Let us mention the head-end configuration enabling arrangement of Internet signal transmission over optical fibre using the above mentioned optical modules, a head-modem BSR-2000 and a subscriber modem SB5101 (Motorola).
The transfer ratio from electrical input of an OM-RT optical node to the electrical output of an OMS-R: 0dB (not including voltage dip in optical fibre). All the signals shaped by the head modem or the subscriber modem (in an interactive cable network) can have a high level, about 118dB/µV. The subscriber modem has output signal dynamic of ≈50dB. The head modem has output signal dynamic of ≈15dB. The signals received by subscriber modems with account to noise of ≈10dB/µV and the necessary store above the noise of about 30dB (25dB for a reverse channel) have a lower value of ≈40dB/µV and an upper value of 75dB/µV. (The head modem controls subscriber modem transmission level maintaining signal at its input within the limit of 65dB/µV to 70dB/µV). All the specified figures depend on a modulation kind, an emission frequency band and definite equipment, i.e. the figures are approximate.
Let us consider an example of a reverse channel data transmission over a fiver-optics communication line using a head modem BSR-2000 and a subscriber modem SB 5101 operating in the DOCSIS standard.
Establish the set-up. (fig.2 2) and provide such an amplification in the OM-RT optical node reverse channel (with the help of a tunable attenuator), so that a transfer ratio of the system: OM-RT - fibre-optics communication link - OMS-R (fig.3) is within the limit of 0dB (0...-5 dB).
The head modem BSR-2000 maintains the reverse channel signal level of about 5dB/µV with account of back-coupling (see fig. 2. Thus the signal at the OM-RT input will be of about 11 dB/µV (the modem SB 5101 itself is able of changing its level form 8 dB/µV to 58dB/µV). As follows from the above loss in a fibre-optics communication link between the OM-RT and the SB0501 can be from 0 to 42...47dB [it is 42dB when the transfer ratio of the system OM-RT - fibre-optics communication link - OMS-R is -5dB, it is 47dB when the transfer ratio of the system OM-RT - fibre-optics communication link - OMS-R is 0dB].
fig. 2 fig. 3
The set-up established according to the fig. 2 provides a stable transmission of reverse channel when having attenuation in a fibre-optics communication link from 0 to up to 10dB. Signal attenuation in current communication link can be from 0 to minus 47dB.
It is evident that data can be transferred from the SB 5101 to the BSR-2000 when having an optical communication link attenuation of higher than 10dB but due to a probable attenuation range in the current communication link.