How about some fiber?...
Is there fiber in your future?
Skip Pizzi
12/30/97
Broadcast Engineering
Copyright 1997 Intertec Publishing Corporation, a PRIMEDIA Company. All rights reserved.
The wide bandwidth and long-distance capabilities of fiber-optic interconnections have established the technology as a fundamental piece of today's telecommunications infrastructure. Over the last two decades common carriers have been installing fiber to expand and extend their networks throughout the world.
Fiber-optic technology has improved during this time. From plastic to glass, from multimode to single-mode, from LED to laser drivers - each improvement has brought higher capacity and/or greater distance capability.
Meanwhile, the explosion of industrial computer connectivity demands and the growing popularity of the Internet have mandated immense bandwidth increases for telecom carriers. Therefore, these improvements in fiber capacity have been welcomed and quickly implemented. Such rapid acceptance has, in turn, driven down the cost of fiber-optic terminal equipment and cable, putting it within reach of an ever-widening group of potential users.
Once the sole province of common carriers, fiber is now being installed by end-users between buildings on corporate and academic campuses, and between rooms in media production and broadcast facilities.
Fiber basics The actual data capacity of a fiber-optic cable is an elusive quantity. It is usually cited as a compound function of bandwidth over distance - usually extremely long distance. For example, today's single-mode fibers can carry gigabits per second over many miles. The actual throughput of any fiber path is always the result of a combination of limits from the fiber itself, the terminal equipment and, in the case of computer interconnections, the networking hardware and software.
For the short paths used within facilities, even the less-expensive multimode fiber and LED drivers can provide adequate bandwidth for some broadcast applications. The most popular application of fiber in broadcast facilities, however, has been for multiplexing several channels of audio and video between adjacent or nearby buildings in a multistructure installation. These are often bidirectional links. In remote applications, the space, weight and labor and time savings of using fiber for signals paths between the production truck and sites within the venue also have become well-known.
More recently, the idea of running fiber to each room of a broadcast operation has been incorporated into facility designs. Home-runs of single or multiple fibers between master control and each production room have been laid into these new facilities, often reserved for unspecified future uses.
In addition to providing adequate bandwidth for high-quality digital signals, fiber-optic lines can also save conduit space and labor, given that one fiber can replace numerous coax cables or dozens of twisted pairs. The higher cost of the fiber and its terminal equipment generally overshadowed these other advantages in the past, but as fiber implementation costs decline, the value of optical interconnection is being rethought.
Dedicated video and audio systems Many companies make audio, video and RF links for broadcast-specific applications of fiber-optic paths. Among the systems are terminal devices that can carry analog or serial digital video , analog or AES3 digital audio, intercom and auxiliary data in various combinations and multiples of channels. Other devices intended for remote satellite earth-station operation carry multiple RF signals between the dish and a non-co-located control facility.
Most of these systems are aimed at dedicated, point-to-point applications, as opposed to switched routing arrangements. As such, their intrafacility uses are limited to multipurpose, bidirectional connections between studios in separate buildings or for long, contiguous runs in a single, large building.
Hybrid applications For switched systems, fiber-optic interfaces can be added to a facility's digital router. This makes sense when many discrete channels must be added to a room and existing coax paths are insufficient. Such a hybrid coax/fiber approach may also be worthwhile when one room is a long distance from the router hub and most others are nearby.
Because most broadcast and teleproduction facilities' routing systems are designed to keep audio, video , control and communications signals on separate layers, and because most cable runs in these facilities are fairly short (less than 1,000 feet), coax remains the pathway of choice, even with digital signals. For these applications, fiber becomes worthwhile only when it can offer extended path lengths or where size and weight of the cable bundle is critical. In all but the most extreme cases of path length, multiple coax paths are replaced by a smaller number of fibers, so size and weight of the connection paths will always be reduced with fiber. In any case, the extended path-length and/or reduced density must still be worthwhile after including the incremental cost of fiber-optic terminal and/or multiplexing hardware, plus the differential cost (if any) of fiber vs. coax paths.
Another way to use dedicated fiber paths involves a fiber-optic patchbay. This allows flexibility in routing high-bandwidth signals through a facility via fiber-optic paths without high cost. Careful design can allow such a patchbay to be integrated with an existing coax routing system.
Even though full-blown optical switching systems are available, their cost is generally prohibitive to broadcast facilities. At present, these remain primarily the province of telco operations.
The hybrid fiber/coax approach may warrant revisiting in future facilities, however, where the more common use of embedded routing may replace the use of separate switching layers (such as in facilities where work is largely pass-through switching with little or no production). Fiber is even more likely to find increased application with the increased bandwidth requirements of HDTV signals. These needs, coupled with the continuing reduction of fiber systems' cost, may affect where the crossover point from coax to fiber is drawn in tomorrow's intrafacility routing.
Circuit vs. packet switching Where fiber begins to earn its keep for intrafacility application today is in packet-based switching. For broadcasters, this applies wherever computers are used, and, therefore, wherever file-based transfer is required.
These applications replace routers and dedicated paths to/from audio and video devices with LAN interfaces to/from computer platforms. Although separate, dedicated paths could be used to home-run (or "star") each device to a central hub, the more commonly used - and less-expensive - approach is a loop architecture ("ring").
