xDSL handshake

The ITU-T has standardized a handshake and activation method for xDSL modems, called the g994.1 aka G.Hs.  Its basic function to provide a consistent way of initiating the various types of xDSL modems available now and in the future.

Before g994.1 each of the various xDSL standards used different tones or signals at different frequencies in order to indicate to the opposite end that modem startup (activation) was desired. Additionally, various modem options were negotiated by using different combinations of tones.   This clearly became a problem as xDSL evolved, expanding the number of options and flooding the market with many types of xDSL modems, so that unused frequency spectrum became a valuable resource in order to identify initialization at each end of the line.

A similar situation happened in the early 1990's when voiceband modems were in use, and each often contained several different voiceband modulation Recommendations (V.22, V.32, V.34).  Instead of using different tones and frequencies, a protocol named V.8 was created, in which a simple digital communication path is created between the 2 end-points, followed by a process of messages transfer in order to identify and select both the appropriate modulation technique used, and the available options.

g994.1 borrows the basic idea behind V.8 and does basically the same for xDSL modems, only it uses somewhat a bit more complicated communication set-up and messaging mechanism due to the extensive nature of xDSL.

Spectrum Allocation
V.8 used two different frequency bands for the digital communication path. Data was transferred in one direction in a spectrum band centered around 1080 Hz and in the other direction in a spectrum band centered around 1750 Hz. It was rather easy to select those frequencies since all voiceband communications occurred in the spectrum from 300 to 3700 Hz.
In xDSL however, the spectrum is not limited, and indeed each region in the world chose to use a different spectrum.  Moreover, DSL lines are exposed to interferences on certain frequencies making them unusable.  As a result, several simultaneous upstream and several simultaneous downstream frequencies were needed in order to initialize communication.  Different carrier sets were designed for different regions in the world, each carrier set contains 3 frequencies. a handshake procedure may start off with as many nine simultaneous upstream tones and ten simultaneous downstream tones. This allows for the greatest possibility of detection at the opposite end of the cooper loop.
As for ADSL, the 4.3125 kHz signaling family has been chosen, meaning signals frequencies retain value of N * 4.3125 kHz, where N is a positive integer.  Carrier sets are designated for 3 different types of ADSL, each correspond to a different region in the world using ADSL.  Figures are described in table 1 below.

Carrier Set Designation	ADSL type	Primary Region	Upstream (maximum power level of -1.65dbm)	Downstream (maximum power level of -3.65dbm)
			Frequency Indices (N)	Frequency Indices(N)
A43	G.992.1 - Annex A, G.992.2 Annex A/B	North America	9      17    25	40    56    64
B43	G.992.1 - Annex B	Europe	37    45    53	72    88    96
C43	G.992.1 - Annex C G.992.2 - Annex C	Japan	7       9	12    14    64

Table 1. carrier sets corresponding to ADSL type

Duplexing and Initial Signals
As mentioned before,  either end of the copper loop may send the first signal of initialization,  and can operate both in duplex and half duplex, depending on the xDSL Recommendation.  We will focus on the duplex system, which is used by ADSL.  In any case, the first signals in all fourth methods are identical allowing complete interoperability between all four cases.
When an xTU-R/C wishes to initiate G.994.1, it delivers a request from the HSTU-R/C which begins the initialization procedure by sending several signals and flags to the other side, as described in figure1.