, to define the exclusive sector coverage. It is easy to use the terms sector and antenna face interchangeably, but the sector is the geographic region of coverage and the face is the antenna configura- tion defining the sector. Each antenna face is connected to a collection of radio equipment. The configuration and cost of this equipment can influence base station design. If the traffic engineering algorithms described in Chap. 23 tell us that 17 radios serve the demand and if 16 ra- dios fit on a radio frame, for example, then we might want to think about a small quality-of-service decrease to save the cost of an entire radio frame. 5.2.3 Server groups and extra cell radii When we discuss cellular growth principles in Sec. 27.7, we deal with varying reuse dis- tances in one channel set, as shown in Fig. 27.18. This is done by defining more than one cell radius, which we call a server group. When we draw the extra hexagonal lines in Fig. 27.18, the cellular engineer defines large-cell and small-cell server groups and then calculates the radio power level thresholds for handoffs between inner and outer cells. The extra, smaller cell radius is called an overlaid cell, a smaller cell colocated with the larger cell. Each cell radius is a server group. When this is done to a sectorized cell, the intersection of a face and a server group is called a logical face. The cell shown Fig. 5.2 has three faces, two server groups, and six logical faces. By using server groups and logical faces, the cellular engineer can choose some radio channels for small-cell reuse and leave others for large-cell reuse. Also, since each 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. Basic Wireless Telephony Key Radio Concepts Figure 5.2 Overlaid cell with six logical faces. radio can have its own independent radio threshold values, one radio frame can have radio channe