This revised edition of "SIP: Understanding the Session Initiation Protocol" gives you a thorough and up-to-date understanding of this revolutionary protocol for call signalling and IP telephony. The second edition includes brand new discussions on the use of SIP for wireless multimedia communications. It explains how SIP is powerful "rendezvous" protocol that leverages mobility and presence to allow users to communicate using different devices, modes and services anywhere they are connected to the Internet. You learn why SIP has been chosen by the 3GPP (3rd Generation Partnership Program for wireless cell phones) as the core signalling, presence and instant messaging protocol.