E2-E3 CONSUMER FIXED ACCESS CHAPTER-4 OVERVIEW OF OFC NETWORK (Date Of Creation: 01-04-2011) Page: 1
Overview Of OFC Network Learning Objective: Optical Fiber concept & types OFC route and optical budget Important OFC Test instruments Use of OFC in BSNL & laying practices Fiber Characteristics: The main job of optical fibers is to guide light waves with a minimum of attenuation (loss of signal). Optical fibers are composed of fine threads of glass in layers, called the core and cladding that can transmit light at about two-thirds the speed of light in a vacuum. Light is either reflected (it bounces back) or refracted (its angle is altered while passing through a different medium) depending upon the angle of incidence (the angle at which light strikes the interface between an optically denser and optically thinner material). Total internal reflection happens when the following conditions are met: Beams pass from a denser to a less dense material. The difference between the optical density of a given material and a vacuum is the material s refractive index. The incident angle is more than the critical angle. The critical angle is the maximum angle of incidence at which light stops being refracted and is instead totally reflected. The principle of total internal reflection within a fiber core is illustrated in Figure given below. The core has a higher refractive index than the cladding, allowing the beam that strikes the surface at more than the critical angle to be reflected. Page: 2
An optical fiber consists of two different types of highly pure, solid glass (silica) the core and the cladding that are mixed with specific elements, called dopants, to adjust their refractive indices. The difference between the refractive indices of the two materials causes most of the transmitted light to bounce off the cladding and stay within the core. The critical angle requirement is met by controlling the angle at which the light is injected into the fiber. Two or more layers of protective coating around the cladding ensure that the glass can be handled without damage. Multimode and Single-Mode Fiber There are two general categories of optical fiber in use today, multimode fiber and singlemode fiber. Single Mode: The single-mode fiber has a much smaller core that allows only one mode of light at a time through the core (see Figure below). As a result, the fidelity of the signal is better retained over longer distances, and modal dispersion is greatly reduced. These factors attribute to a higher bandwidth capacity than multimode fibers are capable of. For its large information carrying capacity and low intrinsic loss, single-mode fibers are preferred for longer distance and higher bandwidth application including DWDM (Dense Wavelength-division multiplexing). WDM is the practice of dividing the wavelength capacity of an optical fiber into multiple channels in order to send more than one signal over the same fiber. This requires a wavelength division multiplexer in the transmitting equipment and a wavelength division demultiplexer in the receiving equipment. Using WDM technology now commercially available, the bandwidth of a fiber can be divided into as many as 80 channels to support a combined bit rate into the range of terabits per second. Cladding Core Figure: Single Mode Fiber Page: 3
Single-Mode Fiber Designs: Designs of single-mode fiber have evolved over several decades. The three principle types and their ITU-T specifications are: Non-dispersion-shifted fiber (NDSF), G.652 Dispersion-shifted fiber (DSF), G.653 Non-zero dispersion-shifted fiber (NZ-DSF), G.655 Multimode: This type of fiber has a larger core than single-mode fiber. It gets its name from the fact that numerous modes, or light rays, can be carried simultaneously through the waveguide. Figure 2-7 shows an example of light transmitted in the first type of multimode fiber, called step-index. Step-index refers to the fact that there is a uniform index of refraction throughout the core; thus there is a step in the refractive index where the core and cladding interface. Notice that the two modes must travel different distances to arrive at their destinations. This disparity between the times that the light rays arrive is called modal dispersion. This phenomenon results in poor signal quality at the receiving end and ultimately limits the transmission distance. This is why multimode fiber is not used in longer distance applications. To compensate for the dispersion drawback of step-index multimode fiber, graded-index fiber was invented. Graded-index refers to the fact that the refractive index of the core is graded it gradually decreases from the center to outward of the core. The higher Page: 4
