User's Manual

UL-801 Installation Manual
Page 28
23-20-06 15 December 2011
DSP which then sends the uplink messages to a designated UniLink system or aircraft. The
message is then processed by UniLink or, when the message is meant for a peripheral unit, is
passed on to the appropriate peripheral unit.
(1) ACARS Uplink Message Receiving
When UniLink receives an uplink message from the ground system, it performs a
Block Check Sequence (BCS) error check. If the BCS message is valid, UniLink
accepts the message and generates a positive acknowledgment for transmission to the
ground. If the uplink message contains an error, UniLink generates a negative
technical acknowledgment. Upon receipt of the negative technical acknowledgment,
the ground system should retransmit the uplink message.
(2) ACARS Uplink Message Log
UniLink maintains an uplink message log capable of storing at least 40 AOC uplink
messages. UniLink stores only those uplink messages that are defined in the database
as requiring storage in the log. UniLink allows users to delete or print specific
messages from the message log. When the message log is full and a new message
requiring storage is received, UniLink will delete the oldest read message in the log.
When the log is full of messages that have not been read and a new message requiring
storage is received, UniLink will delete the oldest message.
(3) ACARS Downlink Message Origination
There are several actions that can generate a downlink. The crew may do so via the
CDU/MCDU and route the message to the UniLink for transmission, or UniLink may
automatically generate a downlink as a response to an event having occurred, such as
weight off wheels or an automatic position report.
(4) ACARS Downlink Message Queue
Once a downlink message is encoded, it is queued until the message can be
transmitted to the ground. UniLink is capable of maintaining at least 70 messages in
the transmission queue. UniLink allows users to delete or print specific messages
from the message log.
(5) ACARS Downlink Message Transmission
When a valid communication link exists, UniLink transmits the message having the
highest priority. If more than one message shares the highest priority then UniLink
selects the oldest from that group of messages. Upon receipt of the downlinked
message, the DSP performs a BCS error check and, if the BCS is valid, it routes it to
the proper destination. The ground system also generates a positive technical
acknowledgment for the message and transmits it to the aircraft. Upon receipt of a
positive technical acknowledgment, UniLink considers the message successfully
transmitted.