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17 Jun 2002
The idea of lasers on roof-tops, transmitting data through the air has always been met with scepticism. What about the weather? What happens if wind blows the transmitter off its line? What about safety issues? Ian Murphy reports.
From Opto & Laser Europe March 2001
Cities around the world are beginning to use roof-top systems to solve the last-mile bandwidth bottleneck that many network operators experience.
Dubbed optical wireless, the technology offers a number of advantages over optical-fibre networks: greater bandwidth, cheaper installation, and capacity can be added temporarily. It also has advantages over traditional radio-frequency wireless systems - it does not suffer from interference and there are no regulatory barriers.
Wireless is rapidly taking over from fixed lines, according to analysts' reports. Many European economies predict that there will be fewer fixed-line customers than wireless customers by 2002. However these predictions don't tell the whole story. While mobility is a major requirement throughout Europe, so is bandwidth and it is bandwidth that is driving the newest technology - optical wireless - to the fore.
Stephen Patrick, director of Cablefree in the UK, is clear about how the demand for bandwidth has
changed. "From early 1997 we were selling point-to-point links where 10 Mbit/s was 75% of the
deployment. A year later, 100 Mbit/s was cutting a much larger slice. By 1999 100 Mbit/s commanded 48%
of all of our sales, with Gigabit Ethernet accounting for approximately 4%. That was the year that the
standard was ratified and we saw the demand increasing for faster bandwidth."
Patrick said: "In the UK we attempt to keep transmission distances to less than 2 km to provide
telecoms-grade reliability, because the criticism that is always thrown at optical wireless is that it does not
work in bad weather. It's no good having high-bandwidth if it's not dependable, so we've kept the
transmission distances short."
Temperature issues also need to be dealt with. In cold environments it is important that the systems
have some form of heating, while in places where it is hot, cooling is required. Heating and cooling devices
need power, which can lead to interference. Also, the system's casing must be able to withstand the external
weather conditions.
Patrick believes that Cablefree has the solutions to these problems. "We use solid-state cooling so
we should get exactly the same mean time between failure in London as in Beirut. We've also addressed
extreme cold. If the unit temperature drops below a threshold value, heaters will kick in."
Another issue that faces optical wireless is the need for an accurate beam and clear line-of-sight. In
places like London this is achievable given that more than 80% of the buildings there are less than seven
storeys high. Cablefree customer SOHO Net has several systems in place that dogleg the signal across a
building and join two devices back to back using optical fibre. With a divergence on the beam of 0.5º,
Cablefree believes that this can increase the laser's immunity to interference and even allows beams to
criss-cross in the air.
By concentrating on a low-power model, the company has been able to gain health and safety
accreditation, which means that it can mount units inside buildings behind window panes. The lowest-power
device is 20 mW. It has a large emission area and, because the power is averaged, laser-safety certification
can be guaranteed.
The point-source method suffers greatly from birds flying through the beam. The large output area
and divergence (at 1 km there is a 10 m footprint) can cope with any movement due to wind and vibration.
This means that there is no need to use complex and unreliable autotracking devices to keep beams aligned.
Cablefree expects to release a 2.5 Gbit/s product by mid-2001 and has a 10 Gbit/s solution under
development. The plan is to track the progress of bandwidth standards and ensure that they can provide a
solution as soon as a standard is approved.
German company GoC is a major player in the free-space arena. Unlike Cablefree, GoC uses a
narrow laser beam that has a divergence of 3 m over 1 km. A tight beam means that less power is needed to
transmit over the same distance and there is even less chance of the signal being intercepted. However, a
narrow beam with a divergence of 30 cm over 100 m is affected by building movement in high winds.
GoC's systems use a multibeam technology with four lenses to receive and four vertical-cavity
surface-emitting lasers to transmit a signal. The lasers send the same signal four times, so each device can
be used with a lower output power. This method reduces the effects of turbulence and scintillations in the
atmosphere.
The lower power is important because Germany has some of the most stringent emission laws in the
EU. The lasers emit at between 2 and 2.5 mW (class 3A), which means that they can be sited indoors and
operate through a window pane.
Other start-ups in the field include PAV Data-Systems, which is based in Windermere in the UK. Its
devices can transmit over a distance of 6 km. In trials in Moscow, when freezing fog restricted visibility to
850 m, the PAV boxes achieved 2 Mbit/s transmission over 1.6 km. PAV says that its main competitor is
the Israeli manufacturer Jolt, which was acquired last summer by MRV Communications of the US.
Optical wireless generated a flurry of interest among the large telecommunications-equipment
manufacturers last year. Lucent Technologies took a 30% stake in Seattle-based firm TeraBeam; and Nortel
Networks announced a reseller agreement with San Diego start-up AirFiber. Terabeam's product delivers
point-to-multipoint connectivity between a base station and a number of end users.
Other manufacturers producing point-to-point links include CBL and Optel of Germany, and US
manufacturers AstroTerra, Canon and Plaintree. In fact the optical wireless scene is crowded and is likely to
remain so, owing to the simplicity, low cost, ease and speed of deployment of the devices and above all their
high bandwidth.
Point-to-point transmission of data from roof to roof could be an ideal alternative to laying optical fibre.
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