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Optical/IP

Optical Crossconnects

Before reading this you may find the following tutorials useful:
Optical Networks, Wavelength Division Multiplexing (WDM)

Huge amounts of information traveling around an optical network need to be switched through various points known as “nodes.” Information arriving at a node will be forwarded on towards its final destination via the best possible path, which may be determined by such factors as distance, cost, and the reliability of specific routes. The conventional way to switch the information is to detect the light from the input optical fibers, convert it to an electrical signal, and then convert that back to a laser light signal, which is then sent down the fiber you want the information to go back out on.

It all sounds unnecessarily complicated, don't you think? It's as if to give money to a friend, you would have to convert it into Euros, then back into your own currency before your friend can use it. Now, even though your information is safer in electrical form than your money may be in Euros, it still seems a strange way to work. What if we could just move the light itself around, without all this ridiculous conversion to electric signals? Well, my friend, what you need is an optical crossconnect (OXC).

The advantages of being able to avoid the conversion stage are significant. “Optical switching” should be cheaper, as there is no need for lots of expensive high-speed electronics. Removing this complexity should also make for physically smaller switches. Additionally, optical switches are relatively future-proof. An electrically based switch will have electronics designed to detect the incoming light signal. If you increase the speed at which the light signals operate (increasing the “bit-rate”) then the electronics will need to be upgraded to handle the faster speeds. If you are just rerouting light, however, it doesn't matter how fast the data is coming — so you can accommodate any future upgrades of bit-rate without needing to upgrade the switch (this is called “bit-rate transparency”). Optical crossconnects are just now coming onto the market with these benefits and more.

Optical crossconnects are very much designed with simplicity in mind. You've got some light in one fiber that you want to move to a different fiber, so just redirect the light somehow and that's all you need — it's child's play. Unfortunately, the technologies used seem to come out of science fiction rather than a child's bedtime story. There is a wide range of wild and wacky ways to switch light between optical fibers. Semiconductor amplifiers, liquid crystals, holographic crystals, and tiny moveable mirrors are just a few. Truly buttock-clenching switching developments are anticipated in the future. One of the most common techniques being developed is that of the tiny moveable mirrors known as micro-electro-mechanical systems (MEMS).

MEMS Mirror MEMS Optical Cross Connect MEMS consist of mirrors no larger in diameter than a human hair. They can be arranged on special pivots so that they can be moved in three dimensions, and several hundred such mirrors can be placed together on mirror arrays no larger than a few centimeters square. Light from an input fiber is aimed at a mirror, which is directed to move the light to another mirror on a facing array. This mirror then reflects the light down towards the desired output optical fiber. It perhaps sounds a little bizarre, but it does seem to work. MEMS mirror arrays are even used successfully in some of the modern digital projectors used for computer-based presentations.

Key Points

  • Crossconnects forward signals to their destination by specific routes
  • Traditional crossconnects convert light to electricity then back to light
  • Optical crossconnect advantages include cost, size, and bit-rate transparency
  • Redirecting light from one optical fiber to another, without electrical conversion
  • Most advanced optical switching technology is MEMS, tiny moveable mirrors


Further Reading

Fiber Bragg Gratings (FBGs), Arrayed Waveguide Gratings (AWGs), Semiconductor Optical Amplifiers (SOAs)

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BuzzLightwave 12/5/2012 | 12:54:11 AM
re: Optical Crossconnects My understanding is that an AON works on a TDM basis. At a preconfigured time slot T, traffic is allowed to go from source port A to destination port X and from source port B to destination port Y. At time slot T+1, traffic is allowed to traverse from port A to port Y and from port B to destination port X. This makes makes switching rather simple - each MEMs based switch just needs to know where to aim its mirrors at each time slot. There is theoretically no need to even parse the packet headers to determine the destination.

While the switch configuration, i.e. the mapping of time slots to mirror positions, is electrical, the packets themselves never leave the optical realm.

This approach limits network utilization, since source the amount of bandwidth between a given source and destination will usually be a fraction of the available capacity when the network is otherwise idle. In the above example, if there is no traffic between B and X during time slot T+1 but port A does have traffic for port X (but none for Y), then the available bandwidth to port X at time slot T+1 still cannot be used - it is wasted. That's a drawback of the OOO switches. An OEO switch can parse the packet, determine that there is no other traffic for the destination X, and send it on. The price you pay is latency.
manoflalambda 12/5/2012 | 12:54:10 AM
re: Optical Crossconnects BuzzLightwave:
My understanding is that an AON works on a TDM basis. At a preconfigured time slot T, traffic is allowed to go from source port A to destination port X and from source port B to destination port Y. At time slot T+1, traffic is allowed to traverse from port A to port Y and from port B to destination port X. This makes makes switching rather simple - each MEMs based switch just needs to know where to aim its mirrors at each time slot.


