3Com CoreBuilder 7000, Руководство по эксплуатации

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ATM Layer 227
The ATM cell header consists of the following fields
:
The ATM Layer ATM is based on a layered architecture. In the protocol stack, the ATM
layer sits directly above the physical layer. Many physical layers can be
specified, including several for 100 to 155 Mbps. In the protocol
hierarchy, the ATM adaptation layer (AAL) sits above the ATM layer. LAN
Emulation (LE) sits above the AAL5.
The ATM layer manages the header for the ATM cell. It accepts the cell
payload from a higher layer, appends the header, and passes the resultant
53-byte cell to the physical layer. In addition, it receives cells from the
physical layer, strips off the header and passes the remaining 48 bytes to
the higher-layer protocols. The ATM layer provides for the transparent
transfer of fixed size ATM layer service data units between
communicating upper layer entities, implementing flow control over the
UNI.
Table 51 ATM Cell Header Fields
Field Full Field Name Description
GFC Generic Flow Control This field has local significance only and provides standardized local
functions (such as flow control) on the customer site. The value encoded
in the GFC is not carried end-to-end and may be overwritten by ATM
switches. It protects the header of the ATM cell, in addition to being used
for cell delineation. This defines where cells begin in a stream of cells, for
example, on the SONET interface
.
VPI/VCI Virtual Path Identifier/
Virtual Channel Identifier
The VPI/VCI value allows the network to associate a given cell with a given
virtual channel connection, so it can be routed to the correct destination.
The VPI/VCI is only meaningful in the context of the given network
interface (UNI or NNI).
PTI Payload Type Identifier This is a 3 bit field used to indicate whether the cell contains user
information or connection associated layer management information (F5
flow). The PTI also indicates a network congestion state or is used for
network resource management, and is used to mark the end of frame for
AAL5 adaptation layer.
CLP Cell Loss Priority In traffic management, cells with this bit set should be discarded before
cells without it set. The CLP bit can be set by the end station or ATM
switches for internal network control, or for VCI/VPI with low quality of
service cells that violate traffic management.
HEC Header Error Control The HEC field is used by the physical layer for detection and correction of
bit errors in the cell header. It is also used for cell delineation.