Comtec CDM-840 Installation And Operation Manual page 268

CDM-840 Remote Router
Appendix G
transmissions from this timing reference, randomly picking one of the Set Aloha Slots (SAS)
presented by the TAP message. The transmit "on" time is of a fixed duration, allowing sufficient
time for the receiving CDD-880 to acquire and pass the management signaling messages. The
CDM-840 will continue to attempt access on cadence intervals using random backoffs, selecting
a different SAS until receiving positive acknowledgement from the HCC.
Upon valid reception, the HCC processes the Remote Identification (RID) packet containing the
CDM-840 IP Address, TAP (Group) ID, and ECM state flags. Each CDM-840 sign-on IP Address is
added to a list of CDM-840(s) that are queued and await assignment.
Depending on the ECM state flag, the CDM-840 either remains idle in the channel or is assigned
one of the next available time-sensitive slots. Each slot, labeled as a Set of Registration Slot
(SRS), is also assigned through the TAP and allows the CDM-840 to complete any necessary
transaction without further contention.
The CDM-840 then holds an SRS until it receives its assignment into dSCPC or determines that all
necessary transactions are complete; at this time, it releases the slot by signaling the HCC
through the RID message.
Upon release of the slot, the HCC re-assigns it to the group of contention slots. This cycle
repeats until all CDM-840s either have been switched to dSCPC channels or require no further
interaction.
Once the CDM-840 indicates that registration with VMS is complete (via one of the flags in the
RID message), on behalf of the CDM-840 the HCC requests dSCPC assignment by placing the
CDM-840 IP Address into the "switch pending" list and sending an ECM type switch request
message to the VMS. The VMS, upon receipt, either grants the request and signals the HCC to
remove the CDM-840 from its list, or ignores the request and leaves the CDM-840 in the "switch
pending" list. The HCC repeats this request at 12-second intervals up to five times before it
removes the CDM-840 from the "switch pending" list. While the CDM-840 is in this "switch
pending" ECM state, it continues to send SUM health status update messages to the VMS on
timed intervals. Since all ECM transmissions from the CDM-840 contain a RID, this causes the
CDM-840 to be placed back in the "switch pending" list. This cycle continues until the CDM-840
switches, or is set to ECM 'Wait' state, or goes offline.
The VMS is the last step in the entry process that provides network registration, and grants or
ignores dSCPC resources. If resources are limited because of bandwidth, hardware, or
unavailability, the CDM-840s will remain in the entry channel pending assignment.
The VMS plays no role in ECM timing or control.
G–4
MN-CDM840
Revision 3