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P300H P300 Series Modem Installation and Operating Handbook Page 148
8.7.2 Closed Network Plus ESC via the Custom Service menu
Early versions of the embedded code for the P400 Series of satellite modems did not directly provide a
Closed Net Plus ESC option, although the feature is still available (under a different name) from the
Custom Service menu dialogue.
If you wish to use Closed Net Plus ESC on a P400 series modem it is probably easier to load the modem
with the latest embedded code (downloadable from our Web site) and use the Closed Net Plus ESC facility
directly. This latest P400 software will also provide the facility on P420's and P440's which are fitted with
an ASIC IFU ("IFU:2", see Appendix C1), which previously could not provide the facility as they had no
"Custom" option on the service menu. The Custom Menu however has always been available on P460's
and P480's.
If you do wish to use the Custom menus to define Closed Net Plus ESC operation, the table below specifies
the parameters to enter at each menu which follows the Custom option from the Tx/Rx Service selection
screen.
Main Data Rate Based On TS32 MF Backward Alarm No. of Back' Alm Overhead Mode
>= 32kbps IBS Normal User choice One Min O/H
< 32kbps IBS No TS 32 MF User choice N/A (no TS32 MF) Min O/H
In both cases under the ESC, DEFINE dialogue set the proportion of the overhead assigned to the async
ESC as Maximum (all remaining) and set the Aux channel to Off, so that as much as possible of the
overhead is used for the ESC reducing the total overhead added to the data. The final satellite data rate
including overhead can be displayed on the INFO, VIEW CONFIG, TX/RX screens.
Note: As the overhead used relates directly to the data rate, ESC & backward alarm settings, then for two
modems to interwork the following parameters must match on both modems:
Main data rate.
Async ESC Baud Rate and Character length / Parity.
Backward Alarm option (ie both enabled or both disabled).
Baseband modes (ie both Continuous Data or both Drop/Insert).
A Closed Net Plus ESC facility as described above (using the Custom Service options) is identical with the
Closed Net Plus ESC facility which may be directly activated in software V1.50 or greater from the Service,
Closed Net, ESC menu.
8.7.3 ESC channel with mis-match configurations at Absolute Minimum Overhead (0.39%)
Summary (fairly technical)
In Closed Net Plus ESC mode (or Custom IBS modes with minimum overhead selected) the modem adds
the minimum possible overhead to accommodate the required ESC channel. The overhead is variable from
1/1 (100%) to 1/256 (0.39%) which covers a large range of requirements, however for very high bit rates
with very low ESC requirements, 0.39% may still provide an ESC channel capable of more than the
required Baud rate. For example with a 2048kbps closed network link with a 2400Baud ESC channel
selected, the minimum 0.39% overhead could actually support an ESC of 6135Baud.
This is not normally an issue as the ESC UART is set to the 2400Baud the user has specified, and the fact
that the channel could provide more is masked by the throughput limitation of the 2400Baud async link
outside the modem. However under certain scenarios discovered recently (Nov 2002) it is possible in this
absolute minimum overhead mode, to get the ESC channel operate strangely. Specifically it is possible to
get the ESC channel to cause a delay in transmission of up to 3 characters (as if they were `stuck in the
pipe`), which only appear out of the ESC channel when `pushed through` by subsequent characters. This