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C
C-DTE
C-DTE stands for Character
Mode Data Terminal Equipment. It is used to characterize personal computers
and terminals that use asynchronous mode for data communications.
C Band
Satellite
communications systems transmit signals from earth stations to satellites
located in space. Antennas located on the earth are pointed at the
geostationary satellite (also called geosynchronous) and microwave signals
are sent up to the satellite (uplink) where the satellite repeats the
signal and sends it down (downlink) to a remote receiving antenna. There
are three frequency ranges that satellite systems use, C band (4GHz
downlink, 6GHz uplink), Ku band (11GHz downlink, 14GHz uplink) and Ka band
(20GHz downlink, 30GHz uplink).
Cable Categories
There are
several cable categories used to describe the different types of
twisted-pair networks. These are listed below:
- Categories 1 and 2: Used for low-speed data
transmission and voice.
- Category 3: The most common type of network cabling
in usage today. It is used for 4Mbps and 16Mbps Token Ring networks
and 10Base-T networks.
- Category 4: Used for voice and data
transmission rates of 20MHz.
- Category 5: Used for voice and data
transmission rates at 100MHz. It is the most popular type of network
cabling being used in new installations.
Cache
Cache memory is
memory that is readily accessible to a computer's central processor. Disk
cache memory is a portion of RAM (random access memory) which is available
for storage and retrieval of commonly used data. Using disk cache speeds up
computer operations because accessing data stored in RAM is much faster
than accessing data stored on a hard drive. When a process running in a
computer needs access to data, cache memory is checked first. If the data
is present it is retrieved.
Callback Modem
A
callback modem is a modem that calls you back after you initially dial into
a network. When you dial this type of modem, it answers the phone, asks you
for your telephone number, asks you to hang up the telephone, and then
calls you back. The primary reason for using a callback modem is for
security purposes, since the modem can track where each call is placed.
Capacity
Capacity is the
ability of a data communications component to carry out its intended
function. It is typically used to describe the capability of a
communications channel or link. The capacity of a T1 channel for instance
is 64Kbps. This does not mean that the channel always contains 64Kbps of
data, but that it is capable of carrying 64Kbps of data.
Carrier
A carrier is a
company which provides communications circuits. Sometimes the digital
communication channels, designated T1, T2, etc., are referred to as
carriers as well. Note that various carriers are implemented with various
transmission media (copper wire, coax cable, fiber optic etc.) and employ
varying digital signal encoding formats, as appropriate for the medium.
Carrier Detect
Carrier
Detect, or CD, is a modem signal that indicates that a carrier frequency
has been detected. In other words the modem is operational and connected to
another modem.
Carrier Sense Multiple Access
CSMA/CD
stands for Carrier Sense Multiple Access with Collision Detection. The
inventors of Ethernet chose this technique for controlling access to the
medium (the bus).
CBS
There are two
important terms that you need to understand when it comes to Frame Relay.
They are Committed Information Rate (CIR) and Committed Burst Size (CBS).
CIR is the guaranteed average data rate for the particular service and CBS
is the number of bits that can be transferred during some time interval.
The relationship between CIR and CBS is T (time) = CBS/CIR. When purchasing
frame relay service these values are important. For example, a CIR of
256Kbps and a CBS of 512 kilobits means that the network will move 512
kilobits in any given 2-second period. This guaranteed rate is for periods
of congestion, under lightly loaded networks the actual throughput will be
greater than the CBS. This is referred to as the EBS or Excess Burst Size.
CCITT
CCITT stands for
the Consultative Committee on International Telegraphy and Telephony. It is
a member of the ITU (International Telecommunications Union), a specialized
body within the United Nations. CCITT supports many recommendations dealing
with data communications, telephone switching, digital systems and
terminals.
Central Office
A
central office (CO) is a telephone company facility where local loops are
terminated. The function of a CO is to connect individual telephones together
through a series of switches. COs are tied together in a hierarchy for
efficiency in switching. Other terms for a central office are local
exchange, wiring center, and public exchange.
