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RCS20 M:N Redundancy Switch
User Interfaces
When Radyne ComStream, Inc. equipment is queried for bulk information (Query Mod, Query
Demod, etc.) it responds by sending back two blocks of data; a Non-Volatile Section (parameters
that can be modified by the user) and a Volatile Section (status information). It also returns a count
value that indicates how large the Non-Volatile Section is. This count is used by M&C developers to
index into the start of the Volatile Section.
When new features are added to Radyne ComStream, Inc. equipment, the control parameters are
appended to the end of the Non-Volatile Section, and status of the features, if any, are added at the
end of the Volatile Section. If a remote M&C queries two pieces of Radyne ComStream, Inc.
equipment with different revision software, they may respond with two different sized packets. The
remote M&C MUST make use of the non-volatile count value to index to the start of the Volatile
Section. If the remote M&C is not aware of the newly added features to the Radyne ComStream,
Inc. product, it should disregard the parameters at the end of the Non-Volatile Section and index to
the start of the Volatile Section.
If packets are handled in this fashion, there will also be backward-compatibility between Radyne
ComStream, Inc. equipment and M&C systems. Remote M&C systems need not be modified every
time a feature is added unless the user needs access to that feature.
4.7.7 RLLP Summary
The RLLP is a simple send-and-wait protocol that automatically re-transmits a packet
when an error is detected, or when an acknowledgment (response) packet is absent.
During transmission, the protocol wrapper surrounds the actual data to form information packets.
Each transmitted packet is subject to time out and frame sequence control parameters, after which
the packet sender waits for the receiver to convey its response. Once a receiver verifies that a
packet sent to it is in the correct sequence relative to the previously received packet, it computes a
local checksum on all information within the packet excluding the <SYN> character and the
<CHECKSUM> fields. If this checksum matches the packet <CHECKSUM>, the receiver
processes the packet and responds to the packet sender with a valid response (acknowledgment)
packet.
The response packet is therefore either an acknowledgment that the message was received
correctly. If the sender receives a valid acknowledgment (response) packet from the receiver, the
<FSN> increments and the next packet is transmitted as required by the sender.
If an acknowledgment (response) packet is lost, corrupted, or not issued due to an error and is
thereby not returned to the sender, the sender re-transmits the original information packet; but with
the same <FSN>. When the intended receiver detects a duplicate packet, the packet is
acknowledged with a response packet and internally discarded to preclude undesired repetitive
executions. If the M&C computer sends a command packet and the corresponding response
packet is lost due to a system or internal error, the computer times out and re-transmits the same
command packet with the same <FSN> to the same receiver and waits once again for an
acknowledgment.
TM056 - Rev. 2.3
4-17
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