Intel IXP400 manual Dependencies, Timing and Idle Cells

Intel® IXP400 Software

Access-Layer Components: ATM Transmit Scheduler (IxAtmSch) API

The client calls the VC queue update interface whenever the user of the VC submits cells for transmission. The structure of the VC queue update interface is compatible with the requirements of the IxAtmdAcc component.

The client calls the schedule-table-update interface whenever it needs a new table. Internally, IxAtmSch maintains a transmit queue for each VC.

IxAtmSch also provides a “VC queue clear” interface for use when the client wishes to cancel pending demand on a particular VC. This interface is useful, for example, when the client wishes to remove a VC from the system.

6.5.3Timing and Idle Cells

IxAtmSch does not rely on a hardware clock for timing. Instead, the component derives timing information from the supplied port transmit rate for each modeled ATM port. IxAtmSch assumes that T = 1/R seconds pass for sending every ATM cell. IxAtmSch also assumes that all cells scheduled in a schedule table are transmitted immediately following the cells previously scheduled by the scheduler on that port. (No cells — other than those scheduled by IxAtmSch — are being transmitted on the port.)

The client is responsible for calling “update table” in the following timely fashion, if the demand is always there. Suppose the “update table” calls for a port corresponds to time spending T(1), T(2),…, where one T(n) is the time needed to transmit cells scheduled in the n’th updated table. Then, if the demand is always there, the client must call the n’th “update table” before T(1)+T(2)+…+T(n-1) has passed, assuming the client’s first such call is at time 0. This can be easily achieved by making sure that port transmission is never empty when the demand is continuously pouring in.

When all registered VC transmit queues are exhausted, an empty schedule table is returned by the ixAtmSchTableUpdate interface. It is assumed that the client will instruct the lower layers to transmit idle cells until new cells are submitted for transmit on a registered VC. IxAtmSch is not aware of the number of idle cells transmitted in this situation and will reset its internal clock to its starting configuration when new cells are queued.

A further interface is provided to allow the client to update the transmit port rate of an ATM port which has already been registered with the IxAtmSch device, and may have established VCs with pending transmit demand. This interface is provided to cater for the event of line-rate drift, as can occur on transmit medium.

In the event that the new port rate is insufficient to support all established VC transmit contracts, IxAtmSch will refuse to perform this modification. The client is expected to explicitly remove or modify some established VC in this event, such that all established contracts can be maintained and then resubmit the request to modify the ATM port transmit rate.

Note: If UBR VCs are registered and they specify a PCR that is based on the initial line rate and the line rate subsequently changes to below the PCR values supplied for the UBR connections, the scheduler will still allow the port rate change.

6.6Dependencies

The IxAtmSch component has an idealized local view of the system and is not dependent on any other IXP400 software component.

April 2005	IXP400 Software Version 2.0	Programmer’s Guide
84	Document Number: 252539, Revision: 007