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Glossary

nondesignated port

A switch port that will not forward frames in order

to prevent a switching loop. Spanning Tree Protocol (STP) is responsible for
deciding whether a port is designated (forwarding) or nondesignated
(blocking).

non-stub area

In OSPF, a resource-consuming area carrying a default

route, intra-area routes, interarea routes, static routes, and external routes.
Non-stub areas are the only areas that can have virtual links configured
across them and exclusively contain an anonymous system boundary router
(ASBR). Compare with: stub area. See also: ASBR and OSPF.

NRZ

nonreturn to zero: One of several encoding schemes for transmitting

digital data. NRZ signals sustain constant levels of voltage with no signal
shifting (no return to zero-voltage level) during a bit interval. If there is
a series of bits with the same value (1 or 0), there will be no state change. The
signal is not self-clocking. See also: NRZI.

NRZI

nonreturn to zero inverted: One of several encoding schemes for

transmitting digital data. A transition in voltage level (either from high to
low or vice-versa) at the beginning of a bit interval is interpreted as a value
of 1; the absence of a transition is interpreted as a 0. Thus, the voltage
assigned to each value is continually inverted. NRZI signals are not self-
clocking. See also: NRZ.

network termination: A point in an ISDN network. See: NT1 and NT2.

NT1

network termination 1: Is an ISDN designation to devices that under-

stand ISDN standards.

NT2

network termination 2: Is an ISDN designation to devices that do

not understand ISDN standards. To use a NT2, you must use a terminal
adapter (TA).

NVRAM

nonvolatile RAM: Random-access memory that keeps its con-

tents intact while power is turned off.

Optical Carrier: A series of physical protocols, designated as OC-1,

OC-2, OC-3, and so on, for SONET optical signal transmissions. OC signal
levels place STS frames on a multimode fiber-optic line at various speeds, of
which 51.84Mbps is the lowest (OC-1). Each subsequent protocol runs at a
speed divisible by 51.84. See also: SONET.

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