AV Technology - May 2008 - (Page B4)

VIDEO ON THE NETWORK There are many applications of network video in the corporate world. Today, corporate IT is often responsible for the transport of training videos. The IT network also frequently needs to carry executive presentations that are considered essential to communications strategies. Unfortunately, not all forms of video are alike, nor do they have the same impact on the network. The type of video transferred across the Internet is often based on TCP (Transmission Control Protocol) rather than UDP (User Datagram Protocol). TCP was created primarily to transfer large bulk data files, so it attempts to consume as much bandwidth as the connection makes available. It is likely to have a big impact on other applications that share network resources. Videoconferencing is now commonly transported over IP, often in the standard MPEG transport stream. Such a transport stream is becoming the universal format for most forms of video. Unfortunately, many of your network troubleshooting tools may not work here, because the tools look for a “call set-up” procedure. If they see it as H.323 or SIP, they often assume the stream is a voice stream rather than MPEG video. Unlike digital audio networks, conventional video over IP follows standards closely, and can most easily be dealt with in an IP network. And the other forms of video are beginning to use MPEG transport stream format. One of the differences between how video is delivered on networks today versus ten years ago relates to how it analyzed and managed. In an analog video world, evidence of problems included ghosting or scattered interference patterns. In a digital non-IP world, the signal is either perfect, or not received at all. But BY PHIL HIPPENSTEEL Once the MPEG payload is inserted, three or four packet headers are added. When the packets are completed, they are typically about 1,350 bytes each. Since these packets have uniform size, they will be handled somewhat efficiently by switches and routers. Play out should be smooth, unless you mix in standard data traffic, like Internet applications, which are mostly TCP-based with variable length packets. This can wreak havoc. When small packets need to get out of a switch before a video packet, there is little delay. However, when a large packet is queued in front of a video packet, there is much more delay. This introduces network jitter, and it forces the STB to compensate for the unpredictable delivery times, causing packet loss and, as a result, tiling. Phil Hippensteel (pjh15@psu.edu) is an industry consultant. He is an assistant professor at Penn State University. with video over IP transport, the most common problem is “tiling.” Small rectangular parts of the screen are distorted due to “packet loss.” When the encoder receives the video input, it separates the video portion from the audio portion. To prepare the video for transport, it takes a single frame (1/30 second) and compresses it. Rectangular blocks of pixels are compressed separately. The resulting data becomes the payload of an MPEG packet. Subsequent MPEG packets are computed or partially computed using that first block. Seven sequential MPEG packets become the payload of one IP packet, and the encoder sends it onto the network. If the network drops any of the IP packets, the portion of the recreated frame will be distorted or black. Video requires large amounts or bandwidth. For example, standard definition video encoded as MPEG2 will need 3 to 5 Mb/s. High definition (HD) will take more — much more. Video Encoder Frames I B B P 184 184 184 184 184 MPEG Header 184 184 184 MPEG Header 184 MPEG Header bytes MPEG Header 184 184 7 MPEG packets The IP packet The video encoder typically compresses each frame of video (1/30th of a second) and compresses it into an I, B, or P payload frame. Then it attaches a 4-byte header. Seven of these MPEG packets become the payload for the IP packet. 4 GUIDE TO NETWORKED AV

Table of Contents Feed for the Digital Edition of AV Technology - May 2008

AV Technology - May 2008 Contents Precedent Corporate: How to Qualify an AV Supplier Education: Working Smarter AV Technology's Guide to Networked AV Government: IPv6 — Why You Should Care Calculating ROI on Your Digital Signage Investment Enhancing Audio in the Classroom Minimizing Equipment Theft Smart AV The Tribe Has Spoken AV Enables Interactive Research AV Image Enhancement Product Spotlight Tech Horizons Product Review New Products AV MO

AV Technology - May 2008

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