Just wanted some clarification on the physicalities of fibre. Googling seems to bring up loads of science about refraction but not the basics. For instance, if we install an eight core fibre cable, does each pair of cores provide one connection, i. e. 4 in total? I had sight of an install recently and it seemed that a switch connection has a Gbic type module with 2 connectors on the end each fed by one fibre core (hence my assumption that 2 cores are required per "connection"). Maybe for 1Gbit you need 2 fibre cores and for 10Gbit you need 4 cores.
Yes, 2 fibres for each connection, one in and one out.
Light source at one end connects to light receiver at the other. A single direct connection therefore needs to be crossed over and I believe that is the way they are shipped. When patch panels are used sometimes need to (un)cross a cable. You need good nails to un-clip the fibre from the holder.
10Gbit may need a different fibre type, single or multi-mode.
Most of the stuff uses one transmit fibre and one receive fibre. Its just easier that way.
It is possible to use a single fibre with different wavelengths (colours) for transmit and receive but I haven't seen it used in kit you can buy on Amazon/ebuyer.
You only need two fibres for 10Gbit (and a lot faster).
Single mode and multi mode refer to the path the light can take through the cable. It mainly decided by the core diameter. Thin cores are single mode and can take the signal much further before it degrades (not loss of power). they cost more and need more expensive optics to drive them.
Multimode can't go as far as the signal degrades due to the multiple paths the light can take. However the optics needed to drive it are cheaper due to the core being much bigger so its easier to get the light into the fibre.
You do need to match the drivers to the cable if you want the system to work near the limits.
However, it is routinely used by BT for their fibre to premises connections. I suspect the motivation for this is that they must pay business rates on "lit" fibre, but not on "unlit" fibre. So the fewer the fibres they actually use, the better.
The equipment BT use (FSP150CP) is often to be found on ebay, complete with the optical transceiver which uses one fibre and two wavelengths (one for each direction)
Single mode fibre will transmit further than multimode (60km or so vs. a lot less :-) before amplification is needed. However as others have pointed out, the interfaces are rather more expensive. Undersea cables use pumping techniques to boost the signal without breaking into the cable. Two fibres are needed to transmit in the two directions.
However, if you're willing to spend a bit of money, you can actually send a shitload more than 10Gbps down a single pair. I've been out of the business for eight years now, but at that time, you could typically (in principle) send 960 Gbps down a fibre pair, made up of 96 separate
10Gbps channels. This is done using 96 different wavelengths of light, each separated by 50GHz. Typically you'd have a large router (we used Juniper, but Cisco made similar stuff) with a number of 10Gbps interfaces connected to optical kit (made by Alcatel) which took the separate separate data streams and merged them onto one fibre-pair going to the next city. Actually not merged really; each light frequency went down the fibre independently of all the others.
Most of the data analysis for large experiments at CERN is done off-site at other centres. Typically each centre would have a private
10Gbps channel to CERN, not going through any routers at all, just connected optically. So the UK customer had a fibre-pair to our London centre, and their data along with ordinary IP traffic would go to Paris and then to Geneva.
The fibre-pairs between cities were rented from the likes of COLT, or T-Systems or Telia. They took care of the light amplification aspects too.
I believe BT invented the concept, decades ago. Also that of using many different colours down a fibre, each carrying a different signal, for much higher bandwidth. These came out of Martlesham Heath.
A group of us (from BT) went there once to see a 'How to splice fibre optic' demo. Very interesting place and demo and pretty specialised work at the time.
Now it's just something you use to hook up your HiFi. ;-)
There's a world of difference between splicing mono-mode high bandwidth optical fibres using low loss joints on fibre segments measured in kilometres to tens of kilometres and plugging in a nice fat and very cheap multimode plastic 'optical fibre' interconnecting cable to bridge a metre or three gap between your Hi-Fi 'separates'.
Incidentally, on long haul fibre optic circuits, each fibre can be used for both directions simultaneously using special couplers, generally on modest length under-sea cable routes (50 to 100 Km or so unrepeatered) where link capacity demands trump the cheapness of overland cables.
Yes, it's true that doubling up on the fibre bundle in an undersea cable doesn't add that much extra to the manufacturing costs but the extra investment in bi-direction launching couplers at each end of such circuits becomes but a tiny fraction of the total investment involved.
Quite, you didn't have to go on a course for the latter. ;-)
I'm assuming that a cable laying ship laying a (fibre) cable wouldn't have to be quite as 'big' as one laying a power cable ... or do they use the same vessels anyway because of other factors (like the cable drum or accurate positioning stability or accurate sonar etc)?
But the technology exists in the form of GPON (Gigabit Passive Optical Network), it's just not very common in the UK but is in the US.
The each fibre in/out from a launch card is optically split to 32 or
64 individual fibres to be routed to each end point. Range is limited, 10 km or so for 1 Gbps. Knocks spots off FTTC - 78 Mbps to a few hundred metres if you are lucky.
The ONTs (Optical Network Termination) are available in consumer quaitities and prices. Normally have a couple of POTS poerts and optional battery back up.
AIUI GPON at the physical level uses different colours for the up and down links and the total bandwidth of those carriers is shared by TDM across the endpoints.
Virgin are currently trashing all the pavements in the village (funny they didn't extend their network here from the local city for 25 years, then within months of BT providing FTTC they decide to).
Instead of their usual coax for the final drop, they'll be providing FTTH using DOCSIS RFoG (radio frequency over glass) which I'd never heard of before.
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