office called a digital subscriber line access multiplexer (DSLAM), which moves the data off the PSTN onto the PPDN. As a result, PSTN capacity usage was re- duced, and average call length dropped back toward its historical norm. 23.6 Reliability and Redundancy Traffic engineering to meet demand at minimum cost for a growing system is only part of the problem. The other part is making sure the system meets its standard for relia- bility. Customers blocked because facilities are all in use can retry their calls a few min- utes later, but customers can be blocked for hours by equipment failures. The reliability requirement for a telephone system can be a sensitive issue because it depends on how people are using their telephones. As an example from another area, when toy pocket calculators evolved into electronic pocket planners, rigorous backup systems were added. Not having a pocket calculator handy to compute the tip on a restaurant check is in a different class from having a personal digital assistant (PDA) break down and lose all the information in its memory. Many cellular users rely on their mobile telephones, and fixed wireless local loop is typically a primary household telephone with the same reliability standards we came to expect of our landline telephones. This becomes an important issue because reliability is expensive. If a base station stops working, then some of its users will be able to use other base stations, perhaps with a lower quality of service, and the others will lose their service entirely. Either al- ternative means unhappy customers. The equipment reliability modeling concepts for the base station in Sec. 4.8 apply to other equipment. The network itself can be designed for reliability as well. The same traffic overflow that improves blocking probabilities also can improve reliability, but the facilities have to have redundant paths. Installing extra pathways in a telephone network means curring extra cost, so the network designer has to decide how much it is worth to pre- vent a total f