rol. Call control is the means whereby a call, be it voice or video, is negotiated and established; it is initiated when you dial a phone number. Phone numbers mean something to switches in the cloud, and they are used to route your call. In IP telephony, the phone numbers are converted to endpoint IP addresses. The endpoints then negotiate and set up the two real-time data streams for the call itself. Chapter 30: VolP In telephony terms, the control communication is called the data chan- nel, while the RTP stream is called the bearer channel. We often use the term bearer traffic to describe the RTP stream. The actual call is sent through RTP. To be accurate, RTP is also used for many types of media, including video. If you hear someone say RTP stream, he's talking about the actual voice or video traffic, not the call control. From a network standpoint, try to remember this important nugget: call control should be prioritized at IP precedence 3, while the RTP stream should be prioritized at IP precedence 5. See Chapter 32 for an example. Protocols In the Cisco world, you will likely hear the term H.323 thrown around. You may also hear H.225, G.7.11, G.729, SIP, and a host of other numbers and letters bandied about. These are all voice and video protocols used with IP telephony. Let's take a look at the more common ones: H.323 H.323 is an umbrella specification (technically a recommendation) from the In- ternational Telecommunications Union (ITU) that covers voice, video, and data transmission over IP. H.323 signaling is based on the ITU recommendation Q.931, which you may have seen in use on ISDN circuits. H.323 includes device descrip- tions for terminal, gatekeeper, gateway, and border elements, to name a few. H.323 is widely used in VoIP systems and video conferencing. H.323 was designed from a telephony point of view, and may seem complicated to those without a telephony background. H.225 H.225 is a call-control protocol used within the H.323 specification. H.245 H.245 is a protocol within the H.323 used for 