 components to communicate using industry-standard and open interfaces such as SIP. This then allows the service providers to mix and match the best of application servers with the best of MRFP/media servers. IMS applications include voice telephony, video telephony, multimedia streaming, HTTP/TCP/IP browsing, instant messaging, file sharing, gaming, push-to-talk/push-to-media, and presence-based services. Two of the first applications making use of IMS are expected to be push-to-talk and presence-based services, which are discussed in the following sections. Push-to-Talk Thanks to its revenue potential, push-to-talk is one of the most talked-about mobile applications. Push-to-talk, which turns a mobile phone into a walkie-talkie by giving it trunked radio functionality, ended up being one of Nextel's (www.nextel. com) key market differentiators, causing the cellular industry to see it as the next killer application for mobile networks after voice and Short Messaging Service (SMS). Push-to-talk gives end users the ability to quickly find one another and engage in brief, burst-oriented communication. Rather than being a replacement of long, interactive communication, push-to-talk is best suited for quick communica- tion among end users. Due to the fact that it is provided in half-duplex mode (i.e., transmission occurs in both directions, but not at the same time-each party must wait to speak), the users are unable to interrupt each other. Because push-to-talk is by definition a group activity, and not everyone in a group may subscribe to the same mobile network or own the same handset model, push-to-talk absolutely depends on standardization and interoperability. Push-to- Talk over Cellular (PoC) is the official standard that defines how push-to-talk can be deployed over a cellular operator's packet data network using VoIP. The key to PoC is IMS. Although the IMS standard has been completed by the 3GPP in its Release 5 specification, it has yet to pass the trial stage even in existing 3G networks. The PoC standard