refraction at the center of the core slows the speed of some light rays, allowing all the rays to reach their destination at about the same time and reducing modal dispersion. Optical Communications Advantages: Compared with systems based on electrical cables, the approach of optical fiber communications has advantages, the most important of which are: The capacity of fibers for data transmission is huge: a single silica fiber can carry hundreds of thousands of telephone channels, utilizing only a small part of the theoretical capacity. The losses for light propagating in fibers are amazingly small: 0.2dB/km for modern single-mode silica fibers, so that many tens of kilometers can be bridged without amplifying the signals. A large number of channels can be re-amplified in a single fiber amplifier, if required for very large transmission distances. Due to the huge transmission rate achievable, the cost per transported bit can be extremely low. Compared with electrical cables, fiber-optic cables are very lightweight, so that the cost of laying a fiber-optic cable can be lower. Fiber-optic cables are immune to problems that arise with electrical cables, such as ground loops or electromagnetic interference (EMI). Transmission Windows: Attenuation and dispersion depends on the optical wavelength, therefore, wavelength bands exist where these effects are weakest, and thus making these bands, or windows, most favorable for transmission. Optical fiber communications typically operate in a wavelength region corresponding to one of the following Transmission windows : The first window at 800 900nm was originally used. GaAs/AlGaAs-based laser diodes and light-emitting diodes (LEDs) served as transmitters, and silicon photodiodes were suitable for the receivers. However, the fiber losses are relatively high in this region, and fiber amplifiers are not well developed for this spectral region. Therefore, the first telecom window is suitable only for short-distance transmission. The second telecom window utilizes wavelengths around 1.3μm, where the loss of silica fibers is much lower and the fibers' chromatic dispersion is very weak, so that dispersive broadening is minimized. This window was originally used for long-haul transmission. Also, low dispersion is not necessarily ideal for long-haul transmission, as sometime it can increase the effect of optical nonlinearities. Page: 5
The third telecom window, which is now very widely used, utilizes wavelengths around 1.5μm. The losses of silica fibers are lowest in this region, and erbium-doped fiber amplifiers are available which offer very high performance. Fiber dispersion is usually anomalous but can be tailored with great flexibility. These second and Third windows have been further divide and standardized. The current bands defined are the following: Band Description Wavelength Range O band Original 1260 to 1360 nm E band Extended 1360 to 1460 nm S band Short wavelengths 1460 to 1530 nm C band Conventional ("erbium window") 1530 to 1565 nm L band Long wavelengths 1565 to 1625 nm U band Ultra long wavelengths 1625 to 1675 nm Transmission Challenges Transmission of light in optical fiber presents several challenges that must be dealt with. These fall into the following three broad categories: Attenuation decay of signal strength, or loss of light power, as the signal propagates through the fiber Chromatic dispersion spreading of light pulses as they travel down the fiber Nonlinearities cumulative effects from the interaction of light with the material through which it travels, resulting in changes and interactions between light waves Length Of The OFC Route Let us assume that the Transmitter has an output of 6 dbm. The Receiver requires a minimum input of approximately O dbm. Hence a loss of 6 db can be accommodated along the Fiber Optic Cable. If the Cable has a loss of 0.35 db per kilometer, a transmission loss of 6dB can be accommodated. This is referred to as 6dB "Optical Budget" indicating that the system designer can "spend" or utilize upto 6 db of optical power in the distribution network. Cable length in Kms = 6 db/0.35db = 17.14 Kms! In practice, to cover larger distance and as well as to repair the OFC cuts, joints have to be made i.e. OFC has to be joined. This joining is called as Splicing. For this a special machine called as Splicing Machine is used. Moreover some losses do happen at connectors. In some cases, Optical signal has to be tapped i.e. branched. This is done with the help of Optical Splitters. These are available in a range of splitting ratios viz.: 50:50 Page: 6
(i.e. split in 2 equal parts), 60:40, 70:30, 80:20 and 90:10. Therefore many other losses are also part of the distribution network such as due to Optical Splitters inserted in the path and loss due to joints or Splicing. Optical Budget The "Optical Budget" is a term used in OFC route planning taking into account various losses. It can be put down as a simple formula. Transmitter Power = Loss in Cable + Splice loss + Splitter loss + Power required at Receiver. Going back to our previous example, the Transmitter power was 6 dbm. The power at the Receiver was O dbm. Therefore 6 dbm = Loss in Cable + Splice loss + Splitter loss + O dbm Most system designers will add another 0.5 db loss as a "Safety Loss". This "Safety Loss" is to accommodate any unpredicted losses during installation of the system. Optical amplifiers are used at appropriate intervals to further enhance the coverage of OFC routes. Applications: In the long-distance network, the majority of embedded fiber is standard single-mode (G.652) with high dispersion in the 1550-nm window, which limits the distance for STM- 64 transmission. Dispersion can be mitigated to some extent, and at some cost, using dispersion compensators. Non-zero dispersion-shifted fiber can be deployed for STM-64 transport, but higher optical power introduces nonlinear effects. In the short-haul network, PMD and nonlinear effects are not so critical as they