Channel
Generically
speaking, a channel is a communications path between two or more
communicating devices. Channels are also referred to as links, lines,
circuits and paths. In the mainframe environment, the channel is the path
between the host computer and a controller device.
Channel Bank
The
devices that perform the multiplexing for FDM transmission are called
channel banks. Similar devices for multiplexing digitized voice signals are
also called channel banks. Channel banks take many slow speed voice
channels or data links and multiplex them onto a high-speed channel such as
a T1.
Channel, ESCON
The
ESCON channel is the host-based channel connection that supports the ESCON
architecture. It directs the flow of information between I/O devices and
host main storage and provides common controls for the attachment of
different I/O devices. I/O devices are attached through control units to
the channel subsystem via channel paths.
Character
A character is a
symbol, figure, number, and punctuation or control function contained in a
data stream. The character may be referred by its decimal value,
hexidecimal value or symbol. Examples of characters are alphabetic
characters (a, b, c), numeric characters (1, 2, 3) and control characters
(carriage return, line feed, etc.).
Character Oriented Protocol
Character
oriented protocols are protocols which are characterized by the use of 8
bit control characters to perform transmission functions such as start and
end of frame, link management, error control and data transparency. Byte
oriented protocol and character oriented protocol are used interchangeably.
Characters Per Second
Characters
per second is a term typically used to describe the number of characters
that can be printed by a printer per second on a piece of paper.
Character Set
A
character set is all the letters, numbers and symbols that can be used by a
computer or computer application. The most common character set is the
ASCII character set which consists of 128 values. Other character sets are
EBCDIC and BCD.
Channel Service Unit
A
channel service unit or CSU is a device that connects customer equipment to
digital transmission facilities such as a T1circuit. The primary functions
of a CSU are line conditioning, equalization and loopback (for testing
purposes). The local loop for digital service always terminates at a CSU in
the subscribers building. The CSU is the device that actually generates the
transmission signals on the local loop (that is, the telephone channel).
Ciphertext
Ciphertext is the
output of an encryption device. It is the combination of the input data
(plaintext) and the encryption algorithm used to derive the new values.
CIR
There are two
important terms that you need to understand when it comes to Frame Relay.
The are Committed Information Rate (CIR) and Committed Burst Size (CBS). CIR
is the guaranteed average data rate for the particular service and CBS is
the number of bits that can be transferred during some time interval. The
relationship between CIR and CBS is T (time) = CBS/CIR. When purchasing
frame relay service these values are important. For example, a CIR of
256Kbps and a CBS of 512 kilobits means that the network will move 512
kilobits in any given 2-second period. This guaranteed rate is for periods
of congestion, under lightly loaded networks the actual throughput will be
greater than the CBS. This is referred to as the EBS or Excess Burst Size.
Circuit
A circuit is the
physical connection between two communicating devices. See also channel,
circuit switched networks and virtual circuits.
Circuit Switching
Circuit
switched networks establish a physical connection between two nodes, and a
packet is passed between nodes by "switching" it through
intermediate points, either through other nodes or through a host computer.
Cladding
The cladding is
one of two pieces that make up a fiber optic cable. Optical fiber is made
up of two parts: the glass core and the cladding. A plastic coating then
covers the fiber. The glass cladding contains a lower refractive index than
the core and reflects the light signal as it propagates down the fiber.
Class A Address
IP
addresses are normally seen as four decimal integers separated by decimal
points where each integer gives the value of one octet of the IP address
(125.169.85.22). Class A addresses are used on networks that have a large
number of IP hosts (16,777,216 or less). The first byte of a class A
address begins with a number from 1-127.
Class B Address
IP
addresses are normally seen as four decimal integers separated by decimal
points where each integer gives the value of one octet of the IP address
(166.169.85.22). Class B addresses begin with a number from 128 to 191.
Class C Address
IP
addresses are normally seen as four decimal integers separated by decimal
points where each integer gives the value of one octet of the IP address
(205.169.85.22). Class C addresses have three bytes that represent the
network portion of the address. Class C addresses can identify from 1-256
unique hosts on a single network.
