Routing systems
Nov 1, 2008 12:00 PM, By Phil Cianci
Distributed routing and media networks will change the way content is moved within a facility.
Figure 1. Centralized routing system architecture
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With the wide variety of audio and video formats in a digital broadcast facility, routing systems have evolved beyond their fundamental function: to allow switching of real-time signals from any source to any destination. Conversion between analog and digital, embedding and de-embedding audio, up- and downconversion, transitions, and auto failover capabilities can now be incorporated into routing equipment, simplifying workflows and reducing the need for discrete conversion equipment.
Yet, with the use of compression technology, traditional baseband facility signal routing has been augmented by the addition of a new dimension: file-based audio and video routing over a media network. In fact, with file-based acquisition, multicast channels and Web distribution, content may never exist in the uncompressed domain. This integration of IT and broadcast systems has extended the routing and distribution infrastructure; media networks must be considered in audio and video routing system design.
System evolution
Early SDI routing systems were designed for SD 601 serial digital signals. As HD-SDI found its way into broadcast operations, the first HD-capable routers only supported the 1.485Gb/s HD data rate; separate SD and HD distribution was required. Similarly, SDTI, SMPTE 310 and ASI routing was either not possible or required a dedicated routing infrastructure.
Figure 2. Spoke and hub, local and centralized routing architecture
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Over time, auto-sensing capabilities were implemented, and SDI routers became SD/HD-agnostic. Router I/O cards became available to support ASI, SMPTE 310 and SDTI.
SDI speeds continue to increase. 1080p60 has spawned 3Gb/s standards. Vendors have addressed the 3Gb/s requirement by exploiting the modular design of their routing systems. Existing routers with backplane and connection schemes capable of supporting 3 Gb/s bandwidth can be upgraded by replacing input, output and cross-point circuit boards in existing frames.
A limitation to increased data rates, however, is the existing cable infrastructure. Due to eye pattern degradation, existing coaxial cable run lengths may not support 3Gb/s data rates. This may create problems if a facility migrates to 1080p60-based production or desires to implement faster than real-time HD-SDI signal distribution.
1080p60 signals may be able to be distributed over existing cabling. HD-SDI capable cable lengths will be cut in half for 3Gb/s signals. Installation of reclocking distribution amps may not be possible for existing cable runs. Additionally, these are RF signals in the L and S bands, where existing cable crimps, wire nicks and tight bends can easily degrade a signal.
Routing topology
Figure 3. Distributed routing topology scenario
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Maximizing source availability is desired in broadcast operations. This has led to a distribution infrastructure where one large house router is fed every source and feeds every destination, shown in Figure 1. Frequently, the physical router is partitioned into physically dispersed frames, with redundant signal paths. In this way, if one portion of the router fails, signals can be routed through a secondary path, ensuring uninterrupted operations.
There is a trend to augment the centralized router with small local routers that serve control rooms, quality control, ingest, edit and graphic areas. (See Figure 2.) Local control panels are configured with limited source and destination cross-point control.
Taking this approach further leads to distributed baseband routing systems where there is no centralized house router. Instead, many smaller routers are strategically interconnected based on workflow and scheduling. Figure 3 illustrates a brute force implementation of distributed routing. The local routers are connected in a fully meshed network. The number of dedicated interconnections grows exponentially as the number of local routers increases. The result is the significant decrease of the number of source/destination connects because of the need for ports dedicated solely to router interconnection — not a very real-world implementation.
Let there be light
Consider a distributed routing system where each SDI input and output port can handle a full 1.5Gb/s SMPTE 292 serial signal. Now add an “uplink” capability, a fiber port with data rates of 10Gb/s, 40Gb/s or higher.
Rather than dedicated baseband interconnection, Figure 4A on page 44 shows how the use of a dedicated high-speed meshed optical core can solve the interconnect problem. All ports on the router are now source/destination; only the uplink handles inter-router distribution. The key is to guarantee data rates by using time division multiplexing (TDM). In this way, each group of data (cell) from an HD-SDI signal is assigned a data slot (time slice) in the data stream over a single light frequency. Multiple streams of aggregate HD-SDI can be Coarse Wave-Division Multiplex, increasing the number of real-time HD-SDI signals that can be transferred between distributed routers.
As a backup in case the high-speed core becomes saturated, a few router ports can be dedicated to baseband interconnect of the local routers. Figure 4B shows a ring interconnection topology. In a worst-case scenario, baseband signals can be routed anywhere over the ring
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