Glossary : O
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Optical Carrier-3. Refers to a 155 Mbps circuit capacity in SONET. There are
higher speed circuits in SONET, i.e. OC-12 (622 Mbps), OC-48
(2.5 Gbps), and OC-192 (10 Gbps) and a lower speed circuit, that
is OC-1 (51.8 Mbps).
Also see SONET.
Orthogonal Frequency Division Multiplexing. A technique in which an RF carrier is modulated by a baseband signal
that consists of a number of orthogonal sub-carriers (i.e. each
sub-carrier is independently modulated by its own data).
Sometimes, OFDM is referred to as DMT.
OFDM is used in 802.11a,
Also see modulation,
Open Systems Interconnection. OSI model is a
7-layer protocol stack model that describes procedures for
data transfer in a network. It was defined by
The OSI model has been used as a guideline for network designers and
manufacturers in developing network products that guarantee interoperability
among various technologies. However, the networking industry doesn't
strictly follow the OSI model. For
instance, functions that relate to several layers in the OSI
model may be performed by one protocol in a particular network technology.
The OSI model defines how one layer interacts with another
layer in the 7-layer protocol stack. When two computers
communicate, information is sent down through the protocol stack
to the physical layer (e.g. cable), and travels up through the
protocol stack to the application layer for processing by the
receiving computer. Below are the layers in the OSI
Layer 1: Physical Layer
Layer 2: Data Link Layer
Layer 3: Network Layer
Layer 4: Transport Layer
Layer 5: Session Layer
Layer 6: Presentation Layer
Layer 7: Application Layer
To easily memorize the OSI model, you can use this funny mnemonic:
People Don't Need TheSe Protocols
Anyway (from Layer 1 to Layer 7)