ers at any given time. OFDMA allows multiple users to transmit simultaneously on the different subcarriers per OFDM symbol. off-hook A state in which a telephone set is in use (i.e., the handset is removed from its cradle). offline (1) Pertaining to equipment or devices not under direct control of the central process- ing unit. (2) Used to describe terminal equipment that is not connected to a transmission line. (3) Not controlled directly by or communicating with a computer. offload To move data or programs out of storage. (outgoing trunk) A one-way trunk that carries only outgoing traffic. OLT (optical line termination) A switch that sends traffic downstream to subscribers and that also handles the upstream traffic. ones density rule A scheme that allows a CSU/DSU to recover the data clock reliably. The CSU/DSU derives the data clock from the data that passes through it. In order to recover the clock, the CSU/DSU hardware must receive at least one 1-bit value for every 8 bits of data that pass through it. Also called pulse density. one-way trunk A trunk between a switch (e.g., a PBX) and an exchange or between exchanges, where traffic originates from only one end. on-hook A state in which a telephone set is not in use (i.e., the handset is resting in the cra- dle). online (1) Being controlled directly by or directly communicating with a computer. (2) Connected to a computer so that data can pass to or from the computer without human intervention. (3) Directly in the line loop. online services Computer functions offered to end users who do not own host computers. Online services include time sharing, archival storage, and prepared software programs. ONP (Open Network Provision) A pan-European standard that ensures the provision of the network infrastructure by European telecommunications administrations to users and com- petitive service providers on terms equal to those for the administrations themselves. ONU (optical network unit) ONU (optical network unit) A device in which optical-to-electrical conversions t