Recovering 33.6 kbit/s from a Software Modem on a Packet-Switched Path

A dial-up ISP whose modem termination is software could not sustain rates above 14,400 bit/s despite the datapump implementing V.34. The cause is a carrier-loss watchdog inside the closed vendor object that ends a call after one unbroken second below a signal threshold. On a packet-switched path the threshold is selected by a lookup table indexed in dBm that cannot be indexed, so it returns its out-of-range clamp — a value chosen for no deployment. The criterion is shorter than the protocol-legal quiet intervals the path produces. The defect was located by static analysis of the vendor object and corrected by link-time symbol interposition, modifying no vendor instruction. In a blocked, interleaved A/B with an identical binary in both arms, call completion rose from 19/30 to 27/30 (Fisher exact, two-sided, p = 0.030), with every completed call in both arms reporting 33,600 bit/s at the calling modem. Five secondary defects are documented, four of them in the measurement apparatus rather than the modem, and seven falsified hypotheses are recorded, including three generated and retracted by the analysis itself. The work was performed by an agentic AI system under human direction; Appendix A sets out the division in detail.