are in long-haul systems, where higher speeds (STM-64 and higher) are more common. DWDM systems using optical signals of 2.5 Gbps or less are not subject to these nonlinear effects at short distances. The major types of single-mode fibers and their application can be summarized as follows: Non-dispersion-shifted fiber (standard SM fiber) accounts for greater than 95 percent of deployed plant; suitable for TDM (single-channel) use in the 1310-nm region or DWDM use in the 1550-nm region (with dispersion compensators). This type of fiber can also support 10 Gigabit Ethernet standard at distances over 300 meters. Dispersion-shifted fiber suitable for TDM use in the 1550-nm region, but unsuitable for DWDM in this region. Non-zero dispersion-shifted fiber good for both TDM and DWDM use in the 1550-nm region. Page: 7
Newer generation fibers includes types that allow the energy to travel further into the cladding, creating a small amount of dispersion to counter four-wave mixing, and dispersion-flattened fibers, which permit use of wavelengths farther from the optimum wavelength without pulse spreading. Main Tests On OFC Cable Loss. Splice Loss. Connector Loss. Fiber Length. Continuity of Fiber. Fault Localizations/Break Fault Main Instruments Required Calibrated Light Source Optical Power Meter Optical Attenuator To Simulate the Regenerator Loss at the FDF (Fiber Distribution Frame) To provide Local Loop Back for Testing. To measure the Bit Error Rate by varying the Optical Signal at the Receiver Input. Optical Time Domain Reflectometer (OTDR): This is the main instrument to perform all the tests mentioned above. Laying of Optical Fiber cable: OFC is either buried or laid in Ducts. For buried type, depth of trench is decided on the basis of soil characteristic. Two categories of soils are: Rocky: Cable trench, which can not be dug without blasting and/or chiseling. Non-Rocky: Other than above including murram and soil mixed with stone and soft rock. Pipes for cable Laying and Protection HDPE pipe 75 mm (diameter) length 5m. (approx 18 to 20 ) HDPE pipe 50 mm (diameter) length 5m. (approx 18 to 20 ) PLB pipe (40 mm. outer diameter) length 1km/200m ( town limits with rope) GI pipe for PLP 50 mm dia length 6 meter Page: 8
Measurement of cable depth: All depth should be measured from the top of pipe. However it is acceptable if it is less by more then eight cms. from the specified. Cross country route (normal soil) Above HDPE pipe 1.5 meter Trench depth 1.65 meter In rocky area minimum depth 0.9 m (all cables having depth less then 1.2 meter should be protected by RCC/GI pipes) In built up area (city/town/urban area) OF cable should be laid through exiting duct. GI pipe or RCC pipe at the entry of duct. In non-duct area it should be laid through HDPE pipe/plp pipe at dept 1.5 meter using RCC/GI pipe for protection. Depth in rocky soil may be consider as 0.9 to 1.0 meter On culvert/bridge over river and nallah. At the depth of 1.5 meter below the bed throw HDPE/RCC Pipe. Pipe length should be 2 meter extended at both ends. This should be fixed along the parapet wall/bridge wall when the river or nalla full of water through out year, through fixed GI pipe on wall at suitable height above the water level. Along rail bridge or crossing Through HDPE pipe/plp pipe protected by RCC or iron pipe as per the prescribed by railway authority. On road crossing At a depth of 1.5 meter through HDP pipe enclosed in RCC pipe extended by 3.0 meter to the side end of the read. Indicators along route Route indicator: At every 200 m route length of showing name of route & no of indicators. Joint indicator: At every joint (Splice) generally it is placed at every 2 or 4 Km (Drum length) Branch (Root diversion) indicator: Provided at route diversion or branching from the main root. Page: 9
Chapter 4. Overview of OFC Network Sample Self study questions Q.1 Fill up the Blank : 1) The is the maximum angle of incidence at which light stops being refracted and is instead totally reflected 2) The core has a refractive index than the cladding 3) By using single mode fibre dispersion is reduced. 4) For long distance OF communication type of fibre is used. 5) This disparity between the times that the light rays arrive is called 6) Decay of signal strength, or loss of light power, as the signal propagates through the fiber is known as 7) For Fault Localizations/Break Fault in an OFC cable meter is used. Subjective Questions 1. What are the two types of OF Cable. 2. Write five advantages of OF cable communication 3. What are the three windows of wavelength used for OF communications and the wavelength used. 4. What is an optical Budget. 5. What are the main Test on OFC. Page: 10
Objective Questions 1. Optical fibers are composed of fine threads of glass in layers, called (a).core (b).cladding (c) Conductor (d) a&b 2. First optical window is having central frequency of: a) 850 nm (b) 1550 nm (c) 1310 nm (d)1600 nm 3. Which fiber is designed specifically to meet the needs of DWDM application? (a) Dispersion shifted fiber (b) Non-dispersion-shifted fibre (c)non-zero dispersion-shifted fibre (d) None of these. 4. While laying OFC, route indicators are to be placed every after: a)100meter (b) 150meter (c) 200meter (d) 500meter 5. In an OF communication Optical transmitter uses a) APD b) Diode c) LED /LASER d) None Page: 11