Comtech EF Data CDM-840 Specifications Page 227

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CDM-840 Remote Router Revision 2
Appendix F MN-CDM840
F–7
F.2.3 CDD-880 Hub Channel Controller (HCC)
Chapter 6. ETHERNET-BASED REMOTE PRODUCT MANAGEMENT
F.2.3.1 Tap Message
The TAP message is a standard UDP/IP multicast forward over the outbound channel with a
payload containing the following sections:
Standard Vipersat Header (distinguishes TAP from other Vipersat messages)
Timestamp field (set by CDM-840 [at WAN interface] when TAP is received)
Aloha Channel Frequency Plan (Frequency, Data Rate, Modulation, and Coding)
Slot and Timing Control
Lists of CDM-840s with Assigned Slots (previous transmission succeeded / failed)
List of CDM-840s that have completed their handshake in the last cycle
As noted previously, receipt of a TAP causes all listening CDM-840s to synchronize internal
transmit timing clock in order to minimize collisions in the Aloha channel.
F.2.3.2 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.
F.2.3.3 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
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