P

P Connector - A 25-pair male amp connector. For a photo of the female version, called a C connector, see 25-Pair Connector.

Bold text - A mainframe environment protocol used between host computers and user terminals/workstations.

P1024C - A mainframe environment protocol used between host computers and user terminals/workstations.

PA System - See Public Address System.

PABX (Private Automatic Branch Exchange) - The old name for PBX, Private Branch Exchange.

Pac Bell - The RBOC that operates the public telephone network in the state of California, owned by Pacific Telesis, who was recently purchased by Southern Bell.

Pacific Telesis - The RBOC that owns PAC Bell and Nevada Bell, which was bought out by Southern Bell.

Pacing - See flow control.

Packet - A unit of data at the network layer of the OSI (Open Systems Interconnect) model. Packets have a header that contains control information and a payload with user data. The terms, datagram, frame, packet data unit, message, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various arms of the networking industry.

Packet Assembler Dissembler (PAD) - See PAD.

Packet Buffer - Memory allocated or dedicated to the temporary storage of a copy of a data packet until the original has reached its destination.

Packet Controller - Another name for a packet switch. A packet switch is the central controlling device in a packet-switched network, such as switched Ethernet, switched token ring, or ISDN packet switching.

Packet Data Unit - A unit of data at the application, presentation, and session layers of the OSI (Open Systems Interconnect) model. Packet data units might have a header containing control information. The terms datagram, frame, packet, message, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various networking circles.

Packet Interleaving - To place many data packets from many data packet sources on one transmission channel.

Packet Internet Groper (PING) - A command followed by an IP address that sends an ICMP (Internet Control Message Protocol) echo request to the host specified by the IP address. If the targeted host has an active IP network connection, it will return the message. The length of time this process takes is listed by the local user system as an indication of network speed. Packet Layer - The layer in the X.25 protocol that is equivalent to the network layer in the OSI model. The packet layer does the same functions as the network layer, but it is simply called something different. Specifically to X.25, the packet layer handles the multiplexing of data. It does this by using an address system that is embedded in its control signaling with the packet layer on the other end.

Packet Level - The part in the telecommunications process of the X.25 protocol where network-layer functions are performed, such as addressing, multiplexing, and demultiplexing.

Packet Switching Exchange (PSE) - Part of an X.25 packet-switching network that receives packets of data from a PAD (Packet Assembler/Dissembler) via a modem. The PSE makes and holds copies of each packet, then transmits the packets one at a time to the PSE that they are addressed to. The local PSE then discards the copies as the far-end PSE acknowledges the safe receipt of the original.

Packet Switch - Also called a data packet switch. A device that routes segmented transmissions between end users, using a connectionless protocol, such as X.25, Ethernet, frame relay, token ring, ATM, or TCP/IP. Data packet switching is often performed in levels, with one protocol carrying another. For example, an Ethernet-based transmission from a LAN could be routed over a frame-relay or ATM connection via a data packet switch. Packet switches make up PDN (Public Data Networks) or PSN (Packet Switching Networks), which are the basis of the frame-relay, ATM, and X.25 services that public telecommunications companies offer.

Packet-Switching Network (PSN) - A name sometimes used in place of Public Data Network (PDN). PSNs connect to users via X.25 or frame relay. The Internet is also a type of PSN using TCP/IP packets.

PAD (Packet Assembler/Dissembler) - The device or software program in an X.25 network packet-switching network that takes a large file to be transmitted (or small) and breaks it down into smaller pieces. It gives each piece an identification number in relation to the rest of the pieces (e.g., 387 of 8954) and an address, along with error-checking information (usually CRC) and other HDLC (High-Level Data Link Control) information. The PAD can be a part of an end users computer or a separate device. The PAD sends the packets to a PSE (Packet Switching Exchange) via a modem, where the packets are individually copied and transmitted. The copies are made by the PSE in case a packet needs to be retransmitted because it was lost or corrupted.

PAD Parameters (Packet Assembler/Disassembler Parameters) - Listed are some common configuration parameters for PADs. Packet assembler/disassemblers are based on the X.3 standard. • Recall A parameter for X.25 pads that allows an administrator to set individual channels to 1 (Yes) or 0 (No) to access command mode. If the device connected to the channel is a terminal, the parameter is usually set to Yes. If the device connected to the channel is a printer, then the parameter is usually set to No. In the PAD configuration, the keystroke used to activate this parameter is optional, and is set to ^P (Control-P) by most administrators. • Echo A setting that determines what device (DCE or DTE) is responsible for displaying typed data on the screen of a terminal. If the echo setting of all of the terminals connected to a PAD (or host) is On, then the setting on the PAD (or host) should be Off, or vise versa. If possible, echo should be performed by a host on an X.25 network because echo also controls which device controls editing functions. It is an unruly task to set all of the host-emulation parameters when echo is left to the PAD. In most cases, the PAD cannot emulate them anyway. • Data Forward Signal (FWD <CR>) The parameter that determines which keystroke will send a data packet. It is most commonly set as the carriage return key (<CR>). If the parameter is set to 0 (Off), then no keystroke will initiate a packet transfer. The PAD will only ship out a packet when the packet is completely full or when a time out is reached (see Idle Time Parameter 4). Values on this parameter are limited to the PAD manufacturer’s specifications, and the one used depends on the host application programming. • Idle Timer (Idle Time 1) The parameter that determines the time out for a packet to be shipped out. If a carriage return is not entered within this time in 20ths of seconds (see FWD Parameter 3), then the PAD ships out the packet automatically. Common settings are 1 (for 1/20 second) and 3 (for 3/20 second) seconds. Setting this parameter to 0 typically sets the PAD to send full packets only, or to send only on a specific keystroke. Some hosts are programmed to automatically set this parameter to the application that they are running. • Ancillary Device Control (Flow Ctrl XON/XOFF) A parameter that deals with flow control between a terminal and a PAD. This parameter gives the PAD the ability to pause or stop the terminal from communicating for a moment. A value of 0 usually is a value of No and 1 is usually a Yes. • PAD Service Signals (Svc Sigs YES/NO) This parameter sets the PAD to deliver X.28 service signals to the terminal, such as call connected, cleared, error, etc. • Procedure On <Break> (Break/Reset) A parameter used in flow control between the PAD and host. This parameter is used to tell the PAD how the terminal will stop the PAD’s data flow (between the PAD and the host) and wait for further instructions. There are several ways to do this. Which way is chosen depends on the PAD, the host, and the application. Some examples of valid parameter settings are one or a combination of the following: send interrupt packet, reset the call, escape to PAD command mode, and send special predefined command for break. The signaling for this is done via an interrupt packet sent through the overhead. Whether the data in transit is discarded or saved is determined by the discard/save parameter setting (Parameter 8). • Discard Output (Discard/Save) When a PAD is in break/reset mode (break/reset is Parameter 7), the discard save parameter determines what will be done with the data that is still in transit. • <CR> Padding (<CR> Pad x) The setting that determines the number of “blank” bits that will be sent while the head of a printer returns to home after a carriage return. This parameter is set to a valid value between 0 and 7. The 0 setting is used for terminals, and 1 through 7 are used for printers. • Line Folding (ln fold 80) A printer setting for the maximum number of bytes (or characters) per printed line. If this parameter is set to high, the printer (or terminal) continues to print additional characters on top of each other at the end of each line that does not have a carriage return (<CR>). In many cases, the host is capable of reading this parameter and format the sent data to match the setting. • Terminal Speed (Speed 300) Terminal-to-PAD communication speed. Some device combinations (host, PAD, and terminal) will set this automatically (referred to as autobaud). The speed ranges from 50 bps to 64 Kbps. • Flow Control By Terminal (Pad Flow XON/XOFF) A PAD parameter that deals with flow control between a terminal and a PAD. This parameter sets the ability of the terminal to stop or pause communication with the PAD for a moment. • Line-Feed Insertion (lf CR/LF) A PAD parameter that is used to emulate host editing functions when the PAD is responsible (set to Yes) for the echo parameter. • Line Feed Padding (lf pad 10) A PAD parameter that is used to emulate host editing functions when the PAD is responsible (set to Yes) for the echo parameter. This particular function is to set how many blank data bits will be sent while the printer advances the paper feed one line. • Editing (edit YES/NO) A PAD parameter that is used to emulate host editing functions when the PAD is responsible (set to Yes) for the echo parameter, rather than the host. This is only set when delay times for screen display are crucial or exaggerated (satellite transmission delay, for instance). • Character Delete (char del ^H) A PAD parameter that is used to emulate host editing functions when the PAD is responsible (set to Yes) for the echo parameter, rather than the host. This is only set when delay times for screen display are crucial or exaggerated (satellite transmission delay, for instance). • Line Delete (ln del ^U) A PAD parameter that is used to emulate host editing functions when the PAD is responsible (set to Yes) for the echo parameter, rather than the host. This is only set when delay times for screen display are crucial or exaggerated (take satellite transmission delay for instance). • Line Redisplay (ln rdisp ^R) A PAD parameter that is used to emulate host editing functions when the PAD is responsible (set to Yes) for the echo parameter, rather than the host. This is only set when delay times for screen display are crucial or exaggerated (satellite transmission delay, for instance).

