An error checking method where each byte of data in memory is accompanied by a ninth bit used to check for corrupted data.
PCI (Peripheral Component Interconnect) bus
Introduced in 1995 with the Pentium processor, it connects the CPU, memory, and peripherals to a 32-bit working at 33 MHz. PCI supports bus mastering, IRQ steering, and Plug-and-Play. Later versions defined 64-bit operation and 66 MHz clock but were not widely adopted on desktop PCs.
PCIe (PCI Express)
An expansion bus standard using serial communications. Each device on the bus can create a point-to-point link with the I/O controller or another device.
The link comprises one or more lanes (x1, x2, x4, x8, x12, x16, or x32). Each lane supports a full-duplex transfer rate of 250 MBps (v1.0), 500 MBps (v2.0), or 1 GBps (v3.0). The standard is software compatible with PCI, allowing for motherboards with both types of connectors.
PGA (Pin Grid Array)
A CPU socket form factor where pins are located on the bottom of the processor to fit in the matching holes in the motherboard socket. PGA-type sockets are still used by AMD but Intel has switched to Land Grid Array (LGA), where the pins are located on the socket rather than the chip.
An air handling space, including ducts and other parts of the HVAC system in a building.
A grade of cable that does not give off noxious or poisonous gases when burned. Unlike PVC cable, plenum cable can be run through the plenum and firebreak walls.
PoE (Power over Ethernet)
Specification allowing power to be supplied via switch ports and ordinary data cabling to devices such as VoIP handsets and wireless access points. Devices can draw up to about 13 W (or 25 W for PoE+).
SEE ALSO Ethernet over Power
In TCP and UDP applications, a port is a unique number assigned to a particular application protocol (such as HTTP or SMTP). The port number (with the IP address) forms a socket between client and server. A socket is a bi-directional pipe for the exchange of data. For security, it is important to allow only the ports required to be open (ports can be blocked using a firewall).
A hardware connection interface on a personal computer that enables devices to be connected to the computer.
Port forwarding means that a router takes requests from the Internet for a particular application (say, HTTP/port 80) and sends them to a designated host on the LAN.
The number between 0 and 65535 assigned to each type of network application so that the transport layer can identify it.
Port triggering is used to configure access through a firewall for applications that require more than one port. Basically, when the firewall detects activity on outbound port A destined for a given external IP address, it opens inbound access for the external IP address on port B for a set period.
POST (Power-On Self-Test)
A hardware checking routine built into the PC firmware. This test sequentially monitors the state of the memory chips, the processor, system clock, display, and firmware itself. Errors that occur within vital components such as these are signified by beep codes emitted by the internal speaker of the computer.
Used when an existing switch does not support PoE. When a device is connected to a port on a PoE switch, the switch goes through a detection phase to determine whether the device is PoE-enabled. If not, it does not supply power over the port and therefore does not damage non-PoE devices. If so, it determines the device's power consumption and sets the supply voltage level appropriatel
The maximum power output available from a PC power supply, measured in watts, calculated as voltage multiplied by current.
PPP (Point-to-Point Protocol)
Dial-up protocol working at layer 2 (Data Link) used to connect devices remotely to networks.
Often used to connect to an ISP's routers and out to the Internet. PPPoE (PPP over Ethernet) or PPPoA (PPP over ATM) are used to provide broadband connections (over DSL or cable Internet, for instance).
IP addresses in ranges defined by RFC1928 which are not allowed to route traffic over the Internet, with those addressed being confined to private LANs.
PRL (Preferred Roaming List)
A database built by CDMA service carriers to indicate which radio bands should be used when connecting to a cell tower.
A server that mediates the communications between a client and another server. The proxy server can filter and often modify communications as well as provide caching services to improve performance.
PSE (Power Sourcing Equipment)
Network switches that provide power through the Ethernet cable to connected devices.
PSTN (Public Switched Telephone Network)
National telecommunications systems have evolved and combined over the years to create a global (and indeed extra-terrestrial) communications network This is referred to as the Public Switched Telephone Network (PSTN) but it is capable of carrying more than simply voice-call services. The basis of PSTN is a circuit-switched network, but the infrastructure can also carry packet-switched data services.