Video Call Concepts

Using video endpoints in the Cisco CallManager system is, from the perspective of a user, as easy as placing a telephone call. Video terminals can be either H.323- or SCCP-controlled and are able to communicate with each other regardless of the protocol controlling the device. All video devices use the same dial plan as all other devices in the Cisco CallManager system. For Cisco CallManager, video devices are treated like voice devices, with the same configuration options. For the Cisco VT Camera, the same DN as the associated IP Phone is used. There is no additional number to remember to be able to reach someone over the video system. The video signaling is transparent to the user, and the devices negotiate their video capabilities, such as codec, format, and bit rate used for the video call, with no additional user action.

Cisco CallManager Involvement in Video Calls

A video telephony call can be established between any two video-enabled endpoints. If one of the endpoints is not video-enabled, the call is set up as a normal voice call. When both endpoints are video-enabled, Cisco CallManager signals the voice and video capabilities to both endpoints, as shown in Figure 28-1, and the endpoints directly build two RTP streams, one for voice and one for video. Call control for video calls operates the same way as the call control that governs voice calls.

Figure 28-1. CallManager Video Involvement

Tip

The term "video call" is sometimes confused with the term "videoconferencing." A videoconference is a video call with at least three participants.

For videoconferences in the Cisco CallManager system, extra hardware is required. The device that mixes the video streams is an MCU.

Video Call Flow

The typical video call includes two RTP streams in each direction. A basic video call includes two unidirectional RTP streams for voice and two unidirectional RTP streams for video. The call can include these stream types:

• Audio These are the same codecs as used in audio-only calls, G.711 and G.729, plus additional codecs, G.722 and G.728. These additional codecs can be used when communicating to third-party equipment. Because Cisco IP Phones do not use these codecs, transcoding is necessary.
• Video The video codecs used are H.261, H.263, H.264, and Cisco Wideband codecs. The video codec is a software module that enables the use of compression for digital video. There is a complex balance between the quality of the video, the video call bit rate, the complexity of the encoding and decoding algorithms, the robustness to data losses and errors, the ease of editing, the random access, the state of the art of compression algorithm design, the end-to-end delay, and a number of other factors.
• Far-end camera control (FECC) FECC is used only for H.323 devices and is optional. FECC enables a user to control the camera of the far side during an active video call. This feature must be supported by both video-enabled H.323 devices. When FECC is used, two more unidirectional RTP streams are sent between the video devices. The two additional RTP streams are used for the camera control parameters. This functionality is typically used for security cameras.

In the example shown in Figure 28-2, the video-enabled endpoints report their video and audio capabilities to Cisco CallManager. Cisco CallManager now treats the endpoints simply as video phone devices. When a call is placed or received between two video-enabled devices, Cisco CallManager signals for both audio and video streams. First the audio capabilities, such as audio codec information and audio bit rate, are signaled and negotiated, and then the audio stream is set up. After the audio stream is set up, the video capabilities, such as video codec information and video channel bit rate, are negotiated, and the devices exchange their video streams separately from the audio. In this example, the video call has four RTP streamstwo unidirectional RTP streams for voice and two unidirectional RTP streams for video transmission.

Figure 28-2. Video Call Flows

Video Codecs Supported by Cisco CallManager

Cisco CallManager supports several standard video codecs and a Cisco proprietary video codec. H.263 is a video codec specified by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) as a low-bit-rate encoding solution for videoconferencing. It was first designed to be used in H.324-based systems (PSTN and other circuit-switched network video environments) but has since found use in these other solutions as well:

• H.323 (IP-based videoconferencing)
• H.320 (ISDN-based videoconferencing)
• Real-Time Streaming Protocol (RTSP)
• Session Initiation Protocol (SIP)

H.263 was developed as an evolutionary improvement based on experience with H.261, the previous ITU-T standard for video compression, and the Moving Picture Experts Group-1 (MPEG-1) and Moving Picture Experts Group-2 (MPEG-2) standards. The first version of H.263 was completed in 1995, and it provided a suitable replacement for H.261 at all bit rates. H.263 was further enhanced in H.263 version 2 (H.263v2, also known as H.263+ or H.263 1998) and H.263 version 3 (H.263v3, also known as H.263++ or H.263 2000).

The next enhanced codec specified by the ITU-T after H.263 is the H.264 standard. Because H.264 provides a significant improvement in capability beyond H.263, the H.263 standard is now considered primarily a legacy design (although this is a recent development). Most new videoconferencing products include H.261, H.263, and H.264 capabilities.

The video codecs supported by Cisco CallManager Release 4.1 include H.261, H.263, and H.264. These codecs exhibit the parameters and typical values listed in Table 28-1.

The Cisco Wideband codec is a proprietary codec that is a fixed-bit-rate codec and runs on a PC that is linked to a phone. It enables the PC to associate with a call that the phone receives and can only be used by the Cisco VT Camera.