Datacom DM991C, Руководство по эксплуатации продукта, Страница: 16 / 89

Product Manual - DM991 Series V Family - 204.4092.08 16
The cabinets destined to the DM991S/SE models usually have primary protection devices, so these
models do not come with additional sparklers.
3.1. Interface Characteristics
ITU-T Recommendation G.991.2 describes a transmission method for data transport in
telecommunication access networks over twisted pairs in the full-duplex mode, with echo canceling.
The interface operate in lines of type 135ohms balanced.
The line coding is 16-level TC-PAM type (16-TCPAM).
The table below shows the line encoding and power transmitted to each annex.
A 3 a 36 16-TCPAM 13,5
3 a 31 16-TCPAM 13,5
32 a 36 16-TCPAM 14,5
Power
[dBm]
Modulation
B
Number of
channels [n x
64kbit/s]
Annex
Table 7. Modulation and power on the line
Connection is done by the basic phases: Handshake, Training, and Data Mode.
3.2. Handshake (Pre-activation)
The pre-activation phase follows ITU-T Recommendation G.994.1, which describes the handshake
procedures for xDSL transceivers.
While on this phase, the two equipments exchange information and they negotiate the parameters that will
be used for the connection.
The ends implement a 12 kHz DPSK modem for the NTU and 20 kHz one for the LTU to perform the
handshake. The Recommendation predefined messages are exchanged and the equipments agree to a
common mode of operation.
In this phase, many parameters are set: the final transmission rate, Annex to be used (A or B), type of
information to transport (TPS-TC - Transmission Protocol-Specific Transmission Convergence), type of
transmission frame (plesiochronous or synchronous), and some others.
If the interfaces do not converge to a common configuration, the two equipments abort the transmission
and they do not pass to the next stage: they will retry after some moments.
For the DM991 the user equipment (NTU) will always accept the configuration sent to it by the office
equipment (LTU), easing installation.
During handshake, the DSL interface LED stays mostly off, briefly blinking once a second.
3.3. Training (Activation)
In this phase the two modems test the transmission line using the rate that was agreed during handshake
to determine which coefficients they shall employ for the receive and transmit digital filters.
During training, the equipments use the normal line coding (TC-PAM) and they no longer use DPSK
encoding.
At first, both modems test the line. Then the precoder coefficients are exchanged and used during data
transmission.
Two things can occur at the end of this stage: the modems do pass the training phase, they determine the
adequate coefficients for the line, and then they enter the data mode or, the modems cannot determine
the coefficients, for some reason (maybe the line is too long, or there is too much noise, or during training
some very strong perturbation renders the convergence not viable, etc.) and they abort transmission.