آپٹکس پر عمومی سوالات: مختصر جوابات

Forty questions that come up at the bench, in the rack and in support tickets, each with a two-sentence answer and a link to the full explanation. Product questions about the CodingBox application itself are in the product FAQ.

Modules and form factors

• Is SFP the same as SFP+? Same shape, different speed: SFP is 1G (up to 4.25G), SFP+ is 10G. An SFP fits an SFP+ port and usually works at 1G; an SFP+ in a 1G port does not run 10G (Form factors).
• Can I put a QSFP28 module into a QSFP+ port? Mechanically yes; it will run at 40G only if the module supports 40G (most 100G modules do not) (Port types).
• Will a 10G module work in a 25G port? Usually yes, if the port supports 10G and the speed is set; port groups may force all four ports to the same speed (Speed & rate).
• What is a DAC, and when is it better than optics? A passive copper cable with SFP/QSFP plugs, zero power, up to 3–5 m; best inside a rack. Beyond that: AOC or transceivers (DAC vs AOC).
• What is QSFP-DD vs OSFP? Two 8-lane 400G/800G form factors; QSFP-DD accepts QSFP28 modules, OSFP does not (it takes more power and has its own heat sink) (QSFP-DD, OSFP).
• How do I tell a multi-mode module from a single-mode one? Label and wavelength: 850 nm = multi-mode (SR/SX), 1310/1550 = single-mode; latch black vs blue/red on SFPs (Labels & colour codes).

Compatibility

• Why does the switch say "unsupported transceiver"? Its firmware compared vendor name and part number against a list and found no match; the module may be perfectly standard (Vendor lock).
• Are third-party modules safe to use? Electrically and optically they are the same MSA parts; the risk is support policy and untested corner cases. Test on a spare port, keep an inventory (Third-party optics).
• What does "coded for Cisco" mean? The module's EEPROM carries a Cisco-style vendor name, part number and, where needed, the vendor-specific check, so the switch accepts it (Vendor lock & coding).
• Does recoding change the optics? No — only the identity bytes; wavelength, power and reach stay what the hardware is. Coding an SR module as LR makes the switch print "LR" and then fail to link (EEPROM recoding).
• Will a firmware upgrade break my third-party modules? It can: vendors change checks between releases. Read release notes and test on a canary switch first (Compatibility matrices).
• Which vendors are strict and which are permissive? See the table in How each NOS validates a module.

Reach, wavelength, fibre

• Can I use a 10 km LR module on a 500 m link? Yes; on very short links check that the received power is below the receiver overload (add an attenuator for 40/80 km modules) (Rx power & budget).
• Can an SR module do 1 km on OM4? No — 400 m at 10G, 100 m at 25G+; modal bandwidth, not attenuation, sets the limit (Reach tables).
• Can I connect an SR module to an LR module? No: different fibre types and wavelengths (Physical mismatches).
• Do BiDi modules have to be bought in pairs? Yes — one transmits at λ1/receives λ2, the other the reverse; two identical BiDi modules never link (Wavelength bands).
• What is the difference between CWDM and DWDM? Channel spacing: 20 nm (up to 18 channels, uncooled lasers) vs 0.8/0.4 nm (40–96 channels, cooled/tunable lasers) (CWDM, DWDM).
• Can I mix OM3 and OM4 patch cords? Yes, both are 50 µm; the link reach is that of the OM3 portion. Never mix 50 µm with 62.5 µm OM1 (Connectors & fibre).
• UPC or APC? Blue UPC for almost all transceivers; green APC for PON and some DWDM plant. Mating them damages both endfaces (Connectors & fibre).
• What is MPO polarity? Which fibre lands on which position at the far end (Type A/B/C trunks); a wrong method swaps Tx and Rx on parallel optics (Breakout & MPO).

DDM and diagnostics

• What is a normal received power? Around −3 to −10 dBm for short SMF links, −1 to −8 dBm for MMF SR; anything below the module's sensitivity (−12 … −18 dBm) is marginal (Typical values).
• Rx power shows −40 dBm or "−inf" — is the module dead? It means no light: fibre unplugged, far end Tx off, wrong fibre pair. The module is probably fine (No link).
• How accurate is DDM? ±3 dB on power and ±3 °C on temperature by specification; good modules are within ±1 dB (Accuracy & limits).
• Why does Tx bias increase over the years? Laser ageing: the control loop raises current to hold power. A rise of 30–50 % over the initial value is a replacement signal (Tx bias & ageing).
• What temperature is too hot? Case above 70 °C for commercial modules; DDM reads a few degrees higher than case. Above the high-warning threshold, check airflow (Temperature grades).
• Why does my copper SFP show no DDM? Most 1000BASE-T SFPs have no laser to monitor and report no diagnostics or fixed values (Copper & DAC).

Speed, FEC, link

• Link is down but light levels are good — why? Almost always FEC or speed mismatch at 25G+, or auto-negotiation left on for an optic (FEC & AN, Port configuration recipes).
• Do I need FEC? Required by the standard for 25G SR, 100G SR4/CWDM4/DR and all PAM4 PMDs; optional for 10G and 25G LR on some platforms. Both ends must match.
• Why does the link flap every few minutes? Dirty connector, marginal power, or a module near its temperature limit; check DDM and error counters (Link flapping).
• What is breakout and does every module support it? A 4/8-lane port split into 4/8 ports; only parallel optics (SR4, DR4, DAC) and only on ports the switch allows (Breakout & MPO).

Power and hardware

• The module is recognised but stays dark — why? Held in low-power mode: power class above the cage limit, or a CMIS bring-up that stalled (Power & thermal, CMIS issues).
• How much power does a module draw? From under 1 W (10G SR) to 15–25 W (400G/800G coherent) (Power & consumption).
• Is the laser dangerous? Pluggable modules are Class 1 (eye-safe), but never look into a fibre or port with magnification, and cap unused ports — mainly to keep them clean (Labels & safety marks).
• How should I handle modules? ESD strap or grounded surface, keep dust caps on, never touch the ferrule, clean before every mating, store in the original ESD bag (Connectors & fibre).
• How long does a module live? Lasers wear out over 10–20 years at normal temperature; heat and high bias shorten it. Most field failures are connectors, not modules (Failures).
• How do I spot a counterfeit? Label vs EEPROM mismatch, implausible serial/date formats, wrong latch colour, poor print, DDM that never changes (Labels & colour codes).

Memory and coding

• What is the EEPROM in a module? A small memory (256 bytes per page) with identity, capabilities, thresholds and live diagnostics, read over I²C (Memory map).
• What are A0h and A2h? The two I²C addresses of SFP-class modules: identity (A0h) and diagnostics/control (A2h) (SFF-8472).
• What is a checksum error? CC_BASE or CC_EXT does not match the bytes it covers — a half-finished edit or a corrupted write; the switch treats the module as invalid (Checksums).
• Why do I need a password to write? Vendors protect the identity area; the password unlocks it via A2h bytes 123–126 (or a vendor scheme) (Passwords, Write protection).
• Can I brick a module by writing to it? You can make it unrecognised (bad checksums, wrong identifier); with a backup of the original image it is recoverable. Firmware areas are a different matter — do not write blindly (Read & write errors).
• Should I back up before coding? Always: read and save the full A0h/A2h (or CMIS pages) image first (EEPROM editor).

In CodingBox

Most answers above start with "read the module": CodingBox shows identity, compliance codes, power class, DDM and thresholds in one screen, which resolves the compatibility, reach and diagnostics questions before the module reaches the switch (Check transceiver).