Iridium SBD

Short Burst Data over an Iridium 9603N modem, usually a RockBLOCK. This is the bearer that works when nothing else does, and the one that costs real money per message.

The constraint that shapes everything

Maximum message out (MO)	340 bytes
Maximum message in (MT)	270 bytes
Serial	19200 baud, AT commands

340 bytes is the whole message, not the payload. Anything longer is fragmented by the bridge and reassembled at the far end, and every fragment is billed as a separate message. This is why the router tries free bearers first and why text is compressed with SMAZ2 before it is sent.

Fragmentation

The fragment header is two bytes: a packed index and total in the first, a per-device message id in the second. The format is shared with the Hub and the Android app, so a message fragmented by one is reassembled by the others.

Cost

An Iridium line costs tens of euros a month plus credits per message, billed in blocks. Give one to every person in a team and the bill scales with headcount. Sharing one link across a mesh is the point of the product.

You bring your own Cloudloop or Rock7 account; the Hub does not resell airtime. See Provider accounts.

What the modem needs

Sky view. A 9603N will not get a fix or a pass indoors. First activation on a new line can take ten to thirty minutes with a clear view of the sky.