Networking video
Mar 1, 2009 12:00 PM, By Brad Gilmer
Efficiency
Figure 1. Illustrates how to calculate the theoretical efficiency of video transport over Ethernet using UDP/IP
As you may know, standard Ethernet frames are a little more than 1500 bytes long (1538 to be exact). The Ethernet payload is 1500 bytes, with the rest taken up by Ethernet headers. Typically, video over Ethernet uses UDP over IP. UDP/IP headers consume 28 bytes of the 1500-byte Ethernet payload. So typical video transport, ignoring collisions, other network traffic and a host of other factors is around 96 percent efficient. (See Figure 1.)
While this may appear to be a small amount of overhead, when you are sending hundreds of thousands or millions of packets, decreasing the overhead seems like a good idea. And of course, engineers cannot resist making things better. Many years ago, the idea of Ethernet jumbo frames was introduced. The idea was simple — to allow Ethernet payload sizes to be increased for large payload types, which would make networks more efficient.
Bill Fink, the author of nuttcp — a networking test tool — has calculated that the throughput of GigE with jumbo frames set to 9000 bytes instead of 1500 bytes is about 99 percent efficient. On the surface, using jumbo frames seems like a great idea, especially for video applications moving a huge amount of data.
There is no denying the math. Jumbo frame networks are more efficient. The problem is that they may not be supported by the switches and routers in your network. And while most equipment supports jumbo frames, it only takes one switch somewhere in the network to disrupt the jumbo frame transmission. If you decide to use jumbo frames, test the network before relying on it.
Other issues to consider
There are several other issues related to moving video over networks that bear special attention. One issue is that when streaming video, everyone watching the video requires a separate connection back to the originating server. To tackle this, people have built content delivery networks (CDNs), which deploy many servers throughout the world, capable of replicating streams in the network themselves. This reduces the overall load on the originating server. CDNs coupled with multicast technology allow the delivery of a large number of streams in a role similar to over-the-air broadcasting.
Another issue that relates to the transmission of video over networks is the mind-set of the people who maintain the networks. For people who deal with packetized networks, minor service interruptions are the norm as they go about their maintenance tasks. But video users are extremely sensitive to outages, so it takes a partnership between the maintenance people and those using the networks to keep interruptions to a minimum.
Moving video over IP networks is done successfully every day. But having an understanding of the issues and potential solutions concerning networked video will help you do a better job as a broadcast engineer.
Brad Gilmer is president of Gilmer & Associates, executive director of the Video Services Forum and executive director of the Advanced Media Workflow Association.
| TV standard | MB per frame | MB/s | Size of 30-second file |
|---|---|---|---|
| 525/29 | 1.126MB | 33.75MB/s | 1.0GB |
| 720p60 | 3.093MB | 105.58MB/s | 5.6GB |
| 1080p60 | 6.187MB | 371.22MB/s | 22.3GB |
Send questions and comments to: brad.gilmer@penton.com
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