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Glossary
UM1600-2-A November 2002
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PSTN (Public Switched Telephone Network)
General term referring to the variety of telephone networks and services in place
worldwide. Sometimes called
POTS .
PVC( Permanent Virtual Circuit, or connection)
Virtual circuit that is permanently established. PVCs save bandwidth associated with
circuit establishment and tear down in situations where certain virtual circuits must exist
all the time. In ATM terminology, called a permanent virtual connection.
Rack mount
A structure that houses shelves (usually a maximum of four). The unit or container that
houses the internal modular circuitry. The shelf consists of slots that hold each module
and a backplane that interconnects all modules.
SAR
Segmentation and reassembly. One of the two sub-layers of the AAL CPCS,
responsible for dividing (at the source) and reassembling (at the destination) the PDUs
passed from the CS. The SAR sub-layer takes the PDUs processed by the CS and,
after dividing them into 48-byte pieces of payload data, passes them to the ATM layer
for further processing. See also
AAL and ATM .
SDU (Service Data Unit)
Unit of information from an upper-layer protocol that defines a service request to a
lower-layer protocol.
Signal Noise Ratio (SNR)
This is a DSL transmission parameter, measured in dB, which indicates the
Signal-to-Noise (S/N) ratio at a receiver point.
SNAP
Subnetwork Access Protocol. Internet protocol that operates between a network entity
in the subnetwork and a network entity in the end system. SNAP specifies a standard
method of encapsulating IP datagrams and ARP messages on IEEE networks. The
SNAP entity in the end system makes use of the services of the subnetwork and
performs three key functions: data transfer, connection management, and QoS