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P Packet The fundamental unit of information transmitted over a network or over a digital communication link. Packets usually contain a header with control information about the packet type, source address, and destination address. They can also contain error-checking information. Packets have a logical structure based on the protocol used, but the general structure of a packet includes a header followed by a payload (data) and an optional trailer (footer). Packets can also have different sizes and structures depending on the underlying network architecture. A packet might also be called a datagram, a frame, or a cell. Packet assembler/disassembler (PAD) A telecommunications device that breaks a data stream into individual packets and formats the packet headers for asynchronous transmission over an X.25 network. It also accepts packets from the network and translates them into a data stream. Packet assembler/disassemblers (PADs) are a form of data communications equipment (DCE) for connecting asynchronous data terminal equipment (DTE) such as computers and dumb terminals to the X.25 packet-switching service. Packet filtering The process of controlling the flow of packets based on packet attributes such as source address, destination address, type, length, and port number. Packet forwarding The process of a networking component accepting a packet and transmitting it to its destination. For example, a router receives packets from hosts on one attached network and forwards them to hosts on another attached network or to another router for further forwarding. How a packet is forwarded is based on a comparison of the packet’s destination address with the routing table stored in the router. Each act of forwarding performed by a router is called a hop across the internetwork. Packet switching The process by which a networking or telecommunications device accepts a packet and switches it to a telecommunications device that will take it closer to its destination. Packet switching allows data to be sent over the telecommunications network in short bursts or "packets" that contain sequence numbers so that they can be reassembled at the destination. Wide area network (WAN) devices called switches route packets from one point on a packet-switched network to another. Data within the same communication session might be routed over several different paths, depending on factors such as traffic congestion and switch availability. Packet switching is the transmission method used for most computer networks because the data transported by these networks is fundamentally bursty in character and can tolerate latency (due to lost or dropped packets). In other words, the transmission bandwidth needed varies greatly in time, from relatively low traffic because of background services such as name resolution services, to periods of high bandwidth usage during activities such as file transfer. This contrasts with voice or video communication, in which a steady stream of information must be transmitted in order to maintain transmission quality and in which latency must remain minimized to preserve intelligibility. The Internet is the prime example of a packet-switched network based on the TCP/IP protocol suite. A series of routers located at various points on the Internet’s backbone forward each packet received on the basis of destination address until the packet reaches its ultimate destination. TCP/IP is considered a connectionless packet-switching service because Transmission Control Protocol (TCP) connections are not kept open after data transmission is complete. X.25 public data networks are another form of packet-switching service, in which packets (or more properly, frames) formatted with the High-level Data Link Control (HDLC) protocol are routed between different X.25 end stations using packet switches maintained by X.25 service providers. Unlike TCP/IP, X.25 is considered a connection-oriented packet-switching protocol because it is possible to establish permanent virtual circuits (PVCs) that keep the logical connection open even when no data is being sent. However, X.25 can be configured for connectionless communication by using switched virtual circuits (SVCs). An X.25 packet-switched network typically has a higher and more predictable latency (about 0.6 seconds between end stations) than a TCP/IP internetwork. This is primarily because X.25 packet switches use a store-and-forward mechanism to buffer data for transmission bursts, which introduces additional latency in communication. In addition, X.25 uses error checking between each node on the transmission path, while TCP/IP uses only end-to-end error checking. Frame relay (also called fast packet switching) is another connection-oriented packet-switching service that gives better performance than X.25. It does this by switching packets immediately instead of using the store-and-forward mechanism of X.25 networks. Frame relay also eliminates flow control and error checking to speed up transmission. This is possible because frame relay networks use modern digital telephone lines, which are intrinsically much more reliable than the older analog phone lines on which much of the X.25 public network still depends. Frame relay supports only connection-oriented PVCs for its underlying switching architecture. Finally, Asynchronous Transfer Mode (ATM) is another packet-switching service in which small fixed-length packets called cells are switched between points on a network. Packet-switching services Telecommunications services provided by telcos and long distance carriers that route packets of data between local area networks (LANs) in diverse geographical locations to form a wide area network (WAN). Packet-switching services are used to connect multiple LANs into a point-to-multipoint configuration, usually called a multipoint WAN. Password Authentication Protocol (PAP) A clear-text authentication scheme used in Point-to-Point Protocol (PPP) connections over WAN links that is outlined in Request for Comments (RFC) 1334. Password Authentication Protocol (PAP) is not a secure form of authentication because the user’s credentials are passed over the link in unencrypted form. For this reason, Challenge Handshake Authentication Protocol (CHAP) or some other authentication protocol is preferable if the remote client supports it. If the password of a remote client using PAP has been compromised, the authentication server can be attacked using replay attacks or remote client impersonation. Patch cable A short cable, usually unshielded twisted-pair (UTP) cabling, that connects a port on a patch panel to a port on a hub or a switch. Patch cables are usually terminated at both ends with RJ-45 connectors. Cable vendors usually supply patch cables in fixed lengths such as 1, 3, 6, 10, 25, 50, and 100 feet, and also in custom lengths. Patch cables usually come in various colors, which can be helpful in organizing the cabling joining devices on your equipment racks and avoiding "spaghetti." You should use patch cables that meet the requirements of the equipment you are using. Category 5 patch cables, which are certified to 100 MHz, or enhanced category 5 patch cables, which are certified to 350 MHz and higher, are generally recommended. Patch panel A rack-mounted panel with a series of connectors that provides a branching-out point for network cabling to leave the wiring closet and make horizontal runs to wall plates in the work areas. Path The route that a user or application follows to locate a file in a file system, an object in a directory, a server on a network, or some other kind of resource in a hierarchical system. A path to an object can be one of the following:
