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Glossary
61200051L1-1D ISU 2x64 Dual Port ISDN Service Unit User Manual 139
DDS
dataphone digital service. AT&T private line service for transmitting data over a
digital system. The digital transmission system transmits electrical signals direct-
ly, instead of translating the signals into tone of varied frequencies as with tradi-
tional analog transmission systems. Digital techniques provide more efficient use
of transmission facilities, resulting in lower error rates and costs than analog sys-
tems.
digital
Referring to communications procedures, techniques, and equipment where infor-
mation is encoded as either a binary 1 or 0, the representation of information in dis-
crete binary form, discontinuous in time, as opposed to the analog representation
of information in variable but continuous waveforms.
DSU
data service unit. A device providing interface between a data terminal or other
data communications device and a digital access line.
DTE
data terminal equipment. The portion of a data terminal that interfaces to the end-
user’s equipment. The main difference between DCE and DTE is that pins 2 and 3
are reversed on the EIA-232.
Frame
A group of bits sent serially over a communications channel. Generally a local
transmission unit sent between data-link-layer entities that contains its own con-
trol information for addressing and error checking. The basic data transmission
unit is employed with bit-oriented protocols, similar to blocks. In video transmis-
sion, a set of electron scan lines that comprise a television picture (usually 525 in
the U.S.).
in-band signalling
Signalling made up of tones which pass within the voice frequency band and are
carried along the same circuit as the talk path being established by the signals. Vir-
tually all signalling (request for service, dialing, disconnect, etc.) in the U.S. is in-
band signalling. Most of that signalling is MF (multi-frequency) dialing. The more
modern form of signalling is out-of-band.
information element
The name for the data fields within an ISDN Layer 3 message.
interface
A common boundary between two systems over which the inter-system commu-
nication occurs.
interworking
Communication between two types of networks or end equipment. This may or
may not involve a difference in signalling or protocol elements supported.