Question

What are the different ways of designing 100 line exchange using uni selector and two motion selectors? Show at least three variations. Which is the best option?

Answer 1

Here three different designing methods for 100 line exchange are discussed: Design 1: Here, Strowger switching system is designed using one two-motion selector for each subscriber. A subscriber is assigned a number in the range 00-99, and the same number is used to identify the two-motion selector assigned to him. The 100 outlets of each two-motion selector are numbered as per the scheme given in table. The corresponding outlets in all the 100 two-motion selectors are commoned and folded back to the corresponding inlets. For example, a subscriber with 87 as his number is assigned the two-motion selector 87. The outlet 87 which corresponds to this subscriber is connected to the 7 th contact in the 8th vertical position of all the two-motion selectors and folded back to his inlet. The arrangement is shown in Fig. If subscriber 23 dials 87, his two motion selector 23 would step vertically 8 times corresponding to the first digit and would step horizontally 7 times to reach the contact to which the subscriber 87 is connected. This switch is non- blocking and uses only one stage of switching elements. Since the two-motion selector is activated by the calling party, the call is terminated only when the calling party disconnects the line. If a two-motion selector goes out of order, the subscriber connected to it will not be able to make any outgoing calls but can receive incoming calls. The design parameters of this switch are:

S =100, SC =50, K = 1, TC =1,

 EUF = 0.5, C= 200, CCI =25, PB =0. 

 

Design 2: Instead of 100 two-motion selectors as in the case of Design 3, let us consider only 24 two-motion selectors. In the case, 24 simultaneous calls can be put through the switch. The 24 two-motion selectors are shared by all the hundred users. The corresponding outlets of the two-motion selectors are commoned as in the previous case. It is implicitly assumed here that the average peak-hour traffic is 24 simultaneous calls. Typically, a 24-outlet uniselector is used as a selector hunter. Each of the 24 outlets is connected to one two-motion selector. Thus, a subscriber has access to all the 24 two-motion selectors in the system. The corresponding outlets of all the selector hunters are commoned and thus, all subscribers have access to all the two-motion selectors. This scheme is shown in Fig. The call establishment in this scheme takes place in two steps. Firstly, when the subscriber lifts his receiver handset, his uni selector hunts through the contact positions and seizes a free two-motion selector. At the next step, the two-motion selector responds to the dial pulses for appropriate connection. The design parameters of this system are: 

 

Design 3: In this design, there are 24 line finders. If any of the 100 subscribers has to get access to any of the 24 two-motion selectors, it is essential that every line finder is capable of reaching any of the 100 subscribers. In other words, each line finder must have 100 outlets. For this purpose, two motion selectors have to be used as line finders. The configuration is shown in Fig. The corresponding outlets of the line finders are commoned. Similarly, the outlets of the numerical selectors are also commoned. 

When the start condition is received, the line finder is caused to hunt vertically until the wipers reach a marked level. The vertical hunting is then stopped and the horizontal hunt commences to find a particular marked contact in that level. It may be noted that in the extreme case, the line finder may have to take 20-steps -10 vertical and 10 horizontal –before a line is found. The line finders are made to step automatically, using interrupter contact mechanism. When the line finder locates the subscriber line, the start condition is removed, the allotter switch steps on to the next free line finder in readiness for further calls, and the numerical selector sends out the dial tone to the subscriber in readiness to receive dialing pulses. Thereafter the establishment of the connection proceeds in the usual manner. Obviously, Design 3 is by far the best for a 100-line exchange. If we had used uni selectors as line finders, it would have been necessary to divide subscriber’s line into small groups of, say 24 each. Such designs involving groupings, function efficiently only under certain specific traffic conditions and generally lead to higher blocking probabilities.