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 Ethernet ((once upon a time you could get ATM versions but who cares about that now?))frames 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:
- makes installing your own fibre quite difficult
- 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).
|You want to reduce CapEx||Unlike radio equipment, Lasers do not need specialist installers. Although, for longer distances you will need someone who understands physical mounting and stability issues (but it’s worth it). For short distances of up to 500 metres or so, a good handyman is all you need.|
|Reduce OpEx||Once the lasers are mounted and pointed they require no more maintenance.|
|You can’t use wireless or radio equipment||Certain physical areas do not allow wireless, even 2.4 Ghz to operate. The Ethernet over Radio systems are often high powered and create significant spectrum pollution issues for personal radios, pagers and industrial equipment|
|High Security||Even a very focussed radio signal will have sidebands or signal radiation that can be “snooped” with the right type of wireless equipment and not within line of sight. Although you can “snoop” a laser, you would need to be within the physical space of the transceivers which is likely to be a restricted space. Otherwise, you would interrupt the signal and service loss would stop you from capturing information.|
|Electrically noisy environments||Not so common today as radios have improved, but some industrial plants and manufacturing can create a lot of radio interference.|
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.
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.
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.
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. ((I often put a 10Mb wireless in as a backup or out of band circuit))
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.
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.