C

C2 security
C2 is one level of security standards in a seven-level range defined by the U.S. Department of Defense. The C2 rating specifies the policies and practices necessary to achieve a level of computer security that is reasonable for most companies.

cable categories
There are several cable categories used to describe the different types of twisted pair networks as follows:

• Categories 1 and 2 are used for low-speed data transmission and voice.

• Category 3 is the most common type of network cabling in use today. It is used for 4-Mbps Token Ring and 10BaseT networks.

• Category 4 is used for voice and data transmission rates of 16 Mbps.

• Category 5 is used for voice and data transmission rates up to 100 Mbps. It is the most popular type of network cabling being used in new installations. Category 5E (enhanced) provides data rates up to 200 Mbps through more precise manufacturing techniques.

carrier sense multiple access with collision detection (CSMA/CD)
CSMA/CD is set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD. This standard enables devices to detect a collision. After detecting a collision, a device waits a random delay time and then attempts to retransmit the message. If the device detects a collision again, it waits twice as long to try to retransmit the message. This is known as exponential backoff.

central processing unit (CPU)
A CPU controls the operation of a computer. It interprets and executes instructions to carry out computer-related tasks.

channel
Channel refers to the physical link that provides connectivity between communicating devices. Channel and link are often used interchangeably.

checksum
The number of bits in a transmitted unit of data is referred to as the checksum. A checksum is appended to the data unit as a simple error-detection method. The receiving node counts the data bits and compares the result to the checksum, to see whether all bits have arrived. If the numbers match, the transmission was probably complete.

circuit switching
Circuit-switched networks establish a physical connection between two nodes, and all packets in one transmission are passed between the same nodes by "switching" them through intermediate points (other nodes or a host computer). Circuit switching guarantees that all packets in the same transmission will sequentially travel over the same physical transmission path. However, each transmission is limited to one path, and cannot achieve greater transfer speed by using multiple parallel paths.

class of service (CoS)
In general, CoS (also called quality of service, or QoS) measures different characteristics of a transmission service. In the context of the OSI reference model, users of the Transport Layer specify QoS parameters as part of a request for a communication channel. These parameters define different levels of service based on the requirements of an application. For example, an interactive application that requires good response time would specify high QoS values for connection establishment delay, throughput, transit delay, and connection priority. However, a file transfer application requires reliable, error-free data transfer more than it needs a prompt connection, thus it would request high QoS parameters for residual error rate/probability.

client
The client portion of a client/server architecture is any node or workstation used by a single user. If multiple users share resources on the same workstation, it becomes a server. Examples of clients are Microsoft Windows NT Workstation and Windows 98.

client/server
Client/server (or client server) is a model in computer networking where individual PCs can access data or services from a common high-performance computer. For example, when a PC needs data from a common database located on a computer on a LAN, the PC is the client and the network computer is the server.

cluster controller
A cluster controller is an IBM device used to control communications between an IBM mainframe and a terminal device (IBM 3270 or ASCII terminal). It is also referred to as a communications controller.

coder/decoder (codec)
A codec is a hardware/software device that takes an analog video signal and converts (codes) it to digital format for compression and transmission. On the receiving end, the digital signal is put back (decoded) into the original analog signal. See PCM.

collapsed backbone
Collapsed backbone is a network topology that uses a multiport device, such as a switch or router, to carry traffic between network segments or subnets. This differs from the original Ethernet backbone, which consisted of a single common bus cable to which nodes and subnets are connected.

collision
A collision occurs in an Ethernet network when two frames are put onto the physical medium at the same time and overlap fully or partially. When a collision occurs, the data on the physical segment is no longer valid.

compiler
A compiler is a software program that takes source code from a programming language such as C++, and converts it into machine readable, executable code to be run on a computer.

complex instruction set computer (CISC)
See reduced instruction set computer (RISC).

Copper Distributed Data Interface (CDDI)
CDDI is a version of FDDI that runs on copper wiring such as twisted pair.

co-processor
A co-processor is a secondary computer processing chip that is optimized for a particular type of operations, such as graphics rendering or mathematical computation. If an application has been written to take advantage of a co-processor, then those operations can be processed there instead of the main CPU.

cross-connect
The term cross-connect refers to a high-speed switch that does not error check frames, but simply passes them to the appropriate destination.

crossover cable
A crossover cable is a UTP cable with RJ-45 connectors at both ends, designed to directly connect the NICs of two computers. The pins on each connector are attached to different wires, thus signals flow from the output of one NIC to the input of the other, and vice-versa. See patch cable.

cyclic redundancy check (CRC)
CRC is the mathematical process used to check the accuracy of data transmitted across a network. Before transmitting a block of data, the sending station performs a calculation on the data block and appends the resulting value to the end of the block. The receiving station takes the data and the CRC value, and performs the same calculation to check the accuracy of the data.