Telco Glossary
Telecommunication Glossary
AM Amplitude modulation is one of the two most common forms of modulation, the other being FM, or frequency modulation. Modulation is the process of having a baseband voice, video or digital signal modify another higher-frequency signal called a carrier. Three basic characteristics of the carrier can be changed by the modulating information signal: amplitude, frequency and phase. In AM modulation, the baseband information signal varies the amplitude of the higher-frequency carrier. AM modulation produces both upper and lower sidebands, which are located above and below the carrier signal in a spectral representation of the signal (electrical power as a function of frequency).
ADM Add/Drop Multiplexer, a multiplexer capable of extracting or inserting lower-bit-rate signal from a higher-bit-rate multiplexed signal without completely demultiplexing the signal.
Add/Drop without Collision The ability to access, retrieve and add data from the fiber-optic network in a seamless fashion without disturbing other data streams.
ANSI American National Standards Institute, the coordinating body for voluntary standards groups within the US; ANSI is a member of ISO, the International Organization for Standardization.
AON All-optical network is a term, thus far reserved for describing WDM networks, which means the exploitation of multiple channel wavelengths for switching, routing or distribution, using light to the almost total exclusion of electronics. AONs are classified as singlD-Eop or multiplD-Eop, defining whether every node in a network is one- or many-hops from every other node in the network. Examples of multi-hop networks are the deBruijn network, the shuffle network and, its bi-directional equivalent, the Banyan network.
APD Avalanche photo diode are semiconductor devices that convert light (visible or IR) into electrical current ,and while providing internal current gain similar in spirit to photomultiplier tubes. APDs require very few photons (the units of light delivery) to produce useful current outputs.
ASK Amplitude-shift keying, a form of modulation in which the amplitude is modulated while keeping the carrier frequency and phase constant. In most practical situations, the amplitude is set to zero during transmission of "0" bits. This form of ASK is called on-off keying (OOK) and is identical to a similar modulation scheme commonly used for noncoherent (IM/DD, see below) digital lightwave systems. A reference figure (presumably worth 10,000 words) is given here illustrating ASK, PSK and FSK. (See below for definition.)
ATM Asynchronous transfer mode, a cell-oriented communication protocol that has the capability to make may logical connections onto one physical link. ATM is one of the two major protocols used for high-speed data transfer on the fiber networks. The other major protocol is Ethernet TCP/IP. One of the major limitations of both of these techniques is that the entire network traffic must be processed over each node. This has thus far limited the processing speeds to that imposed by electronics (about 10 Gb/s) not the higher capacities represented by optics.
Backbone The high-speed channel or channels forming the primary path for information transport between network segments.
Bandwidth, which is usually measured in hertz (Hz), is that portion of the electromagnetic spectrum occupied by a signal or group of signals. It is one way of characterizing an information-carrying channel. Bandwidth is also related to the bit-carrying capacity of an information channel and its internal noise characteristics through the famous Shannon-Hartley information and channel-capacity formula:
Channel capacity [b/s] = BW[Hz]*log2(1+S/N[dB]),
in which BW is bandwidth of the channel measured in Hz, S/N is the information signal to noise ratio measured in dB and the channel capacity is measured in bits per second[b/s]. The noise present in the channel is assumed to be Gaussian (normal).
BER Bit error rate or ratio, the probability of incorrect identification of a bit by the decision circuitry of the receiver. A BER of 2*10-9 corresponds to on average 2 errors per billion bits received. Receiver sensitivity is often defined by the minimum average received optical power, Prec[W], required by the receiver to operate at this BER.
Bit A binary (base 2) digit. This is the smallest unit of information and is usually annotated by a 0 or a 1. Eight bits of information is equal to one byte.
