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P
PABX
PABX stands for
Private Automatic (or Automated) Branch eXchange. It is the automated
version of the PBX (private branch exchange). All PBXs today are automatic.
Early PBXs were simply more sophisticated PBXs, providing additional voice
services (such as call waiting) for analog extension loops. Today's PBXs
are fully digital, not only offering very sophisticated voice services,
such as voice messaging, but also integrating voice and data. A PABX can
multiplex both voice and data onto T-carriers and can support both voice
and data communications from one extension over a single pair of wires.
Packet
A packet is a
unit of information that can be transmitted over a network. A packet is
generated at the Network Layer of the OSI model protocol stack. Information
contained in the header of a packet is sufficient to get the packet from
the sending node to the receiving node, even when the packet must traverse
through intermediate nodes. A packet can be an entire message or it can be
a segment of a much larger message generated at the Application Layer.
Packet Assembler/Dissassembler (PAD)
PAD
stands for packet assembler/disassembler. ITU-T developed a set of
standards, informally called the Interactive Terminal Interface (ITI)
standards, meant to provide access for terminals and DTEs that cannot
execute the layers of X.25. The standards are X.3, X.28, and X.29. The ITI
standards collectively define a "black box" called a packet
assembler/disassembler, or PAD. A PAD "assembles" a stream of
bytes originating from an asynchronous DTE (for example, from a personal
computer) into X.25 packets and transmits them on the X.25 network. Of
course, it performs the reverse operations for data sent back to the DTE.
Packet Level
The
packet level refers to level three, the Network Layer of the OSI protocol
stack, or the X.25 protocol suite packet layer.
Packet Switching
Packet
switching is what occurs as a packet is traveling from the source node to
the destination node when there are intermediate nodes between source and
destination. When the packet travels through the intermediate node (a
packet switch), it must "switch" the packet to the next node in
sequence.
Packet Transfer Mode
Three
transfer modes were considered as possible candidates for B-ISDN. These
three transfer modes are:
- Synchronous Transfer Mode (STM)
- Packet Transfer Mode (PTM)
- Asynchronous Transfer Mode (ATM)
PTM uses variable size packets of data and is
well-suited for data communications. However PTM is not well suited for
voice and video since the transmission of long packets may delay
transmission of time sensitive information. Packets are usually
connectionless in nature, the header of the packet contains enough
information to get the packet through the network. No set physical paths
used in this mode.
Packets Per Second
Throughput
is the ability of a network to efficiently carry data of value between
network users. One measure of effective throughput is TRIB, or throughput
rate in information bits. Effective throughput and response time is
directly related. The higher the effective throughput, the better the
response time. Effective throughput and throughput are often used
interchangeably.
Throughput
is most often used to describe the overall performance of a network. It is
measured in packets per second (PPS), bits per second (BPS) or transactions
per second (TPS). Many factors impact throughput such as:
- protocol efficiency
- server/workstation CPU type
- NIC type
- LAN/link capacity
Transactions per second is probably the best measure of
throughput. An example of a TPS measurement would be 7200 TPH or 7200
transactions per hour (or 2 TPS). Knowing the TPH is not enough to get a
good handle on overall performance, you must also know the average
transaction size and the TPH over different blocks of time.
Parity
Parity refers to the
process of determining if data has been corrupted during asynchronous
transmission of data. This is done by adding an additional bit (called a
parity bit) to each character transmitted. There are two types of parity,
odd parity and even parity. With even parity the additional bit combined
with the character makes the total number of ones an even number. With odd
parity the additional bit that is appended makes the total number of one
bits odd. For example if the ASCII character "A" is going to be
transmitted the seven bits that make up the "A" character 100
0001 will be transmitted. If even parity is used the appended eighth bit
will be a "0" keeping the total number of ones at two. If the
parity is odd the eighth bit will be a "1" making the total number
of ones three.
Payload
Payload refers to
the carrying capacity of a particular protocol. For example, the maximum
payload for an Ethernet frame is 1500 bytes.
PCM
The most basic
aspect of digital telephony is the conversion of an analog voice signal to
a digital signal that can be translated accurately back into a voice signal
after transmission. Two methods are in use: Pulse Code Modulation (PCM) and
Delta Modulation. Delta modulation is used primarily for military
applications. PCM is used by the public telephone systems.
The
device that converts an analog signal to PCM is called a codec (for
coder/decoder). The codec first samples the voice signal. Several thousand
samples per second are taken. Next, each sample is converted to a binary
number that expresses the amplitude of the sample in a very compact form.
These binary numbers form the digital bit stream that comes out of a codec.
