Fiber Optic Communication

In fiber optic communication, information is exchanged between two devices by transmitting pulses of light, typically infrared, through optical fiber cable. Light is the electromagnetic carrier wave for data transmission and is modulated to carry original data. Fiber optic cable has high bandwidth and better immunity to electromagnetic interference; it provides longer distance compared to coaxial or twisted copper pair cable. Researchers at Bell Labs (Alcatel Lucent) have achieved speeds of over 100 Petabits per second kilometer using fiber optic communication.

1. Components and Techniques

Transmitters: Light emitting diodes (LEDs) and laser diodes are the most commonly used optical transmitters in fiber optic communication. Power, speed, ruggedness, and cost are among the common factors that help frame the selection of transmitter. The optical transmitters used must be compact, efficient and reliable, while operating under standard conditions. The LEDs produce incoherent light and are used in small distance and low bandwidth applications. Surface emitting types of LEDs are simple and reliable, but they have limited frequency modulation and broader spectral width. Edge emitting LEDs have overcome these limitations and have better power rating. Laser diodes, thanks to high power, speed and narrower spectral bandwidth characteristics are preferred for long distance and high data rate applications; the disadvantage is that they are nonlinear and sensitive to temperature variations.

Receivers: A photodiode, typically a semiconductor-based photodetector, is the main part of an optical receiver which converts light into electricity with the photoelectric effect. Various types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes and metal- semiconductor-metal photodetectors.

Low Loss Optical Fiber: Fiber optic cable is made of high-quality silica glass or plastic; it is flexible and contains one or more optical fibers. The three main types of optical fiber cable are, single-mode fiber (SMF), multi-mode fiber (MMF) and plastic/polymer optical fiber (POF). The SMF allows at a time one mode of light to propagate thorough its small core and requires costlier components and interconnection systems, but on the brighter side allows much lengthier and better performing links. MMF having a larger core allows propagation of multiple modes of light at a given instant and needs less precise and cheaper transmitters, receivers and connectors. The disadvantage of MMF is that it introduces multi-mode distortion, resulting in limited bandwidth and distance of the link. Furthermore, they are usually expensive and exhibit higher attenuation. POF replaces the glass in conventional optical fiber with plastic or polymer. POF has higher degree of robustness under bend, stretch or stress, much larger core diameter and lower cost. The disadvantage is that it supports lower data rates and distance.

A fiber optic cable has its core made of glass or plastic, cladding facilitating total internal reflection, plastic coating, strengthening fibers and outer jacket. The core is a cylindrical shaped strip whose diameter depends on the application. As a result of the internal reflection, the light moving inside the core reflects at the core-cladding boundary. Cladding is again an optical material that surrounds the core and reflects light back to the core. When light from the dense core enters into the less dense cladding, it gets reflected back to the core. The plastic coating surrounding the cladding acts as a reinforcement to the core in case of shocks and excessive bends in cable. The strengthening fibers are another layer of protection against excessive forces on the fiber, particularly during installation. The outer jacket is the sheath to protect the cable from environmental factors. The jacket is usually color coded to distinguish between various types and applications (Figure 3.14).

Wavelength division multiplexing (WDM) is a technique in which multiple light beams of different wavelength, each modulated with separate information, are transmitted through a single optical fiber to form multiple channels of information. This in effect multiplies the available capacity of optical fibers. To implement WDM, a wavelength division multiplexer at the transmitter side and a de-multiplexer at the receiver side are required; both are satisfied by arrayed waveguide gratings.

A fiber optic communication system has interfacing circuits at transmitting and receiving points, light source and light detector as shown in Figure 3.15. With the input data given, the interfacing circuit at the transmitter side together with the light source generates light beam according to the received electrical signals. The fiber optic cable carries this light beam to the destination where the receiver-side photo detector along with an appropriate electronic circuit converts the information back to the original data bits (Figure 3.15).

The distance over which a fiber optic communication system can transmit data is influenced by fiber attenuation and fiber distortion among other factors. Solutions to these problems are optoelectronic repeaters and optical amplifiers. The optoelectronic repeaters convert the light signal into an electrical signal, boost it and then send it using an optical transmitter at a higher intensity than it was received. Optical amplifiers amplify the optical signal directly without converting it. A common optical amplifier is an erbium-doped fiber amplifier (EDFA) developed by doping a fiber with erbium. It sends light from a laser with a smaller wavelength than the communication signal.

