Comtech EF Data CDM-625A Installation And Operation Manual page 655

CDM-625A Advanced Satellite Modem
Appendix M
The WAN FIFO (the size of which is configurable in the IP Switch setup) produces two control
signals that enable and disable the sending of Ethernet Pause Frames. A Pause Frame is an
Ethernet frame designed to implement flow 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 the diagram it can be seen that when the WAN FIFO 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 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 ACM
controller reduces bandwidth.
M.11 Using ACM with AUPC
The adaptation algorithm has been extended so that, in a fading environment, AUPC (Automatic
Uplink Power Control) is the first method used to protect the link. When the increase in power
reaches a defined maximum, power no longer increases with increasing fade depth, but remains
constant, at which point the algorithm switches to ACM in order to protect the link.
The AUPC scheme is, in most respects, identical to the 'standard' AUPC found in the CDM-625A.
The signicant difference is that it does not require an additional overhead mode to be selected
(such as EDMAC) in order to transmit SNR metrics to the other end of the link. This saves
between 1.6 and 5% of the occupied bandwidth.
See the preceding section Configuring VersaFEC ACM in the CDM-625A for details on setup.
You can select up to 9.9 dB of AUPC range before the algorithm switches to ACM.
M.12 Notes and Recommendations
•
VersaFEC ACM is for point-to-point applications. It is required that both directions on the
link run in IP-ACM mode, although the symbol rates do not need to be equal. If you wish
to constrain one direction to run in CCM, simply set the Min and Max ModCod to be
equal.
•
ACM constitutes a closed-loop control system (very similar in concept to AUPC) and 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 SNR to the
required accuracy, and the transport delay over the satellite. Realistically, it can cope with
fading and other link impairments that do not exceed 1dB/second (more if Target Eb/No
margin is increased).
• The ACM controller algorithm that resides in the CDM-625A modem exhibits a different
response depending on whether SNR is increasing or descreasing. If the SNR is improving,
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