Clear Channel
The
term clear channel refers to a transmission path where the entire bandwidth
of the channel is available for use. Channels that are not
"clear" require a portion of the bandwidth to be used for channel
control or channel framing purposes (in band signaling).
Clear to Send
Clear
to Send, or CTS, is the RS-232 pin used by a receiving modem to signal the
sending modem that it is ready to accept data.
Client
The client
portion of a client/server architecture is any node or workstation that is
used by a single user. If multiple users also share the same workstation,
it then becomes a server. Examples of the most popular client operating
system are listed below:
- Windows
- Windows NT
- DOS
- OS/2
- MacOS
- UNIX
A client typically runs most programs locally, accessing
server functions over a LAN when it is necessary to save or retrieve files
or print to a common printer.
Client/Server
Client server or
client/server is a mode in computer networking where individual PCs can
access data or services from a common high performance computer. For
instance when a PC needs data from a common database located on a computer
on a local area network, the PC is the client and the network computer is
the server.
CLNP
CLNP stands for
Connectionless Network Protocol. It is an open layer 3 protocol (Network
Layer).
Cloud
A cloud is a term
that refers to a network where individual users accessing the network do
not have insight into the individual components of the network. It is often
used as a description of a packet network or an X.25 network.
CODEC
CODEC is a
contraction that stands for coder/decoder. It is a hardware device that
takes an analog signal and converts it to a digital representation.
Collision
A collision
occurs when two or more nodes transmit data at the same time on the same
physical media. This can occur in Ethernet LANs using a bus topology where
all nodes share the same physical media.
Collision Avoidance
Collision
avoidance means that the protocol attempts to minimize the occurrence of
collisions on the link. A collision occurs when two or more nodes transmit
data at the same time.
Collision Detection
A
collision occurs when two or more nodes transmit data at the same time.
Collision detection is the ability of a node to detect that a collision has
occurred on the physical link.
Committed Burst Size (CBS)
There
are two important terms that you need to understand when it comes to Frame
Relay. The are Committed Information Rate (CIR) and Committed Burst Size
(CBS). CIR is the guaranteed average data rate for the particular service
and CBS is the number of bits that can be transferred during some time
interval. The relationship between CIR and CBS is T (time) = CBS/CIR. When
purchasing frame relay service these values are important. For example, a
CIR of 256Kbps and a CBS of 512 kilobits means that the network will move
512 kilobits in any given 2-second period. This guaranteed rate is for
periods of congestion, under lightly loaded networks the actual throughput
will be greater than the CBS. This is referred to as the EBS or Excess
Burst Size.
Committed Information Rate (CIR)
See CBS above.
Common Channel Signaling
Control
signaling anywhere within the same voice channel proved undesirable for digital
voice networks for a variety of reasons. Phone systems are gradually
converting to what is called common-channel signaling (CCS) which dedicates
a separate communication channel to control signaling, eliminating the
problems associated with control signaling within the voice channel. CCS
has culminated with the ITU-T Signaling System Number 7 (SS-7, also called
SS #7), which will gradually be adopted by most networks.
Compression
The term
compression refers to the process of reducing the number of bits required
to represent data without altering the meaning of the information being
conveyed by the data. The primary reason for using compression techniques
is to optimize the use of the communications channel. It is a function of
the Presentation Layer of the OSI model.
Conditioning
Conditioning
refers to the process of modifying transmission facilities to reduce noise.
This is typically done on leased lines for the purpose transmitting data,
which is more susceptible to noise than voice signals.
Connection Oriented
The
term connection oriented refers to a protocol mode used in data
communications networks where the send and receive stations in a network
remain connected for the duration of the session. A session is established
between the sending and receiving stations for the duration of the session.
The stations stay in contact during the session while the datagrams are
being sent back and forth.
Connectionless
Connectionless
transmission refers to a protocol mode where each individual packet (or
datagram) is an independent unit. The network is responsible for getting
each datagram to the final destination. The network does not have to
establish and maintain a session before sending and receiving datagrams. A
network may be connectionless or connection oriented depending on the
particular protocol used.
