 The physical layer of ADSL technology is again very important to be understood because it deals with the frequencies and the modulation types which are used in ADSL. The frequencies are usually leased out to different service providers. And the choice of the frequency is a very strong determining factor in overall efficiency and utilization of a certain technology. So ADSL frequency bands are used for providing internet connectivity to a broad range of users. That's why it is important to understand this. The physical layer, first of all, needs to be understood in conjunction with voice. The telephone lines primarily are meant for voice. The voice is usually band limited to 300 Hz to 3.4 to 3.5, 3.6 kHz. So that actually gives us some room because in the earlier module we said that the frequency band for voice is limited to the upper boundary of 4 kHz. So the frequency range between 3.4 to 4 kHz serves as the guard band. And this is according to the Modem specifications, specifically V.90. ADSL to provide internet connectivity is two-way and it is a full duplex technology. It means that both the uplink and downlink transmissions can take place at the same time. The variants of this duplex modes of communication can be the frequency division duplex, time division duplex, and the echo cancellation duplex. But frequency division duplex is the one most widely used. The ADSL spectrum is divided into certain ranges, both for uplink and downlink. Broadly speaking, the overall range for the initial versions of ADSL because ADSL has also grown and some newer spectrums have been assigned for ADSL. So we'll talk about the initial versions or variants of ADSL. The spectrum actually was from 25 kHz to 1104 kHz or 25 kHz to 1.104 MHz. The lower band that is from 25 kHz to 138 kHz is meant for upstream. Now, if you may have already noticed, this choice of assignment, the lower frequencies being assigned to upstream, essentially implies that first the lower frequency band has lower capability to carry or modulate information because you can carry more information on a higher spectrum because you'd have more frequency assignments possible in the higher frequency band. So on the higher side, we have 138 to 1104 kHz band that is for the downstream. Now it is up to every vendor to have appropriate hardware that allows these frequency bands, both for upstream and downstream, to be used more effectively. Regarding the modulation or the line coding, the initial mechanism or the initial modulation mechanism was the carrier less amplitude phase modulation. Both the amplitude and the phase are modulated and the carrier was not transmitted. So this was initial, but these days as per the ANSI as well as the ITUT recommendations, discrete multi-tone is used. If you look at the overall spectrum, let's appreciate it. We have the telephone call, the PSTN frequency band limited to 4 kHz. Then we have gap, a very broad around 21 kHz band gap so that there is no interference whatsoever between the ADSL frequencies and the voice signals. So we have from 25 to 138 we've got upstream and then we have from 138 to 1104 kHz we have downstream. Although it's already shown to you here, but it would be appropriate to talk about it at a later stage that the frequency band actually has been extended. In fact it's been doubled from 1104 to 2208. So naturally if we keep the modulation scheme same as discrete multi-tone, we are expecting double bandwidth or double data rate for the ADSL to and ADSL to plus downstream transmissions. So this carrier less frequency amplitude phase modulation actually divides the spectrum into three bands. Again for voice, the lowest one then for upstream from 25 to 160 kHz and then it gives a band gap between the upstream and the downstream and from 240 kHz to 1.5 MHz is the downstream. The discrete multi-tone actually as the name suggests is multi-tone, it means so many sub-carriers. We don't call them carriers, we call them sub-carriers. Now discrete multi-tone modulation was adopted by ITUT as a standard in their variants 991.1 which is ADSL and 992.1 which is G.Light. So basically what happens is that there are around 256 sub-carriers each with a width of 4 kHz. So these carriers are allocated to bit stream. So if a user transmits a certain bit stream for instance we say that let's say user has 256 bits to be transmitted. So each bit is now carried on individual sub-carrier. This is for an example say otherwise of these 256 sub-carriers 32 are allocated for the upstream and the remainder which is around 234 sub-carriers are allocated for the downstream. The discrete multi-tone carriers are modulated according to QAM, the quadrature amplitude modulation. If you recall the i and q, the in-phase and quadrature components were used in QAM. So then we have the variants of ADSL as the name suggests ADSL 2 is a successor. Then we have 992.4, 992.3 is the successor of ADSL as such and its splitter less version where the splitter is all together obviated is done away with is the 992.4 and then we have the ADSL 2 plus that we just saw when we looked at the overall spectrum. The bandwidth has been extended from 1.1 to 2.2 MHz. Let's quickly look at the ADSL standards and their bit rates. Let's just quickly recap ADSL as the name suggests it provides 8 Mbps in the downstream 1 Mbps in the upstream so naturally there is a 1 to 8 ratio as we looked at the carrier assignment as well. Then we have the discrete multi-tone that is provides up to 12 Mbps in downstream and 1.3 Mbps in the upstream. When we look at something interesting we will observe that from ADSL to ADSL 2 plus we are getting 24 Mbps in downstream which is quite a lot. If you are doing internet surfing along with the voice over IP app running and an IP TV running also we can offer interesting services and we can in fact offer digital channels as well in such rate. The upstream is not much significantly growing. You see that it is 1 Mbps in the upstream for ADSL whereas it is 3.3 Mbps for ADSL 2 plus because not much changes on the upstream except for certain interactive applications like voice over IP and for instance Skype like apps. So on the right most side we see the corresponding ITU assigned codes for these different standards.