Packet Switching
A core network transmission technique that involves splitting information into packets of data that are routed independently through the network over different routes to the final destination. Attractive for mobile access because radio spectrum is used only when data is actually being transmitted. The alternate is circuit switching. GPRS is a packet-based technology designed for digital mobile networks.

PAD
Packet Assembler-Disassembler

PAL
Phase Alternate Line - The color carrier phase definition changes in alternate scan lines. Utilizes an 8 MHz-wide modulated signal - Television. Mostly used in Europe.

PAM
Pulse Amplitude Modulation 
Modulations scheme where the modulating wave is caused to modulate the amplitude of a pulse stream.

PAP
Password Authentication Protocol - PPP - RFC 1334

PAT
Port Address Translation
Process where the packet arriving at the interface are checked by port number and the IP address they want to reach.

PBX/PABX
Private Automatic Branch Exchange

PCM
Pulse Code Modulation 
Process by which the high statistical correlation between consecutive voice samples is used to create a variable quantizing scale. ADPCM can encode analog voice samples into high - toll - quality digital signals.
á Used to optimize the cost
á The difference of amplitude is analyzed and compared to the former sample
á Silence are detected.
á ITU-T Norm from G.720 to G.730 

PCN
Path Control Network

PCR
Peak Cell Rate

PDN
Public Data Network

PDU
Packet Data Unit / Protocol Data Unit - ISO

PDU
Protocol Data Unit
Another word for packet as defined by the OSI. Used by bridges to transfer connectivity information.

PEL
Picture Element 

PEM
Privacy Enhanced Mail 
RFC 1421 to 1424


PETABYTE
1024 Terabytes

P/F
Poll/Final In-link protocol

PFM
Pulse Frequency Modulation

PGI
Parameter Group Identifier - ISO

PHY
FDDI Physical Sub-layer
Designation for FDDI fiber-optic cables.

PIGGYBACKING
The process of carrying acknowledgments within a data packet to save network bandwidth.

PING
Packet Internet Gopher 
Use the echo ICMP to see if a workstation is answering.

PLAR
Private Line Auto Ringdown.

PLC
This type of service results in a call attempt to some particular remote endpoint when the local extension is taken off-key.

PLC
Power Line Communication 
This technique allows the use of the regular electrical plug to access to the internet.
It uses a higher frequency that the 50 Hz to transmit data, and could go up to 1 or 2 M-bit.

PLCP
Physical Layer Convergence Procedure

PLU
Primary LogicalUnit

PM
Phase Modulation

PMD
Physical Medium Dependent specification - FDDI 
The IEEE 802.3 specification defined a PMD communication protocol that consisted of LLC/MAC layer and the PMD layer. The LLC and the MAC layers correspond to layer 2, the Data Link layer, of the OSI model. The PMD layer corresponds to layer 1, the Physical layer, of the OSI mode.

PNNI
Private Network-to-Network Interface - ATM

PNP
Private Numbering Plan

POL
Path Overhead Layer - Sonet

POP
Post Office Protocol A telecom
Facility through which the company provides local connectivity to its customers.

POP
Point of Presence
Local access provided by and ISP. This allows a company to save cost in building their intranet solution. VPN is then required to encrypt, and for security.

POP3
Post Office Protocol Version 3
TCP protocol working on port 110. 

PORT
A port is a 16-bit number - 1 to 65535 - used by the protocols of the transport layer - TCP and UDP protocols. See Download.

POS
Packet Over Sonet - Cisco ¨

POTS
Plain Old Telephone Service 
Analog service provided by the public telephony. 
Basic telephone service supplying standard single-line telephones, telephone lines, and access to the Public Switched Telephone Network.

PPDU
Presentation Protocol Data Unit

PPM
Pulse Position Modulation

PPN
Public Packet Network

PPP
Point-to-Point Protocol 

PPPoE
Point to Point Protocol over Ethernet 
Point to Point Protocol over Ethernet is a proposal specifying how a host personal computer interacts with a broadband modem -i.e. xDSL, cable, wireless - to achieve access to the growing number of high-speed data networks. Relying on two widely accepted standards, Ethernet and the point-to-point protocol -ppp - , the PPPoE implementation requires virtually no more knowledge on the part of the end user other than that required for standard Dial up Internet access. In addition, PPPoE requires no major changes in the operational model for Internet Service Providers and carriers. The significance of PPP over Ethernet has to do with its far greater ease of use versus competing approaches. By making high speed access easier to use for end consumers, and more seamless to integrate into the existing infrastructure for carriers and ISPs, PPPoE could speed the widespread adoption of high speed access services. 
Also, PPP over Ethernet provides a major advantage for service providers by maximizing integration with - and minimizing disruption of - service providers' existing dial network infrastructures. PPPoE enables rapid service deployment and cost savings. From authentication, accounting and secure access to configuration management, PPPoE supports a broad range of existing applications and services. 
The base protocol is defined in RFC 2516. 

