Comtech EF Data CDM-625 - Manual (Page 408)

CDM-625 Advanced Satellite Modem Revision 15
Adaptive Coding and Modulation (ACM) MN-CDM625
17–12
If you wish to verify that link performance is meeting the required level, the internal BERT tester
is an excellent tool. When using the IP interface it is not an easy matter to verify BER
performance, but the internal BERT will not only do this, but is also tolerant of the change in bit
rate that accompanies a change in ModCod. The BERT can be used to confirm that there are no
sync losses or bursts of bit errors when a ModCod changes.
17.10 ACM Congestion Control
When the ACM controller switches from a lower to a higher ModCod the bandwidth of the
Ethernet link is instantaneously increased. This is not a problem, and the link will adapt to push
more packets/second through the link.
Conversely, when the ACM controller switches from a higher to a lower ModCod the bandwidth
of the Ethernet link is instantaneously reduced. Unless the FIFO in the WAN encapsulator is
configured to be very large, the FIFO will tend to overflow, and packets will be lost before the
network recognizes that there is congestion, and reduces the rate at which packets are sent.
In order to mitigate packet loss when bandwidth is reduced, the CDM-625 ACM system
incorporates a method for congestion control. This is illustrated in Figure 17-4:
Figure 17-4. CDM-625 – 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.