المجموعة الثالثة عشر
المجموعة الرابعة عشر
PCI(R) (Peripheral Component Interconnect) bus: A local-bus standard created by Intel
PCM (Pulse Code Modulation): A modulation technique used to convert analog voice signals into digital form. Used for voice multiplexing on T1 circuits.
PCMCIA (Personal Computer Memory Card International Association): Defines the format and interface for a credit card six device for use with portable or laptop computers. There are 3 types. Type 1, the thinnest at 3.3 mm, is normally used for memory type enhancements. Type 2, which is 5 mm thick, is for modem and LAN enhancements. Type 3, 10.5 mm thick, is for mass storage I/O.
PDN (Packet Data Network): A network established and operated by a PTT, common carrier, or private operating company for the specific purpose of providing data communications services to the public. May be a packet-switched network or a digital network such as DDS.
Peripheral: Any separate device - such as a printer or modem - that connects to and is controlled by a computer.
Physical Layer: Within the OSI model, the lowest level (Layer 1) of network processing, below the link layer; concerned with the electrical, mechanical, and handshaking procedures over the interface that connects a device to a transmission medium; an electrical interface.
PING: (Packet InterNet Groper). A program used to test reachability of destinations by sending them an ICMP echo request and waiting for a reply.
Pinout: A list or diagram that shows how the individual wires in a cable or connector are used.
Plenum rated: In cabling, approved by the NEC(R) for installation in air spaces ("plenums"). Plenum-rated cable does not give off toxic fumes when it burns.
Point-to-point line: A single communications circuit connecting just two locations. Compare withmultipoint.
Port: A point of access into a computer, a network, or other electronic system; the physical or electrical interface through which one gains access; the interface between a process and a communications or transmission facility.
Presentation Layer: In the OSI model, the layer of processing that provides services to the Application Layer, allowing it to interpret the data exchanged, as well as to structure data messages for transmission in a specific display and control format.
PRI (Primary Rate Interface): An ISDN service that operates at a rate of 1.544 Mbps. This service provides 23 B channels at 64 Kbps and 1 D channel at 64 Kbps. The system uses 8 Kbps for framing bits. In Europe the service provides 30 B channels at 64 Kbps and 1 D channel at 64 Kbps, and uses 64 Kbps for framing bits. The service allows for simultaneous transmission over all B channels and the D channel.
Print server: A workstation with an attached printer, a spooler, and software to process and manage printing tasks for all users on the network.
Print spooler: A program or device that acts as a buffer for data going to a printer; see buffer.
PROM (Programmable Read-Only Memory): A nonvolatile memory chip that allows a program to reside permanently in a piece of hardware; compare with volatile memory.
المجموعة الخامسة عشر
Propagation delay: The time it takes a signal composed of electromagnetic energy to travel from one point to another over a transmission channel; usually most noticeable in communicating with satellites; normally, the speed-of-light delay.
Protocol: The procedures used to control the orderly exchange of information between stations on a data link or on a data-communications network or system. Protocols specify standards in three areas: the code set, usually ASCII or EBCDIC; the transmission mode, usually asynchronous or synchronous; and the non-data exchanges of information by which the two devices establish contact and control, detect failures or errors, and initiate corrective action.
Protocol analyzer: A diagnostic tool for displaying and analyzing communications protocols. With this tool, a user can test the performance of network data to make sure that the network and its hardware are operating within the network's specifications. LAN managers use protocol analyzers to plan network upgrades and expansions, and to perform network maintenance and troubleshooting tests.
Protocol converter: A device that translates from one communications protocol into another, such as IBM SNA/SDLC to ASCII.
Punchdown block: Wiring outlet/terminal into which unterminated cable is "punched down." Common types are a 66 block or a 110 style.
PVC (Polyvinyl chloride): The type of plastic from which cable jackets and wire insulation are usually made. Produces toxic fumes when it burns.
Rackmount: Designed to be installed in a cabinet; usually 19" wide but can be 23".
