ABR service parameter

ABR services are services defined by the ATM Forum for the available bit rate (ABR) in Asynchronous Transfer Mode (ATM). The parameters for the ABR services determine the transmission and delay times, the bandwidth allocation and the number of RM cells.

The Trm parameters set the upper limit of the time between RM cells sent from an active source. The range for the Trm parameter is from `100* 2^-7` to 100*20.

The Xrm parameter limits the number of outbound RM cells sent in time. The limitation takes place only if no backward RM cells are sent. The range for the Xrm parameter and the Xrm Decrease Factor( XDF) is between 0 and 255.

The Nrm parameter determines the number of RM cells, which can be between 2 and 256. The Nrm value should be set higher if switches have a limited transmission capacity or if the transmission is real-time and therefore fluctuations in the data rate occur.

The Mrm parameter controls the bandwidth allocation between the RM cells in forward and reverse direction as well as the data cells.

In addition, there are ABR service parameters for increasing and decreasing the data rate and the cell transmission rate, such as the Rate Increase Factor( RIF) and the Rate Decrease Factor( RDF), the Permit Next Increase( PNI), the Time Out Factor ( TOF) and the Transmission Decrease Factor( TDF).

The Rate Increase Factor (RIF) determines how much the cell transmission rate can be increased when receiving an RM (Resource Management) cell. The Rate Decrease Factor (RDF) controls the reduction of the cell transmission rate.