worse than 1 X 10-3 for a period of 10 consecutive seconds. These 10 seconds are considered to be unavailable time. A new period of available time begins with the first second of a period of 10 consecutive seconds, each of which has a BER better than 10-3 (Ref. 2). A common time distribution is 99.99% of a month to be in conformance with ITU- R/CCIR recommendations (ESR should not exceed 0.0032 of any month). This time distribution translates directly into time availability, which is the percentage of time a link meets its performance criteria regarding propagation. 9.2.3.3 Site Selection and Preparation of a Path Profile 9.2.3.3.1 Site Selection. In this step we will select operational site where we will install and operate radio equipment. After site selection, we will prepare a path profile of each link to determine the heights of radio towers to achieve "line of sight." Sites are selected using large topographical maps. If we are dealing with a long system crossing a distance of hundreds of miles or kilometers, we should minimize the number of sites involved. There will be two terminal sites, where the system begins and ends. Along the way, repeater sites will be required. At some repeater sites, we may have need to drop and insert traffic. Other sites will just be repeaters. This concept is illustrated in Figure 9.4. The figure shows the drops and inserts (also called add-drops) of traffic at telephone exchanges. These drop and insert points may just as well be buildings or other facilities in a private/corporate network. There must be considerable iteration between site selection and path profile preparation to optimize the route. In essence, the sites selected for drops and inserts will be points of traffic concentration. There are several tradeoffs to be considered: 1. Bringing traffic in by wire or cable rather than adding additional drop and insert (add-drop) capabilities at relay point. This provides additional traffic concentration. Bellcore, Bell Communications Research, Piscataway, NJ. Now called Te