 In this module, we shall cover public switch telephone networks which form the basis onto which modern next generation networks are laid. Specifically, we shall cover the structure of PSTNs, the services which they offer, and the resultant architecture which offers these services. And we'd also just look at the market which exists as of today starting from the onset of PSTNs. First of all, we look at PSTNs from the components part of view. Broadly categorizing, PSTNs can be defined as comprising the switching elements and the transmission systems. The switching elements you can call them as exchanges are pivotal to the operation of commercial many users to many users PSTNs. And in order to deliver the voice specifically, the transmission systems are integrated along with these exchanges. Let's just look at the PSTNs from an end-to-end structure specifically from the telephony part of view. If we start from the left-hand side, we notice that from the telephone we have a local loop which is the analog transmission line. It could be copper cable either in the form of twisted pair or coaxial cable as well. But mostly it is the twisted pair which is used. That is connected to the end office. This end office could in turn connect to the toll office which is the local exchange either through a digital means or through analog means. Now the toll trunking is the end point or the intermediate point where the transactions are determined on to which the money is charged. That is, it's a billing system. From the toll office, then the switching systems or switching offices are connected to. And you can see from the diagram that the increase in the width of these lines actually shows that the requirement of bandwidth increases as the traffic from individual telephones is aggregated. Now these intermediate switching offices are the exchanges which connect to other exchanges. There is a requirement for very high bandwidth and that is why sometimes fiber optic cable is used and it is being widely used today but back in old days it was also being used. And then you can see the reverse of everything that we've just seen. So it is a symmetric system. That is, if voice is transmitted in analog from the left hand side, it is terminated on to the right hand side in the analog form. And if it's a digital, purely digital system, then the digital call is received on the receiving side. The basic rationale behind the whole infrastructure for PSTNs was to reduce the number of wires. As you can see in A, it's a full mesh. And then you see a star, but when we move from full mesh to star, we do reduce the number of wires, but there is a consequence that the length of each wire becomes uncontrollably or unwieldy. That is why we need to have a hierarchical structure as shown to you in C here. You can see that it is a hierarchical kind of arrangement in which we have local exchanges, we could have regional exchanges and then we could have country level exchanges as well. So this is the biggest rationale on the basis of which the PSTNs are realized. Now if you look at the typical internet connectivity, how the data is delivered from the computer to the ISP and from the ISP server back to the computer, we can see that some additional hardware is incorporated in PSTNs. Let's start from the left hand side once again. We have a computer through which is connected to a modem and this modem is connected to the analog line. Depending upon the availability of the communication medium which is available such as the twisted pair, the modem is geared to that technology. Now the modem is usually connected to the toll office or we say the network side or we can say the exchange through some kind of intermediate transition or what we call as the Kodak. Because sometimes the modem may not be speaking the same language that the toll office may be able to understand. So we need some kind of encoding decoding activity. Here in the toll offices we see again it's a network of exchanges. All these exchanges are connected to one another. Now if you want to communicate something from the computer to the ISP, we would move on to the figure and we'll see that the Kodak that we used on the modem side also needs a sister Kodak here on the ISP side. So we see that the decoding is done here on the top right section where we have the decoding done and it is then sent on to the server of the ISP. Likewise when something is sent back to the computer from the ISP it follows the same procedure. On the right hand side of the figure you can also appreciate that there are different ways through which these servers could be connected to computers because depending on the local network and local infrastructure these ISPs could be connected to a computer which is transmitting through the modem either through specific digital lines as you see for ISP 2 or if the ISP 1 doesn't have the digital line it has to use the modems. So this particular data connectivity is a norm that we have witnessed and we have sometimes experienced it as well. Now coming on to the overall market which exists starting from the early 20th century there has been a consistent rise but we see an exponential growth from 1970s but this growth actually came to a stall somewhere between 2002 to 2005 and that owes to the emergence of mobile communications as an alternate to fixed landline telephones and we have seen that there is a decline from this point onwards. For instance in Pakistan the majority of the users of internet and data networks including the voice as well are mostly comprising the wireless mobile users.