l about. 23.4 Network Routing for Increased Capacity Consider the three nodes in a 2 percent blocking network in Fig. 23.6 with 5.0 erlangs of demand between each pair of nodes. According to Table 23.1, 10 channels are needed to meet a standard of 2 percent blocking for 5.0 erlangs. This means 10 channels on each of the three pathways, a total of 30 channels. Suppose, however, that there is an alternative route, some other path to try. If its di- rect, primary route is not available, then a call is sent on a less direct, secondary route on the other two sides of the triangle. The telephony buzzword for a multiple-link path is a tandem route. Let us now engineer each link for a primary blocking rate of 10 percent and do a "back of the envelope" calculation. Ten percent of the traffic is now using two links in- stead of one, so each link must be engineered for an extra 10 percent, 5.5 erlangs in- stead of 5.0. Ten percent blocking for 5.5 erlangs requires eight channels on each link. When a primary link is blocked, each secondary link is blocked 10 percent of the time, so the secondary route has 20 percent blocking. Thus the network blocking rate is 10 percent for the primary route and 20 percent for the secondary alternative, a 2 Figure 23.6 Three-node network. Downloaded from Digital Engineering Library @ McGraw-Hi (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. Traffic Engineering for Voice and Data Capacity and Quality Principles percent blocking rate, 0.10 X 0.20 = 0.02. The overflow network achieves the same 2 percent blocking rate with 8 channels on each link instead of 10. For the sake of this three-node example, we assumed that the distributions on the three paths are Poisson, that the three links have independent blocking behavior, and that we can add blocking probabilities for tandem paths. All three of these are reason- able enough approximations to make the point that overflowing calls t