Video Routing: A look at what's next
Feb 1, 2009 12:00 PM, By Neil Maycock
Benefits of network routing
Figure 3. The percentage of actual bandwidth used in a crosspoint router follows an inverse trend.
Click to enlarge
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.
Figure 4. A 4 x 4 crosspoint router with equivalent routing on a multicast network
Click to enlarge
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.
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