Super Fiber Optic

Digital Fiber Optic Multichannel V/A/D Transport Systems

Fiber optics is now the dominant medium for terrestrial transmission of digital signals, and digital fiber optic systems are well established for transporting high quality video, audio, and data signals. Systems must make efficient use of optical fiber by transporting multiple channels of video and audio on a single fiber. A digital system working within a digital domain should be capable of expanding, inserting, routing, and switching signals within a network in such a way that video and audio performance is not affected. Of growing importance is the ability of these networks to accept a variety of signal formats and to interface with public television communication networks. Signal formats for transmission of video might include video encoding at various levels of digitizing accuracy, compressed video, advanced or high definition video, as well as digital high speed data. Understanding aspects of multiplexing, modulation schemes, and digital systems are important to implementing a multichannel transmission system.

All video/audio/data transport systems share a number of elements in common that form the basic system building blocks for any v/a/d system. These include: transmitters, receivers, signal regenerators, repeaters, coders, decoders, switches, modulators, amplifiers, A/D and D/A converters, splitters, combiners, signal fanouts, which allow signals to be added and dropped from a network or utilize smaller system components for the signal distribution, A/B switching for redundant circuit protection, network control data interfaces, and synchronizing clock interfaces.

Integrated Services Digital Network (ISDN)

ISDN has been designed to replace the standard telephone system and provide greater numbers of digital services to telephone customers, such as digital audio, interactive information services, fax, e-mail, and digital video. ISDN uses asynchronous transfer mode which can handle data transmission in both connection-oriented and packet schemes. As with regular telephone lines, the user must pay a fee for use of the line. Basic rate ISDN or BRI offers two simultaneous 64 kb/s data channels as well as a 16 kb/s carrier channel for signaling and control information. The combined data rate, 128 kb/s, allows for videoconferencing capabilities. Multiple ISDN-B connections further increase the data rate and the transmission quality. Primary rate ISDN (PRI) offers 30 channels (of 64 kb/s each), giving a total of 1920 kb/s. As with BRI, each channel can be connected to a different destination, or they can be combined to give a larger bandwidth. These channels, known as “bearer” or “B” channels, give ISDN tremendous flexibility.

The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mb/s. B-ISDN requires fiber optic cables and is not yet widely available.