You are probably familiar with Ethernet technology and what it takes to put a network together. It is pretty basic stuff really. You buy a switch or two, wire them together, configure the network parameters of the clients, and you are ready to go. This approach works well for a simple office network, but what if you want to build an Ethernet network primarily intended for video production?
Characteristics of video production networks
There are several things that distinguish a video production network from an office network. Some of these are the size of the files being moved, a requirement to support streaming professional video, nonblocking routing and high network availability (management). Some factors that are important to an office network but may not be important to a video production network include Internet connectivity and e-mail connectivity.
The size of the files and the requirement for support of streaming video create unique demands that are not found in office networks. In a typical office network, a graph of network usage shows many peaks and valleys. Networking technology takes advantage of the valleys (lulls in network traffic) by buffering data at a switch until bandwidth is available. (See Figure 1.) When the network traffic density increases, there is less time for switches to empty their buffers. As a result, low priority packets may be discarded at the switch.
In video applications, a graph of network usage shows a much higher overall density, meaning there are relatively fewer peaks and valleys. This is because video files are large, creating higher overall network demand, and because professional video streaming can require a lot of bandwidth on a continuous basis. If your network is not designed properly, as video-related traffic increases, switches will start to discard packets, and network errors will result. Since we can anticipate that this network will have unusually high overall network traffic density, it is imperative to build a fast network with enough capacity to handle the traffic. Fortunately, this is not as expensive as it once was.
How to build a fast network
There are several things you can do to build fast networks. One solution is to use high-speed connections to the client, but you may want to think this through carefully. Our first reaction as engineers may be that we need to provide the fastest possible connection to every computer on the network. This may not be the case and may actually affect overall network performance. Why? The faster the client connection, the more bandwidth that client will require on the backbone of the network.
A better solution may be to consider what the maximum bandwidth on the backbone of the network will be and then decide what would be a reasonable amount of bandwidth to allocate to each network client. Some switches allow the user to configure the maximum bandwidth allowed over a particular port. Setting this parameter would allow you to use a high-bandwidth media such as fiber, but to limit the ingress traffic from each port so as not to overburden your backbone. This approach allows you to throttle connections at the source rather than indiscriminately discard packets somewhere in the network.
Adding GigE to the mix
In the past, it was common to purchase switches with 10/100 ports and a backbone GigE connection. This allowed the designer to assume that on a six-port switch, for example, the backbone link would never be saturated, because the maximum aggregate speed of all the switch ports (six ports × 100Mb/s = 600Mb/s) would not overburden the backbone connection (1000Mb/s). However, GigE switches are becoming common.
If you have a six-port switch and each port is capable of 1000Mb/s, then the single GigE backbone connection will be overloaded. This is known as a blocking switch. A blocking switch does not have enough capacity to support all ports operating simultaneously at their full speed. Actual throughput of these switches is less due to networking overhead and other considerations.
Before you implement a solution, be sure you actually have a problem. The six-port GigE switch you purchase from an office superstore probably does not have a 6Gb backplane. In fact, the switch may use a GigE as the uplink connection to the backbone. If this is the case, the maximum bandwidth available to the backplane is 1000Mb/s. This does not mean that the switch is internally limited to 1000Mb/s. Check the switch specifications. The switch may be able to support several simultaneous GigE connections between ports within the same switch. With a GigE uplink port, there will definitely be a bottleneck on the way from the switch to the backbone.
Let's assume for a minute that you have a switch that has a high-capacity backplane. Perhaps internally, the switch can handle 4Gb/s. If you only have GigE connections on the switch, how can you get a high-speed connection to your backbone? The answer is port aggregation, sometimes also known as teaming.
Teaming several ports together creates a virtual port that aggregates together the bandwidth of several ports into a single virtual connection. On a 12-port GigE switch, if you aggregate three GigE ports to use as a teamed connection to the facility backbone, you can create a 3Gb/s connection between that switch and the backbone. Now you have a very fast pipe to the backbone. Of course, you may have just moved the problem upstream. Be sure that the backbone switches have the backplane capacity to handle these very high-speed connections.
Monitoring the network
Depending on the nature of the video production network, you may require high network availability, especially for mission-critical applications. Managed switches, especially in the backbone, give you the ability to monitor the network's health and to determine network usage trends. These can be very useful in anticipating outages and taking action before they occur.
For example, if one of the managed backbone switches reports that it is nearing capacity, it is time to budget for a larger switch. Also, if the switch reports an intermittent connection to one of its ports, this could be an early warning that a patch cord is going bad. This gives you the opportunity to replace the patch cord before it fails completely.
Another consideration when designing video networks is what sort of connectivity the network will have to the outside world. In some facilities, the production network is totally isolated from the outside world, meaning there are no Internet connections and no e-mail. In other facilities, Internet and e-mail are a vital part of the overall user experience. Much has been written about Internet, firewalls and e-mail security. I do not have much to add here except to say that if you do decide to connect these networks to the Internet, be sure you take adequate precautions.
Brad Gilmer is president of Gilmer & Associates, executive director of the Video Services Forum and executive director of the Advanced Media Workflow Association.
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