Intel IXP400 manual Connection Admission Control CAC Function

Intel® IXP400 Software

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

6.4Connection Admission Control (CAC) Function

IxAtmSch makes outbound virtual connection admission decisions based a simple ATM port reference model. Only one parameter is needed to establish the model: outbound (upstream) port rate R, in terms of (53 bytes) ATM cells per second.

IxAtmSch assumes that the “real-world” ATM port is a continuous pipe that draws the ATM cells at the constant cell rate. IxAtmSch does not rely on a hardware clock to get the timing. Its timing information is derived from the port rate. It assumes T = 1/R seconds pass for sending every ATM cell.

IxAtmSch determines if a new (modeled) VC admission request on any ATM port is acceptable using the following information supplied by its client:

•Outbound port rate

•Required traffic parameters for the new VC

•Traffic parameters of existing VCs on that port

IxAtmSch works on a first-come-first-served basis. For example, if three existing CBR VCs on the ATM port each use one-fourth of the port’s capacity (PCR = R/4), the fourth CBR VCC asking for 1/3 of the port capacity (PCR = R/3) will be rejected. IxAtmSch issues a globally unique VCC ID for each accepted VCC.

For non-CBR real time VCs — where the SCR and PCR values are different — only the SCR value is used to determine the required capacity for the VC. This is based on the principle that, over a long term, the required capacity of the VC will be equal to the SCR value, even if the VC may burst at rates above that rate for short periods.

Upon a successful registration via the CAC function, each VC is issued a port-unique identifier value. This value is a positive integer. This value is used to identify the VC to IxAtmSch during any subsequent calls. The combination of port and VC ID values will uniquely identify any VC in the processor device to the IxAtmSch component.

Programmer’s Guide	IXP400 Software Version 2.0	April 2005
	Document Number: 252539, Revision: 007	81