3Com CoreBuilder 9000, User Manual

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ATM Layer 439
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 (LANE) 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.
No retransmission of lost or corrupted information is performed by this
layer. The ATM layer also provides the capability to indicate the loss
priority of the data carried in each cell (CLP).
The ATM layer expects the Physical layer to provide for the transport of
ATM cells between communicating ATM entities.
The ATM layer performs the following functions at the UNI:
Multiplexing among different ATM connections
Cell rate decoupling (unassigned cells)
Cell discrimination based on pre-defined header field values
Payload type discrimination
Loss priority indication and selective cell discarding
Traffic shaping
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 is also used to indicate a network congestion state or 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.
Table 71 ATM Cell Header Fields (continued)
Field Full Field Name Description