects directly to a mobile operator's SMSC via the Internet or direct leased line connections. It converts the message format into a format understood by the SMSC, typically this is the SMPP protocol. Direct-to-SMSC gateways are used by SMS aggregators to provide SMS services to their clients. Typically they serve for high volume messaging and require a contract directly with a mobile operator. 
Microsoft Outlook 2007 has native support for sending SMS messages worldwide via the Outlook Mobile Service. There are also plug-ins for Microsoft Outlook that add this functionality.
AOL Instant Messenger
AOL Instant Messenger (AIM) version 5.2 and above has support for sending SMS messages for free. Third-party client Pidgin also supports this functionality through the AIM protocol.
ICQ also has support for sending SMS messages for free. Third-party client Pidgin also supports this functionality through the ICQ protocol since version 2.2.
SMS send platform
Some software providers are proposing a SaaS (Software as a Service) solution to send SMS directly from an internet connected PC. Connecting to these platforms is usually pretty easy as good HTTP API documentation is available..
Windows Live Messenger
MSN Messenger (Windows Live Messenger) 7.0 and up have support for SMS messages.
Skype (Skype) has support for sending SMS messages. SMS sent thru Skype can be configured to appear as originating from the skype user cellphone (a sort of sender autorized spoofing).
Yahoo! Messenger, accessible through a dedicated client application, or through a web site (mail.yahoo.com) have support for SMS messages. Third-party client Pidgin also supports this functionality through the Yahoo! protocol since version 2.6.
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