BL The bit rate-distance product, measured in [Gb/s km], is a quality measure of a communication system in terms of its bit capacity [Gb/s] and its length [km] between repeater stations. This measure has grown several orders of magnitude with the advent of light-wave carriers, replacing microwave relay systems with 100 Mb/s-km efficiencies in the 1970s. The coherent laser and the advent of low-loss fiber-optical cable were both contributing items in this increase. Current optical communication systems approach 100 Tb/s-km BL figures-of-merit using DCF. (See below for definition.)
bps or (b/s) Bits per second, the transmission-rate measure used for digital information. Similar measures are used for high data rate descriptions such as, Kb/s for kilobits (103 bits) per second, Mb/s for megabits (106 bits) per second, Gb/s for gigabits (109 bits) per second and Tb/s for terabits (1012 bits) per second.
BPF Band pass filter, a filter with sharp attenuation characteristics outside a specified frequency range. BPFs are frequently used to eliminate the upper sideband produced in AM modulation, leaving only a single side band (SSB) and a vestigial carrier signal for transmission.
Broadband A data-transmission scheme where multiple signals share the bandwidth (BW) of the medium. Broadband techniques allow the simultaneous transmission of voice, video and data signals over a single fiber. CATV operators use similar techniques to deliver hundreds of TV channels and Internet access to your home.
Campus-Level Network This form of network is an enterprise network(See definition below.) sometimes defined as a Fiber-to-the-Office (FTTO) network.
Carrier 1.) A carrier is typically an analog signal capable of being modified (modulated) in any of three ways: amplitude (AM), frequency (FM) or phase (PM) to carry [hence, the name "carrier"] information. 2.) A carrier is a company which provides communications circuits. Carrier companies are either "private" (which can refuse to service you) or "common" (which cannot refuse to serve you) which are regulated as information utilities and must thus serve the public in order to continue to exist. Same examples of common carriers are: AT&T, MCI US, Sprint, etc. (See also, carrier office, CO)
CATV Common antenna television, a system for transmission and local distribution of numerous TV channels of information often employing fiber-optic and electronic distribution means.
CDM Code division multiplexing, a type of multiplexing using a form of spread- spectrum technique, which allows users to access any channel randomly at an arbitrary time. Each channel is coded in such a way that its spectrum spreads over a much wider region than occupied by the original signal. Spectral spreading is accomplished by means of a code that is independent of the signal itself. The receiver uses the same code for compressing the signal spectrum and recovering the data upon reception. CDM signals are difficult to jam or intercept because of the coded nature of the transmitted signal. The spectrum spreading code is called a signature sequence or a key. One method of spectrum spreading is frequency hopping, where the carrier frequency is shifted periodically according to a preassigned code. This situation differs markedly from WDM in the sense that a fixed optical carrier frequency is not assigned to a given channel.
CDMA Carrier detect multiple access, a form of protocol used in computer-to-computer communication. Also, code division multiple access, a form of spread spectrum communication primarily used in wireless cell phones made by QUALCOMM and in some fiber-optic communication systems. There are very definite transmission speed limits posed by CDMA in the optical fiber domain, especially as the number of users grow on a given fiber interconnect.
Channel (Communication) A communication channel whose function within a medium is to transport sets of information from transmitter to receiver without distorting them. Sometimes multiple channels can be combined within a single fiber medium. Channels are usually assumed to be one-directional.
Chirp With some apology to birds, this is a form of laser "singing." Chirping is the output of multiple optical carrier frequencies due to direct current modulation of the laser cavity and is sometimes called frequency chirping. The net effect is to decrease the distance data can be transmitted without error. Chirping can be avoided by using external laser modulation schemes which also permit modulation with much larger BWs than direct current modulation. Laser amplifiers can also introduce chirping to a fiber-optics communications system.
CLEC Competitive Local Exchange Carriers, are U.S. domestic carriers left over from the break up of Ma Bell such as PacTel, US West, and Bell Atlantic. CLECs have deployed mega-miles of fiber-optic networks.
CNR Carrier to noise ratio [dB], the ratio of the RMS (root mean square) carrier power to the RMS noise power at the output of the photodetector. A formula exists defining the CNR as a function of m, the modulation index for the channel, the detector responsitivity, R, the average received optical power, Prece[W] and the various RMS values of noise currents associated with shot noise (SN), thermal noise (TN), relative intensity noise (RIN) and intermodulation distortion noise (IMD).
