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Fig. 4 X 7798Integrated time-multiplex communieationsystem(The Bell System Technical Journal. July 195 Q )of price than of new and improved functional facilities. A handicap in theintroduction of electronic systems is that they would require entirely newtelephone sets. This might be compensated to some extent if keyset telephoneswere in any case to be made available.The very fact that new telephones will be required, and that no suitablesemiconductor has been found for the switching network, is the reason forBell's production of the ferreed. This is a relay with magnetic lock built ona core circuit of square-loop ferrite (fig. 3). The relay has two make contactsin two separate glass tubes. These relays are used in the switching network,while memory and logic units will be all-electronic. The relay is extremelyrapid and requires only one pulse of about 60 IJ.S to operate, so enables use tobe made of the speed of the electronic marker. At the same time still morerelays must be introduced for transmitter feed, ringing, ring-trip and clearing;the equipment is therefore quite far removed from all-electronic systems.Which way development will take - towards conventional systems with alimited interspersion of electronic elements, bastard systems such as theferreed system, or all-electronic systems - is not easy to say. There is stillanother possibility, namely an integrated switching and transmission system.A system of this kind has been presented by Bell Telephone Laboratoriesunder the name of ESSEX. It has time division multiplex throughout (fig. 4)with line concentrators based on pulse amplitude modulation (PAM). In theline concentrator a change is made to pulse code modulation (PCM), and allother communication in the system is on PCM basis.A more radical system has been proposed from Sweden. This system alsoemploys time division multiplex with PCM transmission between exchanges.In principle the subscriber's line network also works on a four-wire basis. Ongrounds of cost, however, four wires cannot be used physically, but ordinaryanalog transmission is used in one direction and delta modulation transmissionin the other. The delta modulation takes place in a central data processingequipment which stores the last amplitude value for each conversation. Thedifference between the present and the stored amplitudes constitutes the valuein the delta modulation sequence, which is forwarded to line concentrator andtelephone set. The telephone set demodulates the signal and the latter impingeson the receiver. It is thought that the system might embody the TASI methodentirely, so achieving a radical saving in circuits. This system may be said tobe the first integrated system in the full sense.**—00S3S9 111