transmission of nontelephone signals such as encryption, flow control, and jitter management. It is also used for tun- neling H225 and for DTMF (Dual-Tone Multifrequency) Relay. SIP (Session Initiation Protocol) is an IETF-defined call signaling protocol. SIP has gained great prominence in the past few years due to its open nature and, as a result, is the signaling protocol of choice for open source VoIP systems such as Asterisk. SIP is a very robust and configurable protocol. It is text-based, which, though easier to read and understand, can appear excessively wordy when compared with H323. In the 1990s, that might have been a problem with low-powered networking How VolP Works equipment using slow-speed links, but in today's environment where power and speed are abundant, SIP is very popular. SIP was designed for the IP world, and may seem more familiar to those who don't have a telephony background. Skinny Call Control Protocol (SCCP) Skinny for short, SCCP is a Cisco-proprietary protocol used for signaling endpoints with Cisco's Unified Communications Manager (formerly Cisco Call Manager). SCCP is used by the Cisco IP phones in this chapter, the Cisco IP Communicator softphone, Analog Telephone Adapters (ATAs), and other Cisco telephony end- points. This protocol is a matter of some consternation for engineers and executives alike. Executives don't like being forced into buying Cisco IP phones, often quoting the price compared with less expensive SIP phones. The benefit of SCCP is that it is simple to implement and is very lightweight, SO it consumes few network re- sources. If you've got 5,000 IP phones registering with your Call Manager cluster, you'll want those registration messages to be short and sweet. CODEC CODEC stands for COder-DECoder. Although it is not itself a protocol, many of the protocols listed here are CODECs. CODECs describe how data or a signal is translated from one form to another. Common CODECs include MPEG, MP3 (a subset of MPEG), Dolby Digital surround sound, G.711, and G.729. G.711 G