select discrete Broadcast data are sent on output bus SYS5 of the ATSU to the VDR3 port A for VDR3 frequency selection. The ATSU COM2 and VDR/3 buses support:

 -
VDR3 status transmission to the ATSU (voice/data mode, failure...)

 -ATSU status transmission to the VDR3 (primary source/destination, failure...)
 -VDR3 configuration and control by the ATSU
 -ARINC 618 downlink and uplink block exchanges.
 (a) Functional split The functional split between ATSU and VDR3 is the following:
 -
in Voice mode The ATSU controls the VDR3 switching between data and voice mode.

 -
in Data mode

 1_ The ATSU configures the VDR3 in the appropriate protocol
 2_ The ATSU sends ARINC 618 messages in digital format to the
 VDR3
 3_ The VDR3 adds the VHF protocol overhead and performs the
 modulation operation on the VHF signal
 4_ The VDR3 demodulates the received VHF signal, eliminates the
 VHF overhead and sends the received messages to the ATSU in
 digital format. 

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 5_ The ATSU controls the VHF operational parameters of the VDR3
 (frequency...).
 When the ATSU transmits ARINC 618 blocks to the VDR3, it waits
 for the effective transmission to the ground before
 transmitting other ones.
 In case where the transmission is not completed by the VDR3,
 the ATSU sends a 'purge down-link' command, in order to clear
 the failed VDR3 downlink transmission.
 The used System Address Labels (SAL) are:
 - VDR3 SAL is 253
 - ATSU SAL is 304.

 (2)
 Satcom/ATIMS interface Broadcast data are sent on output bus SYS5 of the ATSU to the SDU1. The ATSU COM1 and SDU1 buses support:

 -SDU1 status transmission to the ATSU
 -ATSU status transmission to te SDU1 (primary source/destination, failure...)
 -ARINC 618 downlink and uplink blocks exchanges.
 The protocols used on these buses are ARINC 429, ARINC 618 and ARINC

 741.
 When the ATSU transmits ARINC 618 blocks to the SDU1, it waits for
 the effective transmission to the ground before transmitting other
 ones. In case where the transmission is not completed by the SDU1,
 the ATSU sends a 'purge down-link' command, in order to clear the
 failed SDU1 downlink transmission.
 the used System Address Labels (SAL) are:

 -SDU1 SAL is 307

 -ATSU SAL is 304.

 (3)
 RMP /ATIMS interface (Ref. Fig. 008, 009) The ATSU indicates to the RMPs which system between the ATSU and the RMP is controlling the frequency by means of the port select discrete. The ATSU acquires the remote port select discrete from each of the three RMPs to transmit the pilot request of switching the system controlling the VDR3 frequency. When a switching request is issued, the ATSU activates the VHF voice mode function to determine which system between the ATSU and the RMP will control the VDR3 frequency and in which mode.

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 ATIMS - Switching Request to Data Mode from the RMP
 Figure 008

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 ATIMS - Switching Request to Voice Mode from the RMP
 Figure 009

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 D. Interface with the ground network
 (1) Ground network architecture
R  (Ref. Fig. 010) The airline can exchange data with its aircraft through a ground network which is managed by the following world service providers:
 SITA725 EUROPE SITA550 ASIA/AUSTRALIA (Pacific) AVICOM JAPAN ARINC USA AIR CANADA
 In this ground network, each service provider is responsible for its
 own network.
 The networks are interconnected, therefore the data is transferred
 over any network.
 The aircraft is in liaison with the network through the VHF.
 On the ground, each service provider works on a special frequency:

 -----------------------------------------------DSP FREQUENCY -----------------------------------------------SITA EUROPE 131,725 MHz SITA PACIFIC 131,550 MHz AVICOM JAPAN 131,450 MHz ARINC USA 131,550 MHz AIR CANADA 131.475 MHz
 (2) Data exchange between aircraft and ground network R (Ref. Fig. 011)
 (a)
 Data coding The signal transmitted by the ATSU to the VHF 3 is modulated with the VHF radio signal and then, sent to the ground. Conversely, the signal transmitted by the VHF 3 to the ATSU is obtained after demodulation of the radio signal from the ground. The transmitted data are characters from the ISO alphabet No.5 except the block check sequence field at the end of transmission.
　
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