CO Central Office, the central office of a regulated carrier that allows customer access to the switched network.
CSMA Carrier sense multiple access, a form of network protocol usually used in conjunction with collision detection to connect multiple computers and is used as a common protocol on the Internet.
CW Continuous wave, a term from the early days of radio, was normally used to describe the simplest form of information transmission involving the generation of a carrier (by a RF oscillator or light source). Information to be transmitted via basic CW methods is normally modulated by a key (for the radio) or shutters (on the light source) to be present for a varying amount of time (short duration were "dots," long-duration signals were "dashes"). CW can also be used to describe the use of a continuous wave carrier with other external forms of modulation to encode the information such as AM and FM. It is also used in the optical communication domain to differentiate digital from analog communication techniques. This may be an artificial distinction, since only an analog signal can be sent down an optical fiber on a light carrier.
FDDI Fiber-distributed data interface, the standardized interface for a ring topology fiber-optic LAN. (See below.) The FDDI operates at 100 Mb/s by using multi-mode fibers and LED-based 1.3-mm transmitters.
FDM Frequency division multiplexing is a technique for the transmission of multiple optical channels over the same fiber. The transmission of multiple optical channels using different carrier frequencies is a simple way to exploit a larger part of the 1-THz bandwidth available in the fiber-optic. Channel capacity has typically been limited to 10 Gb/s due to limitations imposed by fiber dispersion and nonlinearity, and the relative slowness of electronic components compared to their optical counterparts.
fs Femtosecond, one quadrillionth of a second (10-15 second). (See also ps/picsosecond)
FET Field effect transistor, a three-terminal (source, gate and drain) or two-port (input and output) device (whichever you prefer) typically constructed as a metal-oxide-semiconductor FET (MOSFET), a junction FET (JFET) or a metal-semiconductor FET (MESFET). Each of the three devices derives its terminal properties from a mechanism called the field effect, which enables the input-port voltage to regulate the output-port current by changing the conductivity of the channel connecting the drain and source terminals.
Fiber-optic A thin (100 mm) fused silica glass cable with an outer cladding material and a 5mm-diameter inner core with a higher index of refraction than the cladding. (A typical index of refraction is 1.443.)
FM Frequency modulation, a form of modulation in which the baseband information signal varies the frequency of the carrier.
FO Fiber-optic (See above for definition.)
FP Fabry-Perot, a mechanism for providing optical feedback provided by placing the gain medium (a doped crystal or semiconductor) inside an optical cavity formed by two mirrors. In the case of semiconductor lasers, external mirrors are not required since the two cleaved laser facets act as mirrors of reflectivity, in which n is the refractive index of the gain medium and k is the extinction coefficient. For semiconductors, n = 3.5 and k = 10-4, typically, resulting in 30% facet reflectivity. Multiple FP cavities can be used to make a very effective interferometer. (See below for definition.)
FSK Frequency-shift keying, a form of modulation in which two frequencies are chosen to represent 0 and 1, respectively, in the data stream. This scheme has been used for many years in telephone modems to convert a binary data stream to a set of analog signals compatible with the audio bandpass of the telephone system, thus allowing computers to send bit streams over the phone system. (See also UART)
FWM Four-wave mixing, a phenomenon resulting from the intensity dependence of the refractive index caused by the third-order nonlinear susceptibility . The finite value of in silica fibers generates a fourth field of frequency 4, given that the three optical fields with carrier frequencies 1, 2 and 3 are copropagating inside the fiber core simultaneously. This fourth generated frequency is related to the other three by a relation: 4=1±2±3. Several frequencies corresponding to different plus and minus sign combinations are possible in principle. Commercial fibers are often designed to have some residual dispersion at the operating wavelength to suppress FWM. FWM is often the limiting factor for multichannel systems making use of wavelength-division multiplexing, WDM. (See below for definition.)
