eaks at both ends. In such cases it is possible to direct traffic SO that peaks of traffic in one area (time zone) fall into valleys of traffic of another area. This is called taking advantage of the noncoincident busy hour. The network design can be made more optimal if configured to take advantage of these phenomena, particularly in the design of direct routes and overflow routes. 1.4 QUALITY OF SERVICE Quality of service (QoS) appears at the outset to be an intangible concept. However, it is very tangible for a telephone subscriber unhappy with his or her service. The concept of service quality must be covered early in an all-encompassing text on telecommunications. System designers should never once lose sight of the concept, no matter what segment of the system they may be responsible for. Quality of service means how happy the telephone company (or other common carrier) is keeping the customer. For instance, we might find that about half the time a customer dials, the call goes awry or the caller cannot get a dial tone or cannot hear what is being said by the party at the other end. All these have an impact on quality of service. So we begin to find that QoS is an important factor in many areas of the telecommunications business and means different things to different people. In the old days of telegraphy, a rough measure of how well the system was working was the number of service messages received at the switching center. In modern telephony we now talk about service observation. The transmission engineer calls QoS customer satisfaction, which is commonly mea- sured by how well the customer can hear the calling party. The unit for measuring how well we can hear a distant party on the telephone is loudness rating, measured in decibels (dB). From the network and switching viewpoints, the percentage of lost calls (due to blockage or congestion) during the busy hour certainly constitutes another measure of service quality. Remember, this item is denominated grade of service. One target figure for grade of ser