QSFP28 100G BiDi: how to pick the right pair
Need 100G between two sites but only have one fiber, or don't want to lease your dark fiber twice? Then you need a 100G BiDi QSFP28. Below: which pair to order, the difference between 20 and 40 km, and what to watch for on a short link.
One fiber, two wavelengths
A 100GBASE-LR4 uses two fibers with a duplex LC. A BiDi QSFP28 transmits and receives over a single single-mode fiber, each direction on its own wavelength, and a built-in WDM filter separates the two. All our BiDi modules have a simplex LC connector: one fiber, one LC.
Always order a pair: U and D
A link only works with a U module at one end and a D module at the other. Two U's or two D's will never give you link. Note: the price is per module. For one link you order two, for example QSFP-100G-BX20U-SO plus QSFP-100G-BX20D-SO.
- 20 km — QSFP-100G-BX20U-SO (transmits 1290 nm, receives 1310 nm) and QSFP-100G-BX20D-SO (transmits 1310 nm, receives 1290 nm)
- 40 km — QSFP-100G-BX40U-SO (transmits 1304.58 nm, receives 1309.14 nm) and QSFP-100G-BX40D-SO (transmits 1309.14 nm, receives 1304.58 nm)
The 40 km wavelengths are two channels from the LAN-WDM grid that 100GBASE-LR4 also uses. They are only 4.56 nm apart, so the filter in the module has to be precise. Never combine a BX20 with a BX40: the wavelengths don't match.
Short link? Do the power maths
The BX40 has an APD receiver and a transmitter up to +7.1 dBm, while its receiver overloads above −2.4 dBm. On a short link you therefore need an attenuator. Only pick the BX40 when the BX20 can't cover your distance or power budget, and read the RX power via DOM after plugging in.
Coding for your platform
Many switches read the vendor ID from the optic's EEPROM and log a warning or keep the port down for a module they don't recognise. So pick your platform under coding: Cisco, Juniper, Arista, Mellanox/NVIDIA or one of the other vendors in the list. We code the module before it ships.
Need less bandwidth? Pick the SFP+ 10G BiDi or the SFP 1G BiDi. Want a spare pair ready for when an uplink drops? Read how to sort spares and same-day replacement.
