ol element in a switch is considerably shorter than that for a trunk, and short sampling intervals (e.g., 10 seconds) or continuous monitoring are used to mea- sure usage. 4.2.1.2 Blockage, Lost Calls, and Grade of Service. Let's assume that an iso- lated telephone exchange serves 5000 subscribers and that no more than 10% of the subscribers wish service simultaneously. Therefore, the exchange is dimensioned with sufficient equipment to complete 500 simultaneous connections. Each connection would be, of course, between any two of the 5000 subscribers. Now let subscriber 501 attempt to originate a call. She/he cannot complete the call because all the connecting equipment is busy, even though the line she/he wishes to reach may be idle. This call from subscriber 501 is termed a lost call or blocked call. She/he has met blockage. The probability of encountering blockage is an important parameter in traffic engineering of telecom- munication systems. If congestion conditions are to be met in a telephone system, we can expect that those conditions will usually be encountered during the BH. A switch is dimensioned (sized) to handle the BH load. But how well? We could, indeed, far overdimension the switch such that it could handle any sort of traffic peaks. However, that is uneconomical. So with a well-designed switch, during the busiest of BHs we can expect moments of congestion such that additional call attempts will meet blockage. Grade of service4 expresses the probability of meeting blockage during the BH and is commonly expressed by the letter p. A typical grade of service is p = 0.01. This means that an average of one call in 100 will be blocked or "lost" during the BH. Grade of service, a term in the Erlang formula, is more accurately defined as the probability of blockage. It is important to remember that lost calls (blocked calls) refer to calls that fail at first trial. We discuss attempts (at dialing) later-that is, the way blocked calls are handled. We exemplify grade of service by the following problem. If