Chapter 3 Switching and switch control 3.1. Introduction

Figure A.3.4 Bandwidth requirements for different telecommunications servicesThe requirement for good network economy applies especially to the capacity of theswitching equipment to handle multiplexed traffic that consists of packets. The packetmodetechnique is a consequence of the requirement for more efficient use of lines.This technique can also be used to integrate services. For example, ATM technologyenables both integrated transmission and integrated switching.Different switching techniques are dealt with in Sections 3.3 - 3.6.3.2. historyA.3.2.1 Manual systemsIn the infancy of telephony, telephone exchanges were built up with manually operatedswitching equipment. The first manual exchange was installed in New Haven, USA, in1878. The operators received calls and switched them manually to the calledsubscriber (the operator set up a "circuit" between two subscribers, hence the termcircuit switching). When the call was complete, the operator released the connection.We might say that the operator was the "control system" of that era. (See note inSubsection 3.1.1.)A.3.2.2 Electromechanical systems

In the years that followed, the manual exchanges were replaced by automaticelectromechanical switching systems. Although these systems required moremaintenance, their overall impact was positive since the number of operators could bereduced significantly (resulting in cut labour costs). The systems provided increasedtraffic capacity at a lower cost, preparing the way for a continued rapid expansion ofthe telecommunications network. The new systems also made it possible to routetraffic more efficiently through the transmission network, reducing the need for cablecapacity.Almon B. Strowger, Kansas City, USA, is regarded as the father of automatic switching.In 1889, he applied for a patent for an automatic telephone exchange. Since then,Strowger's name has been associated with the step-by-step selector (controlleddirectly from the dial of the telephone set) which was part of his idea.Afterwards, developments followed in the direction of register-controlled systems, inwhich the number information does not directly control selector set-up (as in the stepby-stepselector), but is first received and analysed in a register. One of the functionsof the register is to select alternative switching paths, meaning that the transmissionnetwork can be used more efficiently.Examples of register-controlled systems are:• the 500-line selector (1923); and• crossbar systems (1937).3.2.3 Digital, computer-controlled systemsTelephone exchangesAs time went by, transmission as well as switching systems continued to develop,contributing to the total economy. A technique for saving expensive connections wasintroduced into the long-distance networks: frequency division multiplexing (FDM).This technique was developed around 1910, but was not implemented until 1950 whenabout 1,000 channels were transferred on the same cable (the coaxial cable).Digital multiplexing (based on PCM), which was introduced around 1970, also madetransmission networks less expensive while at the same time improving transmissionquality. Costs were further reduced when digital group switches (the actual switchingequipment in the telephone exchange) were combined with digital transmissionsystems, eliminating the need for many relatively expensive analog-digital (A/D)converters. It now became necessary to computerise the control of the exchanges, andbefore long, not only the group switches but the entire exchanges were digital. Thefirst computer-controlled exchange was put into service in 1960 in the US; the firstdigital exchange in Europe was opened for traffic in 1968 (Tumba, Sweden). See alsoSubsection 3.3.1.Today's telephone exchanges use circuit switching technology, just like theirpredecessors at the end of the 19th century.Special nodes for data communication

The strong growth in data traffic and in the number of users of data communicationhas resulted in the development of separate data networks and data switches. In manycases, these can meet users' increasingly stringent quality requirements and the needfor higher transmission rates in a better and less expensive way. Packet mode andframe relay, for example, provide efficient network utilisation, enable packets to beretransmitted when errors occur on a link (applies to packet mode only), and allow forsorting, routing, and buffering.Nodes for N-ISDNDevelopments for providing service-integrated networks (for voice, video, and dataservices) require both public and private N-ISDN nodes (N stands for narrowband). Inprinciple, a complete ISDN node can be seen as a combination of today's telephoneexchanges and packet data switches (circuit switching and packet mode), with animportant sorting function for subscriber traffic.Nodes for B-ISDNThe technology called ATM, which applies the cell-switching technique and which formsthe basis of B-ISDN (B stands for broadband), is not yet completely standardised. Seealso Part G of Volume 2, where ATM and B-ISDN are described.Optical switchesIt is primarily the switching equipment that limits the bandwidth of a connection.Today, we can make use of very high bit rates, up to tens of billions of bits per second(tens of Gbit/s) in optical transmission systems. However, in switching equipment, wemust change over to electrical signals and considerably lower bit rates.The next step is to use optical switching with electronic switch control. And in time, wewill most assuredly have fully optical switching systems. Indeed, in view of theintensive research and development that is being carried out in this area, it should notbe long before the first optical space switches are commercially available.Figure A.3.5 describes technical developments in the field of switching (publicswitching only).

Figure A.3.5 Developments in switching