Comtech EF Data CDM-625A - Manual (Page 699)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix M Revision 3
M–19
Figure M-5. CDM-625A – ACM Congestion Control
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.10 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 significant 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 Section M.6.2 Configuring VersaFEC ACM in the CDM-625A and Section 0 Configuring
VersaFEC ACM in the CDM-625A for setup details.
You can select up to 9.9 dB of AUPC range before the algorithm switches to ACM.