How Calls Work with Cisco VT Advantage

When a Windows PC has Cisco VT Advantage installed, it should be connected to the secondary Ethernet port (that is, PC port) of a Cisco IP Phone 7940G, 7960G, or 7970G (or later) model. In most configurations, the PC will be in a different VLAN than the Cisco IP Phone (located in the voice or auxiliary VLAN). In such configurations, all IP-based communication between Cisco VT Advantage and the Cisco IP Phone has to be routed between the VLANs, and only Cisco Discovery Protocol is exchanged directly. The following list explains the call process shown in Figure 29-1:

1. Cisco Discovery Protocol exchange takes place so that Cisco VT Advantage and the Cisco IP Phone can discover one another. A Cisco Discovery Protocol driver is installed on the PC during the installation of Cisco VT Advantage. This allows the Cisco VT Advantage application to dynamically learn the IP address of the Cisco IP Phone during the Cisco Discovery Protocol exchange, and associate with it. This serves as both an ease-of-use feature for the end user and for security. The use of Cisco Discovery Protocol to facilitate the association process allows it to occur automatically, without the user having to configure the Cisco VT Advantage application. This allows for mobility of the application between different IP phones on the network. The user can plug into the PC port of any supported Cisco IP Phone on the network (if permitted by the administrator) and begin making video telephony calls. Cisco Discovery Protocol also provides a measure of security in that the IP phone will respond only to association messages from a Cisco VT Advantage client that matches the IP address of the device that is connected to its PC port (that is, its Cisco Discovery Protocol neighbor), 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 Cisco Audio Session Tunnel messages on TCP port 4224.
2. After Cisco Discovery Protocol discovery, Cisco VT Advantage and the IP phone exchange Cisco Audio Session Tunnel protocol messages over TCP port 4224. Cisco VT Advantage sends a Cisco Audio Session Tunnel message to the IP phone, which is in a different IP network (VLAN). The packet first travels through the PC VLAN to the default gateway, where it is routed toward the IP phone (using the voice VLAN). The Cisco Audio Session Tunnel protocol allows Cisco VT Advantage 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 Cisco VT Advantage client and the IP phone, the IP phone updates its registration status with Cisco CallManager, advising Cisco CallManager of its video capabilities.
3. When the Cisco IP Phone receives signaling information for video calls, it acts as a proxy toward Cisco VT Advantage for the setup of the video streams. Only the signaling is proxied, but when the RTP endpoints (IP addresses and UDP RTP port numbers) are negotiated, the IP phone specifies the IP address of the PC for the video stream and its own IP address for the audio stream. When Cisco CallManager tells the Cisco IP Phone to open the video channel, (communicating to the IP phone using the voice VLAN) the IP phone proxies those messages to Cisco VT Advantage using the Cisco Audio Session Tunnel protocol. These Cisco Audio Session Tunnel messages have to be routed between the voice and the PC VLAN again.
4. After the voice and video channels have been successfully set up, the audio stream is sent to the IP address of the IP phone (to the voice VLAN), whereas the video stream is sent directly to the PC IP address (to the PC VLAN).

Figure 29-1. Cisco VT Advantage Call Process

Note

Firewalls or access control lists (ACLs) must permit TCP port 4224 to allow the exchange of Cisco Audio Session Tunnel messages.

 

Cisco VT Advantage Video Modes

For privacy, the participants can switch to a mode called receive-only mode to prevent the camera from sending a picture to the other end of the call. Table 29-3 shows various scenarios of the calling or the called party activating or disabling receive-only mode.

Table 29-3. Cisco VT Advantage Video Modes

Cisco VT Advantage Mode on Your PC	Cisco VT Advantage Mode on the PC of the Other User	Result
Enabled	Enabled	When you place or answer a call, two video windows open on your PCyou will see yourself in the Local Video window and the other party in the Remote Video window.
Receive-only	Enabled	When you place or answer a call, you will see the other party in the Remote Video window. The Local Video window will not display. The other party will see a blank image in the Remote Video window.
Enabled	Receive-only	When you place or answer a call, you will see yourself in the Local Video window and a blank image in the Remote Video window. The other party will not see a Local Video window.
Receive-only	Receive-only	No one will see the other party; this mode is similar to a telephone call.

Note

When Cisco VT Advantage is not running on your PC or on the PC of the remote peer, the call functions as a regular telephone call without video.

Configuring Cisco CallManager for Video