ZyXEL Communications VMG8924-B10D User Manual page 289

ATM sets up virtual circuits over which end systems communicate. The terminology for virtual circuits is as
follows:
• Virtual Channel (VC)
• Virtual Path (VP)
• Virtual Circuits
Figure 178 Virtual Circuit Topology
Think of a virtual path as a cable that contains a bundle of wires. The cable connects two points and
wires within the cable provide individual circuits between the two points. In an ATM cell header, a VPI
(Virtual Path Identifier) identifies a link formed by a virtual path; a VCI (Virtual Channel Identifier)
identifies a channel within a virtual path. A series of virtual paths make up a virtual circuit.
F4 cells operate at the virtual path (VP) level, while F5 cells operate at the virtual channel (VC) level. F4
cells use the same VPI as the user data cells on VP connections, but use different predefined VCI values.
F5 cells use the same VPI and VCI as the user data cells on the VC connections, and are distinguished
from data cells by a predefinded Payload Type Identifier (PTI) in the cell header. Both F4 flows and F5
flows are bidirectional and have two types.
• segment F4 flows (VCI=3)
• end-to-end F4 flows (VCI=4)
• segment F5 flows (PTI=100)
• end-to-end F5 flows (PTI=101)
OAM F4 or F5 tests are used to check virtual path or virtual channel availability between two DSL
devices. Segment flows are terminated at the connecting point which terminates a VP or VC segment.
End-to-end flows are terminated at the end point of a VP or VC connection, where an ATM link is
terminated. Segment loopback tests allow you to verify integrity of a PVC to the nearest neighboring
ATM device. End-to-end loopback tests allow you to verify integrity of an end-to-end PVC.
Note: The DSLAM to which the VMG is connected must also support ATM F4 and/or F5 to use
this test.
Note: This screen is available only when you configure an ATM layer-2 interface.
Figure 179 Maintenance > Diagnostic > OAM Ping
Chapter 40 Diagnostic
Logical connections between ATM devices
A bundle of virtual channels
A series of virtual paths between circuit end points
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