Unlike the full-fledged application server approach that many corporate computer networks are implementing, the older file and print style of networking is all that most broadcast production workstations require. Applications reside on each workstation's local drive(s), while data is transferred via the network for single-point storage, file sharing and centrally managed archiving.
Either peer-to-peer or client/server topologies can be used, depending on the scale of the networking needs. Peer-to-peer can work well if only a few workstations are involved; beyond three or four workstations, client/server makes more sense. Archiving and redundancy are also easier to manage in the client/server approach, where a redundant disk array can be used as a central store and full-system backups are performed on a regular schedule.
Fibre Channel For the high-bandwidth needs of digital media on such LANs, a fiber backbone makes perfect sense. Unfortunately, a true optical-switched LAN remains an expensive proposition. But some alternatives exist for relatively inexpensive data-routing that exploit the wide pipe of fiber-optic interconnection. One "off-the-shelf" solution from the computer world that has some application in the broadcast environment is the emerging Fibre Channel standard.
Fibre Channel is suited for large-file transfer and is flexible in covering the range of architectures needed by broadcast and teleproduction facilities. It is also highly scaleable, thereby accommodating the likely expansion requirements the TV industry will soon face with the onset of HDTV production.
One of Fibre Channel's most valuable attributes is its operational "independence." It is merely a method of transferring data between buffers. As such, it can interoperate as a file-transfer medium with a variety of platforms, operating systems and applications. The only prerequisite is the availability of Fibre Channel network interface hardware for the devices to be networked. A substantial amount of this type of hardware is becoming available.
Fibre Channel operates at speeds of 133Mb/s, 266Mb/s, 530Mb/s and 1Gb/s. (These correspond to transfer rates of 12.5MB/s, 25MB/s, 50MB/s and 100MB/s, respectively.) Contrary to its name, Fibre Channel can be implemented on copper (coax or twisted pair) or fiber (multimode or single mode). Of course, the physical interconnection material affects path length limits and throughput speed, as Table 1 illustrates. More important, Fibre Channel is capable of true duplex operation at these speeds, unlike many other network protocols. For example, speed on a SCSI network interface ismeasured in one direction only, while the specified Fibre Channel speed refers t o each direction simultaneously, should the traffic require such bandwidth. Therefore, to count apples with apples, these Fibre Channel speeds could be doubled when comparing them to most other networking systems.
Another advantage is Fibre Channel's versatility in topology. It can work in point-to-point, crosspoint-switched or arbitrated loop installations.
Perhaps most intriguing about Fibre Channel is its ability to emulate a channel-switched and network-switched architecture simultaneously (in crosspoint-switched applications). This means that certain connections can be set up as uninterruptible, dedicated point-to-point channels, ideal for real-time media data transmission. Meanwhile, other stations can operate as "connectionless" traffic on a LAN, retaining Fibre Channel's high throughput and reliable flow control/acknowledgment characteristics. In this way, Fibre Channel provides circuit and packet switching on the same switch fabric (composed of copper or fiber or both). Again, its scaleability allows upgrades over time, accommodating from a handful to thousands of non-blocking channels.
In Fibre Channel jargon, dedicated channels are referred to as Class 1 connections (or "selfish mode"), while packet-switched traffic with receipt-confirmation is called Class 2 (or "unselfish mode"). The use of both modes simultaneously is called Intermix. This mode reserves the entire bandwidth for dedicated-channel connections, but allows Class 2 traffic on whatever bandwidth remains available. A fourth mode, called Class 3 (or "hopeful mode"), allows packet transmissions to multiple locations without confirmation of receipt.
Most popular computer platforms, servers and LANs can be interfaced to Fibre Channel. In fact, because the latest disk drives feature transfer rates that exceed the fastest SCSI interfaces, manufacturers are turning to Fibre Channel as a new standard interface. This also avoids the use of SCSI connectors, which are a frequent source of networking problems.
Fibre Channel networking is defined in five hierarchical layers, FC-0 through FC-4. The lowest layer, FC-0, is physical and includes specifications for electrical and optical parameters (conductors and connectors) at each operating speed. FC-1 defines the transmission protocol for serial coding and error correction. (The Fibre Channel transmission format encodes eight databits into a 10-bit transmission word for increased robustness. A corresponding reduction of 20% in net data throughput is the price paid for this transmission coding.) FC-2 defines data transfer, including framing, sequence management and flow control for the various service modes described earlier. FC-3 covers services for advanced multipoint routing features, while FC-4 defines application interfaces.
Among these FC-4 applications are interfaces for Fibre Channel transport with SCSI, Ethernet, IP, FDDI, HIPPI, ATM and other networking protocols. This adds to the system's versatility and scaleability within the often multiformat world of teleproduction. Because Fibre Channel allows up to 255 different FC-4 interfaces to be specified, many future protocols can be accommodated.
Light at the end of the fiber Whatever the format or protocol, it seems inevitable that intrafacility optical interconnection will have growing importance and widening application for broadcasters. It would be foolhardy not to include fiber in the wiring design of any new facility or renovation project. Even though there may not seem to be a specified or compelling requirement for optical interfaces today, one or more fiber paths running between each of a facility's control rooms and its technical operations center will find no shortage of application in the years to come.Even for relativ ely short paths, increasing bandwidth needs and expanding use of computer-based devices will couple with reduced cost to make in-house optical interfacing more common in future operations. The engineer who runs optical fiber within the broadcast facility today will no doubt be seen as the hero of tomorrow. |
