Optical networking

Optical networking is a means of communication that uses signals encoded onto light to transmit information among various nodes of a telecommunications network. They operate from the limited range of a local-area network (LAN) or over a wide-area network (WAN), which can cross metropolitan and regional areas all the way to national, international and transoceanic distances. It is a form of optical communication that relies on optical amplifiers, lasers or LEDs and wave division multiplexing (WDM) to transmit large quantities of data, generally across fiber-optic cables. Because it is capable of achieving extremely high bandwidth, it is an enabling technology for today’s Internet and the communication networks that transmit the vast majority of all human and machine-to-machine information.

Components

Components of an optical networking system include: • Fiber. Multi-mode or single-mode. • Laser or LED light source. • Multiplexer/demultiplexer, also called mux/demux, filter, or prism. These can include Optical Add/Drop Multiplexer (OADM) and Reconfigurable Optical Add/Drop Multiplexer (ROADM). • Optical switch, to direct light between ports without an optical-electrical-optical conversion • Optical splitter, to send a signal down different fiber paths. • Circulator, to tie in other components, such as an OADM. • Optical amplifier.

Transmission Medium

At its inception, the telecommunications network relied on copper to carry information. But the bandwidth of copper is limited by its physical characteristics—as the frequency of the signal increases to carry more data, more of the signal’s energy is lost as heat. Additionally, electrical signals can interfere with each other when the wires are spaced too close together, a problem known as crosstalk. In 1940, the first communication system relied on coaxial cable that operated at 3 MHz and could carry 300 telephone conversations or one television channel. By 1975, the most advanced coaxial system had a bit rate of 274 Mbit/s, but such high-frequency systems require a repeater approximately every kilometer to strengthen the signal, making such a network expensive to operate.

It was clear that light waves could have much higher bit rates without crosstalk. In 1957, Gordon Gould first described the design of a laser that was demonstrated in 1960 by Theodore Maiman. The laser is a source for such light waves, but a medium was needed to carry the light through a network. In 1960, glass fibers were in use to transmit light into the body for medical imaging, but they had high optical loss—light was absorbed as it passed through the glass at a rate of 1 decibel per meter, a phenomenon known as attenuation. In 1964, Charles Kao showed that to transmit data for long distances, a glass fiber would need loss no greater than 20 dB per kilometer. A breakthrough came in 1970, when Donald B. Keck, Robert D. Maurer, and Peter C. Schultz of Corning Incorporated designed a glass fiber, made of fused silica, with a loss of only 16 dB/km. Their fiber was able to carry 65,000 times more information than copper.

The first fiber-optic system for live telephone traffic was begun in 1977 in Long Beach, Calif., by General Telephone and Electronics, with a data rate of 6 Mbit/s. Early systems used infrared light at a wavelength of 800 nm, and could transmit at up to 45 Mbit/s with repeaters approximately 10 km apart. By the early 1980s, lasers and detectors that operated at 1300 nm, where the optical loss is 1 dB/km, had been introduced. By 1987, they were operating at 1.7 Gbit/s with repeater spacing of about 50 km.^[1]

Optical Amplification

The capacity of fiber optic networks has increased in part due to improvements in components, such as optical amplifiers and optical filters that can separate light waves into frequencies with less than 50 GHz difference, fitting more channels into a fiber. The erbium-doped optical amplifier was developed by David Payne at the University of Southampton in 1986 using atoms of the rare earth erbium that are distributed through a length of optical fiber. A pump laser excites the atoms, which emit light, thus boosting the optical signal.

Wave Division Multiplexing

Using optical amplifiers, the capacity of fibers to carry information was dramatically increased with the introduction of wavelength-division multiplexing (WDM) in the early 1990s. AT&T’s Bell Labs developed a WDM process in which a prism split a beam of light into different wavelengths, which could travel through a fiber simultaneously.^[2] The peak wavelength of each beam is spaced far enough apart that the beams are distinguishable from each another, creating multiple channels within a single fiber. The earliest WDM systems had only two or four channels—AT&T, for example, deployed a 4-channel long-haul system in 1995.^[3] Erbium-doped amplifiers, however, do not amplify signals uniformly across their spectral gain region. During signal regeneration, slight discrepancies in various frequencies introduce an intolerable level of noise into the information-bearing wavelength, making WDM impractical for long-distance fiber communications

To address this limitation, in the early 1990s, General Instruments Corp. began developing components to increase cable bandwidth which it then licensed to engineer David Huber, who co-founded Ciena Corp with Kevin Kimberlin in 1992.^[4] Ciena developed the first dual-stage optical amplifier capable of transmitting data at uniform gain on multiple wavelengths, and with that, in June 1996, introduced the first commercial dense WDM system, a 16-channel system that had a total capacity of 40 Gbit/s.^[5] Recently, the capacity of DWDM systems has increased substantially, with commercial systems able to transmit close to 1 Tbit/s of traffic at 100 Gbit/s on each wavelength.^[6] In 2010, researchers at AT&T reported an experimental system with 640 channels operating at 107 Gbit/s, for a total transmission of 64 Tbit/s.^[7]

Capacity

The bandwidth made possible by optical networking technologies enabled the rapid growth of the Internet and will allow it to continue to grow. The demand for bandwidth is driven primarily by Internet Protocol (IP) traffic, which includes video services, telemedicine, social networking, Web 2.0 applications that are transaction-intensive, and cloud-based computing.^[8] At the same time, machine-to-machine and the scientific community require support for the large-scale exchange of data. The Cisco Visual Networking Index predicts global IP traffic will be more than a zettabyte (10^21 bytes) in 2016. By 2018, the Index predicts, a million minutes worth of video content will cross the network every second, all transmitted by optical networks.^[9]

Standards

The International Telecommunication Union has defined a set of standards that allow interoperability across the network, known as Recommendation G.709 and commonly called an Optical Transport Network.

Protocols

Optical networking uses various standard protocols. These include: • Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) • Asynchronous Transfer Mode (ATM) • Gigabit Ethernet

Other Uses

Other types of optical networks include free-space optical networks, which use many of the same principles as a fiber-optic network but transmit their signals across open space without the use of fiber. These can be used to set up temporary networks, to link LANs on a campus, or to communicate between satellites.

Another variant of fiber-optic networks is the passive optical network, which uses unpowered optical splitters to link one fiber to multiple premises.

References