3Com CoreBuilder 9000, Руководство пользователя

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440 A
PPENDIX
C: P
ROTOCOLS AND
I
NTERFACES
The ATM layer supports fault management, including alarm surveillance
and connectivity verification functions. OAM cells are used for
exchanging operation related information. ATM cells with valid VPI/VCI
values are discarded and Layer Management is informed. Traffic
management at the public UNI monitors traffic on each connection and
takes the proper action to protect the network resources and ensure that
the user-network traffic agreements are respected.
Operation and
Maintenance
(OAM)
The Operation and Maintenance (OAM) functions associated with the
155 Mbps UNI provide for transmission and reception of maintenance
and low level link performance monitoring between ATM devices. The
functions of the OAM are handled at the Physical Layer as well as the
ATM Layer.
The OAM works on a hand-shake basis, where a signal sent from one
end, is received and answered by the other end, resulting in an
end-to-end bidirectional flow.
Physical Layer OAM
The Physical Layer OAM information is carried in the Physical Layer
Overhead Unit together with byte and frame synchronization (F1/F3 as
part of the SONET frame). The Physical Layer OAM provides:
Maintenance signals for the physical layer to indicate the detection
and location of a transmission failure.
A link transmission performance monitoring signal for the physical
layer to detect and report link transmission errors. This signal provides
a low level indication of degraded link error performance.
ATM Layer OAM At the ATM layer, the F4-F5 flows are carried via OAM cells. The OAM cell
flow for end-to-end management functions is carried transparently
through the private ATM switch and made available to the user. The F4
flow is used for segment or end-to-end management at the VP level. The
F5 flow is used for segment or end-to-end management at the VC level
using Payload Type (PT) code points.
The virtual path connection (VPC) operational information is carried via
the F4 flow OAM cells. These cells have the same VPI value as the
user-data cells but are identified by preassigned VCI values. Two unique
VCI values are used for every VPC. The VCI value 3 identifies the
connection between user and network on both sides of the UNI. VCI
value 4 identifies the connection between user end stations.