TRAINING REPORT
SIGNALLING AND TELECOMMUNICATION WORK IN INDIAN RAILWAYS
SUBMITTED BY:GARGEE MEHTA ELECTRONICS AND COMMUNICATION SHAMBHUNATH SHAMBHUNATH INSTITUTE OF ENGINEERING & TECHNOLOGY, (2006 BATCH)
CERTIFICATE This is to certify that GARGEE MEHTA, a III year student of Electronics bran branch ch from from Shamb and Commu Communic nicati ation on Shambhu hunat nath h Insti Institut tutee of Engineering and Technology, Allahabad had completed a 6 week training with North Central Railways (NCR) in the following modules:i) ii) iii)
Solid St State In Interlocking Optical Fiber Commun munication Microw rowave Com Communication
During this period she showed keen interest in every field. We wish her success for his future.
Mr. R. N. Singh Dy.CSTE/C-1 NCR, Allahabad. (For SSI)
Date:- 23rd July, 2005.
Mr. M. Verma ASTE/MW NCR, Allahabad. (For microwave)
Mr. P. Diwedi ASTE/Con NCR, Allahabad (For optical fiber)
TABLE OF CONTENTS
• • • •
Acknowledgement Abstract Introduction Module 1-Solid State Interlocking
Module 2-Optical Fiber Communication
•
•
Introduction A Whistle-stop Tour of Railway Signaling Operation of Solid State Interlocking Overall System Architecture Generic SSI Software
Introduction Optical Fiber Communication System Origin And Characteristics of Optical Fiber Operation of Optical Fiber A Fiber-Optic Relay System Application of Optical Fiber Advantages Of Optical Fiber Disadvantages of Optical Fiber
Module 3-Microwave Communication
Introduction History of Telegraphic Signals Origin of Microwave Signals Microwave Communication Satellites Generation and Frequency Bands of Microwave Signals Microwave and Waveguides Uses of Microwave Signals
ACKNOWLEDGEMENT
Behind the completion of any successful work there lies the contribution of not one but many individuals who may have directly or indirectly contributed to it. I first of all take the opportunity to thank NORT NORTH H CENT CENTRA RAL L RAILWAYS(NCR) for providing me this valuable opportunities to work and learn with them. During this training period everyone there had helped me in every possible way they can. I am also thankful to my parents, my friends and colleagues for their invaluable support. A special note of thanks to Mr. Anil Kumar(NCR), Mr. R N Singh(NCR) and many others for their help and suggestions.
GARGIE MEHTA
ABSTRACT
This report takes a pedagogical stance in demonstrating how results from theoretical computer science may be applied to yield significant insight into the behavior of the devices computer systems engineering practice seeks to put in place, and that this is immediately attainable with the present state of the art. The focus for this detailed study is provided by the type of solid state signaling and various communication systems currently being deployed throughout mainline railways. Safety and system reliability concerns dominate in this domain. With such motivation, two issues are tackled: the special special problem problem of software quality assurance assurance in these data-driven data-driven control systems, systems, and the broader problem of design dependability. In the former case, the analysis is directed towards proving safety properties of the geographic data which encode the control logic for the railwa railway y interl interlock ocking ing;; the latter latter examin examines es the fideli fidelity ty of the commun communica icatio tion n protocols upon which the distributed d istributed control system depends.
INTRODUCTION
Signaling is one of the most important aspects of Railway communication. In the very early days of the railways there was no fixed signaling to inform the driver of the state of the the line line ahead ahead.. Trai Trains ns were were driv driven en “on “on sight sight”. ”. But But sever several al unpl unplea easa sant nt inci incide dent ntss accentuated the need for an efficient signaling system. Earliest system involved the Time Interval technique. Here time intervals were imposed between trains mostly around 10 mins. But due to the frequent breakdown of trains in those days this technique resulted in rear-end collisions. This gave rise to the fixed signaling system wherein the track was divided into fixed sections and each section was protected by a fixed signaling. This system is still being continued although changes have been brought about in the basic signaling methods. Earlier mechanical signals were used but today block signaling is through electric instruments. In the mid 19th century mechanical interlocking was used. The purpose was to prevent the route for a train from being set up and its protecting signal cleared if there was alre already ady anoth another er conf confli lict ctin ing g rout routee setu setup. p. The The most most moder modern n devel developm opment ent in signa signall interlocking is SSI- a means of controlling the safety requirements at junctions using electronic circuits which replaced the relay systems supplied up to that time. In Indian Railwa Railways, ys, first first trial trial instal installat lation ion of SSI SSI was provid provided ed at Sriran Srirangam gam stati station on in 1987. 1987. Nowadays Track Circuits are used wherein the current flow in the track circuit will be interrupted by the presence of wheels and a “stop” signal will be shown. A “proceed” signal will be displayed if the current flows.
