Acm Congestion Control; Figure 7-7. Cdm-750 – Acm Congestion Control - Comtech EF Data CDM-750 Installation And Operation Manual

CDM-750 Advanced High-Speed Trunking Modem
Adaptive Coding and Modulation (ACM) Option
PLFRAMEs are used to keep the demod locked but contain no user data and have a
ModCod value of "00".
If the user wishes to see the exact detail of the ModCod (data rate, modulation, code rate)
then these parameters can be seen under INFO: TX or INFO: RX. Furthermore, if the
user is in a CONFIG: TX or CONFIG: RX screen, both the symbol rate and data rate
are displayed. All of these screens update dynamically, so if a ModCod changes, the
parameters are refreshed.
•
Via the CDM-750 Remote Control and Ethernet Management Interfaces: In addition
to its availability via the CDM-750 front panel, this information is available through the
Remote Control (serial) interface, as well as the Ethernet (Web Server, SNMP and
Telnet) management interfaces.
•
Via Oscilloscope: If the user has access to an Oscilloscope in X-Y mode, the Alarms
connector (J1-3, -11) provides analog voltages to monitor the constellation. The
advantage here is that ModCod changes may be viewed instantaneously.
•
Via Internal BERT Tester: When using the Ethernet IP interface, it is not an easy
matter to verify BER performance. A way to verify that the ACM link is meeting
performance is by using the internal BERT tester. Not only will the internal BERT verify
BER performance, it 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.
7.7

ACM Congestion Control

When the ACM controller switches from a lower to a higher ModCodm, 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.
As shown in Figure 7-7, in order to mitigate packet loss when bandwidth is reduced, the CDM-750
incorporates a WAN buffer. When this WAN buffer cannot accommodate the amount of Ethernet
traffic due to the WAN bandwidth,
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
Figure 7-7. CDM-750 – ACM Congestion Control
the CDM-750 can be configured to initiate a method for congestion
7–16
MN-CDM750
The CDM-750 Ethernet
Switch auto-negotiates
Pause Frame compatibility
Revision 1