PAD Service Signals - A packet assembler/disassembler feature that is activated in the PADs parameter settings. This parameter sets the PAD to deliver X.28 service signals to the terminal, such as call connected, cleared, error, etc.

Page Zone - When attendants use a page feature on a telephone system, they are prompted to input a choice of zone or “area” they want to page. This area is called the page zone. Most PBX systems have three separate page zone options and “all zones” is an option to the attendant as well.

Pager - A small, portable device that receives simplex messages. Pagers are small enough to be worn on a belt or wrist (as a wristwatch-type pager). Pagers come in three types: numeric, alphanumeric, and PCS. The numeric are the older type, receiving numbers that a person desiring a return call inputs to the page signal. The alphanumeric pagers have a larger LCD display and have the ability to receive text messages as well as numeric messages. PCS paging is offered as a service with PCS cellular telephones by many cellular telephone companies.

Pair - Two copper wires, or two optical fibers.

Pair Gain - Usually a reference to a Lucent SLC96 or SLC2000 system. A system in the public network that multiplexes many conversations or phone lines into one or two copper pairs. T1 is a pair-gain system used by public telephone-service providers, such as USWest, PAC Bell, Brooks, ELI, and virtually every other local facilities-based phone company. The photograph shows a pair-gain system outdoor closure/cabinet.

PAM (Pulse Amplitude Modulation) - See Pulse Amplitude Modulation.

Parabola - The curve that all projected objects travel when acted on by a force of gravity. If you watch a baseball that is hit or thrown through the air, it curves during its fall. This oblong curve is called a parabola, and it is very special. Ancient mathematicians discovered that this curve can be duplicated mathematically with trigonometry. Radio communications engineers later used it to focus and guide radio signals because all radio waves from a single point were reflected in exactly one direction. For a diagram, see Parabolic Dish Antenna.

Parabolic Dish Antenna - A directional antenna. This name results from its parabolic shape, which means all radians from a single point are reflected into one direction. For a photo of a parabolic microwave antenna, see Microwave.

Parallel Circuit - A circuit that has more than one path for current through multiple loads or devices. The other type of circuit is a series circuit, which has only one path for current through multiple loads.

Parallel Data - The transmission of data over a media with multiple bits being transferred at one time, such as a whole byte. The other type of transmission is serial data, which sends data one bit at a time. The illustration shows an example of 8 bits being sent in series and in parallel.

Parallel Port - A standard DB25 connector used to connect computer printers. The communication speed of parallel ports is about 1.5 Mbps.

Parallel Printer Cable - A cable that is designed for transmitting multiple bits at one time, used on a parallel-printer port. See Parallel Data.

Parasite - A device that gets its power to operate from the telephone line. The telephone line has a �52-V battery voltage when it is idle and �12 V when it is in use. Standard telephone sets (2500 type) are parasitic telephones. Other devices, such as RFI filters and tapping devices, are parasitic.

Parity - A method of bit-stream checking. Parity is used in error correction. The number of logic “ones” is counted in a bit stream. There is “odd parity” and “even parity.” Which is used depends on if you like odd or even numbers, or if the modem you are trying to connect with likes odd or even numbers. Parity is a part of error-checking protocol. It is simply the part of the protocol where the two devices are told if they are counting odd number bits or even number bits. In odd parity, if the number of ones is an odd number, then a parity bit is set to “one” at the end of the bit stream. This is odd parity because the parity bit is set to one when the number of “ones” is odd. In even parity, the parity bit is set to “one” when the number of “one” bits is even. Odd-parity bit stream: 0 0 1 0 1 0 1 Subsequent parity bit would be 1 because the number of “ones” is odd. Odd-parity bit stream: 1 0 1 0 1 1 0 Subsequent parity bit would be 0 because the number of “ones” is not odd. Even-parity bit stream: 0 0 1 0 1 0 1 Subsequent parity bit would be 0 because the number of “ones” is not even. Even-parity bit stream: 1 0 1 0 1 1 0 Subsequent parity bit would be 1 because the number of “ones” is even.

Parity Bit - A parity bit is a bit added into a bit stream, usually after every seven bits. See Parity.

Parity Check - An older and original method of error correction. Newer methods of error correction include CRC (Cyclic Redundancy Checking), which are more efficient and far more accurate (99% CRC, 50% parity).

Park - A PBX system feature that allows a user to transfer a call to a “ghost” extension. When the call is transferred to the “ghost” extension, it can then be retrieved from any telephone by dialing that extension. The “ghost” extension is thought of as a parking spot in the network.

Part 64 - A reference to the part of the MFJ (Modified Final Judgment) handed to the RBOCs (Regional Bell Operating Companies) by Judge Harold Greene. It specifies the separation of customer-owned equipment (Customer Premises Equipment, CPE) and the telephone companyowned equipment, and Telephone company demarcation.

Part 68 - See Part 64.

Part X - Usually a reference to Part 64 or Part 68 of the MFJ (Modified Final Judgment) handed to the RBOCs (Regional Bell Operating Companies) by Judge Harold Greene. It specifies the separation of customer-owned equipment (Customer Premises Equipment, CPE) and the telephone company-owned equipment, and telephone company demarcation.

Party Line - A telephone line that is shared by multiple residences. A party line is a one-pair circuit that can have as many as eight individual residences, (each with a separate phone number) share that same pair for service. If one residence is using the line, the others can’t. Each residence can have its own phone number, with the use of a SRM (Selective Ringing Module). The SRM is installed in the NI of each residence and contains electronics that can be configured to recognize different ringing formats using DIP switches. Some different ringing formats that an SRM would differentiate are ring voltage on the ring side, ring voltage on the tip side, ring voltage on the ring side with the tip side grounded, and ring on the tip side with the ring side grounded. The selective ringing modules are wired to recognize a certain ring. The central office sends a specific ring to reach a specific number.

Pass-Band Filter - Another name for a band-pass filter. A band-pass filter is used in frequency-division multiplexing as well as the equalizer in your stereo. It is usually a capacitor/resistor/inductor network that has a resonant frequency and a rating of how well it passes one frequency (or a bandwidth of frequencies) and blocks out others (called the Q, quality) of the circuit. The resonant frequency of the circuit is the frequency that the circuit will pass.

