474 GLOSSARY
NOS Network Operating System. Software that connects all the devices on a
network so that resources can be shared efficiently and managed from a
central location. Novell NetWare is one example of a network operating
system.
OEM Original Equipment Manufacturer. The maker of a product or component
that is marketed by another vendor, integrator, VAR (Value Added
Reseller), or reseller.
off-hook The state of a telephone line that allows dialing and transmission but
prohibits incoming calls from being answered. The term stems from the
days when a telephone handset was lifted off of a hook. Contrast with
on-hook.
off-site notification A feature that sends a message to a pager, outside telephone number, or
email account that informs a user of a voice mail message. The user can
retrieve the messages remotely.
on-hook The state of a telephone line that can receive an incoming call.
OSI model A conceptual model of hardware and software layers that define when,
how, and in what order data can be transmitted on a network. The OSI
Model defines seven layers:
out-of-band signaling An extra signal transmitted with the information signal to monitor and
control a transmission. It provides an additional layer of resilience by using
a separate channel.
packet A collection of bits, including address, data, and control information, that
are transmitted together. The terms frame and packet are often used
synonymously.
packet buffer Memory space reserved for storing a packet awaiting transmission or for
storing a received packet.
Layer 7 Application layer
Layer 6 Presentation layer
Layer 5 Session layer
Layer 4 Transport layer
Layer 3 Network layer
Layer 2 Data Link layer
Layer 1 Physical layer
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