Comtech EF Data CDM-760 - Manual (Page 350)

Appendix G Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
G–16
G.4.7 ACM Congestion Control
When the ACM controller switches from a lower to a higher ModCod, the bandwidth of the
WAN link is instantaneously increased. This is not a problem, as 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 WAN link is instantaneously reduced.
Figure G-8. CDM-760 – ACM Congestion Control
As shown in Figure G-16, in order to mitigate packet loss when bandwidth is reduced, the CDM-
760 incorporates a WAN buffer. When this WAN buffer cannot accommodate the amount of
Ethernet traffic due to the WAN bandwidth, the CDM-760 can be configured to initiate a method for
congestion control. The means to do this is by enabling Flow Control.
When Flow Control is Enabled, the WAN buffer produces two control signals that enable and
disable the sending of Ethernet Pause Frames – Ethernet frames 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 Figure G-16, 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 capacity.