FACILITIES ca bling
testing instead of the more commonly used bit error rate testing. Also, frame error tests ran hundreds of times longer than the industry requires.
Double standards
When taking the test results into consideration, it would appear there may well be two kinds of 40G solution in the near future. One family of products will be ‘standards-compliant’, and another will actually surpass these levels. Many of the components available today are signifi cantly better than the existing specifi cations. The higher quality of the current generation of transceivers plays an important part in this. Manufacturers will be able to release products which guarantee performance levels which exceed the standard. That is a totally new and rather exciting development within our industry. The extended reach capabilities clearly demonstrate what can be achieved when you combine outstanding quality optical cable and connectors in support of next generation communications technologies.
So exactly how is the greater range achieved, from a technology point of view? The spectral width of a light signal in a 10G system is 0.25 nanometres. However, for 40G, this is signifi cantly greater, at 0.65 nm. As the width of the signal is larger, it can’t travel as far without introducing interference between adjacent bits. Each single bit may interfere with the bits directly before and after it. However, by using advanced transceivers, the spectral width can be reduced and the possible transmission distance can be signifi cantly and reliably extended. In fact, it becomes possible to narrow down the given 0.65 nm down to the 0.25 nm spec of 10G, allowing light signals to travel greater distances without interference.
Single or multimode?
The extended range has further implications for the choice of single mode or multimode fi bres for data centre applications. 40G multimode cabling is more complex than 10G, because you need to use multimode parallel optics with an MPO connector. However, for 40G single mode, you need only one single fi bre to transmit data and another to receive. This is a relatively inexpensive solution. On the other hand, single mode transceivers rely on very complex wave division multiplex techniques, making them extremely expensive. In fact, the cost can be four to six times higher in comparison to a multimode solution. To summarise: the huge transceiver cost for single mode transceivers make this a viable solution only for very
Why not 100G now?
100G currently requires more than 2x10 fi bres which today is economically not viable and technically rather diffi cult to implement. Standards work is taking place with the aim of replacing this by 2x4 fi bres, as is the case with 40G, but this is still in its early stages. Using 100G transceivers with 20 fi bres, in line with the current standard, would be very expensive since ten VCSELS on each side would be required, as opposed to just four on each side for 40G and the new upcoming standard for 100G.
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long distances. Other important points in the migration from 10G to 40G are the fact that 40G requires a greater amount of fi bre than 10G. With 12 fi bres, you can operate six 10G ports, but you can only operate one single 40G port. Clients need to work out to the best of their ability how many ports they will probably need in future. In some cases it is fairly easy to add fi bres at a later stage, but for other - possibly critical - data centres expansion might not be all that simple. Clearly, it pays to think ahead.
Working with MPO connectors means clients need to be extra attentive and careful, to avoid contamination, which is a key problem with fi bre. This can present quite a challenge initially, but, once they’re aware of how the connectors should be used correctly, it’s really quite straightforward. At present, we’re seeing the fi rst 40G transceivers and switches from all major suppliers reaching the markets. Of course, we are now still only in the very fi rst development phase for 40G and we need to see how the market will develops and what having two different levels of performance might imply for warranties and compliance.
Viable solution
In short: the 40G protocol places constraints on data centre layout, limits the number of cross-connects and decreases channel length, when looking at the offi cial specifi cations. Split data centre rooms are a particular concern. However, recent tests have conclusively shown that networks can deliver a performance guaranteed to be higher than the specifi cation implies. As data centres feel the pressure to upgrade in order to meet the growing requirements imposed by vast FTTH uptake, it would appear that migrating to 40G is a far more plausible solution than it may originally have seemed.
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