DDM (Digital Diagnostic Monitoring, defined by SFF-8472 and also called DOM) lets a switch read an optical module's real-time TX/RX power, temperature, voltage and laser bias over the I²C bus. Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches, and its PON modules — high-power BIDI GPON/XGS-PON optics in Class C+ to C++++, often deployed as a gpon sfp — expose DDM so you can watch link health from the NMS before a fiber actually fails.
DDM / DOM (SFF-8472) reports TX bias, TX power, RX power, module temperature and supply voltage through I²C.
Only modules labeled DDM / DOM have the sensor and diagnostic interface; cheap "dumb" modules report nothing.
RX power is the first thing to watch — it directly shows how much light the link is actually receiving.
Each value carries Alarm and Warning thresholds; crossing them fires an SNMP trap without a site visit.
DDM trends (RX slowly sliding, TX falling while bias rises) warn of degradation or laser aging before the link breaks.
DDM (Digital Diagnostic Monitoring), defined by SFF-8472 and commonly called DOM, lets the sensors inside a ddm optical module be read by the switch over the I²C bus in real time: laser bias current, transmit optical power, receive optical power, module temperature, and supply voltage.
The catch: the module itself must support it. Only optics marked DDM / DOM carry the sensor circuitry and I²C diagnostic interface. A low-cost "dumb" module has none of that and reports zero diagnostics.
| Parameter | What it means | Typical range (example) | Out-of-range signals |
|---|---|---|---|
| TX power | Laser output on the transmit side | +0 to +5 dBm (by class) | Weak light = laser aging |
| RX power | Light the module actually receives | −3 to −25 dBm | Too low = link degradation / fiber issue |
| Temperature | Module junction temperature | 0 to 70 °C (commercial) / −40 to 85 °C (industrial) | Derating, shorter life |
| Voltage | Module supply | Around 3.3 V | Unstable power |
| TX bias | Laser drive current | Tens of mA | Abnormal = laser health falling |
Among these, optical module RX power deserves the most attention — it directly reflects "how much light this link is really getting," and is the first piece of evidence for a weak peer laser, a fiber micro-bend, or a dirty connector.
Every diagnostic value carries two threshold levels: an Alarm threshold and a Warning threshold. For example, when RX power drops below the Alarm threshold, the NMS immediately reports "low receive light" — most often caused by a fiber micro-bend, a dirty connector, or a weakening peer laser.
Thresholds mostly ship as module defaults; some switches allow fine-tuning. Crossing a threshold triggers an NMS trap, so operations knows which port is in trouble without dispatching anyone.
The biggest value of DDM is observable trend. You do not need to haul an OTDR to the site — one glance at real-time RX power from the NMS or SNMP tells the story:
RX slides from −3 dBm toward −22 dBm → the link is steadily degrading; you can intervene before it hard-fails.
TX power drops while bias current rises → often a precursor to laser aging; time to stage a spare.
Temperature riding the upper limit long-term → consider cabinet cooling or derating.
It does not replace an OTDR (only an OTDR locates event points and break distance), but it is the cheap "eye" for daily health monitoring.
Rayin industrial switches and GPON OLT product lines support reading DDM from their ports; paired with SNMP northbound, TX/RX power and temperature can be pulled into platforms like Zabbix for trend curves and threshold alarms. For optical-module selection — BIDI single-fiber GPON/XGS-PON in Class C+ to C++++ — see the Rayin PON solution; switch-side port and monitoring capabilities are covered on the Rayin industrial switches page. For the company background, see About Rayin.
No. Only modules marked DDM / DOM carry the internal sensors and I²C diagnostic interface. A "dumb" module without DDM reports nothing.
No. Optical power is expressed in dBm on a logarithmic scale, so negative values are normal; the closer to 0, the stronger. −30 dBm is far weaker than −10 dBm. The key is whether it sits inside the module's spec and the link budget.
No. DDM shows real-time power trends; an OTDR locates event points and break distance. They do different jobs: daily monitoring belongs to DDM, precise fault location to OTDR.
Modules ship with default thresholds; some switches allow fine-tuning. Crossing a threshold triggers an NMS trap, and operations can wire that to SMS or email per policy.
DDM turns an optical module into an observable device: TX/RX power, temperature, voltage and bias are all visible, and threshold breaches alarm automatically. Operations no longer needs to haul an OTDR to the site — one look at real-time RX power from the NMS reveals a link quietly degrading long before it breaks. That eye is far cheaper than finding out after the fact.
Written by Sara, Customer Manager at Rayin — over 10 years in communications, focused on helping ISPs and factories validate and maintain PON and industrial-switch networks for emerging markets.
About Rayin → https://www.szrayin.com/Profile/
