Comtech EF Data CDD-880, Installation And Operation Manual

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CDD-880 Multi Receiver Router MN-CDD880
Appendix D Revision 2
D–19
• List of CDM-840s that have completed their handshake in the last cycle
As noted elsewhere, receipt of a TAP causes all listening CDM-840s to synchronize internal
transmit timing clock in order to minimize collisions in the ALOHA channel.
D.3.2.1 HCC Configuration
The TAP IP Address specifies the multicast address that all CDM-840s in the group use to receive
the TAP:
• Timing as exposed to the user is message interval;
• Total slot count that derives the Cycle Length is set in milliseconds;
•
All other parameters are calculated based on current rate;
•
The Date Slot Size is fixed at 6, and VersaFEC Blocks and Preamble is computed in
milliseconds based on the specified data rate;
•
Guard Band is determined during terminal commissioning.
D.3.2.2 Hub Operation
After configuration and initialization, the HCC broadcasts the TAP message on cycle intervals
waiting for ALOHA channel RID messages. Successful reception of the RID places the CDM-840 IP
Address along with its ECM state flag (i.e.,
Online
,
Wait
, or
Offline
) into the first queued list. The
controller checks the flag to determine next operation. If the ECM state is Online , an SRS is
assigned for next TAP processing.
•
The CDM-840 receives the TAP with its IP Address in the SRS list and, based on where its
place is in the sequential order, determines the slot assignment. By design convention,
assigned slots are at the end of the cycle and whatever remains is used for contention at the
beginning of the cycle. With the assignment, the CDM-840 transmits management data in its
assigned slot and awaits acknowledgment and next assignment.
•
After the CDM-840 completes its registration protocol, it releases the active flag in the next
RID cycle. At this point, the HCC will remove the processing CDM-840’s IP Address from the
list clearing the SRS allocation. If the CDM-840 is not switched out of ECM mode, it will
proceed to send a Status Update Message (SUM) every minute in order to maintain its
active status with VMS. Typically, a SUM should only require one transmission unless there
is contention, in which case the CDM-840 will go into back-off mode until the CDM-840
received acknowledgment from the HCC that the SUM was successfully transmitted.