Video Protocols Supported in Cisco CallManager

Cisco CallManager Release 4.0 added support for video in both the SCCP and H.323 protocol. You can make calls between SCCP clients, between H.323 clients, and between SCCP and H.323 clients.

H.323 and SCCP endpoints have these characteristics:

• H.323 devices typically register with an H.323 gatekeeper (such as the Cisco IOS gatekeeper). The H.323 gatekeeper maintains the registration state of each endpoint, but it directs all call requests to Cisco CallManager. SCCP devices register directly with Cisco CallManager.
• H.323 devices send the complete dialed number all at once in their H.225 setup messages. SCCP devices send each dialed digit one-by-one as they are entered on the keypad. The difference is subtle but worth noting because it affects the experience of the user. Dialing on an H.323 device is much like dialing on a cell phonethe user enters the entire number and then presses the Call or Dial button to initiate the call. SCCP devices are more like a traditional telephone, in which the user goes off-hook, receives a dial tone from Cisco CallManager, and then starts dialing digits.
• H.323 devices are configured through the user interface of each endpoint. Changes to the configuration or software or firmware loads must be done locally on each endpoint (or, in some cases, through Simple Network Management Protocol [SNMP] or other vendor-specific management applications). SCCP devices are centrally controlled and configured in Cisco CallManager Administration. The configuration and software or firmware loads are then pushed to the endpoints via the TFTP. TANDBERG SCCP devices that receive their configurations via TFTP are the exception; however, software or firmware upgrades must be done manually (or through TANDBERG management applications).
• H.323 devices advertise their capabilities to Cisco CallManager on a call-by-call basis using H.245. SCCP devices advertise their capabilities when they register with Cisco CallManager and whenever their capabilities change. Cisco CallManager then decides, on a call-by-call basis, which types of media channels are negotiated between the endpoints.
• H.323 video devices offer basic call capabilities in interaction with Cisco CallManager. SCCP devices offer PBX-style features, such as hold, transfer, conference, park, pickup, and group pickup.
• For both types of endpoints, the signaling channels are routed through Cisco CallManager, but the media streams (audio and video channels) flow directly between the endpoints using RTP.
• H.323 and SCCP endpoints can call one another. Cisco CallManager provides the signaling translation between the two protocols and negotiates common media capabilities (common codecs).
• SCCP endpoints can invoke supplementary services, such as placing the call on hold, transferring the call, and conferencing with another party. H.323v2 devices support receiving Empty Capability Set (ECS) messages. ECS allows the devices to be the recipients of such features (that is, they can be placed on hold, transferred, or conferenced), but they cannot invoke those features.
• H.323-to-SCCP calls are not the only types of calls allowed by Cisco CallManager. Any device can call any other device, but video calls are supported only on SCCP and H.323 devices. Specifically, video is not supported in these protocols in Cisco CallManager Release 4.1:

  - Computer telephony integration (CTI) applications (Telephony Application Programming Interface [TAPI] and Java TAPI [JTAPI])

  - Media Gateway Control Protocol (MGCP)

  - SIP

Table 28-2 summarizes the key differences between H.323 and SCCP endpoints.

SCCP Video Call Characteristics

If a call is placed from an SCCP phone that reported video capabilities to Cisco CallManager and the other end supports video as well (another SCCP phone with video capabilities or an H.323 device), Cisco CallManager automatically signals the call as a video call. The amount of bandwidth the call can consume is determined by your region configuration. The system does not ask users for bit rate.

In Cisco CallManager Release 4.1, H.264 support was added for SCCP video devices. H.264 contains a number of features that allow it to compress video much more effectively than older codecs. This capability allows H.264 to deliver better video call quality over less bandwidth. Cisco CallManager supports the H.261 and H.263+ codecs as well. In addition, Cisco CallManager Release 4.1 supports the audio codecs G.711, G.722, G.723.1, G.728, G.729, Cisco Wideband codec, and Global System for Mobile Communications (GSM). The wideband video codec of Cisco VT Camera reduces PC CPU utilization, unlike the resource-intensive H.263 and H.264 codecs. However, you will sacrifice a significant amount of bandwidth when using the wideband codec. For this reason, you will use this codec primarily in LANs, not over WAN links.