Interesting. I have have missed this, but how long are the time slots and how much switching time is allowed for the mirrors when you go from T to T+1?

Just curious since some of the mirror systems have a variety of switching times depending on distance to move the mirror, charge-discharge effects,etc.

Salute,
Manoflalambda
debasish71 12/4/2012 | 10:50:09 PM
re: Optical Crossconnects what are the possible implementation scenarios for ooo? how can a lower level tributary like stm-1 be dropped off from an stm-16 or higher optical signal without converting it into electrical signals?
micahvoiers 12/4/2012 | 8:28:06 PM
re: Optical Crossconnects My understanding of a cross connect is that we can map an input to a desired output(route the optical signal from one fiber to another). My question is where is the decision being made to route a signal to a specific output? It doesn't appear that a cross connect is looking at the specific signal. Is this done electronically from another location? How frequently can we expect the mapping to change?

Thanks,

Micah
manoflalambda 12/4/2012 | 8:28:03 PM
re: Optical Crossconnects Micah,

The controller of the OXC decides which input ports are mapped to which output ports. That info comes via some sort of external management/messaging software into the OXC and is stored internally.

Some OXCs also maintain some number of pre-stored maps of cross-connects to handle either pre-configured situations or emergencies.

Salute,
Manoflalambda
ranon 12/4/2012 | 8:28:01 PM
re: Optical Crossconnects OXC's are not really practically viable. There are a large number of Electrical cross connects availaibe.

But the biggest problem with optical cross connects is that they do not have the granularity required. There are chips with 72*72 OC-48 cross connects at STS-1 granularity. While an optical corss sonnect would be at a wavelength granularity (OC-48).
Half-Inch Stud 12/4/2012 | 8:27:59 PM
re: Optical Crossconnects Let's keep clear the OEO Cross-connect medium can be Optical or/and Electrical signals and still be hit the catagory of an OEO OXC with STS-1 granularity. Network system functionality [and revenue] is enables with these approached by adding processing value [protocol stripping and packet streaming].

Yet, the OOO OXC is truely a wavelength path-changer that is limited to the input DWDM granularity of a typical 2.5Gbit/s. Tough to add revenue-enhancing functionality to such a system.
Seems to me the OOO is a hyped-up wavelength groomer that will hit the optical DeMuxs anyhow, before discreet wavelength photodetection.

My bet is Signal/Noise ratios will drive wavelength photodetection to occur as soon as possible, thus showing that added functionality in the Optical [OOO] systems designs to be more expensive and quite simply delaying the inevitable photodetection..

...we'll see.

That what I think and I'm sticking to it.
H.I. Stud
Belzebutt 12/4/2012 | 8:27:57 PM
re: Optical Crossconnects As further reading, I would love to see an article here that talks about possible techniques for designing all-optical wavelength converters for OOO OXCs. Today's OOO OXCs have to be coupled with OEO DWDM line systems in order to switch lambdas, which defeats the purpose of OOO (except for express traffic).

I remember reading a story on LR about a company announcing some all-optical wavelength converter product, does anyone know how these things work in more detail? Perhaps someone at LR could write a piece on this...
opticaltoys 12/4/2012 | 8:27:54 PM
re: Optical Crossconnects At least two companies discuss all optical XC:
Luxcore presented a small 2x2 switch demo, covered by LR:
Luxcore to Demo Optical Switch Advance

Lucent claims to have demonstarted a 100x100 all optical XC based on mems and SOA WC, See OFC 2001 post deadline paper PD16.
OT
optinuts 12/4/2012 | 8:27:51 PM
re: Optical Crossconnects "Today's OOO OXCs have to be coupled with OEO DWDM line systems in order to switch lambdas, which defeats the purpose of OOO (except for express traffic)."


belzebutt,
what other application is there for an OOO other than 2.5G or 10G (or 40G) express traffic. what does wavelength conversion do for an OOO other than find empty slots in a dwdm system. you still need the dwdm system anyway.

i don't understand the comment, thanks.
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