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Glossary 287

Picosecond One trillionth of a second.
Pulse A current or voltage that changes abruptly from one value
to another and back to the original value in a finite length of
time.

Refractive index gradient The change in the refractive index
with distance from the axis of an optical fiber.

Regenerator A device that restores a degraded digital signal for
continued transmission; also called a repeater.

Ring network A network topology in which terminals are con-
nected in a point-to-point serial fashion in an unbroken circular
configuration.
Synchronous Digital Hierarchy (SDH) The ITU-T-defined
world standard of transmission with a base transmission rate
of 51.84 Mbps (STM-0) and is equivalent to SONET’s STS-1 or
OC-1 transmission rate. SDH standards were published in 1989
to address interworking between the ITU-T and ANSI transmis-
sion hierarchies. The European version of the SONET standard
has two major differences: the terminology and the basic line
rate in SDH is equivalent to that of the SONET OC-3/STS-3 rate
(that is, 155.52 Mbps). SDH enables direct access to tributary
signals without demultiplexing the composite signal. The com-
patibility between SDH and SONET enables internetworking
at the Administrative Unit-4 (AU-4) level. SDH can support
broadband services such as a broadband integrated services
digital network (B-ISDN).
Silica glass Glass made mostly of silicon dioxide, SiO2, used in
conventional optical fibers.
Single-mode (SM) fiber A small-core optical fiber through
which only one mode will propagate. The typical diameter is
eight to nine microns.
Slip An overflow (deletion) or underflow (repetition) of one frame
of a signal in a receiving buffer.
Soft-optics The software technologies that package and control
the light, such as the automatic power balancing of lightwave
services, the auto-discovery of optical components and their