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O
OC
The SONET
standard defines a signal hierarchy similar to that which you saw for the
T-Carriers, but extending to much higher bandwidths. The basic building
block is the STS-1 51.84Mbps signal, chosen to accommodate a DS3 signal.
The hierarchy is defined up to STS-48, that is, 48 STS-1 channels for a
total of 2,488.32Mbps-capable of carrying 32,256 voice circuits. The STS
designation refers to the interface for electrical signals. The optical
signal standards are correspondingly designated OC-1 (Optical Carrier-1),
OC-2, etc.
Octal
The octal
numbering system is a positional numbering system just like the decimal
system. In the decimal system (base 10), each position starting from right
to left is 10 times greater than the position to the right. 10 is 10 times
greater than 1, 100 is 10 times greater than 10, 1000 is ten times greater
than 100 and so forth. The octal numbering system is base 8. In the octal numbering
system the positional value is 8. Each time you move one position to the
left the value of that position is 8 times the value to the right. For
example 0010 is eight the value of 0001 (8 vs. 1). 0100 is 8 times 0010 (64
vs 8) and so forth.
Octet
An octet is
another word for an 8-bit byte. It is an 8-bit binary pattern used to
represent a single character. Multiple octets or bytes can be combined to
form larger values.
Off-Line
Off-line is a
term used to describe a condition when a device is operational but is not
actively connected to a network or communications line (online). Personal
printers or network printers are often put into an off-line state when
paper is being added or when a form feed is being performed. In this
condition a computer cannot communicate with the device.
Open Data Link Interface
The
Open Data Link Interface (ODI) is a standard device driver from Novell that
provides an interface between network hardware and the higher layer
protocols. It supports multiple protocols and device drivers allowing
interconnectivity between various types of devices using different
architectures and protocols.
Open Shortest Path First
OSPF
(Open Shortest Path First) is a link-state routing protocol that is
becoming increasingly popular as the routing protocol of choice for IGPs.
OSPF offers some attractive options such as type of service routing. This
allows network managers to define multiple routes to destinations in terms
of service type, such as high throughput or minimal delay. This provides an
effective method of costing routes. OSPF also allows load balancing across
multiple routes. The OSPF protocol requires all exchanges between routers
be authenticated. This authentication guarantees that only trusted routers
propagate routing information.
Open Systems Interconnection
OSI
began as a reference model, that is, an abstract model for data
communications, but now the model has been implemented and is in use in
some data communications applications. The OSI model, consisting of seven
layers, falls logically into two parts-layers one through four, the
"lower" layers, are concerned with the communication of raw data,
and layers five through seven, the "higher" layers, are concerned
with the networking of applications.
Optical Fiber
Fiber
optic cable consists of a light-transmitting core surrounded by a
reflective cladding layer. Information is transmitted by focusing a light
source into the core and then switching it on and off. Light is reflected
off of the cladding layer, so nearly all of the light radiated into one end
of the cable reaches the other end. Fiber optic cable is not susceptible to
loss of signal strength over distance, like copper cable. And, since light
is not appreciably affected by electromagnetic fields, a signal in a fiber
optic cable is not affected by interference. But, by its very nature, a
fiber optic cable must be cut in order to get the received light beam to a
detector, as only point-to-point channels are possible; a bus cannot be
constructed.
There
are two common types of fiber optic transmission techniques, multimode and
singlemode. Multimode uses LED (light emitting diodes) to transmit the
light and is limited to 3km. Singlemode uses laser and is capable of
transmitting distances of 20km and higher.
Overhead
In data
communications overhead refers to the amount of data needed in addition to
the application data, to transmit information from one computer to another.
In the example of the OSI model, this would be the header of each layer and
the trailer of the Data Link Layer. The efficiency of communication devices
is determined by the amount of overhead necessary to transmit user
information. Efficiency typically decreases as the amount of overhead
increases.
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