 Now, we shall see the ADSL technology in a little more detail. We'd look at its architecture and some of the components which are used to realize the ADSL technology Internet connectivity. We look at a very important device called the DSLAM and we'd also look at things called splitters. The ADSL is actually based of course on the transceiver unit which is on the operator side. Then it has a corresponding receiver unit which is another transceiver on the customer side. So these are the two modems which operate at certain frequencies. Then we have splitters. We'd see shortly how these splitters exactly work. Then we have the multiplexers for DSL. Because after all the information which is coming to the exchange from different modems needs to be consolidated, multiplexed and demultiplexed on the exchange side. So we have a device called digital subscriber line access multiplexer. This is a device that works on the operator side. It means the DSLAM works with corresponding modems on the individual user premises. Let's look at the overall architecture. We'll talk about individual hardware components accordingly. First of all, if you look at the overall arrangement of these devices, we have two networks. The Internet represented by the IP backbone on top right. Then we have the voice telephony network, the public switch telephone network on the bottom right. Then we have the devices which constitute the operation as in DSLAM. So let's see what all is there. First of all, if you notice, we have the voice and Internet traffic that is coming from the end users on the left hand side bottom. So the traffic is coming on the exchange side that is on the CO, the central office. Here the splitters are used to filter out the signals which are not meant for a certain network and filter in the frequencies which are meant for that network. So the splitter here successfully use low pass mechanism to relay or transfer the voice traffic to the PSTN and use the pan pass or the high pass filter to relay the frequencies which are meant for the ADSL Internet connectivity. So the traffic actually, Internet traffic goes to the IP backbone. We see some devices, for instance, we see more dams here, we see concentrators here. Now what are these more dams? After all, when the traffic is coming from the user side, it is modulated. It means that at this particular more dam, which is at the central office, all this traffic gets demodulated. We shortly look at what a concentrator is, but for now just keep in mind that concentrator is a device that takes the traffic streams from all individual users, which is the Internet traffic and aggregates or consolidates it onto some broadband long haul multiplexing technology. So this is what concentrator does, but we look at it in a little more detail. So the DSLAM actually, we looked at the operation from in fact the uplink side, but let's look at it step by step. In the downlink, the telephone and ADSL signals are sent towards the customer premises equipment. The splitters are placed on both the sides, that is on the exchange side, that is the central office side, as well as on the user premises side. So on the user premises side, since we are looking at the downlink scenario, the splitters demultiplex the telephone signals and the ADSL signals. The telephone signals are actually band limited to 4 kHz through a low pass filter. The actual range is around 3.5 kHz to 3.6 kHz for voice, but it is actually put at a boundary of 4 kHz to have some guard band as well. Now having the splitter gives us a very inherent advantage that since we have the telephone signals transmitted in parallel to the ADSL signals, so we see that the voice transmission remains unaffected even in the presence of ADSL signals, and even if the ADSL signals are unavailable, because let's say there is some service outage for the internet, if the ADSL frequencies fail, even then the telephone call can still be established. Now let's just quickly look at the uplink once more. The signals which are coming both from the telephone on the customer premises side and the ADSL signals, these two frequencies or the signals are sent simultaneously, so these are actually travelling towards the central office in a multiplexed or interleaved manner. Now the DSLAM receives it, now DSLAM again has splitters on its end as well, so it separates these ports and ADSL signals and then respective outputs are sent both to the telephone exchange and to the internet router. Now as I said, the device concentrator needs some attention as well, concentrators are meant only for the internet traffic. On the uplink side what happens is that these concentrators actually take traffic from individual user streams that is from all DSLAM connected internet users and aggregates this traffic on to the backbone. The backbone could be an all IP backbone, for instance it is a router communicating to another router or let's say it is IP over ATM technology then these individual user streams or user data is concatenated or appended and it is encapsulated into ATM format. Now the concentrator does exactly the reverse of it on the downlink side, now the traffic is separated for the individual ADSL modems by the concentrator. Now the concentrator is concentrator on one end and it is a digital splitter if you may like to call it that way, it is like a digital splitter that splits individual internet streams to their respective ADSL modems. Now this is the job of the concentrator both from the uplink and downlink perspective.