Passive Hub - As opposed to an active hub, a passive hub has no ability to amplify (extend signal transmission range) a signal on an ethernet network, so it needs to be in close proximity to the computers it is connected to. Simply stated, a hub makes a star wiring configuration look like a bus configuration to all the devices connected to it. Hubs are utilized extensively in Ethernet networks. For a diagram of a hub application, see Hub.

Passive Optical Network (PON) - A fiber-optic-based transmission network that contains no electronic devices that require external power. Passive optical networks use the physical characteristics of light to separate different carriers or colors of light. These types of optical networks are relatively inexpensive to implement and maintain, compared to their active counterparts.

Passive Matrix Display - A type of laptop computer display technology where rows of liquid crystal elements are connected as a grid. Those elements are activated by their coordinate (horizontal and vertical) transistor. Passive matrix displays have a slightly fuzzy image during screen movement and screen refresh, although they are less expensive and consume less power.

Patch Panel - A panel equipped with plugs, rather than terminals, for connecting wires or fiber optics. A patch panel can be used to terminate installed wire or be used as a “plug-in” test access point for communications circuits. DS0 and DS3 patch panels are very popular in central offices for testing purposes. Cat 5 patch panels are popular in computer LAN environments for the easy connection of computers to a network of pre-installed wire.

Path -1. The process of aligning a microwave radio link. Two technicians point the dishes at each other while taking AGC readings from the transmission equipment, which is often located in the dish (also called an ODU, Outdoor Unit). 2. The space between two microwave dishes that make a microwave radio link.

Path Cost - A routing metric value, typically based on hop count, media bandwidth, or other measures, that is assigned by a network administrator and used to compare various paths through an internetwork environment. Cost values are used by routing protocols to determine the most-favorable path to a particular destination: the lower the cost, the better the path. See also Metric.

Pause - A feature incorporated with the speed-dial feature of telephones. When speed-dial numbers are programmed, a 1.5-second pause can be inserted by pressing the # key. If a user wants to program a speed dial that rings into a PBX system where an extension needs to be input after an auto attendant answers, the user can input several pauses before the extension number in the speed-dial string that they program on their phone. When activated, the speed-call feature will then dial the number, pause while the auto attendant answers, then dial the extension.

Payload - A transmission signal or packet has two components, the payload and the overhead. The payload carries the customer information, like a B Channel in an ISDN circuit. The overhead carries operational, maintenance, and synchronization information that make the protocol work. An ISDN D channel is an overhead component of an ISDN circuit.

Payphone - A coin operated telephone. Many payphones are owned and operated by local telephone companies, but there are private payphone companies too. Pay or coin-operated telephones can be purchased at telecommunications equipment distributors, such as Graybar and Anixter.

PBX (Private Branch Exchange) - See Private Branch Exchange.

PC (Personal Computer) - A reference to an IBM PC or an IBM clone PC, which is a PC manufactured by some company other than IBM (such as Dell, Hewlett Packard, or Compaq). The older architecture for PCs is the AT (Advanced Technology) architecture. The newer is the ATX architecture developed by Intel. Illustrated is the rear-panel comparison of an AT machine and an ATX machine. Notice that the ATX architecture blocks the serial and I/O connectors (i.e., printer port) together on a backplate. See also AT, ATX, and Personal Computer.

PCB (Printed Circuit Board) - See PC Board.

PC Card - A short name for a PCMCIA card most commonly made for PC laptops. See PCMCIA.

PCI (Peripheral Component Interconnect) - A 64-bit bus standard developed by Intel. PCI is a term that is commonly referred to by PC users that are searching for an expansion board for their PC. PCI is an improvement over the ISA (Industry Standard Architecture) bus. As PCI matures in the industry, it will exceed a throughput rate of 200 Mbps. PCI slots on a personal computer’s motherboard are recognizable by their white or brown color, they are relatively shorter than other sockets/slots, and they have a smaller pin size. See also ISA and AGP.

PCL (Printer Control Language) - A printer-interface communication method developed by Hewlett-Packard, and used widely in the computerprinter industry. PCL translates page data into instructions for a printer.

PCM (Pulse-Code Modulation) - A concept that is similar to that of Morse code, digital signals are sent over a media (twisted copper pairs, radio, fiber optic, coax, etc.) one bit at a time, each bit being represented as a pulse or the absence of a pulse. A typical digital transmission is PCM.

PCMCIA (Personal-Computer Memory-Card International Association) - An organization made up of about 500 companies that developed a standard for small, credit card-sized PC devices. These devices are designed especially for laptop or other portable computing devices, and they are designed to be exchanged without rebooting the PC (although this is not always the case). The first PCMCIA cards were made for additional memory. Subsequent designs included LAN interface cards and modems. There are three types of PCMCIA interfaces. Type-I cards are for memory, and are the thinnest of the three types (3.3-mm thickness). Type-II cards are generally for LAN interface and modem applications (5.5-mm thickness). Type-III cards are the largest in thickness at 10.5 mm thick and are for portable disk drives. All three have the same rectangular dimensions (85.6 by 54 millimeters). Because the rectangular dimensions of the cards are the same, they all fit into the same slots as follows: A Type-III slot can hold one Type-III card or one each of a Type II and Type I. A Type-II slot can hold one Type-II or two Type-I cards. A Type-I slot can hold one Type-I card.

PCR (Peak Cell Rate) - A parameter defined by the ATM forum for ATM traffic management. In Constant Bit Rate (CBR) transmissions, the peak cellrate parameter determines how often data samples are sent. In Available Bit Rate (ABR) transmissions, the peak cell-rate parameter determines the maximum value of the available cell rate. See also ACR, MCR, and CBR.

PCS (Personal Communications Service) - A newer form of cellular communications service that has a different transmission format. Instead of being an analog radio signal, as in cellular, PCS combines multiple customers on each radio channel. PCS operates at a higher frequency than cellular and transmits less power, thus each of the individual cells (geographical cells) are smaller and more compact. This also allows more users in the same amount of airspace. For a photo of a PCS antenna, see Monopole Antenna.

PDN (Primary Directory Number) An ISDN telephone number.

PE (Peripheral Equipment) - Devices that are not a part of a system, but work with it, such as a printer.

Peak Cell Rate (PCR) - A parameter defined by the ATM forum for ATM traffic management. In Constant Bit Rate (CBR) transmissions, the peak cell-rate parameter determines how often data samples are sent. In Available Bit Rate (ABR) transmissions, the peak cell-rate parameter determines the maximum value of the available cell rate. See also ACR, MCR, and CBR.

Peak Power - A method of calculating the power consumption or power output of an electronic/electrical device. Other methods of calculating power include true power, transparent power, and RMS (Root-Mean-Square) power. Most audio applications use either peak or RMS power. A great example to demonstrate the difference between peak power and RMS power is home and car stereo amplifiers. Many people ask which is better, peak power or RMS power? The answer is both. Some stereo manufacturers put peakpower ratings on their products because it sounds better. Some put RMS power on their products because it is closer to the true power of the device. To convert from peak power to RMS power, multiply the peak-power rating by 0.707. The result is RMS power. To convert RMS powr to peak power, divide the RMS power rating by 0.707. The result is the peak-power rating.

Ped (Pedestal) - See Pedestal.

Pedestal (Ped) - Usually a small green box that houses telephone or cable-TV cable splices or terminals.

