Four modems do not give you four modems of bitrate
SRTLA keeps you on air when one link dies. It pays for that with overhead and delay, and nobody tells you the exchange rate before you buy the bag.
SRTLA bonds several connections into one SRT stream. Four modems on a scooter become one link. That is the pitch. It holds up.
What gets skipped is the bill.
How it moves your packets
Your encoder sends across every path at once. A server collects what arrives. It throws away duplicates and fills gaps from whichever link delivered the missing piece. What comes out the far side is ordinary SRT.
The sender watches each connection. Healthy links get more packets. A weak link still carries some traffic. It does not drag the stream down to its speed.
What it costs in bitrate
Budget around 15% overhead. That covers control packets and SRT retransmissions. The rest is waste built into splitting traffic across links with different delays.
So four modems at 5 Mbps each are not 20 Mbps of video. Take the overhead off the top. Then remember the sender holds capacity back to absorb a drop. Bonding is redundancy. It is not free bandwidth.
You are buying reliability. Throughput is what you hand over for it.
What it costs in delay
Set SRT latency between 1500 and 3000 ms. That is far above single-path SRT. It is not optional.
Packets arrive from different paths at different delays. Aggregate jitter beats any single link. The buffer has to be big enough to reorder all of it. Undersize it and you get artefacts on a connection that looks fine.
The window is a trap
SRTLA tunes a window. It decides how hard traffic spreads across links. Get it wrong in either direction and you lose.
| Window | What breaks |
|---|---|
| Too small | Retransmissions pile up. Traffic collapses onto one link and bonding stops happening. |
| Too large | Lost and late packets do not get retransmitted in time. You lose data. |
There is no universal setting. It depends on your links. Your links change every time you turn a corner.
Before you build one
- The reference implementation runs on Linux. Porting it elsewhere is described as straightforward, which is not the same as done.
- You need a server. Bonding is a two-ended protocol and the receiver has to reassemble.
- Your ceiling is the sum of your links minus overhead minus reserve. Plan bitrate against that number, not the marketing one.
- More modems on the same carrier in the same cell is not redundancy. It is one failure wearing four SIM cards.
That last point costs people the most and appears in no spec sheet.
Sources
The Rig Wire reports on streaming technology from public documentation, source repositories and vendor releases. We did not bench-test what is described here. When we have tested something, we say so.