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فزیکل لیئر پر عدم مطابقتیں
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۔
A surprising share of dead links are two perfectly good parts that were never meant to meet: a multi-mode module on single-mode fibre, two BiDi modules of the same colour, an angled connector in a flat port. Nothing is broken, nothing alarms — and the link never comes up. This page lists the mismatches, how each one shows up, and how to catch it before installation.
Fibre type
| Mismatch | What happens | Signature |
|---|---|---|
| Multi-mode module (850 nm) on single-mode fibre | almost no light couples into the 9 µm core | Rx at the floor (−40 dBm) or barely above |
| Single-mode module (1310/1550) on multi-mode fibre | may link over short distances, unstable; modal noise and DMD | link up at 10 m, errors or down at 100 m |
| 62.5 µm (OM1) patch in a 50 µm plant | 2–4 dB loss at the joint in one direction | asymmetric Rx, marginal link |
| OM1/OM2 on 10G+ SR | bandwidth-limited | works at 1G, errors at 10G, dead at 25G |
Check the module's length fields and compliance codes against the plant: an SR module declares OM3/OM4 lengths (bytes 16–19), an LR module declares km (byte 14) (memory map). Fibre types and colours: Connectors & fibre.
Wavelength
| Mismatch | What happens | Signature |
|---|---|---|
| BiDi pair wrong — both ends the same colour (two -U or two -D) | each transmitter lands on a receiver filtered for the other wavelength | Rx −40 dBm both ends, fibre proven good |
| CWDM module in the wrong mux port | the mux filter blocks the channel | that channel dark, others fine |
| DWDM tunable on a channel off the plan | filtered or attenuated | low Rx or dark |
| PON GPON module on an XGS-PON OLT (or vice versa) | different wavelength plans never meet | ONU never sees the OLT |
| 1310 vs 1550 on a long span | 1310 loses ~0.35 dB/km, 1550 ~0.2 dB/km | budget miss on the 1310 side |
The wavelength is in the identity block (SFF-8472 bytes 60–61, SFF-8636 186–187) and on the Check transceiver screen — read it, don't trust the label. Plans: Wavelength bands & channel plans, PON wavelengths.
Connectors and polish
| Mismatch | What happens | Signature |
|---|---|---|
| APC (green) mated to UPC (blue) | angled and flat faces cannot touch | 3–10 dB extra loss, possible end-face damage; high return loss on the UPC side |
| Wrong MPO polarity (Type A/B/C mix) | Tx lands on Tx | link dead both directions on parallel optics, per-lane Rx at the floor |
| MPO pinned to pinned or unpinned to unpinned | cannot mate / no alignment | mechanical — or huge loss |
| MPO-12 cable on MPO-16 optics (or 8-fibre vs 12) | lanes unmapped | some lanes dark |
| Dirty or scratched ferrule | 0.5–2 dB per particle | marginal, flapping (Link flapping) |
400G DR4/DR8 and InfiniBand NDR use APC MPO — mixing in a UPC patch is the classic first-install failure (InfiniBand optics).
Tx/Rx crossed
A duplex link needs Tx→Rx in both directions. If a patch cord or panel is "straight" where it should be crossed, both ends see LOS while each Tx is happily lighting the other Tx. Test: swap the two fibres of the duplex pair at one end. On MPO this is the polarity problem above.
Too much light
Long-reach optics on a short span are a mismatch too: a ZX/ER module rated for 80 km launches +3…+5 dBm; on a 500 m link the receiver sees more than its overload point (typically −3…0 dBm) and errors or refuses to link. Fix with an inline attenuator (5–15 dB) — and read Rx power on the bench first to size it (Receivers).
Port and module class
- 1G SFP in a 10G-only port, or 10G SFP+ in a port that does not do 1G — see Speed & rate.
- QSFP+ in a QSFP28 port works (backward compatible) if the host supports 40G; QSFP28 in a QSFP+ port does not.
- OSFP and QSFP-DD do not interchange mechanically (Form factors).
Catching it before installation
- Read both modules in CodingBox: wavelength, fibre type/lengths, connector code (byte 2 / 130) and rate codes.
- Compare with the plant record: fibre type, connector polish, mux channel plan.
- Label modules by wavelength and polish colour before they leave the bench.
If a mismatch is found after the fact and the hardware is right but the identity is not (e.g. a module coded with the wrong wavelength field for its mux plan), correct the identity in the EEPROM editor — but never code a wavelength the laser does not have.
MPO polarity methods, Base-8/12/16, pinning rules and breakout constraints in detail: Breakout & MPO cabling.
Why 62.5 into 50 µm loses 2–4 dB and single-mode into multi-mode excites modal noise — mode field diameter and coupling loss: How light propagates in fibre; mismatches as one of the plant faults: Fibre plant faults.