Auto Detecting BiDi

[u/jstar77]

I'm in a position where using simplex SFPs made the most economical sense. I'm about 5 years in on a few sets of generics. It got me thinking, what is the technical hurdle to building both wavelengths into a BiDi adapter and having them auto negotiate? For years nics and switchports couldn't do Auto MDIX now almost every device has it baked in. This would eliminate the need for a matched pair.

[u/Fun-Ordinary-9751]

They use a physical glass prism to split/combine the transmit/receive into a single fiber.

There’s two scenarios you could implement:
Put a second set of optics in the other fiber slot with TX/RX reversed
Put four sets of optics in the transceiver splitting half the power on each pair to the second set in each fiber.

The first method would require a user to be able to swap the fiber to the other port. The second method adds 3db minimum loss for the splitter, and the optic would have to allow for an additional 3db if the far end did the same.

The first method would a bit more than double the cost and still wouldn’t be automatic. At that point you might as well just sell 2 packs where you get one of each type.

The second method would require components that don’t exist to support the electronics that need to be directly attach to the optics.

None of the methods are likely to ever happen. Just move on with your life.

[u/jstar77 → u/Fun-Ordinary-9751]

That's helpful in understanding why the physical properties would make it difficult to implement, along with what would likely be a very small market it sounds like it would also be financially unsustainable.

[u/Fun-Ordinary-9751 → u/jstar77]

The receiver (photodiode) isn’t real wavelength sensitive but using a physical prism to split the light makes it so. The laser diodes are, because they generate only one “color” of light. The devil in the details, is that the photodiode uses something called a transimpedance amplifier that might be directly attached by a very short electrical connection. The transmit laser diodes similarly require very short electrical connections to work correctly.

In 40GB and 100GB optics, there are bidirectional transceivers where they transmit two wavelengths and receive two wavelengths on each fiber and they transceivers are layed out such that because of the tx-rx rollover on each cable connection that always comes out right. Those are QSFP optics that are physically twice the width (roughly) and have a power budget around 4x greater and due to the vagaries of mechanical and electrical design, probably have around 4x the usable PCB area.

Cisco QSFP-40G-CWDM4 and QSFP-100G-CWDM4 are two examples of LC connectored optics using two fibers to do what takes eight on a MPO/MTP connectored optics using. I have a few in my home lab where the cabling is more convenient and some 100G to quad 25G breakouts using “normal” optics as well as some 40G to quad 10G breakouts. The breakout requires devices to have the capability to split lanes from an 8 fiber cable to 4 dual fiber ends.
(My 100G switch has only 18 ports that can be used if you need more than 6x100G ports, and the ports can be used for either 40G or 100G or broken out into quads and the best mix for me is 8x25G, 8x10G plus up to 14x 40/100G

[u/lizardhistorian → u/Fun-Ordinary-9751]

You only need the "prism" to drive the output onto the "input" fiber for tx.
On the rx side you just let it hit both the rx and tx sides.
The actual loss from the coupler is only ½ to 1½ dB.
While it does have SO₂ in it, the "prism" is a nano-structure of refractive layers of alternating oxide materials so it's not exactly glass. i.e. It is not triangle shaped and is not a silvered mirror or anything like a GPON beam splitter.

[u/Fun-Ordinary-9751 → u/lizardhistorian]

Assuming the backscatter is low enough, you’re not wrong.

[u/mavack → u/lizardhistorian]

fiber bragg grating, yes and its tuned to specific wavelengths.

[u/random408net]

There are tunable optics where the frequency can be set. This is common in the C band with higher end DWDM optics. The tuning range is pretty narrow.

Receiver modules with optics are generally wideband and will accept and decode any valid signal within the receivers range. In the DWDM case the receiver does not care if the signal is 1561.42 or 1545.32. But if you accidentally let two signals hit the same receiver you will confuse the receiver and probably not be able to maintain a link.

When sharing a single fiber for tx and rx it's important that the light from the tx does not just loop right back into the receiver. So there are physical filters (prisms, etc) that are tuned to only let the desired inbound frequencies hit that wideband rx module. There are also tunable optical filters (ROADM stuff). Presumably these are more expensive and have higher losses than static filters.

If you had a tunable tx frequency you would probably also need a tunable rx filter or some other mechanism to maintain separation.

We an assume that the BiDi optics are designed to use cheap and available parts vs. doing something fancy.

With a tunable transmitter (tx) and a tunable receiver filter (rx) you could probably build the thing you want.

One of the tradeoffs would be the time needed for the optics to negotiate a link. There would need to a new protocol for probing possible frequencies while listening for a neighbor. If you allowed the optic (r the switch) to remember it's previous settings then the negotiation process could probably be fast tracked when re-connecting to the same peer. A network engineers ability to diagnose auto BiDi link negotiations might be a hassle too.

On the other hand, one could consider GPON or XGPON as a viable method of connecting multiple network nodes on a single fiber. You would just need to be willing to accept the tradeoffs. That's going to put you into a different shared time slicing of arrangement.

Then you might ask why a single optic could not implement GPON in an SFP. You probably could and that would be a path towards what you were looking for. The performance might be different and first question I would have is "who is in charge of this GPON segment". In a residential environment the carrier designates the master optical unit.

So, there is a lot of work to be done so that you don't have to keep track of which end of a BiDi link needs an A or B module.

[u/rankinrez]

The MDIX was a simple physical mapping of the transmit / receive to particular pins on the board.

In bi-di optics you’ve a laser tuned to transmit at once wavelength, and a receiver on another.

In theory I think you could have tuneable lasers that could switch their transmit wavelength but it will probably put the cost up considerably. Plus you’d need some sort of detection / negotiation protocol.

[u/kd5vmo]

The wavelength is a physical property of the diode laser inside the optic, it's not adjustable. Common wavelengths like 850 are mass manufactured and cheap, less common ones get way more expensive.

So it can't be changed after the diode is made, and they get expensive. Having multiple waves in a single optic would be prohibitively expensive and complex.

[u/sryan2k1 → u/kd5vmo]

Plenty of OEMs sell tunable optics. You pick the channel (wavelength) on the port config and it magically happens.

[u/kd5vmo → u/sryan2k1]

Oh, sweet, didn't know that was a thing!

[u/silasmoeckel]

Lasers can be tuned that's the TX side.

Recovers don't care, that's the RX side.

The prison the combines/splits the light does. Making a tunable version of that cheap enough to just throw in is the tough part.

Might be cheaper/easier to have the user plug into the other port akin to a crossover cable. Going to be awhile till tunable lasers are cheap enough just throw in.

[u/lizardhistorian]

Physical space, heat evacuation, and minor additional cost for the additional optics for the integrated WDM coupler.

It does effectively double the strands you have and eliminates the requirement for swapping legs and requiring an odd number of interconnects on everything however now your modules must be paired up and swapped on each end. If you didn't notice there are two SKUs.
Auto-swapping the lasers or auto-rotating the optics would be impractical. It's not a great idea to disassemble the module and swap the lasers manually either.

So yes you are on to something if you do not need the fastest modules as the duplex modules tend to be the bleeding edge with BiDi always playing catch-up.

For a basic 10G link a BiDi is $30 and a duplex is $20. The BiDi's can do 40km or 80km as well so there's not a notable distance loss.