SIP is an application layer protocol for establishing, terminating and modifying multimedia sessions within an IP network as defined in IETF RFC 3261 (June 2002). It is destined to be independent of the underlying transport layer that is, it can run on TCP (Transmission Control Protocol), UDP (User Datagram Protocol), or SCTP (Stream Control Transmission Protocol). Another key characteristic of SIP is that it is text-based, allowing for humans to read SIP messages. At the heart of IMS is SIP that provides a centric signaling mechanism for the delivery of multimedia services over IMS. SIP follows a client server model. Diameter: The Diameter protocol is specified in RFC3588 and was chosen for use with IMS as the AAA protocol. Diameter is an evolution of the RADIUS (RFC2865) protocol. Diameter is the base protocol that is used along with the Diameter Applications which extends the base protocol by adding new commands and/or attributes (appropriate within the IMS specifications). MEGACO: Media Gateway Control (MEGACO) Protocol or H.248 is a means for signaling functions to control media serving functions within the IMS environment. It is used between a Media Gateway and a Media Gateway Controller. The protocol was the result of joint work between IETF and ITU. It is defined by both IETF RFC 3525 and by ITU-T H.248.1. COPS: The Common Open Policy Service (COPS) protocol is used within the IMS environment to transport policies between Policy Decision Points (PDPs) and Policy Enforcement Points (PEPs). It is part of the Internet protocol suite as defined by IETF RFC 2748. Services and Applications The main purpose of IMS is to provide end users with rich multimedia services regardless of the access network and device they are using. At a high level, IMS application servers host specific application functionality, for example, video, voice mail, call waiting, IPTV, Push-to-Talk over Cellular, and SO on, and interact with the S-CSCF using SIP. Some key applications whose performance can be enhanced and QoS can be added u