 we know that there are 345 seizures (i.e., lines connected for service) and 6 blocked calls (i.e., lost calls) during the BH, what is the grade of service? Grade of service = Number of lost calls/Number of offered calls = (354 + 6) = 6/360 p 2 0.017. (4.2) The average grade of service for a network may be obtained by adding the grade of service provided by a particular group of trunks or circuits of specified size and carrying a specified traffic intensity. It is the probability that a call offered to the group will find available trunks already occupied on first attempt. This probability depends on a number of factors, the most important of which are (1) the distribution in time and duration of offered traffic (e.g., random or periodic arrival and constant or exponentially distributed holding time), (2) the number of traffic sources [limited or high (infinite)], (3) the availability of trunks in a group to traffic sources (full or restricted availability), and (4) the manner in which lost calls are "handled." Several new concepts are suggested in these four factors. These must be explained before continuing. 4 Grade of service was introduced in Section 1.3.4. 4.2 TRAFFIC INTENSITY DEFINES THE SIZE OF SWITCHES AND THE CAPACITY OF TRANSMISSION LINKS 4.2.1.2.1 Availability. Switches were previously discussed as devices with lines and trunks, but better terms for describing a switch are inlets and outlets. When a switch has full availability, each inlet has access to any outlet. When not all the free outlets in a switching system can be reached by inlets, the switching system is referred to as one with limited availability. Examples of switches with limited and full availability are shown in Figures 4.2a and 4.2b Of course, full availability switching is more desirable than limited availability, but is more expensive for larger switches. Thus full availability switching is generally found only in small switching configurations and in many new digital switches (see Chapter Grading is one method of improving the traf