Gb/s Gigabits per second, 109 bits per second, a measure of channel capacity to transmit information. (See also Mb/s and Kb/s)
GIPR Gigabit Internet Protocol Router, a high-speed Internet hybrid representing an attempt to integrate IP routing and ATM switching technologies. In allowing the IP processing software to contact an ATM switch fabric directly, a GIPR could in theory deliver IP’s interoperability benefits along with ATM’s high speed and quality of service support. The current challenge for GIPR designers is to construct an affordable device that delivers enough CPU and memory cycles to perform all IP packet processing at speeds that keep ATM circuits full and multimedia applications humming.
GVD Group-velocity dispersion, a phenomenon caused by the frequency dependence of the group velocity associated with the fundamental mode of transmission in single-mode fibers due to chromatic dispersion. The result is that different spectral components of a pulse travel at slightly different group velocities, thus spreading their respective arrival times at the terminal end of the fiber. GVD is also sometimes referred to as intramodal dispersion or simply fiber dispersion. It is measured in ps/(km-nm).
Index of refraction A measure of the relative speed of light in the medium compared to the speed of light in a vacuum. The index of refraction is actually a complex number whose real part, n, is the ratio of c/vm where vm is the speed in the medium [m/s] and c is the speed of light in a vacuum, 3.0x108 m/s. The complex part of the index of refraction, k, the extinction coefficient, is also important as a loss measure for fiber-optic components. The extinction coefficient, k, is related to the attenuation coefficient, a [dB/km] by: , where l is the wavelength of the light transmitted down the fiber as measured in a vacuum. Power launched at the input of a fiber of length L[km], say Pin, is diminished to an output power at the other end of the fiber, Pout, given by the expression: The fundamental limit for silica fibers is about 0.15 dB/km at l = 1.55mm.
INTAP Interoperability Technology Association for Information Processing, the technical organization with the official charter to develop Japanese OSI (See definition below.) profiles and conformance tests.
Internetwork A data communications facility involving two or more computers connected by multiple interconnected network types (LANs or WANs).
IP Internet protocol, an agreed-upon communication protocol for the Internet. Fiber networks using IP are the most effective bandwidth conduits in existence and are able to convey a mix of voice, video and data. (See also TCP/IP below.)
ISDN Integrated services digital network, a 144Kb/s duplex digital service on phone lines up to 3 mi. in length.
ISO International Standards Organization, an international standards group that has described in detail a 7-layer Open Systems Interconnections (OSI) model describing all communications systems above the data communications network itself. The seven layers are, in descending order towards the communications network: 1. Application layer; 2. Presentation layer; 3. Session layer; 4. Transport layer; 5. Network layer; 6. Link layer; and 7. Physical layer. The last level describes the mechanical and electrical network interface definitions necessary to physically connect to the data communications network. Above the first layer is the end-user application process.
ITU International Telecommunications Union, an arm of the United Nations, is an international telecommunications governing body, comprised of 185 member nations, which sets standards and requirements over the telecommunications industry.
IXC Interexchange Carrier, typically a large, multi-national telecommunications line service provider such as AT&T, MCI/WorldCOM, US Sprint, Nippon Telephone and Telegraph (NTT), and independent competitive IXCs such as QWEST, BROOKS and WILLIAMS. Long-distance providers (IXCs) were the first to invest in WDM equipment to maximize their existing fiber network capacity.
LAN Local area network, a data communications network, with computers distributed around a single clearly defined geographical location (site) with an extent of a few miles. A LAN generally covers an office building or group of buildings, a campus, a factory or the like, offering many nodes (connection points) and connecting computers and peripherals with high-capacity data links.
LEC Local exchange carrier, a telephone company providing access to the switched network through one of its central office. (CO, see definition above.) Examples of LECs are Ameritech, Bell Atlantic/Nynex, BellSouth, GTE and SBC Communications/Pacific Bell. LECs have deployed and continue to deploy many miles of fiber-optic networks.
