Understanding the network.pdf - Back to Home

endpoints. A Virtual Path Link (VPL) exists between each ATM switch, and
each VPL has its own Virtual Path Identifier (VPI). Contained in the VPL are
the virtual channels set up between the ATM endpoints. Each ATM VC has its
own Virtual Channel Identifier (VCI). As the ATM cells are passed from switch
to switch, each ATM switch constructs its own VPI/VCI translation table to
perform remapping for each cell that passes through. Depending on the ATM
switch type, the switch might use the VPI alone or both the VPI and VCI to
determine what cells are the next destination point in the path.
• Payload Type (PT)—This field indicates if the cell contains control information
or user data. The PT field is 3 bits in size. The first bit indicates the payload,
the second bit indicates the congestion state of the network, and the third bit
indicates management information relating to the cell's data.
• Cell Loss Priority (CLP)—This field indicates if the cell can be discarded in the
event of congestion on the network.
• Header Error Check (HEC)—This check is performed by the physical layer
interface to see if there are errors in the ATM cell header. The checksum of
the header is placed in this field.
As user data flows down from the ATM adaptation layer, the ATM layer constricts the
cell header with the appropriate VPI/VCIs for the first segment of the delivery.
VPI/VCI Translation, Traffic, and Network
Management
After the cells are formatted, they are prioritized in accordance to the VC's QoS
contract, multiplexed into a cell stream, and handed to the PHY layer for
transmission over the physical transport. When they reach the next hop in the VC
path, the cell stream is demultiplexed, and new VPI/VCIs are appended to the cells.
Then they are multiplexed again into a stream and handed back to the PHY layer for
transport. At each junction of the delivery path, the ATM layer must maintain the
QoS requirement for the VC.
The ATM layer is responsible for monitoring the network cell flow and ensuring that
adequate resources exist to meet the QoS requirements of the various VCs. The QoS
contract is established when the VC is set up. This contract defines a set of traffic
parameters that must be maintained to meet the QoS requirements of the VC:
• Peak Cell Rate (PCR)—Defines the fastest rate that cells for the specified VC
can be introduced into the PHY bitstream
• Sustainable Cell Rate (SCR)—Defines the average rate that cells for the
specified VC can be introduced into the PHY bitstream
• Maximum Burst Size (MBR)—Defines the duration of time that a specified VC
can accept cells at the PCR