Broadcast networking
Feb 1, 2008 12:00 PM, By Brad Gilmer
Last month's column covered networking basics. This month, I'll discuss networking for professional video for someone who is new to broadcasting but has experience working with networking technology.
Any of the things you already know about networking are useful, as the broadcast industry employs the same basic networking technology and topology as used in other industries. The broadcast industry includes special applications, though, such as video servers, graphics design workstations and video editors. For the most part, however, these devices use networking technology in the same way as office systems do. Traditional office networks provide all of the support that any business would require.
Learning the lingo
Broadcast networking is frequently deployed as an overlay on top of a traditional office network, with limited connectivity between the two. As Figure 1 shows, broadcast facilities have all of the typical office requirements, but it is common to find another high-speed network associated with specialized broadcast equipment.
Figure 1. Broadcast networking is frequently deployed as an overlay on top of a traditional office network.
Click image to enlarge.
One reason for this is because the broadcast operation is the mission-critical core of the business. Therefore, broadcast networks are kept separate from office networks to prevent a problem on the office network from affecting on-air operations. Another reason for this arrangement is because the servers, graphic design stations and editors probably don't exchange e-mail traffic and Word documents. Instead, they frequently interchange essence and metadata about a commercial, a graphic or an edit session.
In the broadcast environment, essence refers to the pictures, sound, closed captions and other things that are transmitted to the end viewer. All of these are essence, but more specifically, pictures are referred to as video essence, audio as audio essence and closed captions as data essence.
Metadata is information about the essence itself. For example, a commercial has video, audio and data essence, but it also has information about what the commercial contains.
Much of this information is on the slate, which is text that is inserted for 10 seconds before the beginning of the commercial. Usually the slate contains the name of the commercial, the length, identifying code and other information. This metadata is not only conveyed as video at the beginning of the commercial, but it may also be sent as data along with the essence.
Different computer systems can make use of this data as part of their normal operations. Some manufacturers send the essence and metadata in separate files. Usually in this case, the video and audio are sent in a common file, and the metadata is exchanged in an XML file or another metadata format. Other manufacturers send the essence and metadata in one file. When this is done, the file format is known as a wrapper. The most common wrapper format for broadcast content is the Material eXchange Format (MXF).
Identifying video networks
The characteristics are driven by what flows over the networks. Video files can be large, exceeding 1TB for feature-length movies in high resolution. Transferring these files usually requires special protocols because normal FTP limits file transfer size to 2GB. These transfers can quickly saturate your network. They can also make the peak-to-average traffic on the network high compared with office networks, where traffic is more sporadic.
Video networks handle two types of traffic: file transfer and streaming. The difference between conventional network streams and broadcast network streams is the bandwidth. Broadcasters use many low-resolution (conventional) streams on their networks. But sometimes they move high-bandwidth, high-quality video streams on these networks as well. Again, the peak-to-average network traffic can be severely affected by these streams. In some cases, other office-based protocols depend on gaps in network traffic. It is not uncommon to see time-out errors when the protocols and the applications they serve do not anticipate high-network usage applications.
Broadcast facilities frequently include news applications. If the news workflow supports server-based editing and collaborative workflow, then the network requirements can be demanding. Not only will you face the challenges of moving large files and dense streams, but these networks also need to support messaging, e-mail, printing and other conventional office applications.
Server-based editing is where workstations connect to a central repository to access essence for editing. This approach allows users to work collaboratively. Depending on the implementation, however, it may require high-bandwidth connections between the workstations and the repository.
Engineers who design video networks for broadcast should start with a list of requirements for their facility. With some of these categories, the considerations are identical to those in office networks. But there are a few areas where special considerations apply. Those categories are the mix of clients and servers, video application types and the criticality of the network.
Mix of clients and servers
It is important to know the mix of clients and servers. In conventional networks, there are many clients and a few servers. Broadcast networks may have more servers depending on the environment, and as described above, the bandwidth occupied by these servers can be substantial.
It is essential to understand the ratio of clients to servers because servers require high-bandwidth connections to the network. The placement of central servers in the network topology is critical to avoid bandwidth bottlenecks.
Video application types
Fortunately, there are a wide variety of network-aware video applications available today. Many of these applications do not significantly affect network performance. Sending a low-resolution image by e-mail, for example, can be handled by most networks without a problem. However, if a post-production department wants to do effects rendering across the network, or a news department wants to use a centralized storage facility with network editing, then you must consider the extra demand these applications will place on the network.
Study the applications to understand what sort of load they are likely to put on the network. Remember that opening video files across a network for editing requires a huge amount of bandwidth, especially if multiple clients are involved. You may be better off keeping the content local and only exchanging edit information across the network.
Criticality of network
When building a video network for broadcast, one of the most important issues to address is the criticality of the network. It is easy to say, “Of course this network is critical; it serves my broadcast facility.” But are you willing to pay 10 or 20 times more for your network? This is not fanciful thinking. Off-the-shelf consumer networking hardware costs can be extremely low. The cost of enterprise hardware from top-of-the-line manufacturers can be staggering.
The Internet is a necessity
An increasingly important component of any video broadcast network is Internet connectivity. I have found that people are of two minds on the subject, and the issue seems to be driven by user requirements.
On the one hand, there are many times when people need access to the Internet as part of their normal work processes in a broadcast facility. On the other hand, broadcasters are justifiably worried about security issues that come with an Internet connection.
It might be easy to say that Internet connections should never be permitted on video networks for broadcast. But with the advent of commercial and program delivery over the Internet, the increasing use of the Internet in group station environments through virtual private networks and the general ubiquity of the Internet in everyday workflows, outlawing Internet connectivity may not be practical.
Brad Gilmer is the executive director of the Video Services Forum and the Advanced Media Workflow Association. He is also the president of Gilmer & Associates, a consulting firm.
Send questions and comments to: brad.gilmer@penton.com
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