, tenth percentile performance once that average level is set. 27.9 Traffic Engineering Wireless telephony operates at the extreme end of traffic engineering in two important ways. First, our trunk groups are small, and they stay small even when our systems grow large. Instead of having small trunk groups growing large, we have more small trunk groups to more base stations. Second, we have a lot of options for traffic overflow. A call blocked on its primary route often has a secondary option, sometimes one less ef- ficient of system resources. An N = 4, three-sector GSM system with uniform coverage of uniform-density traf- fic has 40 channels serving each sector. As the system grows and cells split, the chan- nel sets are shared by large and small cells. Thus the sectors in a growing system are served by 16, 24, or 32 channels. As a third cell size is introduced, more of the cell sec- tors have smaller number of channels. The overflow possibilities, however, are encouraging. As shown in Fig. 27.21, we can send blocked traffic to other servers in three ways. We can overflow from one antenna face to another at the same base station, sector-to- sector overflow. We can overflow from a small server group to a larger server group, overlaid-cell overflow. We can overflow from one cell to a neighbor cell. Sector-to-Sector Small Cell Neighboring Cell Figure 27.21 Three kinds of traffic overflow. 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. Conventional Reuse Principles Capacity and Quality Principles These overflow techniques can be done actively at call setup by sending the user ter- minal to another server. Or they can be done passively in the handoff process by not handing off a call to a blocked server. As an example of the kind of traffic engineering we do, let us examine a system where half the user terminal traffic can be served on two cell