The Datalink System

A datalink system is a very complex system used by airlines to send data (datalink messages) between their aircraft and the ground.

The virtual datalink system is a simulation of the real world datalink system.

There may be some confusion about ACARS and Datalink. There are no big difference between the two words, actually it is sometimes called an ACARS datalink system. 

There are 3 overall components in the Datalink System:
- Aircraft equipment
- Service provider
- Ground processing

The figure below shows the relation between the elements
in the system:



Aircraft:
The aircraft equipment is explained on the ACARS page, click here to proceed.

RGS (Remote Ground Stations):
The ground stations are placed all over the world. They receive and send datalink messages from and to the aircraft. When a RGS receives a message it will send an acknowledgement to the Control Management Unit (CMU) of the aircraft indicating that the message was successfully received. 

When the RGS sends a message to the aircraft, the CMU sends an acknowledgement to the RGS telling that the message was successfully received.

The RGS’ have either a VHF or a HF radio installed to communicate with the aircraft. The stations have a limited range and are not available everywhere on the earth. Therefore HFDL (High Frequency Data Link) or a satellite system can be used as an alternative for example on trans-Atlantic routes.

Service provider Network:
From the remote ground stations the data is sent via the service provider network to the specific service provider datalink processor. In this case it is the VDLS (Virtual Datalink Systems) main computer. In real world it would be either a ArInc or SITA computer.

If a message is sent from the ground to the aircraft, the message is sent to the RGS from the service provider datalink processor.

Service Provider Datalink Processor:
The service provider datalink processor, the VDLS main computer in this case, identifies, translates and sorts all the received messages, and forwards them to the airlines or to the ATS datalink servers. The processed messages are send trough the service provider connection (VDLS connection).

If a message is going from the ground to the aircraft, the text message is translated into radio format (a kind of morse system) and forwarded to the RGS.

Datalink Servers:
There are two types of datalink servers:
- ATS servers, which are located in airports and in Air Traffic Control stations.
- Airline servers, which are associated with the specific airlines’ computer systems.

When a message from the datalink processeor arrives at the ATS server or at the airline server it is forwarded either to the appropriate end systems or handled by the datalink server immediately.

When a message goes from an end system to the aircraft, the datalink server forwards the message from the end systems to the service provider datalink processor (which translates it into a radio format), which forwards it to the RGS and further to the aircraft.

End systems:
The datalink servers have a number of end systems. These systems can have functions as clearance handling, ATIS delivery etc. The end systems are designed to handle specific message types, for example a ‘request clearance delivery.’ message from an aircraft to for example an ATS server. The end system, in this case the clearance system, will search for the aircraft in their database. Having checked the flight information in the database, the end system generates the departure clearance. The system then sends the departure clearance message to the aircraft via the datalink server, the service provider datalink server, the service provider network, to a RGS and finally to the aircraft.

Examples of other end systems are:
- Flight planning and weather information
- Weight and balance calculations
- Take off data calculations
- Crew scheduling

The Datalink Messages
 
As explained above, messages can be sent in two ways:
- Aircraft to ground, called Downlink
- Ground to aircraft, called Uplink

Messages contain different kinds of information. Examples are:
- ATIS
- METAR
- FLIGHTPLAN
- LOADSHEET
- TODC
- CREW LIST
- MAINTENANCE
- OUT / OFF / ON /IN

These messages are viewed with software developed by the airlines.

Datalink Messenger is a simulation of the software used by the airlines to read the messages
Datalink Messenger can be downloaded for free. Click here to download

Performance

When sending messages, faults and errors can occur. The error rate depends on size, service provider, etc.
Real world message performance:

Downlink success rate is more than:    99%
Uplink success rate is more than:        95%

But as the Virtual Datalink System doesn't use real
VHF frequency communication, the success rate is
100%. 

The message delivery time varies a lot depending on size, service provider, etc.  Real world message performance:

End to end delivery time of uplink messages: 10-20 seconds.
End to end delivery time of downlink messages: 5-10 seconds.