Home > FAQ > Cablefree - Technical Questions and Answers

    What is Cablefree?

    Cablefree products are free-space optical communication systems, offering atmospheric line-of-sight transmission of high-speed digital signals over distances up to and exceeding 1000m. State-of-the art infrared technology offers major performance advantages over traditional cable or radio systems, with huge available bandwidths in excess of 1.5Gbps. Unlike radio and microwave technologies, infrared offers complete freedom from interference or radio licensing regulations.



    What effect does weather have?

    Atmospheric weather effects such as fog, snow, mist, rain, smog, thermal shimmer and sand storms all act to attenuate or disturb the passage of light though the atmosphere. Cablefree systems are designed with a ‘fade margin’ which ensures operation during the normal fluctuations in weather. Extremes of fog or thermal shimmer are investigated during a site-survey to ensure excellent reliability performance is achieved. The availability figure for Cablefree links is typically between 99.997% and 99.7% depending on distance and location.



    What happens if the beam is broken?

    Cablefree systems are normally installed away from ground level to avoid beam-breakage from terrestrial objects such as people or cars. Free-space systems are then of high integrity, with rare events such as birds flying through the beam which disrupts both optical and microwave systems – typically the data path is broken for a small fraction of a second. As the optical beams are relatively narrow, this phenomenon is statistically extremely rare. In a data networking application, network protocols such as Ethernet and Token ring merely re-send lost packets and the result is transparent to the user. E1/G703 systems will experience ‘clicks’ in the audio traffic. CCTV and Broadcast television will experience lost frames, which may not even be perceptible if frame-stores are used elsewhere in the system.



    What are the effects of sunlight?

    Sunlight does not affect Cablefree systems in normal operation – a combination of narrow viewfield, optical filtering to remove out-of-band light and DC offset removal in the receiver ensure that links remain working day and night. In very rare cases, a poorly specified East/West installation could result in the sun rising or setting behind one end of the link, temporarily blinding the remote receiver. Although this condition causes the link to be inoperative for several minutes while the sun passes, no damage occurs and recovery is automatic. This phenomenon affects not only optical systems but other free-space links such as microwave.



    What level of maintenance is required?

    Cablefree systems are extremely reliable in use, and are designed for continuous 24-hour all-year-round operation. Periodic maintenance is minimal but advisable to check integrity of outdoor cables, received signal levels and wipe any dirt or deposits from the front face of the unit. None of these activities requires the system to be disconnected or powered down. Installers and users should be aware that trees or foliage in the line-of-sight path may require periodic trimming.



    Can I upgrade a Cablefree system?

    One of the main features of the Cablefree system is the ability to perform upgrades on-site. Cablefree systems are configured with interchangeable plug-in modules which allow the link to be upgraded to match the rest of a network – a feature not available from competitive systems or leased-line operators. Typically, a 10Mbps Ethernet network can be upgraded to 100Mbps, single-channel 2Mbps E1/G703 to E2 or multiple 2Mbps channels, a CCTV system from single to multiple video channels, or a complete change of use effected. Often this can be done without removing or realigning the link, though a brief verification of link performance is always advised.



    Is this a duplex link? Can I have multiple channels?

    All Cablefree systems are full-duplex links– each unit has an independent transmitter and receiver subsystem. In a data networking or telecommunications application, data flows in both directions across the link. In a CCTV Security or Broadcast Television environment, data, audio or images can be sent in the return direction from the normal video transmission.

    Plug-in modules allow a Cablefree system to support multiple channels of 2Mbps E1/G703 for telecommunications. For CCTV or Broadcast use, up to 4 channels of video with auxiliary sound and telemetry channels can be carried over a single link. External industry-standard multiplexers can of course be used to allow diverse types of data traffic to pass over a single link such as 2Mbps E1/G703 or 155Mbps ATM/STM rates.



    Can I see the beams? Is it safe to do so?

    Transmitted light is visible only when standing directly in line with the link – the beams are typically quite narrow. The infrared wavelengths used by Cablefree systems are just visible to the human eye as red, and are most clearly seen at night. All Cablefree systems are certified to be eye-safe to IEC standards – but tampering or attempting to open the case can give rise to dangerous emissions. These are precision optical instruments, and all Class 3B laser systems should be treated with respect. Viewing with the naked eye is perfectly safe, but avoid staring into the front of the unit at short range using magnifying instruments such as a telescope or binoculars.



    Can I send the beams through windows?

    Yes - infrared beams used by Cablefree travel well through glass, though some of the signal is reflected. The maximum range of the link through glass is therefore slightly less than the free-air value. This can be of use in temporary or covert applications, or for listed buildings.



    Is transmission using Cablefree secure?

    Cablefree systems offer an extremely secure method of transmitting information. The optical beams used are relatively narrow and of low divergence – they cannot be detected outside the footprint of the beam. An attempt to intercept data transmission is likely to partially or completely block the beam, which is detectable by the operator and can be used to halt data transfer – and the attempt is visibly obvious due to the requirement to be ‘in the beam’. Infrared beams cannot be detected using RF meters or spectrum analysers, and require an optical receiver tuned to the appropriate wavelength to decode them. For certain data formats, proprietary line coding techniques used make the data still more secure.



    How does Cablefree compare to Microwave systems?

    Cablefree links use optical frequencies, and therefore can offer higher bandwidths than physically possible with Microwave technology – the trade-off is that receivers are correspondingly less sensitive which makes infrared suited to shorter ranges around 1-2km. Optical systems can be configured to use tighter beams which removes the possibility of interference with adjacent links – and they don’t suffer from signal reflections from nearby objects. Atmospheric effects affect both optical and microwave systems – for infrared, fog is a major source of attenuation, but not rain – the reverse is true for microwave. Optical systems such as Cablefree do not require frequency allocation permits or licenses.



    How do I find out more about Cablefree?

    Cablefree would be delighted to answer your questions on infrared technology and how our systems could suit your application. In addition to the examples given above, Cablefree is suited to a huge and growing range of applications, with some exciting future developments. Cablefree systems can be supplied configured for your application directly from our head office in Surrey, U.K, or from a number of appointed resellers. Contact us for quotation or shipment details.

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