Cisco CallManager uses out-of-band H.245 alphanumeric dual-tone multifrequency (DTMF). DTMF might not work between SCCP and third-party H.323 devices because many H.323 devices pass DTMF in-band.

H.323 Video Call Characteristics

Call forwarding, dial plan, and other call routingrelated features work with H.323 endpoints. During all this, the Cisco CallManager remains the central call-routing intelligence. In Cisco CallManager, H.323 endpoints can be configured as H.323 terminals, H.323 gateways, or H.323 trunks. Many H.323 devices request a gatekeeper for registration.

Some vendors implement call setup such that they cannot increase the bandwidth of a call when the call is transferred or redirected. In such cases, if the initial call is audio, users might not receive video when they are transferred to a video endpoint.

Video H.323 clients that use trunks to other Cisco CallManager systems or other H.323 systems can use the same features as audio-only H.323 clients. Currently, neither video media termination points (MTPs) nor video transcoders exist. If an audio transcoder or MTP is required for a call, that call will be audio only. H.323 video endpoints cannot initiate hold, resume, transfer, park, or offer other similar features. Only if an H.323 endpoint supports ECS can the endpoint be held, parked, and so on.

Dynamic H.323 addressing within Cisco CallManager provides a facility to register H.323 video terminals on Cisco CallManager when the video terminal receives its IP address through a Dynamic Host Configuration Protocol (DHCP) server. Endpoints are tracked based on their E.164 address registration with an adjacent video gatekeeper. The E.164 address is a static identifier that remains constant from the perspective of both gatekeeper and Cisco CallManager. The feature became available with Cisco CallManager Release 4.1. To move to a converged voice and video dial plan, it is highly desirable that Cisco CallManager become the entity that manages call routing and digit manipulation for both the voice and video endpoints, regardless of call-signaling protocol. Earlier releases of Cisco CallManager required that an H.323 video terminal be configured on Cisco CallManager based on static IP address information. As a support and mobility issue, this design could not facilitate a scalable method for endpoint management because configuration information was accurate only as long as the DHCP lease did not expire with the endpoint in question.

Cisco CallManager Release 4.1 supports H.261 and H.263 when using H.323-based video endpoints; H.264 is supported only for SCCP endpoints. H.323 video clients support the voice codecs G.711, G.722, G.723.1, G.728, and G.729. H.323 endpoints can also support far-end camera control (FECC). FECC enables a user to control the camera of the far side during an active video call. For FECC, two separate, unidirectional RTP streams are set up. This feature was introduced in H.323 version 5 (H.323v5) as Annex Q.

Cisco CallManager uses out-of-band H.245 alphanumeric DTMF. DTMF might not work because many H.323 devices pass DTMF in-band. If an H.323 device uses in-band DTMF signaling, Cisco CallManager will not convert it to out-of-band signaling, and the DTMF signaling will fail. Both sides need to use a common scheme; either both devices need to use out-of-band signaling or the H.323 device needs to support both methods and autodetect the method it should use.

SCCP and H.323 in Cisco CallManager

The call-related features of the Cisco IP video telephony integration strongly depend on the protocol in use. FECC, for example, is an H.323 feature supported only by H.323 devices and is not available on SCCP devices. Videoconferencing with three or more parties on a call requires an MCU. The Cisco IP/VC 3540 MCU supports both protocol stacks: H.323 and SCCP. This can be configured in the MCU Administration window.

The Cisco IP/VC 3521 and 3526 videoconferencing gateways bridge the gap between the installed base of ISDN videoconferencing group and room systems and IP-based H.323 systems. The gateways connect H.320 video systems on ISDN to H.323 systems on IP by translating calls initiated from the PSTN to their equivalent on the packet network, and vice versa. To enable SCCP to call H.320 endpoints, an H.323-based video gateway is necessary as well.

Beginning in Cisco CallManager Release 4.1, mid-call video is supported. Mid-call video allows an active voice call to become a video call if the video capabilities are added during the active call (for instance, if the Cisco VT Advantage software is turned on). Cisco VT Advantage will then associate with the phone and try to set up a video stream. If both parties are SCCP video endpoints, the call immediately becomes a video call. If the other party is an H.323 endpoint, the SCCP endpoint tries to request a video channel. If the H.323 endpoint rejects the incoming channel or does not open a channel, the call becomes either one-way video or audio only.