fic-handling capabilities of switching con- figurations with limited availability. Grading is a scheme for interconnecting switching subgroups to make the switching load more uniform. Trunk Inlets Outlets Trunk Figure 4.2a An example of a switch with limited availability. Inlets Outlets Figure 4.2b An example of a switch with full availability. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK 4.2.1.2.2 "Handling" of Lost Calls. In conventional telephone traffic theory, three methods are considered for the handling or dispensing of lost calls: 1. Lost calls held (LCH) 2. Lost calls cleared (LCC) 3. Lost calls delayed (LCD) The LCH concept assumes that the telephone user will immediately reattempt the call on receipt of a congestion signal and will continue to redial. The user hopes to seize connection equipment or a trunk as soon as switching equipment becomes available for the call to be handled. It is the assumption in the LCH concept that lost calls are held or waiting at the user's telephone. This concept further assumes that such lost calls extend the average holding time theoretically, and in this case the average holding time is zero, and all the time is waiting time. The principal traffic formula (for conventional analog space division switching) in North America is based on the LCH concept. The LCC concept, which is primarily used in Europe or those countries that have adopted European practice, assumes that the user will hang up and wait some time interval before reattempting if the user hears the congestion signal on the first attempt. Such calls, it is assumed, disappear from the system. A reattempt (after the delay) is considered as initiating a new call. The Erlang B formula is based on this criterion. The LCD concept assumes that the user is automatically put in queue (a waiting line or pool). For example, this is done, of course, when an operator is dialed. It is also done on all modern digital switching systems. Such switches are computer-based for the "brains" of the control functions and ar