DDM کی درستگی اور حدود

ہماری ٹیم ترجمے پر کام کر رہی ہے۔ یہ مضمون عارضی طور پر اردو میں دستیاب نہیں ہے، اس لیے اسے انگریزی میں دکھایا جا رہا ہے۔

DDM numbers look precise — two decimals of dBm, hundredths of a volt — but the specification behind them promises far less. Knowing what a monitor physically measures, how accurate it is required to be and where it stops working keeps you from chasing a "1 dB drop" that is inside the noise, and from trusting a "normal" reading that a cheap module simply made up.

What the specifications guarantee

Monitor	Required accuracy (SFF-8472 / SFF-8636, internally calibrated)	Resolution (LSB)	Over what range
Temperature	±3 °C	1/256 °C	module's operating range
Supply voltage	±3 % of nominal	100 µV	0 … 6.55 V
Tx bias	±10 %	2 µA	0 … 131 mA
Tx power	±3 dB	0.1 µW	vendor-specified range, typically −8 … +4 dBm
Rx power	±3 dB	0.1 µW	vendor-specified range, typically −20 … 0 dBm or −30 … −7 dBm for APD

Many vendors do better — ±1.5–2 dB is common — but only the datasheet says so. The ±3 dB means two modules on the same fibre may legitimately report Rx values 6 dB apart from each other's Tx minus the real loss. CMIS modules inherit the same monitor formats; their advertising pages may state tighter figures.

What is actually measured

Reported as	Physically	Consequence
Tx power	current of a monitor photodiode catching a fraction of the laser output (back facet or tap)	an estimate scaled at the factory; drifts with temperature and coupling; on some modules replaced by a constant — DDM levels
Rx power	average photocurrent of the receiver photodiode (before or after the TIA)	true average power; independent of data content for NRZ, but responsivity depends on wavelength — a receiver calibrated at 1310 nm mis-reads a 1550 nm signal by ~1 dB
Tx bias	the current the driver sets	usually accurate; on EML modules only the laser section, not the modulator
Temperature	a sensor on the module PCB or in the controller	a few °C above case temperature; not the laser temperature (that is the optional laser-temperature monitor)
Vcc	supply measured inside the module	slightly below the host rail because of the connector and filter

Average vs OMA

Specifications express sensitivity in OMA (optical modulation amplitude); DDM reports average power. For an NRZ signal with typical extinction ratio the two differ by about 2–3 dB (average is lower than OMA + 3 dB… the exact offset depends on extinction ratio). PAM4 makes it worse: average power says little about the eye. A module can therefore be "in range" by average power and still fail sensitivity. Byte 92 bit 3 of SFF-8472 tells which kind of number the module reports (Calibration).

Floors and ceilings

Rx floor — below about −30 dBm (PIN) or −35 dBm (APD) the photocurrent is in the noise; the raw value hits 0 and displays as −40 dBm (10·log10 of 0.1 µW/1 mW). A reading of −40 means "no measurable light", not "−40 dBm of light".
Rx ceiling — above the monitor's range the value saturates; overload of the receiver (errors) may begin before or after the monitor saturates.
Tx — meaningful only while the laser is on; with TX_DISABLE asserted it should fall to the floor. A Tx that stays at its nominal value with Tx disabled is a constant.

Timing

Modules refresh monitors internally several times per second; hosts poll every few seconds; NMS every minutes. A brief event (a flap of 200 ms) may never appear in DDM.
Right after insertion the data-not-ready bit is set (SFF-8472 A2h 110.0, SFF-8636 byte 2.0); values read then are garbage.
Flags are latched: an alarm you see may be from an event minutes ago that has since recovered (Thresholds & alarms).

Where DDM is absent or misleading

Module type	What you get	Note
Passive DAC	nothing — no A2h device	identity only
1000BASE-T / 10GBASE-T SFP	often temperature only, or none; Tx/Rx fields zero or constant	a copper PHY has no optics to measure
AOC	full DDM at both ends	levels are internal to the cable; a low Rx inside an AOC means a failing cable, not a dirty connector
PON OLT module	Rx is a burst-mode average over all ONUs	per-ONU power comes from the OLT's ranging, not from DDM — PON problems
PON ONU stick	correct Rx; Tx meaningful only while transmitting bursts	Tx may read low or zero when idle
Tunable DWDM	Tx/Rx fine; adds laser temperature, TEC current, wavelength/frequency monitors	Tunable maps
Externally calibrated modules	raw values until constants are applied	absurd numbers if the host or tool ignores byte 92 — Calibration
Low-cost or cloned modules	Tx and sometimes bias are constants; Rx usually real	compare against Tx disable and across a batch
Media converters / DDM through a PHY	some devices synthesise values	trust only what the module itself reports on the bench

Practical rules

Treat differences under 1 dB as noise; act on 2–3 dB changes from a baseline (Monitoring).
Compare Rx on both ends and Tx vs Rx across the link, not single numbers.
Verify a suspicious Tx with an external power meter once; if the module lies, note it in the code database and rely on the far end's Rx.
Do not use DDM to certify a fibre plant — that is an OTDR/light-source job. DDM is a built-in health indicator, not a test instrument.
When the numbers are absurd (0 °C, +10 dBm, 0.00 V), suspect calibration, not physics.

Expected windows to compare against: Typical values.

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