Zhone IMACS 8000 User Manual page 773

Non-Return-to-Zero signal format
NT
Network Termination
NTU
Network Termination Unit, connects BRI station equipment to the integrated access system.
Any standard 2B1Q (U interface) device that supports "nailed-up" (one or two B-channels)
connections with no D-channel signaling.
OCU–DP Card
Office Channel Unit - Dataport (one of the data cards for this system).
OOF
Out Of Frame, an indication that the system is searching for the framing bit pattern in the
signal received from the network.
OOS
Out of Service, an indication that a system element has failed.
OSI
Open Systems Interconnection
PAM
Pulse Amplitude Modulation
PBX
Private Branch Exchange, a small phone switch inside a company.
PCM
Pulse Code Modulation
PDU
Protocol Data Unit
ping
Reference Guide E-15