MODULE 1
SOLID STATE INTERLOCKING
INTRODUCTION
Soli Solid d Stat Statee Inte Interl rlock ockin ing g is a datadata-dr driv iven en sign signal al cont contro roll syst system em desi design gned ed for for use use through throughout out the Briti British sh railwa railway y syste system. m. SSI SSI is a replac replaceme ement nt for electr electromec omechani hanical cal interl interlocki ockings ngs-----whi which ch are based based on highly highly reliab reliable le relay relay technol technology ogy-----and and has been been designed with a view to modularity, improved flexibility in serving the needs of a diversity of rail traffic, and greater economy. The hugely complex relay circuitry found in many modern signalling installations is expensive to install, difficult to modify, and requires extensive housing---but the same functionality can be achieved with a relatively small number of interconnected solid state elements as long as they are individually sufficiently reliable. SSI has been designed to be compatible with current signalling practice and principles of interlocking design, and to maintain the operator's perception of the behavior and appearance of the control system.
A WHISTLE-STOP TOUR OF RAILWAY SIGNALING
Railway signaling engineers face a difficult distributed control problem. Train drivers can know little of the overall topology of the network through which they pass, or of the whereabouts of other trains in the network and their requirements. Safety is therefore invested in the control system, or interlocking , and drivers are required only to obey signals and speed limits. The task of the train dispatcher (signalman, or signal operator) is to adjust the setting of switches and signals to permit or inhibit traffic flow, but the interlocking has to be designed to protect the operator from inadvertently sending trains along conflicting routes.
The network can be operated with more security and efficiency if the operators have a broad overview of the railway and the distribution of trains. Since the introduction of mechanical interlocking in the late 1800's, and as the technology has gradually improved, the tendency has therefore been for control to become progressively centralized with fewer signal control canters individually responsible for larger p ortions of the network. In the last decade Solid State Interlocking has Interlocking has introduced computer controlled signaling, but the task of designing a safe interlocking remains essentially unchanged.
panel displays the current distribution of trains in At the signal control centre a control panel displays point switches (points) the network, the current status of {signals}, and sometimes that of point and other signaling equipment. The railway layout is depicted schematically on the panel.
OPERATION OF SOLID STATE INTERLOCKING
There are seven (three aspect) main signals shown here, and three sets of points. It is British Rail's practice to associate routes only with main signals. The operator can select a route by pressing the button at the entrance signal (say, S7 ), ), then pressing the button at the exit signal---the consecutive main signal, being the entrance signal for the next route (S5). S5). This sequence of events is interpreted as a panel a panel route request , and is forwarded to the controlling controlling computer computer for evaluation. evaluation. Other panel requests arise from the points the points keys which are used to manually call (and hold) the points to the specified position, or from button pull events (to cancel a route by pulling the entrance signal button).
Figure: Signals (Si) on the control panel appear on the left to the direction of travel, each signal has a lamp indicator, and each main signal has a button. Switches (points, Pi) show the normal position, and there is usually a points key on the panel so one can throw the points `manually'. Lamps illuminate those track sections ( Ti) over which routes are locked (white), and those in
which there are trains (red).
When the controlling computer receives a panel route request it evaluates the availability condi conditi tions ons spec specif ifie ied d for for the the rout route. e. Thes Thesee condi conditi tions ons are are give given n in a datab databas asee by Geographic Data which the control program evaluates in its on-going dialogue with the network. If the availability conditions are met the system responds by highlighting the track sections along the selected route on the display (otherwise the request is simply discarded). At this point the route is said to be locked : no conflicting route should be locked concurrently, and a property of the interlocking we should certainly verify is that no conflicting route can be locked concurrently.
Once a route is locked the interlocking will automatically set the route. Firstly, this involves calling the points along the route into correct alignment. Secondly, the route must must be proved ---thi ---thiss includ includes es checki checking ng that that points points are correc correctly tly aligne aligned, d, that that the filaments in the signal lamps are drawing current, and that signals controlling conflicting routes are on (i.e., red). Finally, the entrance signal can be switched off when the route is clear of other traffic---a driver approaching the signal will see it change from red to some less restrictive restrictive aspect (green, yellow, etc.), and an indicator on the control panel will be illuminated to notify the operators.
