The Care and Feeding of ISDN4BSD
6.1.6 Configuring Fixed Unit-Length	6.1 The isdnd.rc Configuration	6.2 Command-line Options and

6.1.7 Configuring Variable Unit-Length Short hold Mode for Outgoing Calls

For variable unit-length short hold mode, idle-algorithm-outgoing must be set to var-unit-size and the selected unitlength and idletime-outgoing must be greater than 0 (zero). This short hold mode is suitable when your calls are billed on the elapsed time of the call plus a fixed connection charge. For example British Telecom bill this way.

Each call is divided into two periods, the first is the unchecked and the second is the checked period. The checked period starts 1 second before the first units time expires. During the checked period if there is no traffic for idle-time seconds the call is disconnected.

Experience shows that useful values for idle-time are from 15 to 30 seconds. If idle-time is too short an application that is not yet finished with the network will cause a new call to be placed.