2. Standards and Specification: There exist several industrial standards - such as, enterprise system connection (ESCON)/serial byte connection (SBCON), fiber distributed data interface (FDDI), synchronous optical network (SONET) and Gigabit Ethernet - for realizing an optical fiber communication network. The ESCON/SBCON, first introduced in 1990 by IBM and adopted as SBCON by American National Standards Institute (ANSI) in 1996, is a point to point, bidirectional, fiber optic data link having maximum data rate of 17 Mbyte/s. The ESCON devices can communicate either directly via a channel-to-channel connection, or via a central non-blocking dynamic switch. An ESCON data frame includes a two-byte start of frame delimiter, destination and source address of two-bytes and one byte of link control information for connection request to form the header, payload of data up to 1028 bytes, a trailer of two-bytes cyclic redundancy check (CRC) for errors and an end of frame delimiter of three-bytes. FDDI, proposed in 1980s by ANSI, is a standard for data transmission in LAN - one of the pioneers to specify optical fiber as the physical medium. The FDDI confirms to the architectural concepts of OSI and uses a logical topology of ring-based token network, developed from timed token protocol in IEEE 802.4 token bus. The FDDI names four layers - the physical layer, physical media dependent, media access control and station management. FDDI has bandwidth of 100 Mbit/s, data frame of 4500 bytes, range up to 200 km and has two rings; the second one is a backup if the primary ring fails. FDDI was replaced by Fast Ethernet and later by Gigabit Ethernet. SONET defined by Telcordia and ANSI and synchronous digital hierarchy (SDH) defined by European Telecommunications Standards Institute (ETSI) are the standards used for synchronous transmission of multiple digital bit streams through optical fiber. The framing followed in SDH is synchronous transport module level 1 (STM-1) which has a bandwidth of 155.520 Mbit/s with a frame size of 2430 octets. Nine octets of header termed as overhead is transmitted followed by 261 octets of pay- load or data and this cycle is repeated nine times to complete 2430 octets in 125 ps. The SONET offers an additional basic framing called synchronous transport signal, base level (STS-1) which has a bandwidth of 51.84 Mbit/s with a frame size of 810 octets. Three octets of header termed as overhead is transmitted followed by 87 octets of payload or data and this cycle is repeated nine times to complete 810 octets in 125 ps. Every SDH/SONET connection uses at least two optical fibers. Fast Ethernet increased the data rate from 10 to 100 Mbit/s and Gigabit Ethernet is introduced by IEEE in 1998 as IEEE 802.3z and further ratified as IEEE 802.3ab in 1999 increasing the data rate to 1000 Mbit/s. There are many physical layer standards defined for use in Gigabit Ethernet which uses optical fiber, twisted pair cable and shielded balanced copper cable. IEEE 802.3z standard includes 1000BASE-SX for transmission over MMF and 1000BASE-LX for transmission over SMF along with other definitions (Table 3.2).

TABLE 3.2

IEEE Fiber Optic Standards

Power Line Carrier Communication - IEEE 1901-2010

Power line carrier communication (PLCC) uses the conductors of the regular electrical power transmission and distribution network as the physical medium for data transmission. A variety of power line communication technologies with different data rates and frequencies are needed for different applications, ranging from smart metering to transmission and distribution grid automation.

A typical PLCC network scheme used in power substations is shown in Figure 3.16.

1. Components and Techniques: The system has mainly three parts - namely, line or wave trap, coupling capacitor, and line tuner. Line trap is a parallel L-C filter (normally a band stop filter) that blocks the carrier signal frequencies and allows the power system frequency to pass through and is connected in series with the transmission line. Coupling capacitor acts as the physical interconnection between the electric transmission or distribution line and the PLCC transceiver for relaying the carrier signals. It provides high opposition to power frequency and low opposition to carrier signal frequencies. Line tuner is connected with the coupling capacitor to form a high pass or band pass filter to pass the carrier signal frequency. It provides

FIGURE 3.16 Power line carrier communication.

impedance matching for the PLCC terminal with the transmission or distribution line so as to push the carrier frequency to the electric power line. Drain coil is provided to filter out any power system frequency still present (Figure 3.16).