Peer-to-peer network A network in which the computers are managed independently of one another and have equal rights for initiating communication with each other, sharing resources, and validating users. Permanent virtual circuit (PVC) A form of telecommunications service for wide area networks (WANs) that provides a dedicated switched circuit between two nodes in a circuit-switched network. Ping Stands for Packet Internet Groper, a TCP/IP utility that verifies the integrity of a network connection with a host on a TCP/IP network. The ping command is one of the first commands to use to troubleshoot communication problems on a TCP/IP network. Plain Old Telephone Service (POTS) The basic analog telecommunications service provided by a local telco. Plain Old Telephone Service (POTS) was the only type of telephone service until the 1970s. Point of presence (POP) The local access point for an Internet service provider (ISP). A point of presence (POP) consists of the high-speed telecommunications equipment and technologies that enable users to connect to the Internet via their ISP. The POP might include call aggregators, modem banks, routers, and high-speed Asynchronous Transfer Mode (ATM) switches. A POP has one or more unique IP addresses plus a pool of assignable IP addresses for its permanent and dial-up clients. The actual POP for an ISP might be located within the telecommunications facility of a telco or a long-distance carrier. The ISP rents or leases space in the facility to install the routers and access servers that provide Internet connectivity for clients and for the equipment that provides the ISP with a high-speed T1 or T3 connection to the Internet’s backbone. Point-to-Point Protocol (PPP) An industry standard data-link layer protocol for wide area network (WAN) transmission that was developed in the early 1990s. Point-to-Point Protocol (PPP) allows Remote Access Service (RAS) products and devices from different vendors to interoperate for WAN communication. Point-to-Point Tunneling Protocol (PPTP) A data-link layer protocol for wide area networks (WANs) based on the Point-to-Point Protocol (PPP) and developed by Microsoft that enables network traffic to be encapsulated and routed over an unsecured public network such as the Internet. Point-to-Point Tunneling Protocol (PPTP) allows the creation of virtual private networks (VPNs), which tunnel TCP/IP traffic through the Internet. Remote users can securely access corporate local area network (LAN) resources using the Internet instead of having to use direct modem connections over the Public Switched Telephone Network (PSTN) or dedicated leased-line connections Port In TCP/IP networking, an endpoint of a logical connection between two hosts on an internetwork. Ports are identified by port numbers. A port identifies a unique process for which a server can provide a service or the client can access a service. Ports can be Transmission Control Protocol (TCP) ports or User Datagram Protocol (UDP) ports, depending on the type of service supported. In general networking terminology, a port is a connector for attaching cables or peripherals to a computer—for example, a parallel port for connecting a printer to a computer or a serial port for connecting a serial mouse or modem to a computer. Connectors on networking components, such hubs or routers, are also sometimes called ports, although a better term for a connector on a router is a "router interface." Post Office Protocol version 3 (POP3) An Internet standard protocol for storing and retrieving messages from Simple Mail Transfer Protocol (SMTP) hosts. PPP Multilink A protocol for combining multiple physical WAN links into a single logical pathway to increase bandwidth. Microsoft’s Remote Access Service (RAS) for Windows NT and Window 2000 supports combining Plain Old Telephone Service (POTS) and Integrated Services Digital Network (ISDN) lines using PPP Multilink, as does Windows 98. Primary Rate Interface ISDN (PRI-ISDN) A standard form of communication on Integrated Services Digital Network (ISDN) communication systems. Primary Rate Interface ISDN (PRI-ISDN) connections consist of 24 digital channels divided between 23 B channels and a single D channel. The B channels carry the voice or data between the customer premises and the telco’s central office (CO), while the D channel is used for establishing connections and signaling. PRI-ISDN is often referred to as 23B+D because of the channels that it uses. The bandwidth of each B channel and of the single D channel is 64 Kbps, so the total bandwidth of PRI-ISDN is 1.544 Mbps, which is identical to that of a T1 circuit. By using the bonding protocol, the 23 B channels can be combined to form a single 1.472-Mbps data channel. The European version of PRI-ISDN uses 31 B channels and one D channel, providing the bandwidth of an E1 circuit. Protocol A set of rules for sending information over a network. Protocols can include rules concerning any or all of the following functions:
Protocol converter A general term for a device that enables communication between networks or transmission systems that use different protocols. Protocol converters are often used in mainframe computing environments; they enable one device to emulate the communication functions of another device. For this reason, a protocol converter is sometimes known as an emulator, and it can be either hardware-based or software-based. Proxy cache server A type of proxy server that caches Web pages that users request on the Internet. You can use a proxy cache server like a regular proxy server at the border of a private corporate network in order to cache the Web pages returned from the Internet when users in the private network request them. When users request these pages again, the pages are returned instantly from the cache; a new request need not be sent over the Internet. This speeds up browsing for frequently accessed Web sites and reduces the amount of bandwidth used on the corporate Internet link. Proxy cache servers can also be used at Internet service providers (ISPs) and at strategic locations on the Internet’s high-speed backbone to provide relief to heavily accessed Web servers and to reduce overall backbone traffic. Proxy server A computer that can act on the behalf of other computers to request content from the Internet or an intranet. Public Switched Telephone Network (PSTN) The public telephone network managed by the local telco and long-distance carriers. The Public Switched Telephone Network (PSTN) consists of a digital backbone of switched circuits together with the analog local loop wiring still found in many residences. The PSTN is also known as the Plain Old Telephone Service (POTS), although that term specifically relates to the older, nondigital portion of the PSTN. The PSTN provides the most popular basis for creating wide area networks (WANs) through both leased lines and dial-up lines between local and remote networks. PSTN is often used in wide area networking because of its ubiquitous nature—local loop connections exist almost everywhere in the world.
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