PPSN
Public Packet-Switched Network

PPTP
Point to Point Tunneling Protocol 
VPN protocol used by the Microsoft ¨ operating system to create the encrypted connection between two computers
The data are encrypted, there is no way to see the IP Source and Destination addresses as they are encrypted in the PPTP protocol.
o This protocol allows a remote site to get connected to a host via Internet by building a tunnel and encrypt the data.
The connection to the ISP could be Frame Relay, ISDN or other.
o This is a cheap way of getting connected to a host, although it relies on an external provider which is here the ISP, this solution is secured and quite fast.

PRI
Primary Rate Interface - ISDN 
ISDN interface to primary rate access. Primary rate access consists of a single 64-Kbps D channel plus 23 - in the case of 1.544 Mbps - or 30 - in the case of a 2.048 Mbps - B channels for voice and / or data.

PROXY
A Proxy Server can be used to limit access to some of these undesirable sites. 
A Proxy Server is a World Wide Web server that acts as the sole web server for your entire domain or whatever clients you place behind the firewall, a logical block between your clients and the rest of the Internet. The proxy server usually sits on your firewall and intercepts all web requests coming from clients within the firewall. If the web page request is not on the proxy server's access control list, the request is processed normally and the retrieved web page is sent back to the requesting client. If, however, the requested web page or web site is on the control list, the client instead receives a message indicating that the URL is not accessible or not valid. 
Your network must be set up such that clients needing access control must use the proxy server as their Internet gateway. This can be accomplished through proper router setup, placing all clients needing access control "behind" the firewall. 
A proxy server can also improve your network's performance by functioning as a caching server. Using its cached web pages, the proxy server will serve already-accessed web pages to requesting clients without requiring outside access to the Internet. Consider the situation where a computer lab of twenty client computers is accessing the web under the direction of the teacher. The teacher instructs all twenty students to type in the address of the web page to be accessed. At the moment all twenty ENTER keys are pressed, twenty separate requests for that same web page are initiated, and twenty separate copies of that same web page are retrieved and returned to the classroom. 
Using a proxy server, the same twenty web requests are handled more efficiently. Only the first request to reach the server actually causes that web page to be retrieved, and only if it is not already stored in the server's cache. When retrieved, that web page is sent back to the requesting client and is also cached on the proxy server's hard disk. The remaining 19 clients that requested that same web page are served instead from the proxy server's cache, thus avoiding unnecessary duplicated requests and delays from cyberspace. 
Setting up a proxy server is relatively simple if your server supports proxy operation. However, maintaining the access control list can be a daunting task. While most proxy web servers can accept domain names, individual page names, or wildcard URL specifications, actual identification of inappropriate web sites and pages is like shooting at a moving target. As old, already-documented sites disappear, new ones appear. There are no public clearinghouses, per se, of sites or URL's that contain material that might qualify as "questionable" in the instructional setting. Additionally, the creation of such sites could possibly result in legal challenges. While some vendors market client-based products that will block access to objectionable sites, the updated lists they provide through subscription are designated to work with their product only. 
A sampling of web sites that might qualify for access control lists might be obtained from several, public sources. Classified advertising in many popular computer magazines now includes Internet addresses. Internet yellow page listings sometimes contain references to "AO" material. Usenet newsgroups that specialize in adult topics most likely will contain some references. Cyber surfing is another method of collecting URL's for an access control list. YAHOO <www.yahoo.com> can also provide some sources. 
Proxy servers will not prevent "questionable" material from being downloaded as an e-mail message or as an attachment to an e-mail message. They cannot prevent objectionable material from being transmitted and received during a "chat" session. They cannot filter out material that has been placed on a web server behind the firewall and they cannot filter out material based solely on textual content. 
Proxy servers can, however, provide a greater level of access control than that which exists with unrestricted browsing clients. Proxy servers can also make the process of accessing web pages more efficient for your agency. 

PSDN
Packet-Switched Data Network

PSN
Packet Switch Node 
An Internet packet switch. Also, a switching node in the X.25 architecture. Usually, the PSN is a data communication equipment - DCE - and allows for connection to data terminal equipment - DTE -.


PSPDN
Packet Switched Public Data Network
Public network with packet switching.

PSTN
Public Switched Telephone Networks
The ordinary, wired, telephone network.

PT
Payload Type - ATM

PTE
Path Terminating Equipment 

PTI
Packet Type Indication

PTR
Pointer Reverse

PU 2.1
Physical Unit 2.1 - IBM ¨ 
A type of network node used for connecting SNA nodes in a peer-oriented network. APPN is based upon Node Type 2.1.
Type 2.1 nodes can also be connected into traditional hierarchical SNA network.

PVC
Permanent Virtual Circuit 
Originating refers to the access interface from which the PVC is initiated. Terminating refers to the access interface at which the PVC stops. 

PVPC
Permanent Virtual Path Connection - ATM

PXF
Parallel eXpress Forwarding
Network coprocessor used for Ethernet Gigabit router - Cisco ¨