Design: When to Choose Free Space Optics (Ethernet Lasers) ?

What is Free Space Optics ?

Free Space Optics is the term used to describe using infrared lasers to trans­fer Ethernet across a long dis­tance. A long dis­tance is some­where between 20 and 3000 metres.

Conceptually sim­ilar to Infra Red ports (that we used to have) on our com­puters to con­nect our Palm Pilots and printer except more focussed and more power­ful. These lasers are used to trans­port Ethernet¹frames across a long dis­tance by car­ry­ing the Ethernet sig­nal over a laser bearer.

Why choose Free Space Optics ?

In many designs, lasers are the simplest way to con­nect two build­ings when you can’t use fibre. For example, your com­pany takes another office on the other side of the road which:

1. makes installing your own fibre quite difficult
2. A WAN con­nec­tion costs a lot to install and even more to rent every month. Plus it will take two months to be installed.

A com­mon solu­tion is to use 802.11 style Wireless Ethernet because many people are famil­iar with it but the per­form­ance is not so great. Since the 802.11 2.4 Ghz spec­trum is so crowded these days, using 802.11b/n doesn’t get you much per­form­ance. Maybe you can get up to 30 MBps in most prac­tical situ­ations. Using pro­pri­et­ary 802.11 equip­ment you might get as much as 80Mbps through­put in actual use due to loss and sig­nalling overheads.

Another solu­tion is to use Radio tech­no­logy oper­at­ing in the 3 to 5 Gigahertz fre­quency ranges. Requirements vary from coun­try to coun­try, but you typ­ic­ally need to be cer­ti­fied or licensed by the gov­ern­ment to use that spec­trum in that loc­a­tion. This license will need to be reviewed every year and a fee will be charged. You will also require spe­cial­ist radio installers and reg­u­lar main­ten­ance programs.

Gigabit per­form­ance has its strength and some weaknesses

When I need to con­nect two build­ings at Gigabit speeds, Ethernet Lasers are the first thing that I think of. Then I check how far apart the build­ings are. Because the lasers can sup­port much higher speeds, such as Gigabit, com­pared to Radio at much lower cost, they cost much less and are cheaper to run.

If you plan care­fully, you can also install them your­self. Specialist installers are not a man­dat­ory require­ment (but are usu­ally a good idea).

Useful when .…

The most use­ful fea­ture is Gigabit Ethernet speeds, and using stand­ard fibre optic or cop­per cable to con­nect the devices (some Radio gear needs ATM or other eso­teric interfaces).

Be care­ful of the following

Building move­ment — is not a problem

One con­cern with mount­ing lasers on the top of very tall build­ings is that they move quite a bit. A twenty storey build­ing will typ­ic­ally move 30 cen­ti­metres in either dir­ec­tion in windy con­di­tions and more for taller buildings.

Surprisingly, this doesn’t seem to be a prob­lem. The Laser sig­nal does have some dis­persal and can “fan out” so that the receiver can get enough sig­nal to function.

Some of the newer devices from CanoBeam are able to autonom­ously recal­ib­rate them­selves up to 3.4 degrees, which means even if the build­ing or the mount­ings move a bit, the sys­tem keeps working.

Thermal expan­sion

A com­mon con­cern is that the thermal expan­sion of build­ings or mounts will cause prob­lems. Againm the laser fan out seems to be suf­fi­cient. I haven’t had prob­lems with this over a five year period.

Forms of Interference and Blocking

A laser sig­nal is sub­ject to cer­tain types of inter­fer­ence. Lets sum­mar­ise the key top­ics here:

Fog, but not rain or snow

Lasers are atten­u­ated by rain and snow, but not usu­ally to point where the sig­nal is lost. However, FOG will cause a loss of sig­nal. If you have ever driven a car in foggy weather you will know that your head­lights are com­pletely dif­fused. The same applies to laser beams.

The water particles in fog absorb (or extin­guish) the photons and atten­u­ates the avail­able light power. Once the sig­nal goes below a cetain level the con­nec­tion will be lost.

