Broadcast Engineering - February 2009 - (Page 30)

Feature VIDEO ROUTING Input 1 connects to outputs 1 and 3 Input 2 connects to output 4 Input 3 connects to output 2 Input 4 is not connected 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 on page 32 shows an example of a 4 x 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. Figure 1. A 4 x 4 crosspoint router 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 1200 1200 Available Bandwidth Usage 14% 14% Available Bandwidth Usage 1000 1000 Cost €1000s Cost €1000s 12% 12% 10% 10% 8% 8% 6% 6% 4% 4% 2% 2% 0% 0% 8 816 16 32 32 64 64 128 256 512 1024 128 256 512 1024 800 800 600 600 400 400 200 200 0 0 8 816 16 32 32 64 64 128 128 256 512 1024 256 512 1024 InputInputoutputs and and outputs InputInputoutputs and and outputs Figure 2. The cost of a crosspoint router increases approximately as a square law. Figure 3.The percentage of actual bandwidth used in a crosspoint router follows an inverse trend. 30 broadcastengineeringworld.com | February 2009 http://www.broadcastengineeringworld.com

Table of Contents Feed for the Digital Edition of Broadcast Engineering - February 2009

Broadcast Engineering - February 2009 Contents Viewing the Slowdown in 3-D Examine Workflows Tapeless Technology Digital Audio 24p and 25p Judder Video Routing: A Look at What's Next Managing AFD The Right Connections Solid State Logic's AWS 900+ SE and More... Advertisers Index

Broadcast Engineering - February 2009

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