ELI5: MTP / MPO connectors

[NablaSquaredG]

Hi,

So I've been trying to wrap my head around MTP / MPO connectors and... failed.

I've understood that there's 12 and 24 fibre versions which are not compatible.
There are male and female keyed versions. You cannot directly connect two MTP / MPO plugs (even if male + female), you always need an adaptor.
There seem to be no adaptors to couple male + male or female + female, except if you buy a customised cable.
Transceivers are always male.

What I haven't understood is the polarity and keying thing.
There are three different kinds of polarity: A (straight), B (inverted / crossover, i.e. 1->12, 2->11, etc, for RX->TX and TX->RX) and C (pairwise crossover).
But how does this relate to the keying of the plugs?
And are B polarity cables actually inverted or is the crossover just achieved by the keying somehow?

Please explain like I'm five, because I'm lost, even after reading what felt like half the internet...

 

[alex.forencich]

First, MTP and MPO connectors are the same thing, MTP is just a specific brand of MPO connector.

Like you said, there are 12 and 24 fiber versions, the 12 fiber version has 1 line of 12 fiber cores, and the 24 fiber version has two. If you connect a 12 fiber cable to a 24 fiber cable, none of the fibers will line up and you won't get any light through it.

The male and female keying is simply the presence or absence of two metal alignment pins. The male connector has pins, while the female connector does not. You can't connect two males together as the pins will interfere. You can connect two females together, but without alignment pins, the fiber cores will not be aligned with each other, and you won't get (much) light through. Unfortunately, you can't get a simple adapter to couple male-male or female-female, you have to use a section of cable.

The polarity has to do with how the fibers are connected between the ends of the cables. There are several different styles for different purposes, and it is totally unrelated to the keying of the connectors. I think the "standard" MPO cable is a type B cross-over cable. This enables you to directly connect two QSFP transceivers, as it will cross-connect the transmitters to the receivers. If you use a type A cable in this instance, the transmitters on one end will be connected to the transmitters on the other end, and the link won't work. When you have a coupler, the keys are connected key up to key down, so the fiber order is not swapped (1 goes to 1, 2 goes to 2, etc.). This is in contrast to other fiber connectors - such as LC - that swap the fibers. With LC, the standard fiber arrangement also swaps the fibers, so it works out nicely - transciever -> fiber (swaps RX/TX) -> connector (swaps back RX/TX) -> fiber (swaps RX/TX) -> transceiver. With MPO, the situation is slightly more annoying - transceiver (male connector) -> type B female-female (swaps RX/TX) -> connector (does not swap) -> type B male-male (swaps back RX/TX) -> connector (does not swap) -> type B female-female (swaps RX/TX) -> transceiver. Basically, you always have to go through an odd number of type B cables, alternating between female-female and male-male.

Now, what about type A and type C? I believe these are only used when you're interconnecting patch panels. In this case, you might have 6x LC to MTP on one end, and MTP to 6x LC on the other end. But, you need to use a type C fiber to swap all of the pairs so that the patch panel works correctly, depending on how the LC connectors are connected to the MPO (RX connected to TX, etc.).

IMHO, the design of MTP is kinda screwy. I'm not sure why they went with different male and female connectors, unlike most other fiber types where they don't have this distinction. And I'm not sure why the fiber couplers don't also swap the order, like they do for LC (key up to key up, instead of key up to key down).

 

[NablaSquaredG]

Thanks for your very comprehensive explanation @alex.forencich !

So now about my actual issue:
I've ordered a 3*12 30m (Female on both ends) for very little money... But the breakout on each side of the cable probably isn't long enough and I'm going to need patch / extension cables.
Assuming the cable I ordered is already B-polarity (seller didn't know and I can't find a datasheet) - What patch cables should I get?

Male to Male cables seem to be very rare....

I basically see options:

1. Get two male-male and two female-female cables
   1. Which polarity should I choose for each type?
   2. Advantages: Cable can be reused in other situations
   3. Disadvantages: More expensive, more transmission loss due to more connections
2. Get customised male-female cables
   1. Which polarity should I choose? Both A and B would work, right?
   2. Advantages: Cheaper, less transmission loss
   3. Disadvantages: Cables are non-standard and cannot be reused?

The market for second-hand MTP cables is very sparse...

 

[alex.forencich]

You should be able to check the polarity yourself with a flashlight, assuming the cable is not excessively long (it is fiber optics after all, visible light will propagate, although not as well as the design wavelength). Should be pretty obvious if it's mirrored (B) or not (A) with respect to the key by lighting up half of one end while looking at the other end. Also, if you have SR4 modules and something to put them in, then you can plug it in and see if you get a link (B) or no link (A).

Think of it this way: one B-type cable undoes the swap of an adjacent B-type cable. So two B-type cables end-to-end get you right back where you started. So, you could get either customized A-type male to female cables, or string together a male-male and then a female-female (presumably both B-type, but both A-type would also work). Depends on what you can get your hands on. Main thing to check is that all the fiber cores are the same size, although presumably it's all 50/125 OM-something multimode fiber. Mixing OM2/OM3/OM4 should not be an issue.

 

[NablaSquaredG]

I'll use those "extensions" on both sides, so I could choose from either A and B, right?


Transceiver (Male) - (Female) Customised Extension (Male) - (Female) 30m cable (Female) - (Male) Customised Extension (Female) - (Male) Transceiver

Assuming the 30m cable is B-Polarity and showing the flow from left to right:
Polarity for A custom cables:

Transceiver	Custom Cable	30m Cable	Custom Cable	Transceiver
Normal	Normal	Flipped	Flipped	Flipped

Polarity for B custom cables:

Transceiver	Custom Cable	30m Cable	Custom Cable	Transceiver
Normal	Flipped	Normal	Flipped	Flipped

So both would work I guess.
MTP Cables are quite expensive though, about 58 bucks for one 3m customised cable...

 

[alex.forencich]

Yeah, if you use the same on both sides, then it doesn't matter. The main thing you want to avoid is a pairwise swap.

 

[klui]

As someone who is also considering running some fiber I have viewed a lot of videos and searched for information on higher-bandwidth fiber like MPO/MTP. While these two types are compatible they are not exactly the same. I will leave it to those who want to pay a higher price for MTP cables.

With so many variables it's a good idea to consult with suppliers to understand your options. Personally I don't want to have crosspatch cords on one end and not the other so it makes more sense to have trunks as type B. Then all my patch cords would be type A. You can basically do the same using mating adapters that swaps key-up to key-down so all cables can be type A and with the correct adapter you would in essence turn one segment into type B.

I had a lot of difficulty finding MPO keystones that didn't cost a lot, but they are available.