Praise for Fundamentals of WiMAX

238 Chapter 7 • Networking and Services Aspects of Broadband Wireless7.2.1.3 Other SIP UsesSo far, we have used simple call session setups to illustrate SIP transactions. It should, however,be emphasized that SIP can be used to build much more complex signaling for a wide range ofmultimedia session-control applications.SIP can also be used for QoS signaling and mobility management. For example, invitationsmay indicate in SDP that QoS assurance is mandatory. In this case, call setup may proceed only ifthe QoS preconditions are met. A SIP extended method, COMET, can be used to indicate the successor failure of the precondition. In SIP, users are identified by a SIP URL but are located via anIP address, phone number, e-mail address, or a variety of other locators. This separation of useridentity from contact location address allows SIP to be used to provide personal mobility—a personmoving to a different terminal, but still remaining in contact; session mobility—maintainingan ongoing media session while changing terminals; and service mobility—getting access to userservices while moving or changing devices and/or networks. SIP may be also used, albeit suboptimally,for terminal mobility (maintaining session while the terminal changes its point of attachmentto the network). For example, in the middle of a session, if a terminal moves from onenetwork to another, a SIP client could send another INVITE request to the correspondent hostwith the same session ID but with an updated session description that includes the new IPaddress. SIP does not by itself ensure seamlessness in the sense that packets are not lost or that thetransfer of connection is quick. That depends on lower-layer network detection, selection, andhand-off functions.UserAgentAProxyUserAgentBUserAgentCINVITE100 TRYING180 RINGING180 RINGING200 OKACKINVITEINVITE180 RINGING200 OKCANCELACK180 RINGINGMEDIA FLOWACK200 OK487 Request TerminatedFigure 7.6 A simple call setup and termination using SIP