Network Latency over 4G
I have access to a remote station about 5 miles from home, access is via the 4G cell-phone network - the carrier is EE.
The station consists a K3 and Remote Rig RRC-1258 MK II units - I was curious about delays introduced by 4G so I conducted the following tests:
At home I installed an Elecraft KX3 connected to a simple 10m vertical on a fibreglass pole.
Also at home I have the 'control' RRC-1258 unit with integrated Winkey. Controlling the radio is done with Win4K3Suite software - not great in terms of usability but it works.
I connected the audio output from the RRC unit to the left-channel line input on my PC.
I connected the audio output from the KX3 to the right-channel line input on my PC.
On my PC I ran 'Audacity' - this recording software allows the user to view the incoming waveforms on each channel as a recording is made, and importantly it is possible to see the time-stamp on each part of thewaveform on each channel. Thus I can accurately measure the delay between right channel and left channel.
The screen shot above shows the waveforms for left and right channels as I was sending some dashes from the remote station. Note - the wave-forms show AUDIO output, not RF output. The red vertical line shows the start of the final dash from the sidetone from my KX3. The blue line to the right shows the start of the corresponding dash through the Remote Rig's audio. The delay can be measured on Audacity's timeline.
Delays experienced in various scenarios are detailed below.
It's important to understand at this stage that a PING request from my home station to the remote station takes between 60mS and 90mS about 80% of the time. The other 20% of PINGs can be anything between 100mS and 600mS - so 'jitter' isn't great. However, most of the time the PING timeis around 70mS - this is an 'each way trip' from home to remote and back again over a VPN.
Transmit at Remote Station : How much delay before I hear my transmission at home?
The delay between RRC audio (the CW side-tone as I send) and audio from the KX3 (the received CW audio), the delay was a fairly consistent 125mS.
Transmit from Home: How much delay before I hear the Remote Station receive it?
The delay between KX3 audio (CW side-tone) and the RRC audio (the received CW audio from the remote station), the delay was again fairly consistent at 225mS.
Listening to the same station at home on the KX3 and on the Remote Station: What is the time difference?
I tuned to the same station on my KX3 and my on my remote K3 (listening through the RRC unit at home) and fed the two audio channels into the RRC unit. The remote station was around 165mS behind the KX3.
The same signal being received on the KX3 (top channel) and the remote radio via the RRC-1258 (bottom channel). It's clear from the display that the RRC unit introduces quite a lot of distortion to the audio. The delay between them is represented by the vertical lines.
Marios 5B4WN has an excellent blog on remote station latency <here>.