Peer Communication - The communication of individual layers in a protocol with each other. For instance, the network-layer control signals from the near end are only used by the network layer on the far end. They are not used by any other layer, and are definitely not seen by the user. These types of signals or “control frames” are referred to as peer communications. For an example of a peer communications signal, see SABM and FRMR.

Peer-to-Peer Networking - A local-area network scheme that does not use a server or host. Individual PCs are linked together via network cards and CAT-5 wire or coax. Windows 95 has its own peer-to-peer networking utility built in.

Peg Board - Also called a white board or mushroom board. It is placed between termination blocks (such as 66M150 blocks) to provide a means of support for routing cross-connect wire. For a photo, see White Board.

Performance Management - One of five categories of network management defined by ISO for management of OSI networks. Performancemanagement subsystems are responsible for analyzing and controlling network performance, including network throughput and error rates. See also Accounting Management, Configuration Management, Fault Management, and Security Management.

Peripheral Component Interconnect (PCI) - A 64-bit bus standard developed by Intel. PCI is a term that is commonly referred to by PC users that are searching for an expansion board for their PC. PCI is an improvement over the ISA (Industry Standard Architecture) bus. As PCI matures in the industry, it will exceed a throughput rate of 200 Mbps. PCI slots on a personal computer’s motherboard are recognizable by their white or brown color, they are relatively shorter than other sockets/slots, and they have a smaller pin size. Illustrated is a PCI video card. See also ISA and AGP.

Peripheral Equipment - Devices that are not a part of a system, but work with it, such as a printer.

Peripheral Node - A terminal, printer, or other I/O device on an SNA (IBM System Network Architecture) network.

Permanent Virtual Circuit - A dedicated (private line) channel in a multiplexed transmission or packet network used by telephone companies. Permanent virtual circuits are very common in T3 and SONET carrier networks. A virtual circuit is a switched circuit, like a plain telephone line. A permanent circuit is a dedicated twisted copper pair with a carrier, such as T1 for a private line.

Personal Computer - A small and relatively inexpensive data-processing machine that is designed for one user. Early computers had many terminals and were shared among many users. Personal computers are used for word processing, data-base management, desktop publishing, Internet access, games, composing music, and many other applications. The two major types of personal computers are the Apple Macintosh and the “PC,” which is based on the IBM PC architecture, and frequently referred to as an IBM clone, if manufactured and distributed by one other than IBM. Apple Macintosh computers are frequently referred to as Macs. IBM PCs and IBM clones are referred to as PCs.

Personal Computer Memory Card International Association (PCMCIA) - An organization made up of about 500 companies that developed a standard for small, credit card-sized PC devices. These devices are designed especially for laptop or other portable computing devices, and they are designed to be exchanged without rebooting the PC (although this is not always the case). The first PCMCIA cards were made for additional memory. Subsequent designs included LAN interface cards and modems. There are three types of PCMCIA interfaces. Type-I cards are for memory, and are the thinnest of the three types (3.3-mm thickness). Type-II cards are generally for LAN interface and modem applications (5.5-mm thickness). Type-III cards are the largest in thickness at 10.5 mm thick and are for portable disk drives. All three have the same rectangular dimensions (85.6 by 54 millimeters). Because the rectangular dimensions of the cards are the same, they all fit into the same slots as follows: A Type-III slot can hold one Type-III card or one each of a Type II and Type I. A Type-II slot can hold one Type-II or two Type-I cards. A Type-I slot can hold one Type-I card.

Peta - The prefix for 1,000,000,000,000,000. It would take five million 1GB (one gigabyte) hard drives to have the capacity of one 5PB hard drive. I don’t think we will see hard drives in the PB range any time soon.

Phantom DN (Phantom Directory Number) - Also called a Virtual DN. A directory number or extension on a PBX system that is used to attach a voice mailbox. The phantom DN does not really have a telephone set, but the PBX system thinks it does, so it transfers calls to that DN, which are configured to be forwarded to a voice-mail system. A user of that DN can then dial into the voice-mail system, enter their extension, and receive their messages.

Phase - A reference to a sine wave and its relative cycle to another sine wave or time source. Phase is measured in degrees (0 to 360) or radians.

Phased Locked Loop (PLL) - A very important electronic circuit in the world of FM (Frequency Modulation) and PM (Phase Modulation). Phase-locked loops are used as the detector circuits in FM receivers and to create stable RF references for all types of transmitters and timing circuits.

Phase Modulation (PM) - A method of varying a radio carrier frequency so that a signal (the variations) can ride on it. After the carrier signal has the variations imposed on it, it is amplified and transmitted. The variations in the signal are then detected by the receiver. The variations in the carrier signal are actually voices, music, or whatever is to be transmitted. The other methods of modulating a carrier frequency are AM (Amplitude Modulation) and FM (Frequency Modulation). Phase modulation makes the phase of a carrier frequency change in conjunction with a signal that it is to carry. The color on broadcast television is sent in a PM format. A simple representation is depicted in.

Phase-Shift Keying (PSK) - A method of modulating a carrier frequency by making the carrier signal phase shift in conjunction with the digital input signal. A cosine phase indicates a “1” value and a sine phase indicates a “0.”

Phased Array Antenna - A group of small antennas placed a multiple of a wavelength in distance from each other to create one larger antenna.

Phonetic Alphabet- A set of audibly distinct words that were chosen by the U.S. Army to identify spoken letters and numbers.

Photoconductive Cell - Also called a photoresistor or photosensitive cell. An electronic device that conducts electricity better when it is exposed to light. Photoconductive cells are made from Cadmium Sulfide (CdS) and Cadmium Selenide (CdSe). They are most responsive to green-colored light (5500 angstrom). They react to almost the entire spectrum of light that is visible to the human eye.

Photo-Conductor - A reference to a photo-conductive cell.

Photodetector - A photo-sensitive circuit whose main component is usually a photo-diode or photo-transistor. A photo-detector converts pulses of light into pulses of electricity.

Photodiode - An electronic device that acts as a light-activated switch. It operates similar to a zener diode, except that the reversecurrent effect is controlled by light.

Phototransistor - A transistor that is forward biased (conducts electricity as a switch) when exposed to light. Phototransistors are used the same way as switching transistors, except the base is the photosensitive part of the device.

Photonic Layer - Fiber optic. A reference to the physical layer in the SONET protocol architecture, which is where the type of fiber optic is defined (multimode/single mode).

Physical Address - Another name for MAC address. See MAC Address.

Physical Colocation -A colocation is an interconnection agreement and a physical place where telephone companies hand-off calls and services to each other. This is usually performed between a CLEC and an RBOC. The CLEC installs and maintains interconnection equipment usually consisting of optical carrier (SONET) equipment and a digital crossconnect system. There are other types of colocations. Alarm companies like to have their alarm-signaling equipment located in the local central office for security and convenience of connecting alarm circuits. Longdistance companies colocate with local telephone companies as well.

Physical Layer - A layer in a communications protocol model. In general, the physical layer is the actual media of the communications transmission (twisted-pair wire, coax, air, fiber optic, etc.) It is also the types of connectors used and the pin-outs of those connectors. The 568B wiring scheme for CAT 5 wire is a physical-layer function. The latest guideline for communications protocols is the OSI (Open Systems Interconnect). It is the best model so far because all of the layers (functions) work independently of each other. For a diagram of the OSI, SNA, and DNA function layers, see Open Systems Interconnection. For a conceptual diagram of the OSI model layers, see OSI Standards.

Physical Medium-Dependent Sublayer (PMD) - The ATM physical layer is divided into two parts (sublayers), the TCS (Transmission Convergence Sublayer) and the PMD. The PMD is the lowest physical layer and it determines the types of connectors and the medium used for transmission. It also interfaces the line-coding technique, such as SONET OC-1 or STS-3c/STM1, with the TCS sublayer. See also TCS.

