Definitions and Terms Digital Telephone Switching System Part 3

Some of the definitions and terms associated with Central Office (CO) digital telephone switching systems are defined below:

Definitions and Terms Digital Telephone Switching System Part 3

Delay Distortion: Distortion is an unfaithful reproduction of signals due to changes occurring in the waveform of the original signal. Delay distortion is the delay at one frequency relative to that at another frequency. In a telephone channel, the reference frequency is often taken as 1700 or 1800 Hz. In any channel the reference frequency may be taken as the frequency of minimum delay.
Digital Signal: A signal constrained to have a discontinuous characteristic in time and a set of permitted discrete values.
Digital Switching: A process in which connections are established by operations on digital signals without converting them to analogue signals.
Direct Distance Dialing (DDD): A method of making long distance (national) toll telephone calls automatically on the public switched telephone network by entering the final destination code through the originating stations telephone set dial.
Direct Inward Dialing (DID): Calls can be dialed from a telephone connected to the local exchange (public network) directly to extensions on a PABX.
Direct Outward Dialing (DOD): Calls can be dialed from a telephone connected to the PABX directly to a local exchange (public network).
DTE: Data Terminal Equipment.
Dual Tone Multi Frequency (DTMF) signaling: Generic name for push-button telephone signaling equivalent to the Bell System’s TOUCH-TONE®
Dynamic Range: The range of power levels (minimum to maximum) achievable by a signal or specified for equipment operation.
Echo: A wave that has been reflected or returned with sufficient magnitude and delay to be perceived. (Echo in telephone systems is the return of a talker’s voice).
Echo Canceller: A device that removes talker echo in the return branch of a four-wire circuit by subtracting out a delayed version of the signal transmitted in the forward path.
Echo Return Loss (ERL): A measure of the difference in level between the echo returned and the original signal for frequencies between 500 Hz and 2500 Hz. Singing Return Loss (SRL) is a measure of the difference in level between the echo returned and the original signal for frequencies between 200 Hz and 500 Hz and between 2500 Hz and 3000 Hz. Poor return loss causes singing or near singing of amplifiers as well as echoes which can be objectionable if the magnitude is high enough and the delay long enough. (See also “Singing Return Loss”).
Echo Suppressor: A voice-operated device placed in the four-wire portion of a circuit and used for inserting loss in the echo path to suppress echo.

Elastic Store: A storage buffer designed to accept data under one clock but deliver it under another, so sort term instabilities (jitter) in either clock can be accommodated.
Electronic Switching System (ESS): Designation of stored program control switching machines manufactured by Lucent Technologies.
End Office: Class 5 switching office. Also referred to as a central office.
Envelope Delay: Derivative of channel phase response with respect to frequency. Ideally, the phase response should be linear, indicating that all frequencies are delayed equally (rate of change of phase versus frequency). Envelope Delay Distortion (EDD) is the maximum difference or deviation of envelope delay between any two specified frequencies.
Equalization: The practice of compensating for transmission distortions with fixed or adaptive circuitry.
Erlang: A measure of traffic intensity, expressed as so many hundred (Century) Call Seconds per hour (CCS). 1 CCS = 36 erlangs (1 hour = 3600 seconds). Basically, erlang is a measure of the utilization of a resource (i.e., the average number of busy circuits in a trunk group, or the ratio of time an individual circuit is busy). (See CCS).
Exchange Area: A contiguous area of service defined for administrative purposes that typically comprises an entire town or city and includes the immediate countryside and suburbs. An exchange area may have one end office or many end offices interconnected by trunks and tandem offices.
Expansion: The switching of a number of input channels onto a larger number of output channels. (Increasing the exchange’s capacity with respect to the number of lines and trunks).
Facsimile: The black and white reproduction of a document or a picture transmitted over a telephone connection.
Foreign Exchange (FX) Circuit: An extension of service from one switching office to a subscriber normally serviced by a different switching office.
Four-Wire Circuit: A circuit using two separate channels for each direction of transmission. When wire-line transmission is involved, each direction’s transmission, is provided by a separate pair of wires.
Frame: A set of consecutive digit time slots in which the position of each digit time slot can be identified by reference to a frame alignment signal. The frame alignment signal does not necessarily occur, in whole or in part, in each frame.


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