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Glossary 279

nals and can electronically rearrange (cross-connect) those
channels.

Digital signal An electrical or optical signal that varies in
discrete steps. Electrical signals are coded as voltages; optical
signals are coded as pulses of light.
Dope Thick liquid or paste used to prepare a surface or a
varnish-like substance used for waterproofing or strengthening
a material.
DSX-1 May refer to either a cross-connect for DS1 rate signals
or the signals cross-connected at DSX-1.
DSX-3 May refer to either a cross-connect for DS3 rate signals
or the signals cross-connected at DSX-3.
Exchange Carrier Standards Association (ECSA) An organi-
zation that specifies telecommunications standards for ANSI.
Electromagnetic interference (EMI) Any electrical or
electromagnetic interference that causes undesirable response,
degradation, or failure in electronic equipment. Optical fibers
neither emit nor receive EMI.

Envelope capacity The number of bytes the payload envelope
of a single frame can carry. The SONET STS payload envelope
is the 783 bytes of the STS-1 frame available to carry a signal.
Each virtual tributary (VT) has an envelope capacity defined as
the number of bytes in the virtual tributary less the bytes used
by VT overhead.

E/O Abbreviation for electrical-to-optical converter.
Erbium-Doped Fiber Amplifier (EDFA) A key enabling tech-
nology of DWDM, EDFAs enable the simultaneous amplification
of multiple signals in the 1,500 nanometer region, such as multi-
ple 2.5 Gbps channels, in the optical domain. EDFAs drastically
increase the spacings required between regenerators, which are
costly network elements because they (1) require optical/electri-
cal/optical conversions of a signal and (2) operate on a single dig-
ital signal, such as a single SONET or SDH optical signal.
DWDM systems using EDFAs can increase regenerator spacings
of transmissions to 500—800 km at 2.5 Gbps. EDFAs are far less