Physical Topology - A physical topology refers to the way a Local-Area Network (LAN) of computers is connected for communication. The three different types of physical topologies are: ring, star, and bus. The star and bus topologies work very much the same. The ring topology is also called a token passing topology.

PIC - PIC refers to color-coded cable. Icky-PIC is cable that is color coded and jelly filled to help protect the copper pairs inside from water.

Pico Prefix - for 1 � 10�9 . One picofarad (a capacitor) is equal to 0.000,000,001 farads. It is abbreviated as 1 pF.

Picofarad - A unit of measurement for capacitors. Pico is the prefix for 1 × 10 -9 . One picofarad (a capacitor) is equal to 0.000,000,001 farads. It is abbreviated as 1 pF.

Piggybacking - Process of carrying acknowledgments within a data packet to save network bandwidth.

PING (Packet InterNet Groper) - A command followed by an IP address that sends an ICMP (Internet Control Message Protocol) echo request to the host specified by the IP address. If the targeted host has an active IP network connection, it will return the message. The length of time this process takes is listed by the local user system as an indication of network speed.

Plain-B Wire Connector - Most commonly referred to as beans. A splicing connector used to splice twisted-pair telephone wire. The connectors are crimped onto the wires to be spliced. Inside the connector are teeth, which pierce the vinyl insulation of the wire to make a good connection.

Plant - A reference to telephone company equipment, poles, cable vaults, cable, central offices, and transmission equipment.

Plant Test Numbers - Telephone numbers that when dialed provide a test tone or access to other testing resources, such as a quiet-line or automatic-number identification. Plant test numbers are used by telephone company personnel and are not given to the public.

Plenum - A reference to telephone, communications, or electrical wire that is insulated with polyvinylidene diflouride. It is made with this substance because it does not emit poison gasses when it burns, like PVC (polyvinyl chloride) does (PVC produces chlorine gas when burned). It gets its nickname because it is permitted to be placed in air ducts or plenum spaces in buildings. Plenum wiring or cable is typically three times the cost of PVC jacketed/insulated types.

Plenum Cable - See Plenum.

Plesiochronous -“Almost in time.” Plesiochronous networks are those that the telephone companies use to synchronize T1 and T3 carrier signals. The electronic transport equipment at each end of the transmission does not get timing from the same source (thus being synchronous), but the timing of each individual device is very close. Stratum One clocks are used in this type of communications equipment, which provide a steady timing for each end to transmit and receive signals by. Transmissions between SONET networks over a long-haul circuit could be considered plesiochronous.

PMD (Physical Medium Dependent Sublayer) - The ATM physical layer is divided into two parts (sublayers), the TCS (Transmission Convergence Sublayer) and the PMD. The PMD is the lowest physical layer, and it determines the types of connectors and the medium used for transmission. It also interfaces the line-coding technique, such as SONET OC-1 or STS-3c/STM1 with the TCS sublayer. See also TCS.

POH (Path Overhead) - The overhead that is added to a signal to allow a transport network to carry it.

Point of Interface -See Point of Presence.

Point of Presence - In Internet terms, an access node equipped with modems that allow customers to dial into their ISP. National Internet service providers try to have a point of presence within every area code that a customer could try to access their network through.

Point to Point Usually a reference to a private-line circuit that is leased from the telephone company. It can also be a reference to a switched service, like a plain telephone line where communications links are switched from one point to another, depending on the number dialed.

Pointing Stick - An alternative to the use of a mouse on portable laptop computers. The pointing stick is usually located in the middle of the laptop’s keyboard and resembles a pencil eraser. Other laptop mouse alternatives include the roller ball and the touch/track pad.

Point-to-Point Protocol (PPP) - Also known as dial-up IP. PPP is the successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network-layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP. See also CHAP, LCP, NCP, PAP, and SLIP.

Poisson Distribution - A mathematical formula that used to be used in traffic engineering for calculating the probability of blocked calls in a telephone network. Now we have computer programs (CTI) that provide graphs of trunk groups and their usage. The graphs are much easier to use. When customers don’t have expensive software to manage their telephone networks, they simply add more lines when they get complaints of busy signals.

Poison Reverse Updates - Routing updates that explicitly indicate that a network or subnet is unreachable, rather than implying that a network is unreachable by not including it in updates. Poison reverse updates are sent to defeat large routing loops.

Polarity - A reference to positive or negative voltage potential, or to the polarization of an antenna or dish-type antenna.

Polarization - The pointing of a microwave dish antenna so that the transmission dispersion is in a vertical or horizontal pattern. The headlights on cars are polarized in a horizontal manner so that the light dispersion is spread across the horizontal surface of the road. The other kind of polarization is vertical, where the transmission dispersion is in an up-anddown pattern. The two antennas or dishes used in a point-to-point application need to be polarized the same way.

Pole Attachment - A lease from a utility company (usually power) that permits a telecommunications company to attach their cable facilities to power poles. For more photos of different aerial-attachment hardware, see B Washer, Strand Clamp, Guy Thimble, and Johnny Ball.

Policy-Based Routing - A routing scheme that forwards packets to specific interfaces based on user-configured policies. Such policies might specify that traffic sent from a particular network should be forwarded out one interface, while all other traffic should be forwarded out another interface.

Poll/Final - A bit in the control byte of an X.25 frame. When the two devices on the ends of an X.25 link lose track of where the other is in receiving the transmission, a poll bit will be sent inside the control byte of the next frame. The device on the other end will send a frame with the poll bit set to one that is followed by the number of the nest frame desired (binary one through seven).

Polling - A LAN line-sharing method where a primary network device “asks” all other devices attached to a network if they have data to transmit. This gives each device a fair chance to use the network media. Token ring is a polling-type of network protocol. In contrast, Ethernet is not a polling protocol.

PON (Passive Optical Network) - A fiber-optic-based transmission network that contains no electronic devices that require external power. Passive optical networks use the physical characteristics of light to separate different carriers or colors of light. These types of optical networks are relatively inexpensive to implement and maintain, compared to their active counterparts.

POP - 1. Post Office Protocol. POP is a daemon program that runs on SMTP (Simple Mail Transfer Protocol) servers acting as if it were a mail server in itself. It processes SMTP client’s mail-retrieval requests. Without POP, clients cannot receive mail unless they are online at the time it is sent to them. POP and SMTP have been further enhanced with the MIME (Multipurpose Internet Mail Extensions) standard, which allows users to attach files to their text messages. 2. Point of Presence. See Point of Presence.

Port Number - An identification/reference system that TCP uses to identify applications that it will packetize and send. The port numbering system allows TCP to know how to handle the application for which it is performing communications. A field of the TCP header is dedicated for the port number being utilized in the current transmission. All applications have standard port numbers assigned to them. Standard port numbers are called well-known port numbers. Some examples of well-known port numbers are: 20�FTP-data, 21�FTP-control, 23�Telnet, 25�SMTP, 53�Domain Name, 161�SNMP agent, and 162�SNMP manager. Further, port numbers can be altered. Network administrators do not always utilize the standard application’s port numbers within intranets as a security design measure. See also Socket.

Portability -A reference to the ability to change telephone companies and take your phone numbers with you.

POST (Power On Self Test) - A set of hardware diagnostic instructions (usually retrieved from ROM) that operates within routers and other traffic-management devices when they are first turned on.

Post Office Protocol POP - is a daemon program that runs on SMTP (Simple Mail-Transfer Protocol) servers acting as if it were a mail server in itself. It processes SMTP client’s mail-retrieval requests. Without it, clients cannot receive mail unless they are online at the time it is sent to them. POP and SMTP have been further enhanced with the MIME (Multipurpose Internet Mail Extensions) standard, which allows users to attach files to their text messages.

