1 March 2007
Connecting in the Broadest Sense
Broad band technology offers potential for plant-wide network connectivity-for less cost
By Kwek Keong
Financial institutions deployed an emerging communication technology, free-space optics (FSO), within hours of the 9/11 terrorist attacks to reconnect with the outside world. Telecom service providers have since adopted FSO to achieve the last-mile solution. Likewise, business owners, campuses, and hospitals are embracing FSO to extend their enterprise networks across premises where the cost and time for road digging and work permits are prohibitive, particularly in urban areas. Yet the question for industrial users remains: Can FSO live up to their expectations in a harsh environment?
Some users have considered FSO, a new line-of-sight (LOS) high-speed broadband technology, as a backup to existing fibers for plantwide Ethernet connectivity. FSO uses infrared light emitting diodes (LEDs) or laser diodes to enable an optical wireless communication of data, voice, and video. It uses up to 10Gbps through the air without the need to lay cables and RF spectrum licenses. The technology was originally developed for U.S. military three decades ago as one of the most secured communications. But commercial viability in recent years has helped FSO to position itself as an alternative for network connectivity by service providers.
FSO, also known as free space photonics or optical wireless, uses infrared (IR) beams to achieve optical wireless communication through the atmosphere. FSO adopts lasers or LEDs to transmit data in free space whereas fiber cable encloses data stream in a glass fiber. Typically, an FSO link comprises two transceiver heads mounted on building roof tops or indoors behind glass windows of two buildings, preferably less than 2 km apart. It interfaces with network bridges, switches, hubs, and routers via media converters and supporting redundant or back up communication through fail-over devices.
The application of FSO resembles that of IR LANs that use light beams and travel through free space as optical links with full duplex capability. The FSO links also behave similar to fiber optic links, which use optical signals from transmitter output focused into light beams. The difference is FSO signals transmit using air as a medium, whereas fiber optic signals transmit via fiber cables buried underground or on cable trays above ground.
FSO uses infrared band
FSO as a physical layer under layer 1 of open systems interconnection operates in an unlicensed IR band with wavelengths ranging from 750 nm to 1600 nm. Most commercialized FSO systems run on two wavelength separate bands, 750nm to 900nm or 1500nm to 1600nm. The reason is these two bands possess better transmission quality through the air. FSO systems use directed IR that requires LOS, whereas IEEE802.11 IR wireless LAN operates with diffuse IR light that does not require LOS. The data rates for FSO vary from 1.5 Mbps for T1 (or 2.0Mbps for E1) up to 10Gbps, although 10Mbps, 100 Mbps, 155 Mbps, and 622 Mbps most commonly deploy for networks with distances from 20 meters to 5 km.
Each FSO head comes with one transmitter where optical light is typically emitted with beam divergence angle of 6mrad (0.343 degrees). It uses more than two receivers to capture optical signals. The general rule is the longer the transmission distance, the more powerful light emitter and accurate receivers required, which means a higher cost.
In selecting FSO products, rate performance on four main parameters at a given data rate: total transmitted power, transmitting beam width, receiving optics collecting area, and receiver sensitivity. You can use a figure of merit to compare competing systems, based on the basic physics of this equation:
Figure of Merit = (Power x Diameter²)/ (Divergence² x Sensitivity)
Power = Laser power in milliwatts
Diameter = Effective diameter in cm (excluding any obscuration losses)
Divergence = Beam divergence in millirad
Sensitivity = Receiver sensitivity in nanowatts
Plant-wide Ethernet connectivity
FSO can also be an alternative or backup to the existing fibers for plant-wide Ethernet connectivity. One advantage of FSO is cost savings on installation without extensive road digging and cabling works. Shortened project duration as deployment takes several hours to complete including FSO tuning. Free license is similar to that of ISM band for RF spectrum, and there are no recurring phone bills in the absence of T1 (or E1) lease line from local service providers. No RF interference and no data encryption are required as optical signals are difficult to intercept and jam. Its high speed and broad band are particularly useful for video streaming and voice over internet protocol (VoIP). There's low power consumption on power-over-Ethernet versions, versatile system configurations as standalones, backup or redundant systems, high scalability for point-to-point, point-to-multipoint and mesh network, and fast payback time within months, which means high return on investment.
FSO for industrial applications
The deployment of FSO for industrial network connectivity is not yet mainstream due primarily to technological gaps between the automation industry and ICT industry. This is driving technological evolutions in tandem with the law of supply and demand.
