Five modems, one DSP
Fitting five concurrent Fax-over-IP sessions onto a single DSP56311 — where protocol correctness and hardware density had to be solved together.
The problem
Fax-over-IP means taking a protocol family (V.17, V.34) designed around the timing assumptions of a dedicated analog phone line, and making it work correctly over IP while running on shared, constrained DSP hardware. Fax handshakes are timing-sensitive by design — that's exactly the property that makes them unforgiving on hardware where you're not the only thing running.
Constraints
- Protocol timing is not negotiable. V.17 and V.34 were designed for point-to-point analog lines; getting them to behave correctly over IP telephony without breaking their timing assumptions is most of the difficulty.
- Shared, constrained hardware. A single DSP 56311 has a fixed amount of compute to divide across every concurrent call — density and per-call correctness pull in opposite directions.
- No easy way to "see" a protocol failure. Fax protocol errors don't announce themselves clearly; without dedicated diagnostic tooling, a failure mode can look like random flakiness instead of a specific, fixable defect.
The decisions
I implemented the Fax-over-IP protocols themselves, but the part of the work with the most leverage was building diagnostic, testing, and validation tools specifically to expose protocol correctness and system stability issues that would otherwise be invisible. From there, the optimization work focused on increasing modem density on constrained hardware — scaling up to five modems running concurrently on the same DSP 56311.
The trade-off
Every additional concurrent modem session on the same DSP is less headroom for each individual call — density work that isn't paired with real diagnostic visibility just trades one hard-to-reproduce failure mode for a more frequent one. Building the validation tooling first made it possible to push density up with evidence that correctness was holding, rather than by hoping it would.
Impact
The system optimized modem density and resource utilization on constrained hardware to the point of running five modems concurrently on a single DSP 56311, backed by diagnostic and validation tooling built to keep protocol correctness verifiable rather than assumed.