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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.
Gateway types 
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(Global System For Mobile Communications) 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 any company to send SMS to their own customers or prospects.
Direct-to SMS gateway 
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 
AOL Instant Messenger 
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.
Windows SMS Sender 
Windows SMS Sender (Windows SMS Sender) is a specialised application for sending text messages directly from Windows PC using CS Networks SMS Gateway.
Skype has support for sending SMS messages. SMS sent through Skype can be configured to appear as originating from the skype user cellphone (a sort of sender authorized spoofing).