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P300H P300 Series Modem Installation and Operating Handbook Page 141
8.2 DETERMINING CLOCKING SCHEMES AND BUFFER SIZE
Please note that unless the clocking scheme has been discussed and agreed by both ends of the link, it
is unlikely to work reliably !
You need to know the following things:
Incoming Rx carrier : 1. Is your Tx clock looped back to you at the distant end ?
2. If not, what is the accuracy of the clock they are transmitting.
Outgoing Tx carrier: 1. Are you looping the incoming Rx clock (Tx Clock = Rx, or clock looped
by the equipment or cables your end).
2. If not, what is the accuracy of your Tx clock
Follow these guidelines:
8.2.1 Clock Loop At One End:
The end with the clock loop requires Buffer disable (Rx Clock = Satellite). If there is either a cabled
clock loop or the equipment loops the Rx clock back to the modem Tx input then set Tx clock = Tx Clk In.
If there is no physical clock loop set the Tx clock = Rx.
The end without the loop requires the buffer active and set for 2 x Doppler shift only as there is no
Plesiochronous effect with a clock loop. The Buffer size required depends on the satellite orbit (or rather
the accuracy with which it's position is maintained), currently 2ms (end to end) buffering is enough for the
Doppler shift from 1 satellite hop on current INTELSAT satellites. For 2 x Doppler shift 4ms is required.
Set the Tx & Rx Clock modes as follows to use the clock listed:
Clock from the interface / terrestrial network
Tx Clock = Tx Clk In, Rx Clock = Tx Clk in
Modem internal clock
Rx Clock = Internal and feeds the clock to the terrestrial equipment. If the Rx output clock is externally
looped by equipment or in cable then set Tx Clock = Tx Clk In. If the Rx output clock is not returned to
the modem Tx input by the equipment or the cable then set Tx Clock = Rx Clk.
Station clock
Rx Clock = Station, Tx Clock as for "Modem Internal Clock" above.
Satellite
Not possible, with clock loop at distant end, Rx satellite clock is already a copy of our Tx clock we
cannot loop it again.
8.2.2 No Clock Loop
If there is no clock loop, then both ends need the Buffer active to cope with the normal Doppler effect, plus
the plesiochronous effect (ie the difference between the clocks at each end). Apply the following rules to
both ends:
Clock from the interface / terrestrial network
If the modem has a station clock (which also drives the network which the Rx output feeds), then select
Rx Clock = Station. If there is no station clock then set Rx Clock = Tx Clk In (which as the Tx clock
comes from the network, is presumably derived from the same master clock which clocks the network).
Set the Tx Clock = Tx Clk In.
Other clock modes