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–3
D.1.2 ECM Theory of Operation
While a CDM-840 is in Entry Channel Mode, it allows the passing of management traffic only – it
will not transmit user data traffic. An ECM-enabled CDM-840 may remain in the entry channel
for an extended period if “online” communications are not required, or if dSCPC ( D ynamic S ingle
C arrier P er C hannel) resources are unavailable. While idle or waiting in the entry channel, the
CDM-840 sends periodic health status messages while it continues to service VMS recovery logic
timers.
Note that a “channel” refers to a fixed carrier slot on the satellite that allows the CDM-840
Remote Router random access (in time) to a readied HCC (CDD-880) for transacting each
acquired transmission. Each CDD-880 supports only one Entry Channel; this channel is always
assigned to Demod #1.
The HCC uses a TAP ( T ransmission A nnouncement P rotocol) message to broadcast a key subset
of transmit parameters that match its receive configuration, but assumes all other site specific
parameters were preconfigured at the time of antenna commissioning. In addition, the TAP
provides timing information in the form of slot parameters that define the required acquisition
time of the receiver and the amount of time allowed for data transmission. The TAP also
contains a list of CDM-840s that successfully transmitted during the previous cycle. The list of
CDM-840s from which the HCC expected – but did not receive – a response is also provided;
these CDM-840s are assigned a slot for the next cycle.
Each associated CDM-840 achieves loose time synchronization via the broadcast TAP message
transmitted at periodic intervals. Since the TAP transmits via the satellite, all CDM-840s receive
it at effectively the same time, with delay differences (due to geographic variation of each
remote site cluster) compensated with a specified guard time. Upon receipt of the TAP message,
the CDM-840 resets its ECM time reference and uses the slot information to determine/select
the next transmit opportunity:
•
If the CDM-840 has transmitted in the previous cycle, and does not indicate it is finished, it
receives an assigned slot as indicated by its IP Address.
• If the CDM-840 does not have an assigned slot, it randomly picks one from the available
contention slots.
•
If no contention slots are available, the CDM-840 waits for the next cycle.
This process allows each CDM-840 to transmit at a discrete time to minimize the chance of
collision. To reduce slot contention further, a random backoff (next slot) algorithm is deployed if
the transmission was not received. Each CDM-840 attempts to enter the network by gauging its
transmissions from this timing reference, randomly picking one of the S et ALOHA S lots ( 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.