The receiving codec reverses the process, using each successive binary
number to control a digital/analog circuit that generates the required
analog wave form on the voice output channel.
PCMCIA
PCMCIA stands for
Personal Computer Memory Card International Association. The PCMCIA slot in
a laptop was designed for PC memory expansion. Network adapter cards (NICs)
and modems can attach to a laptop through the PCMCIA slot.
PDN
PDN stands for
Public Data Network. It is a network available to the public for
transmission of data, usually using the X.25 protocol. Users dial into a
server that allows access to the network. PDNs are especially popular in Europe.
PDU
PDU stands for
Protocol Data Unit. The concept of PDU is used in the OSI reference model.
From the perspective of a Protocol Layer, a PDU consists of information
from the layer above plus the protocol information appended to the data by
that layer.
Peer-To-Peer
Two programs or
processes that use the same protocol to communicate and perform
approximately the same function for their respective nodes are referred to
as peer processes. With peer processes, in general, neither process
controls the other, and the same protocol is used for data flowing in
either direction. Communication between them is spoken of as
"peer-to-peer."
Permanent Virtual Circuit
PVC
stands for Permanent Virtual Circuit. There are two types of virtual
circuits: PVCs and Switched Virtual Circuit (SVCs). A PVC behaves like a
dedicated line between source and destination end-points: when activated, a
PVC will always establish a path between these two end-points. It is
usually used in the context of a packet switching (or cell switching)
network.
Phase Modulation
Modulation
is the process of modifying the form of a carrier wave (electrical signal)
so that it can carry intelligent information on some sort of communications
medium. Digital computer signals (baseband) are converted to analog signal
for transmission over analog facilities (such as the local loop). The
opposite process, converting analog signal back into their original digital
state is referred to as demodulation. There are three basic types of
modulation: frequency, phase and amplitude.
Physical Address
In
data communications the term physical address refers to the address
"burned in" to the hardware of a particular node. For instance
the physical address of a node connected to an Ethernet network is a 48-bit
value that is unique for every Ethernet card purchased.
Physical Circuit
A
physical circuit is the hardware and cabling used to connect devices
together.
Physical Layer
The
Physical Layer is the lowest layer in the OSI reference model. The Physical
Layer is responsible for the transmission of bits across a communications
channel, or "wire," which can range from a coaxial cable, fiber
optic cable, or satellite links to an ordinary telephone wire.
The
Physical Layer processes provide the service of transferring bits across
the physical link. They do this without any knowledge of the meaning or
structure of the bits. They don't know, for example, whether they are
transferring 8-bit bytes or 7-bit octal characters. Some faults, such as an
open connection, can be detected and the error indication passed on to the
higher layers, but most error detection and all error correction are the
responsibility of higher layers.
The
transmission protocol (also called subnet protocol) that the processes of
the Physical Layer use vary according to the nature of the link. The
protocol is concerned with such things as:
- how bits are represented
- how to tell when transmission starts and
ends
- whether bits can flow in one direction only
or in both directions at the same time
Physical Unit
Each
node in an SNA network contains at least one physical unit (PU). The
physical unit implements physical connections to the network. SNA defines
PU types that correspond directly to the node types above: PU 1, PU 2, PU
4, and PU 5. Thus, a Type 2 node, for example, is said to contain a PU 2.
Photonic Layer
The
photonic layer is responsible for the transmission of bits across the
optical fiber. This layer converts electrical inputs signals to optical
signals and vice-versa. A transceiver is the device used to convert
electrical signals to optical signals.
PIM
Protocol
Independent Multicast-Sparse Mode (PIM-SM) is a protocol for efficiently
routing to multicast groups that may span wide-area (and inter-domain) internets.
The protocol is not dependent on any particular unicast routing protocol,
and is designed to support sparse groups.
Ping
Ping stands for
Packet Internet Groper. The ping command is used to determine if a
destination host is reachable from the sender in an IP network. An ICMP
echo packet is sent to a host and if a response is not sent back from that
host within a specified amount of time, the host is considered
"unreachable."
Pipes
Pipes are used in
interprocess communication to provide the mechanism for sending and
receiving messages.
Plaintext
Plaintext refers
to a message that has not been encrypted. A wide variety of methods can be
used for data encryption. In general, they involve using a "key"
to either re-order the bits, bytes, or words of a message (a transposition
cipher) or substituting one or more encryption bytes or words for the
"plaintext" bytes or words of the message (a substitution
cipher).
PLP
PLP stands for
Packet Layer Protocol. PLP manages connections between DTEs anywhere in the
network, accepting packets from the Transport Layer process and taking
responsibility for error-free delivery of the packets to their destination.
