Rllp Summary - Radyne DMD20 Installation And Operation Manual

DMD20 Universal Satellite Modem User Interfaces

4.9.1.8 RLLP Summary

The RLLP is a simple send-and-wait protocol that automatically re-transmits a packet
whenever 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 Link Level Protocol Message Block is shown below.
SYNC COUNT SRC DEST FSN OP DATA CHECKSUM
ADDR ADDR CODE BYTES
4.9.2 Remote Port Packet Structure
:
The Modem protocol is an enhancement on the DMD20 protocol. It also uses a packet structure
format. The structure is as follows:
TM103 – Rev. 1.1 4-55