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IXP400 Software
Access-Layer Components: ATM Transmit Scheduler (IxAtmSch) API
April 2005 IXP400 Software Version 2.0 Programmer’s Guide
82 Document Number: 252539, Revision: 007
6.5 Scheduling and Traffic Shaping
6.5.1 Schedule Table
Once an ATM port is modeled and VCs are admitted on it, the client can request IxAtmSch to
publish the schedule table that indicates how the cells — on all modeled VCs over the port — will
be interleaved and transmitted.
IxAtmSch publishes a scheduling table each time its scheduling function is called by a client for a
particular port. The schedule table data structure returned specifies an ordering on which cells
should be transmitted from each VCs on the port for a forthcoming period. The client is expected to
requests a table for a port when the transmit queue is low on that port.
The number of cells that are scheduled by each call to the scheduling function will vary depending
on the traffic conditions. The schedule table contains an element, totalCellSlots, which specifies
how many cell slots are scheduled in this table returned, including idle cells.
When the client calls the schedule function, the scheduler assumes that all previously scheduled
cells on this port have been transmitted and that it may overwrite the previous schedule table with
the new table. The client, therefore, must not be dependent on the integrity of the previous table
when a request is made for a new schedule table. Additionally, the client should ensure that the
current schedule table has been processed by the transmit mechanism before it requests for a new
table.
Figure 25. Multiple VCs for Each Port, Multiplexed onto Single Line by the ATM Scheduler
IxAtmSch Component
VC 2
Port 2
VC 4
Port 3
Schedule Table for
PORT 1
VC 1
Port 1
Schedule Table for
PORT 2
Schedule Table for
PORT 3
VC 3
Port 1
VC 5
Port 1
VC 6
Port 2
VCs submit demand
for transmit of ATM
cells.
IxAtmSch component
determines when to
schedule each cell
on the physical port.
Cells are queued for
transmission on each
port based on this
schedule table, such
that all traffic contracts
are fulfilled.
B2298-01