Continuing the TelePresence notes series based on Cisco TelePresence Fundamentals by Cisco Press.
TelePresence Network Interaction
As highlighted in each TelePresence system connectivity schematic, the primary codec is the interface between the CTS system and the network infrastructure. The primary codec connects to the network access-edge switch through an RJ-45 10/100/1000 port. The access-edge Catalyst switch that it connects to provides IP services, 802.1Q/p VLAN serv- ices, QoS services, and security services to the TelePresence system.
TelePresence systems use a private network for internal communications between the primary and secondary codecs and between codecs and cameras. By default, the internal address range is 192.168.0.0/24 through 192.168.4.0/24; however, if the TelePresence codec receives a 192.168.x.x address from the network, the internal private net- work switches to 10.0.0.0/24 through 10.0.4.0/24.
Even though only 192.168.0.0/24 through 192.168.3.0/24 are illustrated, 192.168.4.0/24 is reserved within the system for future (internal) use.
Similarly, if the TelePresence system uses 10.0.0.0/24 through 10.0.3.0/24 for its internal net- working address range, 10.0.4.0/24 is reserved within the system for future (internal) use.
Following are three key points regarding the internal networking of TelePresence systems:
- From the network’s perspective, the TelePresence primary codec appears as a single endpoint device with a single IP address. (Remember, the 7975G IP Phone also appears as a separate endpoint device with its own IP address).
- The internal components (such as secondary codecs and cameras) do not receive a default gateway; therefore, they cannot route beyond the primary codec.
- If the primary codec uses 192.168.0.0/24 through 192.168.4.0/24 as its internal networking addresses (which is the default), it cannot connect to external servers or end-points that uses these same addresses (because it will attempt to reach such addresses via its internal network, not its external default gateway). Conversely, if the primary codec has been assigned an IP address from the network in the 192.168.x.x range, it uses internal networking addresses in the range of 10.0.0.0/24 through 10.0.4.0/24 and similarly cannot connect to external servers or endpoints that might use these same addresses.
Cisco TelePresence network control, management, and signaling protocol:
Cisco TelePresence signaling and media paths:
When the TelePresence system completes these protocol interactions, it is ready to place and receive calls. When a call initiates, the following steps occur:
- The Cisco 7975G IP Phone sends an XML Dial message to its primary codec.
- The initiating TelePresence primary codec forwards the request as a SIP Invite message to the CUCM.
- The CUCM, in turn, forwards the SIP Invite message to the destination TelePresence primary codec (or Session Border Controller, in the case of business-to-business calls).
- The destination codec forwards the message as an XML Ring message to its associated 7975G IP Phone. (The TelePresence primary codec can optionally be set to automatically answer the incoming call, in which case the codec answers the call immediately and proceeds to Step 6, which is to send a SIP OK message to CUCM.)
- If auto-answer is not enabled, when the user presses the Answer softkey on the 7975G IP Phone, the 7975G IP Phone replies with an XML Answer message to the destination codec.
- The destination codec sends a SIP 200 OK message to the CUCM.
- The CUCM relays this SIP 200 OK message to the initiating TelePresence primary codec, and the call is established.
- Real-time media, both audio and video, passes between the TelePresence primary codecs over Real Time Protocol (RTP).