Post, Telephone, and Telegraph (PTT) - The general name for the communications service providers in countries that have not yet allowed this service to be controlled by a nongovernment agency or corporation. PTTs are often extensions of the postal service, where telecommunications service is a branch from the original letter-mail and telegraphy service. South American and African countries commonly have a PTT agency, rather than multiple telephone companies.

PostScript - A printer interface communication method developed by Adobe. PostScript incorporates ASCII code and translates it to instructions for printer devices. PostScript is also a reference to an Adobe standard in text fonts.

POT - 1. Abbreviation for potentiometer. Also known as a variable resistor. Many electronic control knobs are connected to variable resistors. Variable resistors are usually made from carbon film. For a photo and schematic symbol of a potentiometer, see Variable Resistor. 2. Plainold telephone, a reference to standard switched residential and business telephone lines.

Potato - (slang) Another name for an aerial service-wire splice. Also called a football. For a photo, see Aerial Service-Wire Splice.

Potential - A voltage difference. Potential is a voltage from one point to another. The voltage potential of a POTs telephone line is �52 volts from ring to tip.

Potentiometer (Pot) Also known as a variable resistor. Many control knobs are connected to variable resistors. Most volume controls are variable resistors. For a photo and schematic symbol of a potentiometer, see Variable Resistor.

POTS (Plain Old Telephone Service) - A telephone line, with a telephone number, like the standard ones subscribed to by residences and many small businesses.

Pound Key - The button on a telephone dial pad with the # on it.

Power - Current multiplied by voltage. Power is measured in watts. If you use a certain amount of wattage over a certain period of time, then you have used energy. Energy is equal to watts multiplied by time, and the unit is joules.

Power On Self Test (POST) - A set of hardware diagnostic instructions (usually retrieved from ROM) that operates within routers and other traffic-management devices when they are first turned on.

Power Supply - A device that converts 120-V or 220-V standard AC power to a voltage that can be useful for an electronic system.

PPP (Point-to-Point Protocol) - Also known as dial-up IP. PPP is the successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network-layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP. See also CHAP, LCP, NCP, PAP, and SLIP.

PPSN (Public Packet-Switched Network) - A reference to Frame Relay.

PPSS (Public Packet-Switched Service) - A reference to Frame Relay.

PRAM (Parameter RAM) - A small battery-powered unit of RAM used in computers to store basic user settings, such as time and date.

Preamplifier - An amplifier designed to amplify the voltage level of a very small signal so that it can be fed to a power amplifier, which amplifies the current aspect of the signal so that it is powerful enough to drive the signal current through a loudspeaker or other device.

Predictive Dialing - Another term for auto dialing or progressive dialing. Instead of telemarketers dialing digits through a list or phone book all day long, the numbers are entered into a predictive dialer system. The system then dials the numbers; when a call is answered the predictive dialer transfers the call and the associated information to the computer screen of the appropriate telemarketer.

Premises Equipment - Also called CPE (Customer Premises Equipment). Telephones, wiring, answering machines, CSU/DSUs, and anything else you might find on the customer side of the network interface.

Premises Wire - The wiring on the customer side of the communications company’s demarcation point (NI, Network Interface). The premises wire is owned by the customer and is the customer’s responsibility to maintain. Many communications companies sell maintenance contracts, which enable them to troubleshoot and repair the telephone wire within your home or business, at no extra charge. Typical maintenance contracts are about $2.00 per month.

Prepaid Phone Card - A card that comes with an 800/888 number that the card owner dials to reach a network that allows them to dial anywhere they like. The service is good for the amount of time that the prepaid phone card says on its face. Prepaid calling cards are becoming bigger and bigger, especially because they don’t cost much more than third-party billing to your home number when you are out of town. Typical prepaid calling cards have a rate of 30 to 35 cents per minute, flat rate, no matter when or where you call within the continental U.S. The way the system works is that a calling card company sets up a data base with card numbers in it and connects it to a callingcard platform. A calling-card platform is a computer that receives a phone call and prompts a caller to enter their calling-card number and the telephone number that they wish to dial. The calling-card platform then checks the card number to see how many minutes it has left on it (it sometimes tells the customer with a recorded message). If time left on the card, the system then dials the number on an outgoing trunk to connect the call. In reality, two long-distance calls are made, one to the calling card platform and one to the number being dialed by the customer.

Prepay - A reference to a coin-operated telephone that requires a coin to be inserted before a number is dialed.

Presentation Layer - A layer in a communications protocol model. In general, the presentation layer performs the function of encoding and decoding the data to be transmitted within the communications protocol. The latest guideline for communications protocols is the OSI (Open Systems Interconnect). It is the best model so far because all of the functions work independently of each other. For diagrams relating to the OSI, see Open Systems Interconnection and OSI Standards.

Pressure Cable - Telephone cable that is equipped with air-pressure equipment. In many cables, nitrogen is used instead of air because it is noncorrosive (air contains humidity, which corrodes copper pairs). Nitrogen is pumped into the cable and the pressure is monitored. If the cable is cut, the pressure drop notifies the telephone company of the cable problem and the nitrogen rushing out of the cable helps prevent any water from entering.

Presubscription - When a customer calls a local telephone company and orders a new phone line, they are asked which long-distance company they would like to subscribe to. When the customer tells them, then the telephone company sets the customers line up in translations so that when the customer dials 1 as a first digit, they are connected directly to the long-distance company that they selected.

Prewire - To install standard wiring into a building or space while it is being constructed. Standard building wiring is that all wiring from each jack terminates to a common location, usually called the telephone closet. Prewiring of buildings is common for telephone and CAT 5 computer LAN wiring.

PRI (Primary Rate Interface) - One of two ISDN (Integrated Services Digital Network) circuit sizes. ISDN first evolved in 1979. It brings the features of PBX systems and high-speed data-transfer capability to the telephone network. The only thing that complicates ISDN is the many available features. The two kinds of ISDN lines are Primary Rate Interface (PRI) and Basic Rate Interface (BRI). ISDN has two types of channels within an ISDN circuit. The B (bearer) channel carries the customer’s communications and the D (data) channel provides control and signaling for the B channels. The BRI ISDN line has two B channels and one D channel. A PRI has 23 B channels and one D channel. The separate control of the ISDN line over the D channel is what enables the broad flexibility and features available with ISDN. When you are talking or sending a data transmission over an ISDN line, the voice and/or data is carried by the B channels. While you are talking on your ISDN line, you can still dial digits (signal the central office) to change or alter the state of your service because of the separate D channel. For example, imagine you want to arrange a meeting with a client. You dial the client’s telephone number on your ISDN telephone to reach the client. While you are speaking with the client, you can dial up an Internet access on your computer and put two baseball tickets in at the ticket counter while on the same BRI line. Then you can fax your client directions by downloading a map provided by the baseball ticket office, disconnect and redial your client’s fax number. All of this occurs while talking to your client the entire time. Through the advanced convenience and flexibility of ISDN, you can send different types of data and messages to different places at the stroke of a few buttons, and at a much faster speed than a regular telephone line. If you are interested in ISDN, call your local phone company. They can help you decide on what kind of terminal adapter (equipment that connects your computer and phone equipment to the ISDN line) to buy and what kind of features to subscribe to. ISDN is not yet available everywhere. For a diagram that compares an ISDN BRI and ISDN PRI circuit, see Integrated Services Digital Network.

Primary Rate Interface (PRI) - See PRI.

