Half Fibre Pair

Can someone explain to me what is a half fibre pair? I took it literally to mean a single fibre strand but someone insisted it was a large quantity of spectrum. Please illuminate. On or off list as you please.



single strand / cwdm optics

Maybe it's like half a pair of glasses, the perfect accessory for the
one-eyed man who's king.

Seriously though, it sounds like a bad language construction. If a
vendor is offering you that, I'd ask for clarification. Are they
leasing a dedicated strand of fiber end-to-end? Dedicated wavelength
directions delivered by fiber? Something else?

If you're thinking of offering it, find better words.

Bill Herrin

I’d internet that to be a really weird way to describe a single strand as well, but I could see a confused person asserting it’s 44 out of 88 wavelengths? I’ve never heard that.

Ms. Lady Benjamin PD Cannon, ASCE
6x7 Networks & 6x7 Telecom, LLC
"The only fully end-to-end encrypted global telecommunications company in the world.”

FCC License KJ6FJJ

Back in the day, there were these things called half-circuits or half-cables.

Telephone companies in different countries would “share” a cable under the ocean, where the company in each country would own “half” the cable - i.e. from their shore to the middle of the ocean.

I have no idea what the context here is, but ….

Actually it is standard language in the undersea cable world for a large spectrum purchase. Sometimes a fiber pair on a system may be too much, but the buyer still wants many terabits of capacity. "

The Half Fiber Pair is the same as 10*MSUs in a virtual fiber pair, either in C-band or L-band. I believe these are primarily used in transocean routes."

This is what I have learned so far.

Now that deep sea cables are being deployed with as many as 24 pairs, there will be more players doing fractional purchases.

You can see the terminology getting referenced in articles such as this one from Telegeography:


“Further PLCN incorporates Spectrum Manager, that allows the C+L band to be sliced into blocks of spectrum that can be independently assigned to separate SLTE that is owned, operated and upgraded by the party leasing the spectrum. PLCN has productized it in terms of Minimum Spectrum Unit (MSU) which is 5% of the total C+L band capacity i.e. 5% of 240x100G or 1.2 Tbps. It’s extended further as virtual fiber pair with Quarter Fiber Pair as 5xMSU or 6 Tbps and Half Fiber Pair as 10xMSU or 12 Tbps. This notably is not unique to PLCN and is supported by most [of] the cable systems designed post 2015 or supported with upgrades.”

Depending on the context it could be a single fibre strand for use with bi-directional transceivers.

I am guessing it’s not commonly used though, just based on nobody suggesting this.


It is language used in the submarine world, where a member of the consortium may not be able to afford a full fibre pair on the system. Don’t take it literally :-). Mark.

How much spectrum is a half fibre? It must be standardized in some fashion.



It would be based on the amount of capacity each fibre in the overall system can carry across a given line system span. So say a cable system is able to carry 960Gbps per fibre pair, and it has 5 fibre pairs, that means a half fibre pair purchased by one of the consortium members would be 480Gbps. It is also possible for a consortium member to own a full + a fractional fibre pair, e.g., two and a-half fibre pairs. In such a contract, for example, say a 24 fibre-pair system could carry 1.2Tbps per fibre pair, that member would have 3Tbps of capacity. Mark.

What is interesting is this new deep sea design. In the old days cables had 4 to 8 pairs max. Now I am seeing Orange talking about 18 pairs and 24 pairs. With more widely regeneration.


Orange takes a leading role in the US to Europe route with two new generation submarine cables linking the East Coast to France | Orange Com
After the landing of the Dunant cable, a Google project announced back in March 2020, Orange announces it is now ready for service for its wholesale and business customers. With 12 fibre pairs with over 30 Tbps of capacity each, multiplying by three the previous generation of transatlantic submarine cables capacity. Orange also announces the signature of a partnership on the AMITIÉ cable …

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I believe strand counts were small because the power needed for that many amplifiers was too much to bear for budgets.

I suspect it’s a combination of more power efficient amplifiers and a greater willingness to bear the extra costs to get the capacity that hyperscalers need.

Have many of those higher strand count cables been proposed that have any distance to them that don’t have a variety of hyperscalers in the anchor tenants? It’s a lot cheaper to power a 300 km cable than a 3,000 km cable.

Also, I don’t just mean in the MRC, but in the NRC of the plant needed to supply and transmit that much power.

That was just an example to illustrate the commercial contracting, not the technical capabilities. Cables currently being laid in the sea are going in at anywhere between 16 and 24 fibre pairs. The design capacity per fibre pair of these systems is 20Tbps, with a possible 480Tbps of Shannon limit capability. Mark.

Because of the way current submarine cables are being built (mainly by the content folk, less by traditional telco’s), limiting the number of fibre pairs does not make sense anymore, especially for them since they would be partnering with some carriers along the route. I mean, you’re already laying it. Fibre, itself, doesn’t cost that much… the cost is elsewhere. Mark.

Also because the amount of capacity we are talking about nowadays, driven by the content folk, is something telco’s could only (and still) dream of. You guessed it… it’s not traditional telco’s pushing cable builds anymore. It’s not uncommon to have multiple fibre pairs on shorter spans and fewer on longer/express ones. But yes, longer systems cost a lot more money; for everything, not just power. Mark.

I asked a submarine guy how much the fibers can carry because this sounded low to me. His response:

it depends on the type of cable. Older cables (with embedded dispersion compensation) have a lot less capacity and I have seen some as low as 1Tb/s per fiber pair and some as high as 10Tb/s per fiber pair. All newer D+ Cables that have been deployed in the last 5 years and will be the only cables deployed going forward can easily carry 20Tb/s of capacity per fiber pair. Something Like Havfrue can support 22T per fiber pair and there are 8 fiber pairs for a total of 176T.