Mb/s Megabits per second, a measure of communications channel data transfer capacity equal to 106 b/s.
MAN Metropolitan Area Network, a term describing short-haul or short-distance telecommunications provided by a regional local exchange carrier. (LEC, see above for definition.) MANs can also be called metro, inter-office, or local loop telecommunications.
Mode hopping The sudden shift of the laser diode output from one longitudinal mode to another during what should be single-mode operation. (See also "chirping") This only occurs during direct-current modulation of the laser cavity, and not at all when external modulation is used with the laser operating in CW mode.
MUX Multiplexer, a device allowing several data streams to be sent over a single communications medium, typically a single COAX line or fiber, simultaneously. A MUX can be used to connect several links from one layer (ISO) to the next higher layer in a communications system using only one connection. In DWDM systems, separate lasers at different wavelengths are MUXed onto a single fiber. At the receiving end of a DWDM system, each separate laser wavelength is "deMUXed," or separated optically and electronically, to deliver the individual data streams to the user.
OC Optical carrier, a designation used as a prefix denoting the optical carrier level of SONET data standards. OC-1/STS-1, OC-3/STS-3, OC-12, OC-48 and OC-192 denote transmission standards for fiber-optic data transmission in SONET (see definitions for Sonet and STS ) frames at data rates of 51.84 Mb/s, 155.52 Mb/s, 622.08 Mb/s, 1244.16 Mb/s and 2.48832 Gb/s, respectively.
Optical modulator An external modulator usually made with LiNbO3 or GaAs bi-refringent materials in which an optical wave guide (laser light path) has been defined and closely coupled with some form of surface electrodes, allowing input of a modulating RF electronic baseband information signal. Such external optical modulators vary the intensity of the transmitted laser light beam (an analog operation, not digital) by varying the relative amount of the ordinary and extraordinary index of refraction in the material. This is accomplished by coupling the electronic field in the RF information signal to the optical wave traversing the wave guide.
OSI Open systems interconnect, a reference model that embraces the network environment and adds additional application-oriented protocols and standards to allow end systems (computers) to communicate with one another in an open way. (See also ISO)
PBX Private branch exchange, a smaller version of the switching system (or switch) found at the telephone company’s central office.
Photodiode A two-electrode, optical-radiation-sensitive device using a semiconductor junction (Si, Ge, GaAs, InGaP, etc.) to convert incoming photons (optical power) into electron-hole pairs (electrical power). The resulting reverse current varies in direct proportion to the illumination level measured in watts. The performance of photodiodes is often measured in terms of their power conversion efficiency in A/W, where the resulting current in amps is from a given illumination level in watts. Another measure of performance may be made in terms of the device’s rise time (usually measured in ps) vs. a short pulse of incoming light (say, of a few fs duration) or its bandwidth (BW) measured in GHz.
Photonics The science of using photons in place of electrons. Many photonic devices are based on photonic wells and photonic wires that strongly confine light in one and two dimensions, respectively. Such devices include micro-ring lasers and micro-cavity resonators. Photonic switches and branching elements as well as computer logic blocks have been made and patented. Speed improvements in the order of 10,000x are possible in the light vs. electronic domain.
PON Passive optical network, a WDM computer network in which a passive star coupler is used to deliver all the data traffic to each node, thus eliminating the need for active switching. In WDM optical networks, channel wavelength is used for switching, routing or distributing each channel to its destination, resulting in an all-optical network (AON).
ps Picosecond, 10-12 seconds, a measure for very short time intervals. (See fs above.)
PSK Phase shift keying, a digital modulation scheme in which the source information is carried by the phase of a sinusoidal waveform.