Prime Line - A key telephone system and hybrid key telephone system feature. The feature enables a user to select the line that a key system connects to a telephone set to when the receiver is lifted. If you don’t want people in the office using the main telephone line in the office to make outgoing calls, then don’t select that line as a prime line for any of the telephone extensions.

Primitives - The IP layer in the TCP/IP layer operates with the smallest actions or instructions that can be initiated. These small-size actions are called primitives. Some examples of standard IP primitives are: Receive Datagram, Send Datagram, Select Source Address, Find Max Datagram Size, Advise on Delivery Success, Send ICMP (Internet Control Message Protocol) message, and Receive ICMP Message.

Print Server - A computer dedicated to fielding, managing, and executing (or sending for execution) print requests from other devices (such as servers or workstations) on its network.

Printed Circuit Board (PC Board)- The green- or brown-colored board that has copper-conductive tracks etched onto its surface. Electronic components are soldered onto these boards by hand or by a method called flow soldering. Some PC boards are layered or sandwiched, with conductive tracks inside them and on both sides.

Private Branch Exchange (PBX) - A telephone system used to maximize use of telecommunications services purchased from a telecommunications company. A PBX simply takes telephone lines from the outside world and makes them accessible to extensions within a certain building, home, or office. PBX systems are available in many sizes, with many software and feature options. PBX features include call forwarding, speed dial, internal/external paging, and call-detail recording (call accounting). The larger PBX manufacturers are AT&T, Northern Telecom, Siemons, Toshiba, Iwatsu, NEC, and Rolm. PBX systems have six main parts: the cabinet-backplane (also called a KSU, Key Service Unit), the station/telephone connectivity, the trunk/telco connectivity, the power supply, the telephones/extensions, and the administrative access. • Cabinet/KSU The cabinet of the system contains the electronics that make the PBX system work. The backplane (for a photo of a backplane see, Backplane) that interface cards plug into is located here. The CPU or core processor (for a photo, see CPU) is located in here as well. Many PBX cabinets are designed to allow for additional circuit cards (trunk interfaces/trunk cards and telephone interfaces/station cards) to be added or plugged in later on as the system grows. These spaces are called expansion slots. • Station-telephone connectivity This wiring runs from each office or telephone location to the location of the PBX cabinet. Four-pair wiring is most popular because it is inexpensive and contains enough wire to add additional lines or telephones in the future (or additional wire if one or two should go bad). This wiring is installed in a “home run” method, which means that every wire installed runs directly from a jack (usually an RJ-11) directly to the location of the PBX cabinet. Next or near to the PBX cabinet, the individual pairs are neatly terminated and labeled on 66M150 or AT&T 110 (one-ten) blocks. • Trunk-telco connectivity This is similar to the station connectivity, but it needs to be separately labeled from the station connectivity. This is the point where cross connects will be run from the telephonecompany demarcation (or NI, Network Interface) to your PBX system. • Power Supply The power source for the phone system is a very important consideration. If the power is interrupted, the PBX system will cease to function unless its power supply is incorporated with a UPS system or rectifier/battery system. The best way to go for power is the rectifier with battery back-up (a heavy-duty UPS system especially designed for telephone equipment). Different PBX systems can be ordered to run on 120V AC or –48V DC. The –48-V DC system is designed to be powered by a rectifier. The 120V AC system is designed to run on standard outlet power or a UPS system. • Telephones The telephones for each individual PBX system will work only with that system. They will not work if they are plugged into a regular telephone line. Each phone will determine what features can be implemented. The features are enabled or disabled by the programming or administration done on the PBX system. Some systems have an interface (SDI, Serial Data Interface) for a computer or terminal and some are simply programmed by using the telephone stations. • Administrative Access The administrative function of a PBX system can be performed by the user or a telephone-equipment service company. The administrative responsibilities of a PBX system include changing extension numbers, moving phones, changing name displays, and other programming of the system. It also includes maintaining the CallDetail Reports (CDR) of the system. The call-detail reports are reports output by a call-accounting system, which is offered as an extra by virtually every PBX manufacturer. Call-detail reports summarize numbers dialed, length of calls, and incoming calls, caller ID, and their duration.

Private Carrier - A telecommunications company not regulated by the rulings of the PUC; however, they are regulated by the Telecommunications Act of 1996.

Private Line - Also called a leased line or leased circuit. A leased line is a telephone service that is permanently connected from one point to another. Leased circuits include 56K analog and DS1. A leased circuit acts like a pipeline that carries data from one point to another. If you put a bit in one side, the same bit pops out on the other side. It can carry data across town, across the country, or around the world. Leased lines are relatively expensive. Because leased lines have been offered, new services, such as frame-relay and switched 56K services have evolved. Frame relay does the same job as a private line, except that it is not isochronous (real time), and you need a private line to put your frame-relay service on. Frame relay is a cost-effective solution for longhaul/long-distance data-transfer applications.

Programmable Read-Only Memory (PROM) - Electronic memory comes in two families, ROM (Read-Only Memory) and RAM (RandomAccess Memory). Memory devices are made from two different technologies: Bipolar (TTL) and MOS (Metal-Oxide Semiconductor). Memory is stored by a technique called “writing” and is retrieved by a technique called “reading.” ROM devices can only be read and are programmed during manufacture. PROM devices can be programmed at a later date by an electronics reseller or electronic assembler for a special application using special equipment. Special ROM devices called EPROMs (Erasable Programmable Read Only Memory) can be electronically erased and re-used. RAM has read and write capability. The term random access means that any memory address can be read in any order at any time. The two types of RAM are static and dynamic. Static RAM can hold its memory even when power is removed. Dynamic RAM needs constant power to refresh its memory. For a diagram of the different types of dynamic memory, see Memory.

PROM (Programmable Read-Only Memory) - See Programmable ReadOnly Memory.

Prompt - A message from a computer or interactive device that indicates that it is time for a user to input a decision, choice, or other response. Many PBX systems have software that is a “prompt response” style of programming. For example, when the user inputs extension 255, the system responds with “Hands Free?” The user then responds with “YES” or “NO.”

Prompt, Response IO - See Programming PBX.

Propagation Time - The time for an electrical, optical, or radio signal to travel from one point to another.

Propagation Velocity - The speed that a communications signal travels from one point to another. Electromagnetic waves (radio), electricity, and light approach 300,000,000 meters per second, which is about 160,000 miles per second.

Proprietary - Specially made. All PBX equipment and other premises telephone equipment is proprietary. Northern Telecom telephones will only work with Northern Telecom PBX systems. The same goes for Lucent, Mitel, and other specialized telephone equipment manufacturers.

Protector Block - A block that has many lightning protectors, used to terminate telephone cables. A protector is a device used in telephone company network interfaces that provides an easier path for lightning to travel to ground, compared to a telephone user or inside wiring. Before lightning protectors, houses sometimes burned down because of lightning striking the telephone lines. The two types of lightning protectors are carbon and gas. The carbon protectors are simply a piece of carbon that connects tip and ring to ground. The gas protectors are the same, only they are a gas instead of solid carbon. The good thing about gas lightning protectors is that after they are hit by lightning, they do not need to be replaced.

Protocol - The organized processes and rules that communications equipment use to transfer bits and bytes (data). The many communications protocols and layers of protocols that carry other protocols (called protocol stacks), include ISDN, Ethernet, token ring, POTS signaling, DS1, ATM, frame relay, and SONET.

Protocol Analyzer - A test device that can plug into a hub or communications port on a LAN and monitor any address on that LAN at any protocol level. Protocol analyzers are useful for verifying that an address is good through a network. Most networks are not so complex as to need a protocol analyzer to troubleshoot them.

