ing it, and subscribers do not have to travel far to be outside their own MSC regions. This may not be roamer service on the subscribers' cellular telephone bills, but it is roamer service so far as the system is concerned; the call comes into one MSC and is served by another. These MSC-to-MSC connections may have dozens of dedicated DS-1 links to avoid pay- ing PSTN connection rates. We have to remember, in designing these connections, that high blocking rates may be perfectly acceptable. The issue is pure economics rather than customer satisfaction, and the minimum-cost solution may have 10 percent of the calls on the PSTN. Design- ing an MSC-to-MSC link with 10 percent blocking may make a telephone engineer un- comfortable, but it will be the right decision in many of these connections. 26.3.2 Adjusting capacity for mobile-to-mobile calls The case of calls from one wireless user terminal to another is not, by itself, a wireless system function. A subscriber calls another subscriber, and the system has no obliga- tion to handle the call in a special way. The call can be sent to the PSTN for normal handling. This is in contrast to roamer terminations, where the wireless telephone sys- tem has to handle the call in a special way. Mobile-to-mobile calls represent an oppor- tunity to save a lot of money in PSTN charges. If a call is between two subscribers at the same MSC, an intra-MSC call, then it re- ally seems obvious that the call should not be routed through the PSTN. The MSC has loopback trunks so that the calls can go out and come back in without going anywhere 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. Capacity Calculations for Cellular Networks Capacity Calculations for Cellular Networks and without incurring PSTN charges. 9 The MSC could be designed with special soft- ware for intra-MSC calls, but it is easier to use loopback t