Moving video on Ethernet can be like trying to cut wire with a screwdriver.
Increasingly, users have a requirement to interconnect professional video equipment via Internet Protocol (IP) over Ethernet. Many times it's because this is the most readily available technology. But Ethernet connectivity is also generally low cost, and it is ubiquitous, not just between facilities but within facilities as well. Equipment manufacturers have responded by adding Ethernet connections on almost all professional video devices. At first, these connections were not used to transfer video of any kind. Then manufacturers added the capability to move video files on and off of devices via Ethernet. Now it is becoming more common to stream real-time video using Ethernet. However, users have encountered problems transferring video over Ethernet.
What is the problem?
In short, the problem is reliability. As the professional user comes to rely more on IP-based technology, limitations of this technology raise reliability issues that must be addressed. In a way, we are victims of our own success. Someone tries moving video files or real-time video over a network and succeeds. Others then do the same thing, and quickly, the network is overwhelmed by the amount and type of traffic video represents.
The root of the problem is that moving video on Ethernet was never foreseen. We are trying to cut wire with a screwdriver; while you can successfully do it, a screwdriver may not be the best tool for the job. To be fair, it is likely that IP will be the predominant method of video transport going forward, and network engineers have addressed the fundamental issues. That said, it is difficult to achieve reliable professional video transport on generic Ethernet networks.
A brief review of traditional IT QoS
I wrote about this topic in the March column, but let's have a refresher. Using traditional IT QoS, you can guarantee the level of performance across a network. But you should know that QoS is not a thing; you cannot buy QoS. You also cannot set the “QoS switch” on your router and get guaranteed delivery.
Instead, QoS is a framework of policies and technologies that can be used to control the quality of service that your network delivers. At a high level, QoS works by controlling traffic admitted to the network, marking traffic by type (voice, video, data, etc.), and establishing priorities for traffic types so that lower priority traffic gets dropped first when the network gets busy. QoS can also establish guaranteed routes for particular flows so that all packets in a stream flow from source to destination via the same path or reserve bandwidth during a specific time. Other QoS tools exist as well.
The IT world has spent a lot of time and effort to address QoS on IP over Ethernet. It is possible to provide reliable professional video transport across Ethernet networks. But in order to do this, you will need to implement a QoS framework on the networks. For a more detailed discussion of QoS, read the book “Video Systems in an IT Environment” by Al Kovalick.
Other QoS options
When addressing the QoS issue, it may help to stand back from the problem and think outside the box. Do you really need to use IP over Ethernet in this application? If you are trying to cut wire, perhaps you can find a wire cutter rather than use a screwdriver. Here are some other options that can move video from one place to another without the QoS issues associated with creating a managed IP over Ethernet network:
There is not much to say here that has not already been said. SDI and SDTI are well known to the professional video community. SDI routers are recognized quantities. They are nonblocking switches, meaning that if you use every input and every output on a video switch, things continue to function; this cannot be said of almost all Ethernet switches when they are running at the maximum bandwidth on all ports. Furthermore, SDTI can be used to transmit all sorts of data, not just video. And at 1.5Gb/s, SDTI provides high data transfer rates. SDI/SDTI can be a great choice for video transport inside a facility.
- Fibre Channel
This technology was developed for high-density, high-speed data transfer. It provides nonblocking transport without the need to deploy QoS technologies. It is known within the professional video and IT communities. Unfortunately, Fibre Channel is nowhere near as ubiquitous as Ethernet or SDI. It also has technical and financial limitations that make it unrealistic to deploy across an entire facility.
Several years ago, asynchronous transfer mode (ATM) looked like it was going to be the preferred choice for professional video applications. ATM provided guaranteed high-bandwidth connectivity between devices, was widely used in other industries, supported large file sizes and high speeds, and large switches were available to interconnect the number of devices typically found in a television plant. However, high costs initially limited ATM installations. ATM never really hit its stride in the professional video environment. The issue was not entirely cost-related. In point-to-point applications, SDI was firmly entrenched. In networking applications, Ethernet quickly dominated, eclipsing ATM in all but a limited number of applications.
- Ethernet without QoS
Ethernet may be used in professional video applications without a QoS framework. In this case, the lowest cost and easiest implementation in a professional video environment is to simply overprovision your network. Overprovisioning means to provide much more bandwidth than you would ever expect to be used. While overprovisioning would seem to logically take care of any issues you might have with moving video over a computer network, there are times when, even with a vastly underutilized network, video transfers will fail. For example, FTP transfers using generic FTP clients are guaranteed to fail if the delay on the network is high enough, regardless of the amount of available bandwidth.
Determining the optimum QoS solution
Implementing QoS is a trade-off. Whether you choose to use SDI or ATM, which have high QoS built-in, or whether you decide to use Ethernet networks and deploy a QoS framework that delivers guaranteed performance, the final choice will depend upon a number of factors.
As Figure 1 illustrates, you can think of QoS as a pyramid. At the bottom of the pyramid is the best effort Internet. At the top of the pyramid are technologies such as SDI. QoS improves as you go from the bottom of the pyramid to the top. Varying levels of complexity are used to address QoS in the levels of the pyramid. However, it is interesting to note that in the pyramid there is no specific QoS framework at either the bottom or the top of the pyramid. In both cases, the QoS is implied. At the bottom, you get best effort. (If the packet makes it, great. If not, oh well.) At the top using SDI, the video is guaranteed to make it by virtue of the technology employed.
It is up to the facility engineering team to determine the appropriate level of QoS required and to balance the trade-offs between ubiquity and the near universal connectivity of Ethernet, and the implicit extremely high QoS of SDI. As a final thought, it is likely that both technologies will co-exist in major professional video facilities.
Brad Gilmer is president of Gilmer & Associates and executive director of the Advanced Media Workflow Association.
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