Building a TCP/ IP Network Over Dark Fiber?

1101z asks: "Well I work for a public access station in a city where a second cable/phone/internet company has moved just started operating. Part of there deal with the city was to let us have (for free) dark fiber links between several location in the city and our studio, so that we would be able to cablecast live from those locations. As the computer guy I would like to be able to interconnect computer networks that already exist at several of those locations, when we are not using the fiber for cablecast. The question is what is the cheapest way to build a TCP/IP network over this dark fiber." I wonder if the fiber being used is related to this story, from a month ago?

Comments

You can't do it.

[MarkusQ]

Goal: TCP/IP over dark fiber.

So far as I can see, you can't do it.

As soon as you try, the fiber won't be dark anymore, invalidating one of the conditions. You can have dark fiber, or fiber with TCP/IP (or just pretty lights for that matter) but not both at the same time.

-- MarkusQ

you guys all suck

[Twirlip of the Mists]

I know it's a Friday night (in some places) but man, the responses on this Ask Slashdot really suck so far.

Short answer: you can set up a TCP/IP network over a dark fiber link for as little as a few hundred bucks, if you can find equipment for a good price. Here's how.

I'm going to make a couple of assumptions here; correct me if I'm wrong. I'm going to assume, first of all, that each link you've got access to is actually a pair of links; that's the way dark fiber is almost always sold. Second, I'm going to assume that you've actually got a dark fiber link, as opposed to buying a lambda. (Buying a lambda means that the telco is letting you use one frequency of a dense wave division multiplexed [DWDM] link. Not the same as dark fiber in the literal sense, but the same in most practical senses.) Finally, I'm going to assume that the telco has provided you with the necessary repeaters on the line so that you can actually push light from one end to the other without any additional hardware. If your telco has sold you (or given you, whatever) "dark fiber," chances are that all three of these assumptions are true.

If all of those things are true, then you're in a really good position. You can run anything across these fiber links that you could run across a shorter length of optical cable: FDDI, Ethernet (any speed), Fibre Channel, FireWire, HIPPI, whatever you want.

You said "cheapest," and what's cheap depends on what's available. If you can get your hands on a couple of old Ethernet switches with 10BASE-F or 100BASE-F (which are simply 10 Mbit and 100 Mbit Ethernet over fiber optic cable instead of copper cable) you're in business. Just plug the dark fiber into a switch at each location and poof! A single TCP/IP network running across the fiber to both sites, at 10 or 100 Mbps depending on what you can find.

My last company had, among other things, some Bay Networks (now Nortel, I think) stackable Ethernet switches with 24 100BASE-T ports and two 100BASE-F ports. I think they sold for about $2,500 when new (in 1998 or so), but should now be available for a lot less used. If you can find some of those used you'll be in good shape. Asante also makes switches like these; I've never used them, so I won't vouch for them, but you can buy them.

Another option would be to bridge Ethernet to FDDI; switches that do this should be available for really cheap, if you can find them, because FDDI fell completely out of favor in the mid-1990's. FDDI runs at 100 Mbps, just like 100BASE-F, but it has to be bridged, and sometimes this can cause problems with packet splitting and MTU sizes, especially on Cabletron switch gear. Unless you're looking at an absolutely killer deal, avoid the FDDI option.

If you want to go with something more up-to-date, you can run Gigabit Ethernet over the fiber links. It'll cost more, but you'll get better bandwidth. A good idea might be to buy a couple of cheap 100BASE-T switches with 1000BASE-T gigabit uplink ports (about $150 each), then equip each switch with a 1000BASE-T to 1000BASE-SX media converter (as little as $200 each).

Any of those solutions-- 10BASE-F, 100BASE-F, 1000BASE-SX, bridged FDDI-- would require nothing more than a switch with the right media type at each end; you wouldn't have to mess with routers or anything, and you wouldn't have to do anything fancy with your IP network. In fact, you wouldn't be limited to running just IP. You could run anything that can be carried over Ethernet: AppleTalk, NetBIOS, whatever.

If you get the gear for a reasonable price, you can run any of those networks for really, really cheap. When the links aren't being used for video, plug 'em in to the switches and go to town. When you're ready, just unplug 'em and go back to video. The link will be down, but neither the switches nor the computers will care.

Re:Nothing to do with dark fiber

[Twirlip of the Mists]

He hasn't even explained what sort of fibre it is! Single mode? Multi mode? Frequency division?

It doesn't matter. When you buy dark fiber from a telco, you're given an MMF connection for each end. The link behaves just like it's a nice short run of multi-mode fiber. What actually happens in between-- DWDM, repeaters, microwave links, whatever-- is irrelevant. In fact, if everything is working properly, you'll never even be able to tell that there's anything going on in the middle at all.

I guess a lot of people are confused by the term "dark fiber." It's hardly ever literally true. When you buy "dark fiber" from a telco, what you're getting is an analog optical link to do with what you will. You can run anything over it that you can run over ordinary MMF. Is it ever actually, literally, "dark?" Hardly ever.

Can he afford a fibre ring?

Read the submission again. The telco is providing these links in a hub-and-spoke topology for free. "Can he afford a fiber ring" is a completely irrelevant question.

What sort of redundancy does he need?

None. They're going to use the links for IP traffic only when they're not being used for video. It'll be an ad-hoc network.

How long are the fibre runs?

It. Does. Not. Matter.

There are plenty of solutions here - ATM between sites with LANE, GigE into some 3550s, 10baseF into tranceivers

Fortunately, the submitter gave you a hint. He said "cheapest." Would ATM with LANE be "cheapest?" Of course not. Would Cisco gear be "cheapest?" Of course not. You're not even trying to be helpful, are you?

This guy obviously doesn't even know the extent to which he's in over his head.

You're just trying to make it sound more complicated than it is. Dark fiber is, far and away, the simplest form of long-range communication known to man. Well, maybe smoke signals or cups-and-string are simpler. Shine a blinky light down one end, and it'll come out the other. The question before the group is what's the cheapest way to turn Ethernet into blinky light and back again.