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PVC—permanent virtual circuit. 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.
QoS—quality of service. Measure of performance for a transmission system that reflects its
transmission quality and service availability.
SAR—segmentation and reassembly. One of the two sublayers of the AAL CPCS, responsible for
dividing (at the source) and reassembling (at the destination) the PDUs passed from the CS. The
SAR sublayer 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.
SONET—Synchronous Optical Network. High-speed (up to 2.5 Gbps) synchronous network
specification developed by Bellcore and designed to run on optical fiber. STS-1 is the basic building
block of SONET.
SSCS—service specific convergence sublayer. One of the two sublayers of any AAL. SSCS, which
is service dependent, offers assured data transmission. The SSCS can be null as well, in classical IP
over ATM or LAN emulation implementations.
SVC—switched virtual circuit. Virtual circuit that is dynamically established on demand and is torn
down when transmission is complete. SVCs are used in situations where data transmission is
sporadic. Called a switched virtual connection in ATM terminology.
T3—Digital WAN carrier facility. T3 transmits DS-3-formatted data at 44.736 Mbps through the
telephone switching network.
UBR—unspecified bit rate. Quality of Service (QoS) class defined by the ATM Forum for ATM
networks. UBR allows any amount of data up to a specified maximum to be sent across the network,
but there are no guarantees in terms of cell loss rate and delay.
UNI—User-Network Interface. ATM Forum specification that defines an interoperability standard
for the interface between ATM-based products (a router or an ATM switch) located in a private
network and the ATM switches located within the public carrier networks. Also used to describe
similar connections in Frame Relay networks.
VBR—variable bit rate. QoS class defined by the ATM Forum for ATM networks. VBR is
subdivided into a real time (RT) class and non-real time (NRT) class. VBR (RT) is used for
connections in which there is a fixed timing relationship between samples. VBR (NRT) is used for
connections in which there is no fixed timing relationship between samples, but that still need a
guaranteed QoS.
VC—virtual circuit. Logical circuit created to ensure reliable communication between two network
devices. A virtual circuit is defined by a VPI/VCI pair, and can be either permanent (PVC) or
switched (SVC). Virtual circuits are used in Frame Relay and X.25. In ATM, a virtual circuit is called
a virtual channel.
VCI—virtual channel identifier. 16-bit field in the header of an ATM cell. The VCI, together with
the virtual path identifier (VPI), is used to identify the next destination of a cell as it passes through
a series of ATM switches on its way to its destination. ATM switches use the VPI/VCI fields to
identify the next network virtual channel link (VCL) that a cell needs to transit on its way to its final
destination.
VCL—virtual channel link. Connection between two ATM devices. A VCC is made up of one or
more VCLs.
VPI—virtual path identifier. 8-bit field in the header of an ATM cell. The VPI, together with the
VCI, is used to identify the next destination of a cell as it passes through a series of ATM switches
on its way to its destination. ATM switches use the VPI/VCI fields to identify the next VCL that a
cell needs to transit on its way to its final destination.
Voice over ATM Switched Virtual Circuits on the Cisco MC3810 31
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