PLP handles the multiplexing of packets across a link. It establishes
virtual circuits and routes packets across those circuits. Many virtual
circuits can share a link, so this results in the multiplexing of packets.
Point-to-Point
A network
connects nodes, some of which are hosts to which terminal nodes attach, in
two different ways: point-to-point and broadcast. Point-to-point networks
fall into two classes: circuit-switched networks, in which a connection is
formed between the nodes, as in a telephone network; and packet-switched or
connectionless networks, in which packets of data, or datagrams, are passed
from node to node until they reach their destination, like telegrams.
Point of Presence
A
subscriber can connect to the IEC's facilities only at a point of presence
(POP). Typically, this connection is made by a line provided by the LEC
from the subscriber's premises to the POP. Alternately, the subscriber can
bypass the LEC and connect directly to the POP. For example, the subscriber
could install a microwave link to the POP (B).
Polling
Polling is a
method used to control communication between a master and slave node in an
unbalanced data communication configuration. In an unbalanced configuration
the master "polls" the slave to ask if it has data to send or if
it is in a state to receive data. An example of a protocol that uses a polling
technique would be synchronous data link control (SDLC).
POP
A subscriber can
connect to the IEC's facilities only at a point of presence (POP).
Typically, this connection is made by a line provided by the LEC from the
subscriber's premises to the POP. Alternately, the subscriber can bypass
the LEC and connect directly to the POP. For example, the subscriber could
install a microwave link to the POP (B).
Port
There are two
primary ways the term port is used in networking. The first is referring to
a physical port in a device such as an I/O port on a switch. Ports also
relate to processes running on nodes connected to a network. Ports are also
referred to as sockets. If a machine is running multiple processes, each
process must be uniquely identified if it is to communicate. Well-known
ports in the TCP architecture, such as FTP and PING,
are examples of port addresses.
Port
Mirroring
Also
known as a roving analysis port, is a method of monitoring network traffic
that forwards a copy of each incoming and outgoing packet
from one port
of a network switch
to another port where the packet can be studied. A network administrator
uses port mirroring as a diagnostic tool or debugging
feature, especially when fending off an attack. It enables the
administrator to keep close track of switch performance and alter it if
necessary. Port mirroring can be managed locally or remotely.
PPP
PPP stands for
Point-to-Point Protocol. PPP is a protocol that allows a computer to use
the TCP/IP via a point-to-point link. PPP is based on the HDLC (High-Level
Data Link Control) standard that deals with LAN and WAN links and operates
at the Data Link Layer of the OSI model.
PPS
PPS stands for
packets per second. It is a measure of the number of data packets that can
be transmitted over a given link during a one-second interval. If a circuit
is capable of transmitting data a rate of 56,000bps (bits per second) and
the average packet size is 70 bytes (560 bits), the maximum PPS that can be
transmitted is 100 in
one second.
PPTP
PPTP (Point
to Point Tunneling Protocol) allows PPP to be channeled through an IP
network. It uses a client-server architecture to decouple functions which
exist in current Network Access Servers and support Virtual Private
Networks. It specifies a call-control and management protocol which allows
the server to control access for dial-in circuit switched calls originating
from a PSTN or ISDN, or to initiate outbound circuit switched connections.
PPTP uses a GRE-like (Generic Routing Encapsulation) mechanism to provide a
flow- and congestion-controlled encapsulated datagram service for carrying
PPP packets.
Presentation Layer
The
Presentation Layer handles the representation of data as they flow between
nodes. The lower layers provide the service of transferring data between
nodes in an orderly fashion and ensuring that what is received is what has
been sent. The Presentation Layer provides services that relate to the way
data are represented.
Primary Rate Interface
The
ISDN Primary Rate Interface is also called 23B+D. It is a circuit
consisting of 23 64Kbps channels for carrying voice, data, and video and a
data channel used for carrying signaling information. It is similar to T1
signaling.
Primary Station
In a
data communications network there are three types of stations. These terms
are normally used in reference to devices running HDLC or a subset of HDLC.
- A primary station sends command frames and
accepts responses (it would be considered a "master" node).
- A secondary station accepts commands and
sends responses (it would be considered a "slave" node).
- A combined station can send or accept
commands and respond appropriately.
Primary Wire Center
Early
phone systems connected each subscriber with a central switching office
("exchange"), where an operator provided manual switching between
subscribers (today we would call this a star network). Since a major cost
of a telephone system was the wire connecting the subscriber to the
exchange, exchanges covered relatively small geographical areas and were
interconnected by "trunk" lines that could be shared by all
subscribers.
