Q. What is an ERTS or Emergency Roadside Telephone System ?
R. In order to securitized motorway a specific telephone network is fitted along the roadside in a protected area. An adapted and continuously verified telephone assumes a permanent link with operator of the motorway company or security services. It helps the operator to engage the good assistance to road user. The new generation of ERTS are designed to use digital transmission and optical fibers. ( See ERTS on ERECA web site )

Q. Why digitizing video for transmission ?
R. Analog video transmission has the disadvantage to damage its characteristics proportionally to the signal intensity regression versus the distance in the fiber. By digitizing video stream this disadvantage disappear, as optical dynamic range allows to the receiver detecting digital signal the video is keeping the original characteristics.

Q. How to realize bi-directional transmission on only one fiber ?
R. Transmission are often bi-directional for remote control or audio signal. This function can be realized on only one fiber, following two modes, the most popular is to use two different wavelength (850 and 1300 nm for multimode fiber or 1310 and 1550 nm for singlemode fiber), the second mode can be used exclusively with single mode fiber and consist in using the same wavelength for both direction transmission. To perform this design the link won’t be reflective, the use of fusion splice and angle connectors are highly recommended.

Q. How to secure a fiber optic cable ?
R. The better manner to secure a strategic link is to double the transmission path by using 2 optical cable. In one end an optical coupler is used to distribute the signal in two way (normal and safety way). At the other end an optical jumper will allow switching to the safety way manually or an optical switch can be fitted to make an automatic switching.

Q. What are attenuation on optical fiber ?
R. Attenuation of optical fiber is depending of the wavelength and the type of fiber chose. Multimode fiber, common average value used to estimate a link budget are:
·     850 nm linear attenuation is 3,5 dB/Km with 50/125 µm
·     850 nm linear attenuation is 4 dB/Km with 62,5/125 µm
·     1300 nm linear attenuation is 1 dB/Km with 50/125 µm
·     1300 nm linear attenuation is 1,5 dB/Km with 62,5/125 µm
Single mode fiber, common average value used to estimate a link budget are:
·     1310 nm linear attenuation is 0,45 dB/Km with 9/125 µm
·     1550 nm linear attenuation is 0,28 dB/Km with 9/125 µm.

Q. Bandwidth of the optical fiber ?
R. Used bandwidth is proportional to quantity of information to transmit. The largest capacity is used by real time video, digitizing and multiplexing of real time 25 images/ second video image require a bandwidth greater than 1 gigabit/s. Today telecom and computering meet this level of need and only singlemode fiber is able satisfying this demand.

Q. Which connector must we use ?
R. A lot of very different connectors were created with the evolution of fiber optic network. But today the experience give the way to select for multimode “ST” connector and Push pull “SC/APC 8°” for singlemode fiber, this particular one protects against reflection to answer to stability required for high datarate transmission or very high bandwidth analog transmission.

Q. What is multiplexing ?
R. For transmission two types of multiplexing exists:
·     Electric mupltiplexing, composed by Time division multiplexing realized by electronic circuitry (TDM) and frequency modulation using division of the bandwidth with subcarrier (e.g. SPFM Sygma Pulse Frequency Modulation).
·     Optical multiplexing is done with different layer of optical wavelength call WDM (Wave Division Multiplexing).

Q. Which wavelength must we used ?
R. Wavelength usable with telecommunication optical fiber are separated in 2 area, depending on multimode and singlemode optical fiber. Wavelength are identified with measurement from 780 to 1300 nm for multimode fiber and 1280 to 1640 nm for singlemode. The judicious choice of the wavelength optimises distance of the transmission.

Q. What is the difference between multimode and singlemode optical fiber ?
R. Multimode optical fiber is the first and of course oldest but its used for short distance application of poor bandwidth on 2 Km. Singlemode fiber is more and more used, the cost is reduced and technical specifications are better.