nternal or external disturbances, thus improving reliabilities (and surviv- ability) from the levels on the order of 90% to 99% to levels on the order of 99% to 99.9% or better. Switching cannot carry out this task alone. Constraints have to be iter- ated or fed back to the transmission systems, even to the local area. The transmission system must not excessively degrade the signal to be transported; it must meet a reliabil- ity constraint expressed in MTBF (mean time between failures) and availability and must have an alternative route scheme in case of facility loss, whether switching node or trunk route. This latter may be termed survivability and is only partially related to overflow (e.g., alternative routing). The single transcontinental main traffic route in the United States suggested earlier has the drawback of being highly vulnerable. Its level of survivability is poor. At least one other route would be required. Then why not route that one south to pick up drops and inserts? Reducing the concentration in the one route would result in a savings. Capital, of course, would be required for the second route. We could examine third and fourth routes to improve reliability-survivability and reduce long feeders for concentration at the expense of less centralization. In fact, with overflow, one to the other, dimensioning can be reduced without reduction of overall grade of service. 8.3.3 Link Limitation From a network design perspective a connectivity consists of one or more links in tandem. 2 We define a link as the transmission facilities connecting two adjacent switches. CCITT in Rec. E.171 (Ref. 1) states that for an international connection there shall be no more than 12 links in tandem. This is apportioned as follows: 4 links in the calling party's country; 4 links in the called party's country; and 4 international links. This concept is illustrated in Figure 8.3. Complete international telephone connection Local Local system Four-wire chain system Subscriber's set Four-wire switching exchange Internatio