Basics of ADSL

Today’s lection is devoted to basics of ADSL.

ADSL is the most popular technology among all xDSL technologies. The main reason of such popularity of this technology is its asymmetric property, that is downstream rate is significantly more than upstream rate. And this asymmetrical property is corresponding to the asymmetrical nature of Internet traffic, because quantity of data for request is much less than quantity of downloaded data.

There are several types of ADSL technology given in the table.

Types of ADSL technology	ITU-T Recommendation	Maximum data rate (DS/US), kbit/s	Maximum frequency Fmax	Maximum number of carrier (subcarrier) Ncar
ADSL	G.992.1	6144 / 640	1104	256
ADSL G.Lite	G.992.2	1536 / 512	552	128
ADSL2	G.992.3	8000 / 800	1104	256
Splitterless ADSL2	G.992.4	1536 / 512	552	128
ADSL2+	G.992.5	16000 / 800	2208	512

As seen from this table, there are 256 carries in ADSL signal and last carrier corresponds to frequency 1104 kHz. Thus frequency spacing between adjacent carriers is equal to Fmax / Ncar = 1104 / 256 = 4,3125 kHz. For another types of ADSL technology the frequency spacing is the same: 552 / 128 = 2208 / 512 = 4,3125 kHz.

I am reminding you that the DMT modulation is used in all types of ADSL technology. And now I’ll explain you what is sense of DMT modulation. I’ll draw the simple scheme of DMT modulation.

    The bit stream, intended for transmission, come in serial / parallel convertor, where this bit stream is divided to several bit streams with lower bit rate. And then each of these bit streams modulates certain carrier ‑ f₁, f₂, …, or f_n. And at last signals on these carriers are summed and come in to channel.

The equation for DMT signal for one clock period is next:

where l – number of carrier, a_l and b_l are information parameters, mapped from transmitted on l-th carrier bits, w₀ is cyclic frequency of first carrier, w₀ = 2pf₀, f₀ = 1/t₀, where t₀ – duration of orthogonality interval.

Note. Two signals S₁(t) and S₂(t) are orthogonal if 

Duration of clock period T is set more than duration of orthogonality interval: T > t₀. And difference between T and t₀ is equal to guard interval t_g: T > t₀ = t_g. Guard interval is used for mitigate interference between signals from different clock periods and different carriers.

For each concrete carrier, DMT modulation is the same as QAM.

The superframe for ADSL consists of 69 frames, one of them is synchronization frame and last of them are data frames. The duration of superframe is 17 ms. Thus duration of one frame is 17/69 ms » 0,246 ms. And frequency of frames is equal to 69/ (17 ms) » 4,059 kHz. Frequency of information frames differs from frequency of data frames and equal to 68 / (17 ms) = 4 kHz. In ADSL system the duration of guard interval is equal to 1/16 of orthogonality interval duration, thus carrier spacing is equal to 69/ (17 ms) × 17 / 16 = 4,3125 kHz.