Check out this blog about Direct Push and how it works.
Read the whole post here:
http://msexchangeteam.com/archive/2006/04/03/424028.aspx
Background
Exchange 2003 introduced the Always Up To Date notification feature (AUTD) that kept devices up to date by sending SMS triggers to the device. The triggers were sent from the enterprise as SMTP messages to the SMTP front end at the mobile operator. They were then sent through the SMS gateway as SMS messages to the device. This approach had some limitations since not all mobile operators did the SMTP to SMS conversion. Even when they did, there was latency involved with SMS messages and there were end-to-end reliability issues. Also some mobile operators charged for each incoming SMS message so that added an extra dimension to the cost of staying up to date. To alleviate these issues, Exchange 2003 SP2 introduced Direct Push.
Direct Push Architecture
Direct Push is a client initiated HTTP connection to the server where the device opens a connection to the Exchange Server and keeps it alive for a duration known as the heartbeat interval. Basically the client sets up the connection, chooses the appropriate heartbeat interval and tears down and reestablishes the connection if and when necessary. The server sends notifications about new items over this connection and the client synchronizes to get the new items.
A new AirSync command called PING has been introduced for Direct Push. This command is sent as part of the POST request from the device.
Continue at source
- Joel