Page 523

Glossary

491

payload compression

Reduces the number of bytes required to accu-

rately represent the original data stream. Header compression is also pos-
sible. See also: compression.

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

PDU

Protocol Data Unit: The name of 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.

PFC

Policy Feature Card: The PFC can be paralleled with the NFFC used

in Catalyst 5000 switches. It is a device that is capable of examining IP and
Ethernet headers in order to establish flow caches.

PGP

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

application offering protected transfer of files and messages.

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, Ses-
sion 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.

www.sybex.com