Comtech EF Data CDM-840 - Manual (Page 227)

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