 Let's continue with the discussion of the evolution of the traditional telecom market. The first natural understanding of the telecommunication market comes from the usage of telephones. Now if you look at the telephone market, it has evolved not only in terms of the usage, but it has simultaneously involved in terms of the behavior of the users. So it means that eventually it is customer driven. In this module, we shall look at how it was the need that evolved the technology itself and how the availability of the technology in return changed the behavior of the users. We'd look at some very basic standards which were, although competing with each other, were trying to address the market that had evolved from the users. Let's look at switching first. Traditionally, the telephones right from the time of Alexander Graham Bell had been largely analog. It was thought that the first telephone that was invented by Alexander Graham Bell along with his friend, Mr. Watson, was a simple one, a two-directional link. That is, it was a half-duplex link. But it was never meant to be that limited. After all, the very purpose of doing that invention was to spread it amongst potential users. That has created a multiparty scenario as we see it today. Therefore, the need for switching arose. In original terms, the switching was meant to be a way to adjust the wide variety and the diversity of the users who call or to whom the call is made. These are known as the call party or the calling party. To begin with, mechanical stepwise switching was done, for instance, in stroger switches. But since it was mechanical, it had its own delay and mechanical malfunctioning issues. So eventually, the electrical systems replaced stroger in the form of crossbar. For over 100 years, till early 90s, crossbar switches were still being used. In both of these schemes, it was actually implemented to be an electrical system. But since the voice call was analog and the management of the voice calls through crossbar switches was digital, it was somewhat known as quasi-electronic switching. The next important evolution that took place in telephony was from migration from analog voice circuits to digital voice circuits. As you know, that human voice is sampled, encoded, and eventually digitized for storage and transmission in the form of pulse code modulated signals. Since this digital analog to digital conversion process was cumbersome, so it was not largely available on the local loops. For most part, the caller party and the call party last mile was mainly analog. Initially, the long haul communication, that is, from an exchange to another exchange carrying bulk traffic, that is, a number of users simultaneously utilizing multiplexed common channel was also analog. Resultantly, this caused signal degradation. The repeaters could not be used because repeaters are not known to have a very good performance in terms of not adding noise to the original signal. And lastly, these signals would interfere with each other. Consequently, the first phase of digitalization took place at the exchange level where the analog signals from the customer premises or the last mile were terminate, the digitalization from analog to digital would take place there, and from thence onwards, the digital signals would be sent to the other exchange. There, the digital to analog conversion was then carried out. Then new concept of integrated services, digital network evolved, that was why not to provide digital services right at the doorstep, that is, the local loop also had to be digitized. With the availability of better twisted pair right to the doorstep, that was known as the good pair, it was thought to provide digitized voice service right at the doorstep. For that, two bearer channels of 64 kilobits per second and a D channel or the delta channel of 16 kilobits per second was integrated into a single household, that is per household. Since integrated services, digital network somehow emerged around the same time frame when the internet services were also getting widely popular. As a consequence, ISDN started lagging behind in terms of service provisioning. After all, internet came with a lot of promises from surfing to intelligent browsing to chatting to wash over IP. So ISDN fell short of providing these services at high data rates because it was meant to provide services at 64 kilobits per second each. At the same time, some intelligent people really came up with an idea to digitize the subscriber so that the subscriber actually determines what data rate and what particular service category does it need. So integrated services, digital network was replaced by the digital subscriber line. The digital subscriber line paved way for the worldwide web services inclusive of video and wash over IP. The striking feature of DSL is that it is subscriber centric, it can be either symmetric or asymmetric depending upon the user requirements and the very success of DSL can be determined from the fact that it is still being used as of today.