Notes And Recommendations - Comtech EF Data CDM-750 Installation And Operation Manual

CDM-750 Advanced High-Speed Trunking Modem
Adaptive Coding and Modulation (ACM) Option
control at the MAC layer. A switch supporting 802.3x can send a Pause Frame (with Pause time
set to 0xFFFF) to force the link partner to stop sending data. Devices use the Auto-Negotiation
protocol to discover the Pause Frame capabilities of the device at the other end of the link.
In Figure 7-7, it can be seen that when the WAN buffer reaches a fill state of 87%, it signals the
Ethernet Switch to send Pause frames back to the LAN to inhibit the sending of further data. The
Pause Frames continue to be sent until the FIFO fill state has been reduced to 75%. At this point,
normal operation in resumed by sending a Pause Frame with Pause time set to 0x0000.
This mechanism has been shown to be very effective at mitigating packet loss when the Ethernet
traffic exceeds the WAN capcity.
Refer to Sect. 5.2.2.1.1.2 in Chapter 5. FRONT PANEL OPERATION for further information
Note:
about Flow Control.

Notes and Recommendations

7.8
•
CDM-750 ACM operation is designed for point-to-point operation only. Future point to multi-
point operation will be introduced in a later release of the product. ACM operation and
purchased option are required on all modems running the link (two in non-redundant mode and
four in 1:1 redundant mode).
•
ACM constitutes a closed-loop control system similar in concept to Automatic Uplink Power
Control (AUPC). It should be remembered that, like all control systems, the speed at which the
system can react is governed by a number of factors, including: the time taken to estimate
Es/No to the required accuracy at the far side demodulator; the transport delay over the satellite;
and the processing of messages and associated action taken at the near side modulator.
•
The ACM controller algorithm that resides in the CDM-750 does not have to switch through
ModCods sequentially – it can change, if needed, directly from ModCod4 to ModCod28 (or
vice versa).
•
While ACM can do remarkable things, the fundamentals still apply. The demodulator will not
run at a 16APSK ModCod if the Es/No instantaneously drops to 0dB – the demodulator will
lose lock and the system will recover by switching to ModCod4 (if so configured). The
recommended setting for Minimum ModCod is 4. Comtech EF Data also recommends that
the operator set the Unlock Action to 'Go to minimum ModCod' – this will give the most
robust link availability.
•
Running the ACM link with the Target Es/No Margin set to 0dB will give the best utilization
of link power but, in conditions of fast fading, may cause demod unlock events or highly
degraded BER just prior to the switch to a lower ModCod. In order to mitigate this, use a
increase the target Es/No Margin if the fading events are particularly severe and/or frequent.
•
The value of Max ModCod may be limited by other FAST codes installed in the CDM-750.
Example: Suppose the 15 Msps FAST option is installed, and the symbol rate is set to 15
Msps; the theoretical maximum data rate would be 52.2 Mbps at ModCod 23.
However, if CnC is being used with a 40 Mbps FAST limit, the ACM Max ModCod will be
limited to ModCod 18, or 38.6 Mbps.
If, for a given symbol rate, it is not possible to set Max ModCod to the desired value, the user
should therefore check to determine what other FAST codes may be limiting Max ModCod.
7–17
Revision 1
MN-CDM750