A main contractor on a large scale petrochemical complex received advice to deploy an FSO solution for a DCS Ethernet cabling extending to an adjacent plot of land separated by an existing chemical plant another company owned. The contractor consulted the owner and opted for a conventional method of routing extensive F/O cabling. This involved trenching works of 2 km from the existing plant to bypass the neighboring installation. The project duration took much longer to reach the new site where the storage facilities were built, despite the cost for FSO links of 500 meters. The links would cross over a V shape wharf-a fraction of F/O cabling works with a shorter timeline. In fact, several industrial applications could benefit from FSO. They include:
Plant-wide information access by linking up the existing Ethernet TCP/IP of control systems at various control rooms through point-to-point topology
LAN or WAN redundancy for its exiting network connectivity between control rooms, tank farms, laboratories, warehouses, and administration buildings via either point-to-multi-point or mesh topology
Data, voice, and video communications for offshore platforms within 2 km apart
Remote communications for underground mining between main shafts and production shafts
Broadband video streaming for plant safety, security, and surveillance systems in process areas
Integrated building automation systems for BAS, lighting, energy meters, CCTV, and VoIP
Main office and construction site office communications to transmit engineering drawings and monitoring construction activities remotely via remote cameras
FSO is thus far the most cost effective solution to be deployed within the shortest time frame in view of the heightening homeland security and plant safety requirements. One reason is its higher speed and broader bandwidth are most suitable for plant safety, security, and surveillance systems to share security information. FSO as compared to fiber cables will cost between $5,000 to $20,000 per link to deploy, whereas fiber installation will cost from $50,000 to $200,000 for the same distance in an urban vicinity.
Challenges and countermeasures
One of the most prominent differences between fiber and FSO is fiber is a predictable medium while FSO is an open medium subject to environmental variations. Therefore, FSO networks must overcome the unpredictable nature to avoid degrading the performance of FSO. A comprehensive site survey would be inevitable to minimize any potential pitfalls due to atmospheric attenuations and other environmental constraints. Take a look at some other challenges for FSO to overcome in the industrial landscape.
Dense fog: Dense fog is most detrimental to FSO, especially compared to rain and snow. So, it is important to shorten the communication path to less than 2 km or use a hybrid FSO/RF solution by adding a redundant microwave or WiFi, WiMax for antenna diversity.
Absorption: Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in power density (attenuation) of FSO beams, directly affecting system availability at certain wavelengths. With higher optical power, FSO helps maintain the 99.999 availability service providers require.
Scintillation: Scintillation is heated air rising from the scorching ground and equipment surface to create temperature variations among different air pockets that present around the communication path. It can cause fluctuations in signal amplitude and alter the signal at the FSO receiver end. Multi-beam FSO can help to alleviate the effects of this scintillation.
Building sway/earthquakes: Building sway and earthquakes will upset the alignment between receivers and transmitters. Divergent beams can maintain the connectivity, whereas the more effective mean is to add auto tracking with multiple beams to re-align receivers and transmitters of an FSO system after an incident. The majority of manufacturers produce FSO based on wavelengths of 1300nm to 1500nm with intelligent error correction, such as auto tracking to realign the system results from building swaying and harsh weather conditions.
Scattering: Scattering occurs when the wavelength collides with the scatterer. Rayleigh scattering is when the scatterer is smaller than the wavelength. A scatterer of comparable size is Mie scattering. Non-selective scattering is when the scatterer is much larger than the wavelength. Scattering has no loss of energy but directional redistribution of energy that may have significant reduction in beam intensity for longer distances.
Other atmospheric attenuations and constraints: During the changing seasons, solar shifts of sunrays onto the front of FSO may weaken the optical power. The reflections of sunlight on metallic surfaces and tinted glasses of buildings shining on the apertures of FSO will distort the communication path. FSO based on higher wavelength will outperform the lower wavelength on atmospheric attenuation under clear sky and hazy conditions, however it makes no difference on a foggy day.
About the Author
Kwek Keong SIEW MEngM, MBA, is a board member with ISA's Management Division, president of the ISA Singapore Section, and managing director at PAC Technologies in Singapore. E-mail him at firstname.lastname@example.org.
Free-space optics vs. industrial wireless
Benefits of FSO
Unlike RF wireless, FSO is an optical technology that operates in invisible parts of the optical spectrum at near-infrared wavelengths. Here are some advantages over RF:
Transmission is highly directional
More secure than RF technologies, but requires two points to be connected be within line-of-sight of each other
No RF spectrum licensing
No interference, much lower latency, no rain-fade
No security software upgrades
Immune to radio frequency interference or saturation
Industrial Data Communications (TS05)
Ethernet and TCP/IP on the Plant Floor (FG21C)
Industrial network integrity
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