ATL User Guide
AM200 Modem
134
The nominal DSL frame is always 6ms long, regardless of the transmission rate of the individual channels.
The DSL frame length is adjusted slightly by the use of stuff bits. There may be 4 stuff bits or 2 stuff bits per
frame. The stuff bits are used to adjust the effective payload bandwidth of the DSL frame.
If the user rate clock is slightly quicker than the line rate clock then less stuff bits are sent. Alternatively, if the
user rate clock is slightly slower than the line rate clock then more stuff bits are sent. This mechanism allows
the line rate bandwidth to be adjusted to match the user rate bandwidth.
At the receive end, the rate at which the stuff bits arrive is used to recover the user clock.
G.SHDSL uses Trellis Coded Pulse Amplitude Modulation (TC-PAM). TC-PAM was chosen as the basis for
G.SHDSL due to the low complexity of the algorithms and the low latency required for voice traffic. The use
of TC-PAM provides an additional 'coding gain' that improves the performance of the digital signal in the
presence of interference. Compared to SDSL (2B1Q, or CAP), TC-PAM is more spectrally friendly, ensuring
compatibility with other DSL-based services such as ADSL.
14.3 CONTROL CIRCUIT
The control circuit is based on a micro-controller and determines the operational status of the unit according
to the state of the transmission system, the data interface and the configuration information received from
the terminal.
