AV Technology - February/March 2009 - (Page 52) MAKE ROOM FOR VIDEO ➤ TABLE 2: TRAFFIC CHARACTERISTICS BY TRAFFIC TYPE Traffic Type Data Voice (VoIP) Type 1 Video (broadcast) Type 2 Video (conferencing) Bandwidth Demand highly variable low high moderate Burstiness very high very low very low moderate moderately high Tolerance to Network Jitter high moderate low low high Type 3 low (Internet streaming) ing the video traffic from the data traffic is mandatory. DHCP, ARP, and browsing (server browsing, not web browsing) are based on layer two broadcasts. This broadcast traffic is also very bursty in nature and is processed by every device on the network. So, broadcast traffic is disruptive to video streams that are being transmitted. In this case, a separate VLAN will be very helpful if the switches have the bandwidth to pass both the data and the video without introducing latency. Most modern switches are capable of doing this. Since videoconferencing is usually done across significant distances, WAN circuits are usually involved. The LAN part of the network won’t normally present a problem. However, on the WAN circuits, you’ll need to implement separate logical circuits or use VLANs. For best results, keep internet streamed video and data transfers separated from videoconference traffic. Internet streaming video can generally be treated like data. They are very much like large file transfers. Your biggest concern may be the total consumption of bandwidth. So, if it is possible, find out how the video can be throttled at the server. Generally, video servers will allow limiting the video to some fixed bandwidth, such as 256 kbps. It affects the creation of the video by adjusting the compression algorithm, frame rate, and resolution. If throttling at the server isn’t possible, place rate-limiting devices (or “traffic shapers”) in the path of the video. Most of these look at some combination of IP address, port number, and URL to determine the type of video and limit all traffic matching those identifying characteristics. Most of these devices allow traffic limiting based on a perapplication basis or a per-user basis. As long as the traffic can be identified, the traffic shaper will enforce a maximum bandwidth limitation. NETWORK STORAGE Some network files can be very large. For example, if a feature film is encoded as MPEG-2 in the same format that it might be transmitted over a network, it will need 20 to 30 GB of storage. It will also need to be read from the storage device at 2 to 6 Mbps. While modern disk drives and disk interfaces are normally rated higher than this, the ratings assume that the information on the disk isn’t fragmented. If the server has had a lot of reads and writes, the file to be sent may be highly fragmented, slowing the rate at which it can be presented to the encoder that creates the IP packets. Of course, this process is very dependent on the system processor. An older server with limited memory will likely cre- 52 | AV TECHNOLOGY | february | march 2009 http://www.spcontrols.com
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