that radio conditions may change and drive out one of the users in the sector. One of the frustrations of cellular engineering is that even something as simple as blocking rate is hard to measure. It seems like the simplest thing in the world: Count the number of calls that are blocked, divide by the total call attempts, and multiply by Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Engineering Concepts Telephony Engineering Concepts 100 for a percentage. The problem is that people whose calls are blocked try again and again rather than going away quietly. If the average caller tries three times before giv- ing up, then the measured blocking rate could be three times the true blocking rate. Measuring ineffective attempts in a wireless system is particularly difficult when some of those are due to poor radio signal. If the original call attempt message is lost, then the system does not even detect a call attempt. Also, a cellular telephone with a defective radio or a broken antenna wire will perform poorly, but this is indistinguish- able from a user being almost out of range. Thus it is difficult to distinguish failures user equipment from errors in wireless system planning or engineering. 12.2.2 Lost calls Once a connection is formed, it stays until one party ends the call. In wireless net- works, a changing radio environment or a moving radio telephone can cause signal quality to drop and a wireless call to be lost. As in the case of ineffective attempts, measurement is vague for lost calls. Long calls are more likely to be dropped not only because of their greater time exposure but also because their radio environment changes more. We would expect 8-minute calls to ex- perience more user motion and more radio environment change and SO to be lost more than twice as often as 4-minute calls, but we have no data on this. And a wire