Data Communication Definition of Terms overview:
A problem that evolved over the data communication developmental years is the designation (names and terms) used for the various components that make up the data communications realm. For example: The terms Circuit, Line, Facility, Channel, Connection, Band and sub-band could all have the same meaning or have individually different meanings depending on individual expressions or intended use. The term ‘cable’, as simple as it sounds, may take on a different meaning depending on use. A twisted pair of wires could be a circuit or line and could be part of a cable containing many twisted pairs. Then we have shielded coax cable and fiber optic cable, which could be bundled (more than one) in a cable. If we go back to the Western Union Telegram service, a telegram was referred to as a ‘cable’. Twisted pair wires are used, in the past and today, because it reduces interference and inductance across two adjacent wires. By twisting the two wires any interference caused by the parallel portion of one part is canceled out after the twist because the wires are on opposite sides, a change of polarity. Each section between the twist is usually very short.
Channel’ could refer to: a voice grade channel (4000 Hz); a single channel (sub-channel) within a divided voice grade channel; a single voice grade channel within a divided broadband channel. Many times a broadband channel is divided into voice grade channels (multiple 4000 Hz) because that allows individual voice grade channels to be bundled for efficient use of facilities. To add a little more confusion a channel may be called a ‘band’ under these same circumstances.
The term ‘facility’ has more than one meaning and, depending on an individual’s age and experience, may be used to indicate a telephone channel, a line, a circuit, a building that houses communication equipment, or part of the equipment located within the building.
When referring to the transmission (sending) or reception (receiving) of data many terms are used. These would include message, traffic, signal(s) (signaling), data, frame, segment, block, packet, cell, information, digital or analog.
The same problem occurs when referencing a ‘Multiplex’ unit or device. The terms multiplex, multiplexer, multiplexing or ‘mux’ are used when referencing these devices. A multiplex unit could be a Time Division (DC) or frequency division (AC) device. A multiplex device could be called a ‘Concentrator’, where the term Concentrator really refers to a completely different device, although a multiplex unit could be contained within the Concentrator. The multiplex unit could also be part of a computer for accessing and sharing a single port by many devices.
The Carrier device, which modulated and demodulated an AC (alternating current) signal, was the original frequently used MODEM (Modulator–Demodulator) type device. The AC signal, on Telephone lines, to be modulated, is referred to as an Analog signal or the carrier frequency (measured in Hertz ‘Hz’); or the term Tone or Sine Wave may be used. The telephone line carrier frequency can be modulated by the Carrier device, and this modulated frequency provides an analog representation of the DC pulse(s) (bits) from the transmitting device. The AC signal is measured in frequency levels using the designation Hertz (Hz). The term Hertz was preceded by the term ‘cycles’ per second, i.e. a frequency of 1000 cycles.
The use of the term ‘Carrier’ by various individuals and companies can be confusing, especially if one was around when the Carrier device first came on the scene. There is the ‘Carrier device’ (like a MODEM), the ‘carrier frequency’ (to be modulated) and the ‘Common Carrier’ (Telephone Company, etc.). Also there is the ‘Carrier system’, which can be a network of wires, such as the electrical wiring in a home or the power network grid used by Power Companies, or the equipment designed to operate on a network of wires may be called a Carrier system. Then there is the non-communication use identifying a ‘carrier of goods and services’. There are probably a couple of other ones, but a few blanks are needed for someone to fill in!
Discussions concerning AC, Alternating Current, can involve multiple usages of various terms. AC is sometime called AC voltage and usually this term will be used when referring to house wiring. In communications different terms may be used when discussing AC ground (land line) circuits or wireless (radio) circuits. The terms carrier, analog, frequency, Hertz (Hz), sine wave, band, bandwidth and sub-band are normally used when discussing land line (ground) circuits. When discussing wire-less (radio) the terms frequency, hertz (Hz), tone, band, frequency spectrum, wavelength and wave are normally used. The following picture illustrates a two wavelength signal. The signal starts at 0, then goes to positive polarity, then returns to 0, then goes to negative polarity, then returns to 0, which completes one wave (wavelength), and then continues to repeat this cycle. The length of a wavelength, or time, is determined by the frequency rate; the higher the frequency the shorter the wavelength.
In the year 1962 AT&T (Bell Systems) produced the first widely used MODEM, called a Data Set, which operated at a 300 Baud rate and provided full-duplex MODEM features, such as protocol functions and connection controls in addition to modulation & demodulation functions.
A ‘digital’ MODEM was invented in the 1950’s and was used primarily for the military. The term MODEM is somewhat misused when a digital signal is transmitted as a digital signal, because no modulation of an analog signal occurs. Today all high speed data transmission uses digital devices and circuits, because much more information can be sent digitally per second in comparison to the transmission speed of a modulated analog signal. Also digitalization of a signal (data) improves the signal quality and allows signal compression.
The term ‘Baud’ and Bit per Second (bps), at times, are used interchangeably to identify bit transfer rates, although each may have a different meaning. Baud is usually defined as the measurement of the number of signal level changes per second, regardless of the information content of those signals. If each signal event represents only one bit condition, baud would be the same as bits per second. But, when more than a one-bit transition occurs (combined multiple bits) then the result would be different. For example: a 1200 bit per second MODEM actually operates at 300 baud, but it moves 4 bits per baud (4 x 300= 1200 bits per second). The 4 bit combination is treated as one transition.
The term Baud originated following the development of the Baudot code set and Baud is a contraction of Baudot. Baud was the only term used for many years until more sophisticated equipment, including synchronous operation, was implemented then the acronym bps (bits per second) was used for measuring bit transfer rate. Again the age of an individual will determine which term or acronym might be used. Baud was the first measurement criteria used when character bits included ‘start’ and ‘stop’ bits that were used for all character transfers. Also Words per Minute (WPM) and Characters per Minute (CPM) were used to measure transmission speed prior to the Baud transmission speed measurement tool being implemented. A word length, when referring to WPM, is five characters plus a space, which makes the word 6 characters in time length.
Protocol is a term used to identify the specifications, recommendations, standards and rules for the interface signals between devices and the formatting of control information to insure proper routing and delivery of information (data). The terms: signals, talk to, hand to, handshaking (pre 1980), communicate, exchange and pass to or pass are used when describing the signal exchanges between devices, and information exchange between software (programs). These terms all relate to the necessary protocol standards.
Terminals were named or referred too in many different terms. The term ‘uncontrolled’ or ‘freewheeling’ refers to a terminal that can send or receive without prior notification or selection. The terms ‘dumb’, ‘smart’ and ‘intelligent’ are used for different levels of functional capabilities. The intelligent level would be a computerized terminal, which services other terminals and/or processes information.
Many other terms used, depending on individual expressions, are also interchangeable. The age and experience of an individual plays a major part in the definitions used to describe various parts of the communication media.