Videoconference H.323 over Wireless

Question

Hi everyone
We are looking at the possible creation of a "video only" network for the transport of our clients H.323 videoconferencing traffic throughout our community. Does anyone have any experience transporting H.323 video over wireless networks? Any success?
Specifically we would like to create a central hub site located in the city which all the edge videoconferencing units connect to. We would put an Aironet 1200 centrally located and 1200's at each client site. The radius around the hub is about 5km. All buildings are line of sight. One client is at the 5km mark and will be the main jump point to the surrounding communities where they already have all the IP based videoconferencing systems sitting on their WAN.
I am not sure if the 1200 with the 5Ghz modules are capable of this distance at a reasonable speed? And have not been able to find someone that had tried H.323 video over a half duplex link? Any thoughts would be greatly appreciated!
Thanks,
Ken

Answer 1

We have obtained exceptional video and audio performance with a H.323 Video Conference solution using two Polycom Veiwstations at 768Kbps over an 802.11b (11Mbps) network using 350's, however the key to this type of installation is very high signal to noise and narrow beam point to point links (in areas with lots of public traffic).
The project was for a video conference link between a remand facility and a local courthouse. They found that the costs of transporting prisoners between the remand center and courthouse and the waiting times were far more costly than the implementation of a videoconference facility to do simple remand hearings.
The Courthouse and Remand Center are aproximately 8km distance but have no direct line of sight, so we encorporated a central repeater site. Initially this central site was equiped with an Omni 180deg 15dBi antenna and a 350 series bridge, the two remote sites were equiped with 13dBi 30deg yagi antennas directed towards the central site. We had signal levels of between 13-18 dB signal to noise but found that the performance was not adequate to carry video traffic at sustained speeds. Since the wireless network is half duplex the addition of the repeater site makes the end to end network act like a 'quarter duplex' as it were. (two seperate legs each having to wait for the other to finish before communicating).
Before we were sucessful we tried a number of senarios, including the placement of a video bridge at the central location which enhance the store and forward capability of the video and audio data above that of the 350's data buffers at the cost of a bit of latency. In the end we needed to add a second bridge, to separate the two legs with non-overlapping channels, in the central site and replacing the Omni with two Yagi on the two seperate bridges (the bridges linked via 10Mbps switch) just to get the system up and running by the deadline. A lot of our greif we later determind was due to other wireless networks in the vacinity (keep an eye on carrier tests at all locations durring the installation, and use narrower beam antenna's where aplicable) this coupled with the 'quarter duplex' senario greatly hampered the situation.
In hind site we would definately use as narrow a beam width high gain antenna as possible, like a parabolic dish to point at the central site, eliminating much of the sorounding noise. and try and keep the S/N as high as possible as well.
-We suggest using a well grounded nema enclosure mounted as close to the antenna as possible to eliminate loss on coaxial cabling.
-Pay the extra money for higher quality coax, coax can add as much as 20dB/100ft. attenuation depending on the quality of cable.
-the use of signal splitter/combiners with multiple narrow beam high gain antenna's and amplifiers to increase your power output and receive gain (taking note of maximum ERP allowances in your area of course ;) instead of an omni or sector antenna. This will reduce interference from other radio networks in the vacinity.
These precautions hold true for any installation but considerably so for Full Duplex Constant Bit Rate applications being used over a half duplex system like Video Conferencing which requires low and stable latency.
Feel free to contact me with particular questions in regards to your project.
David Fulgham, AScT
ComGuard CTS Ltd.
867-979-3292