SDH Synchronous digital hierarchy, the European version of the SONET standard with two major differences: 1. The terminology; 2. The basic line rate in SDH is equivalent to that of the SONET OC-3/STS-3 rate (i.e., 155.52 Mb/s). In contrast to the existing plesiochronous digital hierarchy (PDH), SDH allows direct access to tributary signals without demultiplexing the composite signal. As a result, network node costs are reduced because direct multiplexing is cheaper than step-by-step multiplexing. Furthermore, SDH supports advanced operation, administration and maintenance (OA&M) by dedicating several embedded channels for this purpose. SDH also supports a concatenation mechanism so that lower rate payloads can be combined to form higher rate payloads. The frame in SDH is called the synchronous transport module-1 (STM-1). It consists of 2,430 bytes represented as a 9-row-by-270 column structure similar to the frame in SONET. STM-N frames support line rates of Nx155.52 Mb/s. The compatibility between SDH and SONET allows for internetworking at the Administrative Unit-4 (AU-4) level. SDH can support broadband services such as broadband integrated services digital network (B-ISDN).
SONET Synchronous Optical Network, a physical transmission vehicle capable of transmissions in the gigabit-per-second range. SONET is defined by a set of electrical and optical standards. The basic building block of SONET is the synchronous transport signal level-1 (STS-1). This is a 51.840-Mb/s serial transmission using an optical carrier level-1 (OC-1) optical signal. Although SONET is physically serially transmitted, it is conceptually simpler to think of it in terms of bytes. The grouping of a specified set of bytes in the STS-1 is called a frame. An STS-1 frame consists of 6,480 bits, which is 810 bytes. The bytes in a frame are usually represented as 90 columns of 9 rows. The bytes are numbered from left to right across the top row until 90 is reached, then continue on the second row again left to right starting with byte 91. In a given byte, the most significant bit is transmitted first. This group of bytes (the STS-1 frame) is transmitted in 125 ms so that 8,000 frames are transmitted and received per second. All the higher data rate standards are simple integer multiples of the OC-1/STS-1 data rate.
Star coupler In a star coupler topology, all nodes are connected through point-to-point links to a central node called a hub, or simply a star. Such LAN configurations are further subclassified as active-star or passive-star networks, depending on whether the central node is an active or passive device. In an active-star configuration, all incoming optical signals are converted to the electrical domain through the use of optical receivers. The electrical signal is then distributed to drive individual node transmitters. Switching operations can also be performed at the central node since distribution takes place in the electrical domain. In the passive-star configuration, distribution takes place in the optical domain through devices such as directional couplers, thus the power transmitted to each node depends inversely on the number of users and faces additional losses due to the insertion losses of the directional couplers.
STM Synchronous transfer module, a frame defined within the SDH (see SDH). STM is the standard method international carriers use to assign time slots or channels.
Switching and routing Since everything carried on an optical fiber starts out as electrical impulses, such data streams must be converted to optical form. Switches and routes convert the electric impulses by modulating a laser so that the analog variations in the light carry the baseband signal information. Photonic switches perform two types of switching (data-path alteration) operations: 1. Call-by-call switching, which results from real-time input from customers who want a connection made between themselves and other endpoints; and 2. Protection switching, normally performed in a high-speed national network that occurs to restore connections after catastrophic damage in the fiber or cable part of the network connecting switches.
T1, T3 T1 is an older digital transmission service with a basic data rate of 1.544 Mb/s duplex service to 1-mi. distance on phone lines. T3 is equivalent to 28 T1 circuits for a capacity of 44.736 Mb/s.
Tb/s Terabit per second, (1012 bits per second), an information carrying capacity measure used for high-speed optical data systems.
UART Universal asynchronous receiver transmitter, a digital IC used to perform parallel-to-serial and serial-to-parallel data transfers. Most data transfers and operations inside the computer are parallel, but data communications systems are serial in nature. A UART utilizes FSK (see definition above) modulators and demodulators in order to communicate its incoming and outgoing serial data streams to the phone system.
VPN (Internet) Virtual Private Network, a communications system employed by many large and small businesses that exploits the interconnectivity of the Internet with the use of cryptographic techniques to guarantee the privacy of the communications.
WAN Wide area network, a data communications facility involving two or more computers with the computers situated at different sites. (See also LAN and Intenetwork.)