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DMD2401 LB/ST L-Band Satellite Modem and ODU Driver
occurred, the RLLT causes the equipment to issue the appropriate error message(s) in the
pending equipment response frame.
To maintain frame synchronization, the IOPT keeps track of error-laden packets and packets
intended for other equipment for the duration of each received packet. Once the packet is
complete, the IOPT invokes an I/0 wait state and searches for the next <SYN> character.
4.4.8 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. If the checksum values do not match, the receiver replies with a
negative acknowledgment (NAK) in its response frame.
The response packet is therefore either an acknowledgment that the message was received
correctly, or some form of a packetized NAK frame. 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. However, if a NAK response packet is returned,
the sender re-transmits the original information packet with the same embedded <FSN>.
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 B-4, Link Level Protocol
Message Block.
SYNC
COUNT
The RLLP Remote Port Packet structure is as follows:
<SYNC>
Message format header character that defines the beginning of a
message. The <SYNC> character value is always 0x16. (1 byte)
<COUNT>
Number of bytes in the <DATA> field. (two bytes)
<SOURCE ADDR>
Identifies the address of the equipment from where the message
originated. (1 byte)
TM075 - Rev. 1.3
Table 4-6. Link Level Protocol Message Block
SRC
DEST
ADDR
ADDR
FSN
OP
DATA
CODE
BYTES
User Interfaces
CHECKSUM
4-37
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