Q-Flex Satellite Modem Installation and Operating Handbook
7-7
rule of thumb, 2ms end-to-end buffering should be sufficient for one satellite hop and 4ms
for a round trip where twice the Doppler shift is required.
Set the Tx and Rx clocks as follows:
When the clock is provided via the terrestrial interface
Set
Tx clock
to
Tx clk in
and
Rx clock
to
Tx clk in.
Modem internal clock
This sets
Rx clock
to
Internal
and feeds the clock to the terrestrial equipment. If the Rx
output clock is internally or externally looped then set
Tx clock
to
Tx Clk In
. If the Rx output
clock is not returned to the modem Tx input then set
Tx clock
to
Rx Clk
.
Station clock
When
Rx clock
is set to
Station
then set the
Tx clock
as for modem internal clock above.
Satellite
This is an invalid Rx clock setting. With a clock loop at the remotet end the Rx satellite clock
is a copy of the Tx clock and cannot be looped again.
7.1.3.2 No Clock Loop
If there is no clock loop then both ends need the buffer to be enabled in order to cope with
Doppler and the plesiochronous effect (i.e. the difference between the clocks at each end).
Apply the following rule to both ends:
•
If a clock is being provided via the terrestrial interface and a station clock is
available then set
Rx clock
to
Station
. If there is no station clock then set
Rx clock
to
Tx Clk In
(as the Tx clock comes from the network this will be derived from the
network master clock). Set
Tx clock
to
Tx Clk In
.
7.1.3.3 Determining Buffer Size
The following calculation can be used to determine the buffer size required. The required
plesiochronous buffer size (middle-to-end) in seconds is:
Buffer size (Bs) = Ms x 24 x 60 x 60 x (Lc + Rc)
Where:
•
Lc is the local clock accuracy (e.g.
1 x 10
-9
),
•
Rc is the remote clock accuracy (e.g.
1 x 10
-11
)
•
Ms is the minimum-slip period in days (e.g. 40 days)
