Comtech EF Data CDM-625 - Manual (Page 378)

CDM-625 Advanced Satellite Modem Revision 15
IP Sub-Mux MN-CDM625
15–2
This gives you 34 ratios from which to choose. If framing is on, the overhead associated with the
selected frame structure is applied to the primary data only, and the IP ratios above are
fractions of the framed primary data.
IP ratios that put the composite rate above the maximum data rate of the modem cannot be
selected. For example if the primary rate is 8192.000 kbps and the maximum rate of the modem
was 25000.000 kbps, then a ratio above “2” cannot be selected. Also, if Sub-Mux is on and the
primary rate is raised so the composite rate goes above the maximum of the modem, then the
IP ratio will be lowered so that the composite rate stays below the maximum.
15.3 Data Rate vs. Composite Rate
Without Sub-Mux, the CONFIG: Tx and CONFIG: Rx front panel menu screens display the
modem’s transmit and receive data rates, which refer to the terrestrial rate of the primary
interface type. Additions to the primary data due to framing are not shown, but are reflected in
the displayed symbol rates.
Once Sub-Mux is enabled, the data rate display is replaced with the composite rate. This is the
sum of the primary interface’s data rate – which now includes framing overhead – and the IP
rate at the selected ratio. The later is displayed by itself as IP INFO Rate under the CONFIG: Tx 
Tx Data Rate or CONFIG: Rx  Rx Data Rate submenu. Note that Reed-Solomon overhead, if
enabled, is also included in the composite rate calculation.
(See Sect. 5.2.1.3 CONFIG: Tx or Sect. 5.2.1.4 CONFIG: Rx and their respective subsections in
Chapter 5. FRONT PANEL OPERATION for detailed information).
Example: For a G.703 E1 primary interface, with a 4/5 Sub-Mux ratio selected:
Primary data rate = 2048.000 kbps
Unframed
IP info rate
=
2048 x 4/5
=
1638.400 kbps
Composite rate
=
2048 + 1638.4 or 2048 x 9/5
=
3686.400 kbps
Data rate with overhead = 2048 x 29/27 = 2199.703 kbps
Add ESC++ framing (added to El path only)
IP info rate = 2199.703 x 4/5 = 1759.762 kbps
Composite rate
=
2199.703 + 1759.762 or 2199.703 x 9/5
=
3959.466 kbps
Composite rate
=
3959.466 x 126/112
=
4454.400 kbps
Add Reed-Solomon FEC (added after Sub-Mux)
The Sub-Mux frame does contain a very small overhead required for acquisition and
identification of primary vs. IP data: As this overhead is taken from the "IP info rate" rather
than the primary data rate, actual IP throughput will be slightly reduced.