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CUVA leverages several communications protocols, as detailed in Table 19-1.
Table 19-1 CUVA Communication Protocols
Networking
Protocol Description Usage Notes
Cisco Audio
Session Tunnel
(CAST)
Allows communication between the
Cisco IP Phone and associated
software, such as CUVA.
TCP source and destination port 4224.
Cisco proprietary protocol.
CAST is used between CUVA and the
IP phone to build an association. The
PC discovers the IP phone, using Cisco
Discovery Protocol (CDP).
CAST is also used to send signaling
information for video streams from the
IP phone to CUVA.
CAST signaling features include call
video stream start and stop, hold, and
resume.
Cisco Discovery
Protocol (CDP)
Layer 2 Cisco proprietary device
discovery protocol that runs on Cisco
manufactured equipment. Works only
between directly connected equipment.
CUVA uses CDP to communicate its
capabilities to the Cisco IP Phone, and
the Cisco IP Phone uses CDP to
communicate information, such as its
IP address, to CUVA.
Real-Time
Transport
Protocol (RTP)
A standard for using User Datagram
Protocol (UDP) to transport real-time
data, such as interactive voice and
video, over data networks.
RTP is used to encapsulate and stream
the audio (between Cisco IP Phones)
and video (between CUVA endpoints).
Skinny Client
Control
Protocol
(SCCP)
A Cisco protocol using low-bandwidth
messages that allows the exchange of
signaling messages between IP devices
and the CUCM.
Uses TCP port 2000.
CUVA does not use SCCP itself. It
uses CAST to send signaling messages
to the Cisco IP Phone, which acts as a
proxy and passes the signaling
messages to CUCM using SCCP.
492 Chapter 19: Cisco Unified Video Advantage
When a Windows PC runs CUVA using a hardware-based Cisco IP Phone, the PC should
be connected to the PC port of the Cisco Unified IP Phone. In most configurations, the PC
will be in a different VLAN from the Cisco IP Phone. All IP-based communication between
CUVA and the Cisco IP Phone needs to be routed. Only CDP is exchanged directly.
CDP exchange between the IP phone and Video Advantage application takes place so that
CUVA and the Cisco IP Phone can discover one another. A CDP driver is installed and
bound to the TCP/IP stack on the PC during the installation of CUVA. This driver allows
the CUVA application to dynamically learn the IP address of the Cisco IP Phone during
the CDP exchange and associate with it. This feature allows mobility of the application
between different IP phones on the network. The user may connect a PC to the PC port of
any video-enabled Cisco IP Phone on the network and begin making video telephony calls.
CDP also provides a measure of security in that the IP Phone responds only to association
messages from a CUVA client that matches the IP address of the device that is connected
to its PC port, minimizing the risk of someone else associating with your Cisco IP Phone
over the network and receiving video when calls are placed on your IP phone. The Cisco IP
Phone begins listening for CAST messages on TCP port 4224.
CUVA and the IP phone then exchange CAST protocol messages over TCP/IP. CUVA
sends a CAST message to the IP phone, which is normally in a different IP subnet (VLAN).
The packet first travels through the PC VLAN to the default gateway, where it is routed
toward the IP phone. The CAST protocol allows CUVA to associate with the IP phone and
receive event messages from the IP phone when calls are placed or received. After this
association process occurs between the CUVA client and the IP phone, the IP phone
updates its registration status with CUCM, advising CUCM of its video capabilities.
The Cisco IP Phone acts as a proxy toward CUVA for the setup of video streams. Only the
signaling is proxied through the phone. The IP phone specifies the IP address of the PC for
the video stream and its own IP address for the audio stream. CUCM communicates only
with the Cisco IP Phone to set up video calls. The IP phone then communicates the video
setup to CUVA using the CAST protocol. The CAST messages between the IP phone and
CUVA have to be routed between the voice and the PC VLAN again.
Figure 19-2 illustrates the following video call process:
Step 1 CDP runs on both the Cisco IP Phone and the CUVA application on the
desktop PC. The CDP announcement from the Cisco IP Phone includes
the IP address of the Cisco IP Phone.
Step 2 The PC initiates a CAST association to the Cisco IP Phone. Because the
Cisco IP Phone is in a different IP subnet (VLAN), the PC sends the
association to the Layer 2 destination MAC address of the router to be
routed to the Cisco IP Phone’s IP subnet (VLAN). The PC and Cisco IP
Phone create an association to each other.
CUVA Communication Flows 493
Step 3 A user picks up the Cisco IP Phone and dials an extension that is video
enabled and running CUVA. CUCM sends SCCP signaling to the Cisco
IP Phone indicating that it should open a video call. The Cisco IP Phone
contacts the desktop PC to indicate.
Step 4 The Cisco IP Phone opens an RTP bearer audio channel with the
destination phone, and the PC opens an RTP bearer video channel with
the destination PC’s CUVA application.
Figure 19-2 Video Call Setup
The audio channel of a video stream consists of the audio codecs used by CUCM.
The G.711 audio codec bit rate is 64 kbps, and the G.729 audio codec is 8 kbps.
The bit rate available for the video channel depends on the audio codec and the video call
speed. The video channel bit rate is the speed of the video call minus the bit rate of the audio
codec. A 384-kbps video call with G.711 audio will create a video channel of 320 kbps (384
kbps minus 64 kbps). A G.729 audio codec with the same video speed (384 kbps) would
result in a 376 kbps video channel (384 kbps minus 8 kbps).
Table 19-2 lists video call speeds with different audio codecs and the resulting video
channel bit rates.
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