Comtech EF Data DD240XR Rev Е - Manual (Page 54)

User Interfaces DD240XR High-Speed Digital Demodulator
4-34 MN-DD240XR – Rev. E
If an acknowledgment (response) packet or a NAK 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 or a NAK packet.
To reiterate, the format of the message block is shown in Table 4-4, Link Level Protocol
Message Block.
Table 4-4. Link Level Protocol Message Block
SYNC COUNT SRC
ADDR
DEST
ADDR
FSN OP CODE DATA
BYTES
CHECKSUM