Low Bandwidth Connections
Status: Supported · Applies to: FlexRadio
Reduce network traffic and smooth out problems when connecting over metered, congested or bandwidth-limited links such as LTE, satellite, hotel Wi-Fi, SmartLink, or VPN tunnels (ZeroTier, WireGuard, Tailscale).
A normal connection to a FlexRadio carries several UDP streams:
| Stream | Format | Approx. Bandwidth |
|---|---|---|
| RX Audio | uncompressed stereo, 24 kHz | ~1.5 Mbit/s |
| FFT Spectrum | uint16 bins | ~0.5–1.0 Mbit/s |
| Waterfall Tiles | uint16 bins | ~0.3–0.5 Mbit/s |
| Meters | uint16 pairs | negligible |
On a home LAN this is fine. Over a cellular or VPN link the combined 2–3 Mbit/s can cause dropouts, latency and data charges. AetherSDR has several independent controls. Use whichever your link needs.
Using the Low-Bandwidth Options
Audio compression (Opus)
Location: Settings → Radio Setup... → Audio → Audio Compression (SmartLink / tailnet)
| Setting | Behaviour |
|---|---|
| Uncompressed (default) | Uncompressed on every connection |
| Auto | Opus on SmartLink and on a radio reached over a tailnet, uncompressed on the local network |
| Opus | Opus on every connection |
Until you choose, a radio reached over a tailnet gets Opus.
Opus reduces the audio stream from about 1.5 Mbit/s to about 64–128 kbit/s. It applies to both receive audio and PC-microphone transmit audio. On a LAN connection the radio may ignore the request and send uncompressed audio anyway.
When to use: choose Auto if you use SmartLink, or Opus on any metered or limited link.
Low bandwidth mode
Location: connect panel → Connection options for slower links → Use low bandwidth mode
The Connection options for slower links box appears when you choose Remote with SmartLink or Connect by IP. With the box ticked, AetherSDR asks the radio for a low-bandwidth session (client low_bw_connect) as it connects, and the radio reduces the FFT, waterfall and meter data it sends.
- It takes effect when you connect. To change it, disconnect and reconnect.
- The choice is remembered for SmartLink and Connect-by-IP connections. Connecting to a FlexRadio found On This Network always uses full bandwidth.
- On a Hermes-Lite 2, the same option caps the panadapter span at 96 kHz (about 6 Mbit/s).
AetherSDR exposes this as a manual choice because overlay networks such as ZeroTier, WireGuard and Tailscale can make a radio look local even when the real path is cellular or satellite.
Adaptive frame-rate throttle
Location: connect panel → Enable adaptive frame-rate throttle (off by default)
When enabled, AetherSDR automatically lowers the FFT and waterfall frame rate when network quality degrades, reducing the chance of a disconnect on a congested link. It applies to every connection type.
- Toggling it while connected takes effect at the next quality update. To lift a cap that has already been applied, reconnect.
- The Stream Rates page of Tools → Network Diagnostics... charts the frame-rate cap — see Runtime Monitor.
Network MTU
Location: Settings → Radio Setup... → Network → Advanced → Network MTU
Network MTU sets the largest VITA-49 UDP packet the radio sends. AetherSDR's default is 1450 bytes (the radio default is 1500), which suits most VPN and SD-WAN tunnels. The range is 576–9000.
VITA-49 FFT and waterfall packets are about 1436 bytes at an MTU of 1500. Tunnels add overhead that reduces the effective MTU (see Reference). A packet larger than the tunnel MTU is fragmented (adding latency) or silently dropped (losing spectrum and waterfall). Audio packets are smaller and usually get through — which is why you can hear audio but see no spectrum over a VPN.
AetherSDR sends client set enforce_network_mtu=1 network_mtu=<value> to the radio on each connect. It applies to this client's session only and does not permanently change the radio or affect other clients.