RAID (Redundant Array of Inexpensive Disks): A method of storing data on multiple hard-disk drives, for faster access, greater reliability, or both. There are six officially defined "levels," each designed for a specific kind of application.
RAM (Random-Access Memory): Semiconductor read/write volatile memory. Data stored is lost if power is turned off. It is where data is stored while being processed by the microprocessor.
Redundancy: 1) In data transmission, the portion of a message's gross information content that can be eliminated without losing essential information. 2) The technique of building in extra identical components, to be used as backups in case the primary components fail.
Remote access: The ability of a computer in one location to reach a device that is some distance away, perhaps at another site.
Repeater: In digital transmission, equipment that receives a pulse train, amplifies it, retimes it, and then reconstructs the signal for retransmission; in fiberoptics, a device that decodes a low-power light signal, converts it to electrical energy, and then retransmits it via an LED or laser source. Also called a "regenerative repeater." Unlike a digital signal, repeating an analog signal amplifies the noise as well as the signal.
RG58: A standard for Thin Ethernet coaxial cable; it specifies 50-ohm impedance.
RG62: A standard for ARCNET coaxial cable; it specifies 93-ohm impedance.
RGB: Red, Green, Blue; a video standard in which the color signals for red, green, and blue are carried on separate lines, then combined to form a color video picture. Horizontal and vertical sync are imposed on one of the colors, usually green.
RIP (Routing Information Protocol): A protocol that's used by routers on a LAN to exchange routing information amongst participating routers
RISC (Reduced Instruction Set Computing): Internal computing architecture in which processor instructions are pared down so that most can be performed in a single processor cycle, theoretically improving computing efficiency.
ROM (Read Only Memory): Memory chips that store data or software; firmware.
Routing: The process of selecting the correct circuit path for a message.
RS-422 , RS-423: EIA serial transmission standard that extends transmission speeds and distances beyond those of RS-232. RS-422 is a balanced system with a high level of noise immunity. RS-423 is an unbalanced version of RS-422.
RS-449: RS-449 specifies the pinning for RS-422 and RS<\!->423 when a DB37 or DB9 connector is used.
RS-485: EIA serial interface standard for multipoint lines. Unlike its predecessors (RS-232, RS-422, etc.), RS-485 uses a tri-state driver. The third state the other interfaces lack is an off condition that allows equipment to communicate over shared wiring while another device sits quietly. It's tolerant to electrical noise, which makes it ideal for industrial users like manufacturing facilities and airports. And RS-485 supports long cabling distances up to 4000 feet (1219.2 m) at 100 Kbps.
RS-530: Specifies the pinning for balanced interfaces such as RS-422, when a DB25 connector is used. Designed to replace RS-449.
SCSI (Small Computer Standard Interface; pronounced "scuzzy"): A specification of mechanical, electrical, and functional standards for connecting small computers with intelligent peripherals such as hard disks, printers, and optical disks; each device on the bus has a unique identification.
SDLC (Synchronous Data Link Control): The primary data link protocol used in IBM SNA networks. It is a bit-oriented, synchronous protocol.
Serial transmission: The transmission of a character or byte of data one bit at a time. Contrast with parallel transmission.
Session Layer: Layer Five of the OSI reference model; provides protocols for assembling physical messages into logical messages.
Shielding: The process of protecting a cable with a grounded metal surrounding, so that electrical signals from outside the cable cannot interfere with transmission inside the cable. Shielding lessens the chance that the information moving along the cable will interfere with adjacent cables.
Short: An abnormal condition that occurs when there is an unwanted electrical connection between two wires. It results in a flow of excess current.
Simplex transmission: Transmission in only one direction.
Single-mode fiber: An optical fiber that supports only one mode of light propogation above the cutoff wavelength. Core diameters are usually between 5 and 10 microns, and claddings are usually ten times the core diameter. These fibers have a potential bandwidth of 50 to 100 GHz per kilometer