How does short message peer-to-peer-protocol (SMPP) compare to simple object access protocol (SOAP) in terms of complexity to implement and performance?
Short message peer-to-peer, a protocol specifically designed for use on mobile phone networks) can't be directly compared to simple object access protocol-- they serve different purposes. Short message peer-to-peer is analogous to both HTTP and SMTP in that they are all transports for arbitrary messages (though each of these protocols offer different trade-offs and qualities of service). None of these protocols mandate how to structure the message body itself.
Simple object access protocol, on the other hand, only defines how to structure the message body and is totally transport independent. The same simple access object protocol message could be sent over HTTP, SMTP, or even SMPP (though you'd have a tough time fitting a SOAP message within the 161 byte limit of an SMPP message).