The number of trunk lines required to interconnect all end offices in an
area in a full "mesh" network rises geometrically, so for even a
small number of offices, many lines would be required. Yet the utilization
of trunks between end offices that are remote from one another is
relatively low, so end offices were also interconnected in a star to a toll
center. This hierarchy was extended to four higher levels. Today in the U.S.,
any switching office is often called a central office (CO) so a center is
often referred to according to its position in the hierarchy, for example,
a CO3 would be a primary wiring center.
Private Line
A
private line is a telecommunications channel used exclusively by a single
subscriber. Private lines are also referred to as dedicated lines or
dedicated circuits.
Private Network
A
private network is a network consisting of private lines, switching
equipment and other networking equipment that are provided for the
exclusive use of one customer. In other words the general public does not
intend the network and associated services of the network for usage. Most
corporate networks are private networks.
Propagation Delay
All
transmission media have some resistance to the flow of electromagnetic
force. Over a great enough distance, the signal will be attenuated
(reduced) to the point that an unacceptable number of errors are
introduced. Fiber optic transmission is far less susceptible to attenuation
than electrical transmission. Broadband is less susceptible than baseband.
Signal
propagation delay refers to the time that it takes for a signal to
propagate from one end of the network to the other. Signal propagation is
most apparent in networks that use satellite technology.
Protocol
Data
communications involves the transfer of data between computer programs.
Just as humans must share a common language in order to communicate, the
programs must have a common protocol. The protocol simply defines the
format and meaning of the data that the programs interchange.
Protocol Data Unit
The
concept of a Protocol Data Unit (PDU) is used in the OSI reference model.
From the perspective of a protocol layer, a PDU consists of information
from the layer above plus the protocol information appended to the data by
that layer.
Protocol Stack
A
protocol stack refers to the number of protocols needed to transmit data
between communication devices. A typical stack would include physical layer
protocols such as UTP cabling, Link Layer protocols such as Ethernet,
Network Layer protocols such as IP and so forth.
Proxy ARP
The
technique in which one machine, usually a router, answers ARP requests
intended for another machine. By "faking" its identity, the
router accepts responsibility for routing packets to the "real"
destination. Proxy ARP allows a site to use a single IP address with two
physical networks. Subnetting would normally be a better solution.
PTT
PTT stands for
Postal Telephone and Telegraph. These are telephone companies that are
operated and regulated by government agencies. These are found in most
countries other than the United States
and Canada.
In the United States,
telephone service is offered by private, profit making companies that are
regulated by government agencies.
Public Network
A
public network is a network that provides leased lines, packet switching
services and circuit switching services to the general public. It is the
opposite of a private network. The term public switched network however
usually refers to the voice telephone network.
Pulse Code Modulation
The
most basic aspect of digital telephony is the conversion of an analog voice
signal to a digital signal that can be translated accurately back into a
voice signal after transmission. Two methods are in use: Pulse Code
Modulation (PCM) and Delta Modulation. Delta modulation is used primarily
for military applications. PCM is used by the public telephone systems.
The
device that converts an analog signal to PCM is called a codec (for
coder/decoder). The codec first samples the voice signal. Several thousand
samples per second are taken. Next, each sample is converted to a binary
number that expresses the amplitude of the sample in a very compact form.
These binary numbers form the digital bit stream that comes out of a codec.
The
receiving codec reverses the process, using each successive binary number
to control a digital/analog circuit that generates the required analog wave
form on the voice output channel.
Pure Ring
A ring
is made up of a collection of separate point-to-point links, arranged to
make a ring. Each node attached to the ring has one input and one output
connection, so each node is connected to two links. Signals received on the
input connection are passed through, immediately and without buffering, to
the output connection by "repeater" circuitry in each node. Thus,
data flow only in one direction on a ring. Each node has the ability to put
new bits onto the ring to send messages and, if the message is addressed to
that node, to copy bits off of the ring as they go by.
In a
pure ring, if the node fails (for example, if power is removed), then it
doesn't repeat signals from its input. If any node fails to repeat signals,
the ring is broken, and data transfer stops until the failing node is
restored or removed from the ring. As you might imagine, the ability of a
single failing node to bring down an entire ring network is unacceptable.
It also turns out that for technical reasons having to do with
characteristics of data transmission in a ring, a single ring as we have
described above has a limit of only a few hundred nodes.
PVC
PVC stands for
Permanent Virtual Circuit. There are two types of virtual circuits: PVCs
and Switched Virtual Circuit (SVCs). A PVC behaves like a dedicated line
between source and destination end-points: when activated, a PVC will
always establish a path between these two end-points. It is usually used in
the context of a packet switching (or cell switching) network.
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