Contention
Contention in
data communications networks occurs when two stations attempt to send data
over a shared communications media at the same time. The most common
occurrence of this is found with the Ethernet protocol (CSMA/CD). When this
occurs, the data on the bus is corrupted and each station must retransmit
its data.
Control Character
Control
characters are ASCII characters such as carriage return (CR) and line feed
(LF). They are non-printable characters used for control of the flow of
data or the format of data. They are entered from keyboards by holding down
the control key while simultaneously pressing another key. For instance
holding down the control key and pressing M and J keys produces the
carriage return and line feed control characters.
Converter, ESCON
The
ESCON converter is intended as a migration support tool and is available in
two models. The 9034 Model 1 allows parallel bus-and-tag devices to
communicate with the serial ESCON channel and can be attached directly to
the host or to an ESCON Director. The 9034 Model 2 allows ESCON-compatible
DASD devices to communicate with the bus-and-tag channel.
Cooperative Processes
Processes
that communicate with one another obviously must be cooperating in some
sense to accomplish a useful function. But they can cooperate in different
ways. Three important styles of cooperation have been given names:
peer-to-peer, client/server, and master/slave.
COPS
The COPS (Common Open Policy
Service) protocol describes a simple query and response protocol that can
be used to exchange policy information between a policy server (Policy Decision
Point or PDP) and its clients (Policy Enforcement Points or PEPs). It is
designed to be extensible so that other kinds of policy clients may be
supported in the future. The model does not make any assumptions about the
methods of the policy server, but is based on the server returning
decisions to policy requests. Each message consists of the COPS header
followed by a number of typed objects.
CPE
CPE stands for
customer provided equipment or customer premise equipment. It refers to
telephone equipment that resides at the customer site.
CPU
CPU stands for
central processing unit. It is the processor in a computer that processes
the code and associated data in a computer system. A mainframe is a
computer system on a grand scale. Mainframe computers are large and
powerful and attach to networks and high-speed peripheral devices such as
tapes, DASD, and printers. In the IBM world a mainframe is also referred to
as a host computer or CPU. An example of an IBM mainframe would be the
ESCON 9121. Other companies such as Amdahl and Hitachi also build mainframe computers.
CRC
CRC stands for
cyclic redundancy check. It is the mathematical process used to check the
accuracy of the data being transmitted across a network. When a block of
data is about to be sent from one station to another, it performs a
calculation and appends a value on the end of the block. The receiving
station takes the data and the CRC character and performs the same
calculation to check the accuracy of the data.
CSMA/CD
CSMA/CD stands
for Carrier Sense Multiple Access with Collision Detection. The inventors
of Ethernet chose this technique for controlling access to the medium (the
bus).
CSU
A channel service
unit or CSU is a device that connects customer equipment to digital
transmission facilities such as a T1 circuit. The primary functions of a
CSU are line conditioning, equalization and loopback (for testing
purposes). The local loop for digital service always terminates at a CSU in
the subscribers building. The CSU is the device that actually generates the
transmission signals on the local loop (that is, the telephone channel).
CTERM
CTERM stands for
Communications Terminal Protocol. It is DECnet's (DNA) virtual terminal
(VT) service for VMS systems. CTERM is conceptually similar to the TELNET
protocol used in TCP/IP networks. CTERM is an alternative to the Local Area
Transport Protocol (LAT). It can be used over any supported link, whereas
LAT is only available for Ethernet.
CTS
Clear to Send or
CTS is the RS-232 pin used by a receiving modem to signal the sending modem
that it is ready to accept data. Ready to receive is the corresponding
signal in the V.24 protocol.
Cycles Per Second
MHz
stands for megahertz. One hertz is one cycle of a sine wave in one second.
One million Hertz (1 MHz) would be one million cycles per second.
Cyclic Redundancy Check
CRC
stands for cylic redundancy check. It is the mathematical process used to
check the accuracy of the data being transmitted across a network. When a
block of data is about to be sent from one station to another, it performs
a calculation and appends a value on the end of the block. The receiving
station takes the data and the CRC character and performs the same
calculation to check the accuracy of the data.
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