Scintillation or Heat Waves

Scintillation is the term used to describe the effect of heat waves rising of the grounds. This is the com­bin­a­tion of air move­ment and heat pock­ets causes “refer­act­ive tur­bu­lence”. Basically, the hot air pock­ets causes the same idea as a mirage in the desert which impacts the propaga­tion of light wave by mak­ing it devi­ate from inten­ded path or dif­fus­ing it.

Sunlight on Receiver

Direct sun­light on the receiver face may swamp in the input. You need to ensure that units will not get dir­ect sun on the receiver. This is why all the units have a little “roof” that shade them. Water or rain on the lens is not a issue but the roof helps too.

Summary of Interference

My rule of thumb is to con­sider the weather of the pro­posed deploy­ment. Provided you have a mod­er­ate weather sys­tem or cool but not cold win­ters (i.e. no fog or snow) and not desert like con­di­tions in sum­mer (i.e. no heat shim­mer). Not how­ever that heat shim­mer is mit­ig­ated by height.

If you have that sort of weather you can con­sider using Ethernet lasers.

Safety Risks

As with any laser, eye safety is a con­cern. There are two wavelengths of light, 800nm and 1550nm. The 1550nm units are, gen­er­ally, safe due to the fact that the human eye (aqueous lens) absorbs the light energy and no dam­age will be sus­tained to the retina.

The 800nm wavelegth can cause dam­age to the ret­ina. The per­son will not be aware of the dam­age since the ret­ina has no pain recept­ors and invis­ible light does not cause a blink reflex. Therefore 800nm lasers need to be installed care­fully and ensure that human eyes will receive the sig­nal. This is eas­ily done by mount­ing the lasers on a wall.

Manufacturers

Pavdata​.com
Canon Canobeam
Lightpointe

How they integ­rate with your network

Implement Free Space Optics at layer 2 or Layer 3

Some units act as a “patch lead”. The phys­ical inter­faces will go up and down accord­ing to whether the laser con­nec­tion is up or down. These are the easi­est type to install since they can dir­ectly con­nect to routers and fire­walls and the Layer 3 rout­ing will resolve altern­ate paths in the case of fail­ure.²

Other units will act as bridges like a two port switch. Some units have SNMP cap­ab­il­it­ies, but many are dumb units. You can only tell they are up by using CDP or reach­ab­il­ity to the far side of the network.

Using Layer 2 — Unencrypted

Most lasers act as Layer 2 bridges. You can think of them as a two port switch that run span­ning tree. In this case, you can either con­nect the remote site as a Layer 2 (which is OK for a site with a small num­ber of users), or imple­ment Bidirectional Forwarding Detection for fast fail­over.

Layer 3 — Encrypted

For most people, send­ing Ethernet details over a wire­less net­work (whether Radio or laser) would be a secur­ity risk. how­ever high per­form­ance IPsec encryp­tion remains a problem.

From the Cisco Web site for per­form­ance of the ASA equipment:

As you can see, you are going to need a verg large fire­wall to provide encryp­tion at Gigabit per­form­ance, but still using an Ethernet laser at 400 MBps is going to be MUCH cheaper than altern­at­ives. You will need to imple­ment a little bit of QoS pre­clas­si­fic­a­tion if you want to use this option.

Other meth­ods

There are a num­ber of other meth­ods for imple­ment­ing the net­work­ing, but this is enough for now. Leave a com­ment if you would like to see more and I will see what I can do.

Out of Band — use 802.11b

I always install an 802.11b con­nec­tion at the same time as the laser and use this of an out of band man­age­ment con­nec­tion. It pos­sible that the laser can get blocked or fail, and hav­ing an altern­ate path to site is worth the small about money needed.

Get your installer to quote on both the laser and 802.11b link at the same time. It’s well worth the little bit extra.

Wrap It Up

So Free Space Optics have a num­ber of con­straints, and need care­ful con­sid­er­a­tion. But you will not find a more simple and inex­pens­ive solu­tion for high speed con­nec­tions between build­ings. It is much more reli­able than you think — I have a num­ber of install­a­tion that have worked full time for the last four or five years.

Definitely, worth keep­ing in mind when you are design­ing solution.

Footnotes

1. once upon a time you could get ATM ver­sions but who cares about that now? [back]
2. I often put a 10Mb wire­less in as a backup or out of band cir­cuit [back]

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