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IXP400 Software
Access-Layer Components: ATM Transmit Scheduler (IxAtmSch) API
Programmer’s Guide IXP400 Software Version 2.0 April 2005
Document Number: 252539, Revision: 007 81
6.4 Connection 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.