The generic PLCC system uses various frequencies like low and medium frequency (24-500 kHz), high-frequency (>1 MHz), ultra-high frequency (> 100 MHz) for transmission of data over power line for a variety of applications (Table 3.3). The data rates supported vary from a few hundred bits per second to several megabits per second and under special circumstances even up to gigabits per second. There is a wide range of applications that use PLCC outside the scope of smart grid like telemetry, telephony and home automation and networking. Smart metering, home appliance control, protective relay, inter substation and control station communication and wide area monitoring and control are some applications that use PLCC within smart grid domain. The PLCC standards for home applications include X10 protocol and HomePlug; HomePlug and IEEE 1901 are used for long distance communication which includes features from HomePlug and HD-PLC.

2. Standards and Specification: The IEEE 1901-2010 uses frequencies below 100 MHz and has a data rate up to 500 Mbit/s and is used by various broadband over power line (BPL) devices including those used within buildings for LAN, smart energy applications and other smart grid data communication applications. IEEE 1901 supports two physical layers, one being FFT orthogonal frequency division multiplexing (OFDM) modulation derived from HomePlug and wavelet OFDM modulation derived from HD-PLC. Above these two physical layers, two MAC layers are defined to meet the requirements of (i) in-home network and (ii) internet access. IEEE 1901 supports time division multiple access (TDMA) and carrier sense multiple access/collision avoidance (CSMA/CA) to do medium

TABLE 3.3

PLC Technology Specification

access control. Inter-system protocol (ISP) manages the various devices and systems connected via PLCC for the coexistence of different physical and MAC layers and also prevents interference of various devices operated in close proximity.

The X10 protocol developed in 1975 by Pico Electronics uses the existent power lines as the physical medium for communication between smart electronic devices. The data to be communicated is encoded onto a 120 kHz carrier signal that is transmitted as bursts during the zero crossings of the AC power signal with a single bit at each zero crossing. The X10 data exchange happens with four bits house code followed by four bits unit code and finally four bits command code. Usually, X10 devices operate like a controller of operations of a device and there is only one-way communication. Provision exists for two-way message exchange in X10 where the controller queries about the status of a device and the device responds. In X10, bit “1” is transmitted with an active zero crossing (a one millisecond burst of 120 kHz at zero crossing) followed by an inactive zero crossing (no pulse at zero crossing). Bit “0” is transmitted by an inactive zero crossing followed by an active zero crossing. The data frame format includes a start code having three active zero crossings and an inactive zero crossing, four bits house code and five bits function code. The first four bits of the function code form the unit code or command and 0 in the last bit indicates a unit code, whereas 1 indicates a command. Multiple unit codes can be sent before finally sending a command to control multiple devices in the same way. All frames are sent twice, and the effective data rate is around 20 bits per second.

HomePlug is the family name that refers to different PLCC specifications such as low data rate internet content, low power, low throughput, applications such as smart power meters and in-home data exchange between electric systems and appliances. All commercial HomePlug devices satisfy the advanced encryption standard (AES)-128 encryption standard. Most HomePlug devices act as adapters, which are plugged into wall power outlets and provide one or more Ethernet ports. End devices with functionalities found in such standalone adapters are also evolving. This allows a smart device to use wired Ethernet, powerline or wireless communication to facilitate a redundant and reliable failover system in realizing smart homes.

HomePlug 1.0, introduced in 2001, provides a peak physical layer data rate of 14 Mbit/s, has since been replaced by HomePlug AV which offers a peak data rate of 200 Mbit/s at the physical layer and about 80 Mbit/s at the MAC layer. The HomePlug AV2 specification, brought out in 2012, is interoperable with all previous versions and is IEEE 1901 compliant. HomePlug Green PHY with peak rates of 10 Mbit/s falls under HomePlug AV and is intended for use in smart grid applications such as smart meters and small end appliances so as to facilitate data exchange over home network and with the power utility. HomePlug devices normally do not coexist with devices that employ other powerline technologies, but IEEE 1901 with its ISP enables coexistence of all PLCC systems.