SIP is best described by RFC 2543, which states that it is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants.

These multimedia sessions include audio, video, and data and can include multiple partners. SIP enables participants to be invited into an impromptu conference. These multimedia sessions can communicate through multicast, unicast, or a combination of both delivery mechanisms.

Very few implementations of SIP are currently running, although many vendors and customers are interested in using SIP to deploy enhanced services.

See Chapter 11 for more detailed information on SIP. Open Service Application Layer

By far the most interesting layer of any networking protocol is the application layer. Without good applications, the network infrastructure is built for naught. When moving to a new infrastructure, it is not necessary to carry over all the features that are on the old infrastructure. Only the features or applications that customers need are required.

When building a network that has open interfaces from the packet layer to the call-control layer and from the call-control layer to the application layer, vendors no longer have to develop applications. Now, they can simply write to these standard APIs and have access to a whole new infrastructure. When a new packet infrastructure is built, opportunities for new applications become widely available.

Legacy applications such as call-centers for enterprise networks, and standard PSTN applications such as call waiting and call forwarding, must be ported onto a new infrastructure without the end user realizing that the change occurred. After these legacy applications are ported, literally thousands of new enhanced applications can be specifically developed for packet infrastructures. These include (but are not limited to) Internet call waiting, push to talk, find me-follow me, and unified messaging. These applications are discussed in Chapter 6, "Voice over IP Benefits and Applications."

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