VITA-49 RX buffer
Location: Settings → Radio Setup... → Network → Advanced → VITA-49 RX buffer
The operating-system receive buffer for the radio's data stream. The slider snaps to 256 KB, 512 KB, 1 MB, 2 MB or 4 MB (default). A larger buffer absorbs bursts of panadapter and waterfall data so they are not dropped — dropped bursts show up as dips in the network statistics and can trigger the adaptive throttle.
The granted label shows what the operating system actually allowed. On Linux the size is capped by net.core.rmem_max.
Audio smoothing
Two audio settings in Settings → Radio Setup... → Audio help on lossy links even when bandwidth is not the problem:
- Smooth packet loss (on by default) fades over lost audio packets instead of producing a click.
- Audio Buffer (default 100 ms, range 50–1000 ms) — increase it if audio stutters because of jitter.
See Audio Settings.
Reducing panadapter traffic
You can reduce traffic further from the panadapter's Display panel (right-click the spectrum):
- Lower FFT FPS — PANADAPTER section, FFT FPS (5–60, default 25). Try 5–10 on a slow link.
- Slow the waterfall — WATERFALL section, WtrFall Rate.
- Zoom in — a narrower span sends fewer FFT bins.
- Shrink the waterfall — if you only need the spectrum trace, a minimal waterfall height reduces tile traffic.
Diagnosing a link
Tools → Network Diagnostics... shows latency, jitter, packet loss by stream, stream rates and audio-buffer health over time, with symptom search. See Runtime Monitor.
Reference
Recommended settings by connection type
| Connection | Audio Compression | Low bandwidth mode | Adaptive throttle |
|---|---|---|---|
| Home LAN (Ethernet/Wi-Fi) | Uncompressed | — | Off |
| SmartLink over broadband | Auto | Optional | Optional |
| SmartLink over LTE / hotel Wi-Fi | Auto | On | On |
| VPN over broadband | Opus | Optional | Optional |
| VPN over LTE/cellular | Opus | On | On |
| VPN over satellite | Opus | On | On |
Typical tunnel MTUs
| Tunnel Type | Typical Effective MTU |
|---|---|
| WireGuard | 1420 bytes |
| OpenVPN (UDP) | 1400 bytes |
| Cisco SD-WAN | 1400–1450 bytes |
| ZeroTier | ~1400 bytes |
| Tailscale | 1280–1420 bytes |
Recommended MTU settings
| Connection | MTU Setting |
|---|---|
| Home LAN | 1500 (or leave at 1450) |
| VPN / SD-WAN | 1450 (default) |
| Satellite / high-overhead tunnels | 1300–1400 |
Known issues
- On a remote link, receive audio can drop out briefly several times a minute when packets arrive out of order, although none are lost (#6051).
- The network-quality indicator in the status bar is averaged, so it can stay green while the link is bad; use Tools → Network Diagnostics... for live conditions (#2825).
Troubleshooting
Audio works, but the spectrum and waterfall stay empty over a VPN
Spectrum and waterfall packets are larger than the tunnel's MTU, so they are dropped while the smaller audio packets get through.
- Open Settings → Radio Setup... → Network → Advanced.
- Lower Network MTU to suit your tunnel (see Recommended MTU settings).
- Reconnect; AetherSDR sends the MTU to the radio on each connect.
The VITA-49 RX buffer says it was capped
On Linux, the operating system limits the buffer to net.core.rmem_max.
- Raise
net.core.rmem_maxto use a larger buffer. - Check the granted label again.
Audio stutters or clicks on a remote link
Network jitter or lost packets are reaching the audio buffer.
- Raise Audio Buffer in Settings → Radio Setup... → Audio a little at a time.
- Leave Smooth packet loss on.
- Use Opus compression and low bandwidth mode.
Low bandwidth mode has no effect
It takes effect only when you connect, and never on a FlexRadio found On This Network.
- Disconnect.
- Connect again with Remote with SmartLink or Connect by IP, with Use low bandwidth mode ticked.