As more and more program flows involve files rather than traditional video signals, a debate has emerged over how, when or if traditional crosspoint routers should be replaced by IP networks. After all, hardly a day goes by without another task being performed via the mother of all networks, the Internet.
Also, recent major sporting events, such as the UEFA Euro 2008 Football Championships and FIFA World Cup Football, have been covered with high-bandwidth IP-based contribution networks at specific locations. Is it time to adopt the same principle for studio acquisition and program playout? The first step is to examine the benefits and drawbacks of each.
Crosspoint routing
The traditional crosspoint router is a matrix of electronic switches that can connect combinations of inputs to outputs. Typically an input signal can be switched to multiple outputs, making the router distributive. Figure 1 above shows a representation of a crosspoint router with four inputs and four outputs. The crosses represent active switches connecting an input to an output.
For a clean video switch, the devices handling the video have to operate synchronously with the video, switching between sources in a nonvisible part of the video signal. This requires switches to be accurately timed to at least 40ms in the case of standard definition.
Another key requirement for the crosspoint router is to have 100 percent availability for any switch. With a separate electronic switch for every possible path through the router, it is inherent in the design.
However, this guaranteed availability comes at a price. Figure 2 below illustrates how the cost of a crosspoint router increases approximately as a square law (note, this also applies to the physical size), and Figure 3 on the next page shows how the percentage of actual bandwidth used at any one time in the router follows an inverse trend.
Network routing
So how does this compare with a network? A network consists of interconnected components or circuits, or in computing terms, it's a system that delivers a means of transferring data between devices. This is in essence what the crosspoint router does, so does a network offer us an alternative means of performing this task, and are there any benefits?
Almost certainly there are no network topologies that follow the same cost and bandwidth curves shown in Figures 2 and 3. Therefore, there is a potential saving to be made by better use of a system's available bandwidth.
In a crosspoint router, as a switch is provided for every possible route, the system bandwidth is related to the number of crosspoints, which is the number of inputs multiplied by the number of outputs. In a network router, the bandwidth only needs to be related to the number of inputs. So the cost of a network router increases in a linear fashion, as opposed to the square law of the crosspoint topology.
Media networks distribute information by switching packets of data. However, the packetization and switching of individual packets requires data processing at the network nodes, which introduces some form of delay within the network. What these delays might be and how they can be managed are key factors when considering the real-time distribution of video.
Benefits of network routing
There are two potential benefits of network routing. One is that it is truly format and signal agnostic, because the packets of data can be coded representations of whatever we choose. This allows our switching infrastructure to support video, audio and other signals. With a crosspoint router, the electrical characteristics of different signal types have typically meant separate routers for each.
Secondly, while a crosspoint router is a single device with all signals to be switched wired directly to the router, a network is distributed. This means each node can be located near the origin or destination of each group of signals, potentially saving large amounts of cabling in a video installation.
So how can we model the crosspoint router on a network? First, look more closely at Internet Protocol (IP), the dominant network protocol in use today. Basic IP addressing sends each packet of data to a destination defined by a familiar four-field address in the packet header, such as 192.168.0.1.
However, when we consider the distributive nature of the crosspoint router, this point-to-point mechanism already poses a problem, because in a large installation the same source may be routed to many destinations.
A system that transmits a separate packet for each destination would require sufficient bandwidth and processing capabilities at the network input to cope with sending packets to as many destinations as required. When dealing with video, this soon becomes unachievable, or at least very expensive, as the input device bandwidth equals 270Mb/s multiplied by the number of outputs requiring the video source. And that's only for SD. With HD, the figure jumps to 1.5Gb/s, or even 3Gb/s for 1080p.
Multicasting
The multicast ability of IP reduces this overhead, as packets originated by each network node can be sent to multiple destinations by the routers in the network.
Multicast operates by a network source device originating packets with addresses in a reserved IP range of 224.0.0.0 to 239.255.255.255. Network routers recognize these packets as a multicast and forward them to all network devices that are members of the multicast.
Membership of the multicast is initiated by a network node requesting membership via the Internet Group Managment Protocol (IGMP), and the network routers also use IGMP to communicate their requirement to receive multicast data (or not).
Figure 4 shows an example of a 4 × 4 crosspoint router with equivalent routing on a multicast network. Each video source is transmitted on the network with a unique multicast address, and the four destination devices are each subscribed to one of the multicasts, achieving the equivalent end-to-end connectivity of a crosspoint router.
Ethernet
It's worth saying a word or two about Ethernet, which has become the dominant network technology in the IT computing world. Gigabit Ethernet provides the bandwidth required for handling video, and its widespread adoption means that costs are continually falling.
Ethernet does not define any device-to-device timing or flow control mechanisms, as network devices transmit data packets asynchronously to one another. The delays between network nodes are a function of the network loading and the devices in the network, such as switches and hubs. So if network traffic exceeds the bandwidth of a network device and data is lost, Ethernet does not provide any retry mechanisms.
The variable network latency and lack of guaranteed data delivery seem to suggest that Ethernet cannot deliver the quality of service required for video distribution. However, solutions are provided by protocols higher up in the ISO Open Systems Interconnection (OSI) model.
Common examples are Transmission Control Protocol (TCP), which provides acknowledge and retry mechanisms to ensure successful transmission of data, and Rate Control Protocol (RCP). RCP is a member of the IP set that provides packet ordering and timing information relating to the source data being carried in the packet, allowing a solution to be engineered around the limitations of Ethernet.
All of this makes networks a viable option for the distribution of video, and there are technologies that provide an equivalent solution to the crosspoint router for video.
Today, crosspoint routers are still significantly cheaper than a network equivalent, but there are already applications where a hybrid of the two approaches are being used. Typically, the network will provide connectivity over the wide area, with crosspoint routers providing local switching at the network nodes.
So crosspoint routing technology is not dead, but we need to rethink. Historically these switches have been the most economical solution for handling demanding television applications. However, the reduction in cost of network bandwidths means that a network may become a viable alternative.
A nice black box with a row of RJ45 telecomms sockets alongside another row of BNC connectors may sound like the obvious way forward. But this is unlikely to be developed for years, if ever. One problem is flexibility: How many would you specify of each? Another is how you would then deal with legacy systems, and especially distributed router infrastructure.
A more practical system would separate the decision making from the execution. In other words, required tasks would be monitored from a central unit, which would then designate these to different equipment as appropriate.
The idea is to link together cross-point and software-directed signals with a control and command center. The benefit of this hybrid approach is that if a direct link from A to Z fails, it could be dynamically rerouted from A to P, P to T and T to Z. Such flexibility is already happening in specific island installations, but not in a manner that satisfactorily bridges the different worlds of files and video signals.
An important part of all this is that the method of conveyance must be abstracted away from the user, as the last thing we want to do is burden operators with an additional layer of complexity. Far from it in fact, the system must be capable of selecting the best means of signal conveyance in a manner that is completely transparent to the user.
Of course the days when routers were little more than an electronic jackfield have long gone, with the best systems now offering A-to-D/D-to-A and digital format conversion within the router itself.
What we need to do next is consolidate the benefits of both crosspoint and network technology within the same system, and further integrate the roles of routing with automation systems.
Neil Maycock is chief marketing officer at Pro-Bel.
