MMS messages are delivered in a fashion almost identical to SMS, but any multimedia content is first encoded and inserted into a text message in a fashion similar to sending a MIME e-mail. MMS defines a subset of MIME content formats in the MMS Message Encapsulation specification. The message is then forwarded to the carrier's MMS store and forward server, the "MMS relay". If the receiver is on another carrier, the relay forwards the message to the recipient's carrier using the Internet.[1]
Once it reaches the correct MMS relay for the receiver, the content is extracted and sent to a temporary storage server (often the same process as the relay) with an HTTP front-end. An SMS "control message" containing the URL of the content is then sent to the recipient's handset to trigger the receiver's WAP browser to open and receive the content from the embedded URL. Several other messages are exchanged to indicate status of the delivery attempt.[2]
Some installations also include a conversion service that will attempt to modify the multimedia content into a format suitable for the receiver. This is known as "content adaptation", or MMSC.
E-mail and web-based gateways to the MMS (and SMS) system are common. On the reception side, the content servers can typically receive service requests both from WAP and normal HTTP browsers, so delivery via the web is simple. For sending from external sources to handsets, most carriers allow MIME encoded message to be sent to the receiver's phone number with a special domain.
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