is also increases the speed at which new services can be developed and tested as each service no longer requires its own mechanisms to handle charg- ing, authentication, and SO on. Service Delivery Platforms The IMS has a dedicated control layer, which is handled separately from the multimedia traffic. The media traffic can follow a different path from the control traffic, allowing for low-latency multimedia sessions, while the control signal traffic is passed securely through the network operators network. It is also access agnostic, as the service functionality is separated from the access technology. The IMS can be used with a variety of fixed line or mobile access technologies. 8.2.1 Architecture and Signaling As mentioned previously, the IMS uses an open, human readable control protocol that is popular on the Internet. The Session Initialization Protocol (SIP) is used to establish, modify and tear-down any form of multimedia session [6]. It is very flex- ible, and can be used to create sessions for IPTV, video calls, conferencing, and SO on. It is used in conjunction with the Session Description Protocol (SDP), which allows the multimedia to be fully described separately from the session control infor- mation. Together, these protocols allow the session details to be negotiated before the session is established, or renegotiated during a session. This greatly enhances compatibility between different devices and implementations as each side can pro- vide information about their capabilities (e.g., supported video codecs) in order to determine a set of matching capabilities to be used for that particular session. There are four main functional elements in the IMS core. These are the Proxy Call Session Control Function (PCSCF), the Interrogating Call Session Control Function (ICSCF), the Serving Call Session Control Function (SCSCF), and the Home Subscriber Server (HSS). It is important to note that these are logical ele- ments, and are not tied to a single piece of hardware. For example, one can have multiple S