 In this module, we shall start our discussion on some of the most widely adopted and deployed technologies in the broadband side, which is the next generation network backbone, the DSL and the cable access networks. We'll start our discussion with DSL and one of the most important flavors of DSL, namely the ADSL. First, let's look at the penetration of DSL technology as such in the broadband market. Now, the broadband access to the internet is offered to either the end customers, the retail users or the enterprises. The fixed broadband technologies, which are most widely and most profoundly accepted by the user community is both the digital subscriber line and the cable access network. The digital subscriber line as the name suggests is the transmission of digital signals over the telephone line. The variant of DSL that has seen the most success is the asymmetric digital subscriber line or the ADSL. ADSL provides asymmetric bit rates both in the downlink and uplink directions. Now, our understanding of downlink and uplink has to be clear. The uplink is actually the client what it sends to the server while being in the user premises. The client usually sends a request. Likewise, if you look at the server to client communication, it is the downlink communication. It is the reply to the request which is made by the client. Now, this kind of model is very well suited for asymmetry because usually the request is of few bytes or few maybe kilobytes. But the reply could be of the order of megabytes or even the gigabytes. So, it means such asymmetry can be effectively put in the technology that suits certain applications like the IP television in which the client simply clicks on a certain channel and the channel starts to playback and the video streaming. Now, the ADSL actually is meant to have a kind of legacy connection. That is, the ADSL technology supports already implemented runs of underground cable, the UG cable, which was meant primarily for the telephone lines. Now, since in every suburb, in every neighborhood, the cables have been laid, so the access part has been there for quite some time. And upgrading the access part with newer cables and newer technology in terms of medium is not much advisable. So, ADSL came into being as a digital subscriber line technology that best suits the existing infrastructure. Since the telephones actually use a certain band that is from 0 to 4 kHz, we'll go into its more detail at an appropriate time. But for now, just understand that ADSL technology, since it complements and co-exists with the simultaneous telephone calls, so it uses the upper frequency band, which is not used in telephony. So, essentially, there is a requirement to split the frequency band for the telephone, that is the voice call, and for internet connectivity. This is realized through a device called a splitter. The splitter, which is a kind of a filter, both the low pass and high pass filters in the splitter, which allow the distinct frequencies to be sent to their specific hardware that can process these frequencies. Now, with the help of a splitter, there are certain advantages that we get. The first one is there is no interference as such with the existing PSTN. So, the existing telephone lines can continue to service the telephone calls to the end users while the internet connectivity is provided to the interested parties. Now, with this small technological incorporation of the splitters and using a higher frequency band for the digital signals carrying the internet traffic, the ADSL actually can be considered as the pioneer or the winner that can be attributed with broadband development and deployment worldwide. Of course, it is because of the fact that right after the emergence of telephone, the telephone lines were deployed massively all over the world. The initial standard of the ADSL came from the American National Standards Institute. However, it was later adopted by the International Telecom Union, the ITUT, and certain variations in the technology were made. For instance, ITUT calls it the discrete multi-tone G.DMT. And for longer distances, ITU has another flavor of ADSL called G-Lite. G-Lite actually provides slightly lower data rates, say around 1.5 megabits per second downlink, so the uplink is going to be even smaller. But the advantage is that longer distances can be covered. So, this is how the ADSL started to capture the attention of the world. And as we speak today, in so many parts of Pakistan like the rest of the world, we see the ADSL deployed massively.