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User Interfaces DMD15/DMD15L IBS/IDR Satellite Modem
4-54 TM051 – Rev. 5.8
Each piece of supported equipment on the control bus executes a Radyne Link Level Task
(RLLT) in accordance with its internal hardware and fixed program structure. In a flow control
example, the RLLT issues an internal "message in" system call to invoke an I/0 wait condition that
persists until the task receives a command from the M & C computer. The RLLT has the option of
setting a timeout on the incoming message. Thus, if the equipment does not receive an
information/command packet within a given time period, the associated RLLT exits the I/0 wait
state and takes appropriate action.
Radyne equipment is logically linked to the control bus via an Internal I/O Processing Task (IOPT)
to handle frame sequencing, error checking, and handshaking. The IOPT is essentially a link
between the equipment RLLT and the control bus. Each time the M&C computer sends a
message packet, the IOPT receives the message and performs error checking. If errors are
absent, the IOPT passes the message to the equipment's RLLT. If the IOPT detects errors, it
appends error messages to the packet. Whenever an error occurs, the IOPT notes it and discards
the message; but it keeps track of the incoming packet. Once the packet is complete, the IOPT
conveys the appropriate message to the RLLT and invokes an I/0 wait state (wait for next
<SYNC> character).
If the RLLT receives the packetized message from the sender before it times out, it checks for any
error messages appended by the IOPT. In the absence of errors, the RLLT processes the
received command sent via the transmitted packet and issues a "message out" system call to
ultimately acknowledge the received packet. This call generates the response packet conveyed to
the sender. If the IOPT sensed errors in the received packet and an RLLT timeout has not
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 <SYNC> character.
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 <SYNC> 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