The limitations of an IP network
Digital communication is done by sending data packets over an Internet Protocol (IP) network. This presents challenges in communication, challenges that were not previously present in analog transmission and reception. An IP network sometimes causes delays. That means digitized audio must be converted into packets in the very smartest way possible. If you were to split 1 second of audio into 1-millisecond packets, the switch providing the connection might have problems with the amount of packets. Would you send 1 second of audio in one large packet, then you run the risk of latency. In either case, that affects the playback of the audio in a negative way. The ED-137 therefore prescribes that packets of 10, 20 and 30 milliseconds may be sent for digital voice communications. It is up to the engineer to determine what size of packet is desired, in relation to the hardware and software to get an optimal result. MEP has developed specific software to optimize the performance of networks which are fallen below the specifications of ED-138.
How to establish feedback on current transmission with ED-137
Analog transmitters and receivers work with an eight-wire system, for receiving and transmitting audio. It is a continuous two-way traffic where aircraft use the same frequency as air traffic control. The audio transmitted by the air traffic controller is also picked up again by its own transmitter and reproduced in the headset. So the air traffic controller hears himself back and uses this 'off-air sidetone' as a check to make sure the communication is working correctly. This works well for them, since auditory verification of functioning eliminates the need for a visual check to check the connection. So therein lies the problem with digital communications: in the analog world, the 'off -air sidetone' comes back to the headset within 10 milliseconds. In the digital version over an IP network, you spend 10 milliseconds just to pack the audio into a packet; for unpacking, depending on the system, you need 40, 50 or sometimes as much as 80 milliseconds.
This results in a delay that makes hearing yourself back at such an interval rather annoying. It does not come across as natural and interferes with the air traffic controllers work, since you constantly are hindered by your own voice. However, this problem can be solved. With digital communication you basically can't hear anything interchangeably -as is the case with analog- therefor we have developed a system where we create the 'local sidetone' digitally and locally. This ensures that air traffic controllers hear their own voice and know that it is transmitted correct. Failures in transmission are digitally monitored within ED-137 and the 'digital created local sidetone' will be interrupted to feedback a failure to the controller. Compared to systems which can do 'off-air sidetone' the controller is unable to hear a stepped on transmission while transmitting. The controller can not hear when a pilot is transmitting at the same time. This is specific an issue for busy areas /sectors.
Conclusion
We always recommend to inform all stakeholders about the advantages and disadvantages of implementing IP systems. In some cases (for example local airports with radios, VCCS and Tower located in the same building), analogue connections might be a better choice, if they remain available. The prerequisite is that VCM and transmitter are in the same room. If there is a large distance between VCM and transmitter, an IP connection must still be used. We can always help to optimize the performance of your VCCS given the connections available to you.
