SOAP is the protocol that underlies the built-in support for Web Service applications. SOAP marshals method calls using an XML encoding. SOAP connections use HTTP as a transport protocol. 

SOAP connections have the advantage that they work in cross-platform applications because they are supported on both the Windows and Linux. Because SOAP connections use HTTP, they have the same advantages as Web connections: HTTP provides a lowest common denominator that you know is available on all clients, and clients can communicate with an application server that is protected by a "firewall." For more information about using SOAP to distribute applications, see Using Web Services.. 

As with HTTP connections, you can't use callbacks when the connection is formed via SOAP.