Dark Fiber

FloOz

Before starting my new job I had never really worked with dark fiber before. Now in my current environment we have many dark fiber links that connect between many of our locations. I have done some research on my own about the topic and it is really interesting. However I have some questions that I can not find a straight answer for. Basically dark fiber is a cable run that is completely dedicated to you. So if you're not passing traffic then no light ever is on that link. My question is how many channels(lamdas) can you have on one dark fiber? What is the bandwidth on a dark fiber link? Does it vary depending on what you're paying for?

For example can you have a 100Gb dark fiber run that breaks off into ten 10Gb channels?

Sorry if these are noob questions.

it_consultant

The short answer is yes. It is called wavelength multiplexing. Most optics will operate on one wavelength and you get one link and you are good to go. ISPs and other people (Colorado DOT for example) will use something like a ciera switch which can utilize coarse fiber, they can get 10 100GB links (depending on the optic) off of one pair. The ciera switch does something that resembles a hybrid between SONET rings and ethernet over fiber which gives you the nice logical ethernet feeling with the redundancy of a ring architecture. Using that, you can send 100GB across the ring in one direction and 100GB in the other and have what will appear to be a 200GB link without using any form of trunking.

If you already have dark fiber run, then it is almost always less expensive to light your dark fiber and buy optics as its needed than it is to get into this mess because the costs we are talking about are astronomical. It is still cool to talk about though.

Wavelength-division multiplexing - Wikipedia, the free encyclopedia

stryder144

You mention that dark fiber is completely dedicated to you. That is not strictly true. While your building may have had fiber laid between them and a LEC, gateway, etc, dark fiber is essentially unused fiber optic cable that can be turned on when necessary. Often times, telecoms will pay to have their fiber optic conduit buried in the ground/strung over power cables and place more capacity than they currently need in order to reduce one of the largest single costs. It usually cost a little more to bury twelve conduits than just one or two. So, the majority of fiber optic cable (one source says 90%) is dark.

An interesting read, and the source I mentioned above, is Optical Networking: A Beginner's Guide by Robert Elsenpeter with Toby J. Velte.

FloOz

Thanks for the replies guys. Here's a great presentation that I found for those interested.
https://www.nanog.org/meetings/nanog48/presentations/Sunday/RAS_opticalnet_N48.pdf

DevilWAH

stryder144 wrote: »

You mention that dark fiber is completely dedicated to you. That is not strictly true. While your building may have had fiber laid between them and a LEC, gateway, etc, dark fiber is essentially unused fiber optic cable that can be turned on when necessary. Often times, telecoms will pay to have their fiber optic conduit buried in the ground/strung over power cables and place more capacity than they currently need in order to reduce one of the largest single costs. It usually cost a little more to bury twelve conduits than just one or two. So, the majority of fiber optic cable (one source says 90%) is dark.


An interesting read, and the source I mentioned above, is Optical Networking: A Beginner's Guide by Robert Elsenpeter with Toby J. Velte.

You are right in that ISP do lay more cable than needed and bring it on line as required. But if an ISP does assign you Darkfibre, this does mean they are essentially saying you use this fibre as you wish, no one else will put light down it or interferer with what you use it for. So this means that depending on the optics you install at each end you control the bandwidth and speed, you are provided a purely physical cable with out any ISP active equipment running it.

Like IT_consultant said, you can get pretty nifty hardware, like fibre switches that using optics can demutiplex mutiple wave lengths travelling along a single core and split them in to a number of new separate fibres. This all works with out having to convert the signal from light to electrical and back to light. however it is serious money and not the kind of thing most companies can think or running. The noramly way to break a 100gb link in to separate 10gb links would be to have active equipment at the junction. Basically a switch with Fibre modules in it. Where the 100gbx link goes in one side and mutiply 10gbs connections come out the other.

First think to ask is what fiber is installed, Single/multi mode and the specification and core size oh and the distance between end points. This will tell you the maximum speed it can carry and what optics you need. In answer to you question about lamdas and bandwidth it does not vary by what you pay for, but is limited by the grade of fibre you are provided. and if it is truly a "dark" fiber.

deth1k

Well if you stop thinking of fiber in terms of bandwidth i.e 100G fiber is nothing more than a string of glass and 100G DWDM kit that terminates at the end or this bad boy: 1830 Photonic Service Switch | Alcatel-Lucent

DevilWAH

FloOz wrote: »

Thanks for the replies guys. Here's a great presentation that I found for those interested.
https://www.nanog.org/meetings/nanog48/presentations/Sunday/RAS_opticalnet_N48.pdf

nice little presentation there

FloOz

deth1k wrote: »

Well if you stop thinking of fiber in terms of bandwidth i.e 100G fiber is nothing more than a string of glass and 100G DWDM kit that terminates at the end or this bad boy: 1830 Photonic Service Switch | Alcatel-Lucent

Yep we have a bunch of those models in our offices. And yeah it hard to not look at it from a bandwidth perspective since that is how I have always looked at links.

it_consultant

You also have to look at it from a perspective of how you are going to signal your fiber. If you have dark fiber (and you own it, like we do) then you have a lot of options open to you. Our second datacenter has one pair dedicated to 10GB ethernet and 2 pair dedicated to 16GB fiber channel. We have 6 dark pair. You end up with a lot of dark fiber because, as someone mentioned, the cost is minimal to run say 64 pair from 32 pair since you already did all the work of trenching and laying conduit. We are redoing our fiber over 47 miles and we are pulling 96 strands which is WAYYYYY overKILLLLL.