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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 loopback
Technique for testing the digital processing circuitry of a communications device.
May be initiated locally or remotely via a telecommunications circuit. Device be-
ing tested will echo back a received test message after first decoding and then en-
coding it. The results are compared with the original message (compare with
analog loopback).
DSU
Data service unit. A device providing an interface between a data terminal or oth-
er 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 RS-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.).
four-wire circuits
Telephone lines using two wires for transmitting and two wires for receiving of-
fering much higher quality than a 2-wire circuit. All long distance circuits are 4-
wire. Almost all local phone lines and analog phones are 2-wire.
in-band signaling
Signaling 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 signaling (request for service, dialing, disconnect, etc.) in the U.S. is in-
band signaling. Most of that signaling is MF (multi-frequency) dialing. The more
modern form of signaling is out-of-band.
information element
The name for the data fields within an ISDN Layer 3 message.
61202.086L1-1
ISU 512 User Manual
Glossary
101
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