Protocol Converter - A network device or software that converts packeted or framed data from one format to another. This is accomplished by recognizing the initial packet or frame format, removing the data from the packet or frame, and adding new frame or packet headers to the data that are conforming to the new protocol. In older network architectures, protocol converters were an individual entity on a network that existed as a device or software. In newer network architectures, routers and data switches perform the protocol-conversion function.

Protocol Data Unit - Another variation of the term Packet Data Unit (PDU). See Packet Data Unit.

Protocol Stack - A set of related communications-control programs (software) that work together and as a group. Each individual software program is called a protocol. A protocol stack can control communication processes at some or all of the seven layers of the OSI reference model. Not every protocol stack covers all layers of the model. Often, a single protocol in the stack will control a number of layers at once. Some protocols within a stack exist only to provide services or specific support functions for other protocols. TCP/IP is a typical protocol stack and ARP (Address-Resolution Protocol) is one of the protocols within that stack.

Protocol Translator - A network device or software that converts packeted or framed data from one format to another. This is accomplished by recognizing the initial packet or frame format, removing the data from the packet or frame, and adding new frame or packet headers to the data that are conforming to the new protocol. In older network architectures, protocol translators were an individual entity on a network that existed as a device or software. In newer network architectures, routers and data switches perform this function.

Provisioning - A term that refers to the process of allocating copper pairs, central-office ports/equipment, and programming of central-office equipment. This is what happens before a telephone company network technician installs a telephone service, such as a POTS line or a high-capacity digital service line.

Proxy- An entity or device that, in the interest of efficiency, essentially stands in for another entity.

Proxy Address-Resolution - Protocol A variation of the AddressResolution Protocol (ARP) in which an intermediate device (for example, a router) sends an ARP response on behalf of an end node to the requesting host. Proxy ARP can lessen bandwidth use on slow-speed WAN links. See also Address-Resolution Protocol.

Proxy Server - A network server that is loaded with software and equipped with hardware to interface a LAN, MAN, or WAN to the Internet. Proxy servers make up the hardware part of a firewall, which is software that protects the LAN’s interworkings from being accessed by strangers/unwanteds/hackers on the outside. Although firewalls are expensive and abound everywhere, hackers still manage to get through them.

PSC (Public Service Commission) - See Public Service Commission.

PSE (Packet Switch Exchange) - Essentially, a data packet network access or relay device in an X.25 network, such as the Nortel DPN100.

PSI - 1. Pounds Per Square Inch, a unit of air pressure. Telephone cables (pulp-insulated cables) that are pressurized with nitrogen are kept at a pressure of 10 to 15 PSI near the central office. 2. Packet-Switching Interface gives a customer a means to connect with a packet switching network, such as frame relay.

PSK (Phase-Shift Keying) - See Phase-Shift Keying.

PSN (Packet Switching Network) - A name sometimes used in place of Public Data Network (PDN). PSNs connect to users via X.25 or frame relay. The Internet is also a type of PSN using TCP/IP packets.

PTN (Public Telephone Network) - Also called PSTN (Public Switched Telephone Network) and PSN (Public Switched Network). The telephone network that we know today provides us with an openended dial tone, the ability to dial a telephone anywhere we wish.

PTT (Post, Telephone, and Telegraph) - The general name for the communications service providers in countries that have not yet allowed this service to be controlled by a nongovernment agency or corporation. PTTs are often extensions of the postal service, where telecommunications service is a branch from the original letter-mail and telegraphy service. South American and African countries commonly have a PTT agency, rather than multiple telephone companies.

Public Address System (PA System) - There are different types of PA systems. High-fidelity PA systems are used in studio recording andconcert productions and simple systems are used for paging/intercom and loudspeaker systems. The two main components of a PA system are the amplifier and the speakers. Different components can be attached to the input of a PA system. The PA amplifier input is a high-impedance circuit (this means that it does not draw a lot of electrical current from the source, thus transferring maximum voltage). Common source (signal input) devices include microphones, musical instruments (electric), and the paging output of telephone systems. If an amplifier is used to drive external speakers (rather than the ones inside telephones), then it is called an external paging amplifier or PA amplifier. The question that most people have about PA amplifiers is which one to buy. The answer is that the majority of the cost in a paging system is usually the wiring and the speakers. Most paging amplifiers are equipped with multiple inputs so that different areas or “zones” can be paged individually. The factor that affects the price of a PA amplifier the most is the power output rating. The more power that an amplifier is capable of pushing through the speaker network, the more expensive it is. A good general rule is to allot 5 watts of RMS power per speaker in an office environment and 10 to 25 watts of RMS power per speaker in an industrial environment. The crucial factor in designing the speaker network is that the impedance (Z) of all the combined speakers matches (or is equal to) the output impedance of the PA amplifier, which is usually 8 or 16 Ω. If the impedance is not matched, there is a possibility of over working the amplifier and causing it to fail or having a poor performance and sound quality. Quality PA amplifiers/paging amplifiers have instructions on how to wire and arrange the connections of speakers. See the drawing for an example of how a 16-Ω output amplifier is matched with four 16-Ω speakers.

Public Data Network (PDN) - Also called PSDN (Packet-Switched Data Network) or PSN (Packet-Switching Network), a reference to public-network X.25 services. This type of service, when available, eliminates the need for a private line connection beyond the local telephone company, which is cost efficient for users who send very little data.

Public Service Commission (PSC) - The watchdog for the Public Utilities Commission. The Public Utilities Commission regulates the telecommunications companies under federal judgments (which change from time to time), and other utility companies. For a telecommunications company to be regulated, it must have a minimum number of customers. All the RBOCs are regulated by the PUCs of their area.

Public-Switched Digital Service - A general name for switched 56K service from a local or long-distance telephone company.

Public Switched Network (PSN) - Also called PTN (Public Telephone Network) and PSTN (Public Switched Telephone Network). The telephone network that provides open-ended dial tone, the ability to dial a telephone anywhere we wish.

Public Utilities Commission (PUC) - The governing body of regulated public utility service companies and the Public Service Commission (PSC) that watches over them. The Public Utilities Commission regulates the telecommunications companies under federal judgments (which change from time to time) and other utility companies. For a telecommunications company to be regulated, it must have a minimum number of customers. All the RBOCs are regulated by the PUCs of their area.

PUC (Public Utilities Commission) - See Public Utilities Commission.

Pulling Strength - A cable specification. The maximum pulling force that can be applied to a strength member of a cable without voiding the warranty.

Pulp Cable - Telephone cable used in outside plant applications that uses paper insulation on the twisted copper pairs. The other kind of widely used cable is pick cable, which has color-coded plastic-insulated pairs. For a photo of pulp-insulated cable, see Lead Jacket.

Pulse Amplitude Modulation (PAM) - See PAM.

Pulse-Code Modulation (PCM) - See PCM.

Pulse Density - See Ones Density.

Punch-Down Block - A 66M150 block, AT&T 110 (one ten) block, crone block, or other wire-terminating device. A punch-down block provides connections to neatly connect and label wires.

Punch-Down Tool - A tool that is used to terminate telephone wires onto punch-down blocks.

PVC - 1. Permanent Virtual Circuit. A logical connection made between two end-communicating devices on a packet network. PVCs are set up by using Committed Information Rates (CIRs), as in frame relay. 2. The substance with which common telephone wire is insulated. PVC wire is available in many colors. The other more expensive option for telephone wiring is Plenum. Plenum wiring is required in many newer buildings because when it burns, it does not emit poison gasses (PVC produces chlorine gas when burned). Plenum wiring is made from polyvinylidene diflouride, and costs about three times as much as PVC does.

PVDF (Polyvinyl Diflouride) - Better known as plenum wire.