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Glossary

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PAT

port address translation: This process allows a single IP address to rep-

resent multiple resources by altering the source TCP or UDP port number.

PCM

pulse code modulation: Process by which analog data is converted

into digital information.

PCR

peak cell rate: As defined by the ATM Forum, the parameter speci-

fying, in cells per second, the maximum rate at which a source may transmit.

PDN

public data network: Generally for a fee, a PDN offers the public

access to a computer communication network operated by private concerns
or government agencies. Small organizations can take advantage of PDNs,
aiding them to create WANs without investing in long-distance equipment
and circuitry.

PDU

Protocol Data Unit: The processes at each layer of the OSI model.

PDUs at the Transport layer are called segments; PDUs at the Network layer
are called packets or datagrams; and PDUs at the Data Link layer are called
frames. The Physical layer uses bits.

PGP

Pretty Good Privacy: A popular public-key/private-key encryption

application offering protected transfer of files and messages.

phantom router

Used in a Hot Standby Routing Protocol (HSRP) net-

work to provide an IP default gateway address to hosts.

Physical layer

The lowest layer--layer 1--in the OSI reference model, it is

responsible for converting data packets from the Data Link layer (layer 2)
into electrical signals. Physical-layer protocols and standards define, for
example, the type of cable and connectors to be used, including their pin
assignments and the encoding scheme for signaling 0 and 1 values. See also:
Application layer, Data Link layer, Network layer, Presentation layer,
Session layer, and Transport layer.

PIM

Protocol Independent Multicast: A multicast protocol that handles the

IGMP requests as well as requests for multicast data forwarding.

PIM-DM

Protocol Independent Multicast Dense Mode: PIM-DM utilizes

the unicast route table and relies on the source root distribution architecture
for multicast data forwarding.

PIM-SM

Protocol Independent Multicast Sparse Mode: PIM-SM utilizes

the unicast route table and relies on the shared root distribution architecture
for multicast data forwarding.

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