The operation of Solid State Interlocking is organized around the concept of a polling cycle. During this period the controlling computer will exchange messages with each piece of signaling equipment to which it is attached. An outgoing command telegram will drive the track-side equipment to the desired state, and an incoming data telegram will report the current state of the device. Signaling equipment is interfaced with the SSI modules. A points module will communications system through track-side functional modules. report whether the switch is detected normal or normal or detected detected reverse depending depending on which, if either, of the electrical contacts in the switch is closed. A signal module will report the circuit in the signal: if no current is flowing through the lamp status of the lamp proving circuit in filaments the lamp proving input in the data telegram will warn the signal operators about the faulty signal.
Other than conveying status information about points and signals, track-side functional modules report the current positions of trains. These are inferred from track circuit inputs circuit inputs to the modules. Track circuits are identified with track sections which are electrically insulated from one another. If the low voltage applied across the rails can be detected, this indicates there is no train in the section; a train entering the section will short the circuit causing the voltage to drop and the track section will be recorded as occupied at the control centre. Track circuits are simple, fail-safe devices, and one of the primary
safety features of the railway.
All action actionss perfor performed med by Solid Solid State State Interl Interlock ocking ing-----whe whethe therr in respons responsee to period periodic ic inputs from the track-side equipment, a periodic panel requests, or in preparing outgoing command telegrams---are governed by rules given in the Geographic Data that configure each Interlocking differently.
OVERALL SYSTEM ARCHITECTURE
SSI is a multicomputer system with two panel processors, a diagnostic processor, and thre threee cent centra rall inte interl rloc ocki king ng proce process ssor orss which which opera operate te in repa repair irab able le trip triple le modul modular ar redundancy. Higher-order control devices such as route planning and automatic route setting computers are not part of SSI, but they can be interfaced with the system.
The central interlocking processors are responsible for executing all signaling commands and producing correct system outputs, and operate in TMR to ensure high availability and single fault tolerance in the presence of occasional hardware faults. These are the safety critical elements of SSI. A TMR system has been implemented for hardware reliability:
each subsystem is identical, and runs identical software. All outputs are voted upon, redundantly in each interlocking processor, and the system is designed so that a module will be disconnected in the event of a majority vote against it---SSI will continue to operate as long as the outputs of the remaining modules are in agreement. A replacement module is updated by the two functioning modules before being allowed online. (In the sequel we usually usually refer to the central interlocking interlocking processors processors collectively collectively as the SSI , or the Interlocking .) .)
The panel processors are responsible for tasks which are not safety critical such as interfacing with the signal control panel, the display, and other systems such as automatic route setting computers. These processors are run in duplex `hot standby' for reasons of availability. The diagnostic processor is accessible from a maintenance terminal (the technician's techni cian's console console)) through which the system's performance and fault status can be monitored, and whereby temporary restrictions on the Interlocking's behavior can be introduced. In the latter case this is a provision for temporarily barring routes, locking points, or imposing other restrictions that are not directly under the control of the signal operators (for example, at times when there is a need for track maintenance).
A central feature of SSI is that the controlling computer is directly connected to trackside side equi equipm pmen entt by mean meanss of a dupl duplex ex dat data a hig highway hway carrying carrying discrete discrete signallin signalling g information. Track-side functional modules (TFMs) interface with signals and points to provide power switching under microprocessor control. Here, duplication of the hardware has been designed to ensure safe response to failures, but not fault masking: the TFM will set its outputs to the most restrictive state (e.g., signals at red) whenever a fault is detected or the duplicated control paths are found to diverge. One points module may be connected to two to four point switches, and can report up to four track circuit inputs. A signal module is usually connected to one signal and several nearby track circuits, but is flexible enough for any other desired function.
Figure: Schematic overview of the main features of SSI.
The operation of Solid State Interlocking is organized around the concept of a major cycle.. During this period the central interlocking will address each of the track-side cycle functional modules, and expect a reply from each in turn. A maximum of 63 TFMs can be connected to one SSI, and the major cycle is consequently divided into 64 minor cycles.. In the zeroth cycle data are exchanged with the diagnostic processor. In each cycles minor cycle the central
interl interlocki ocking ng will will decode decode one incomi incoming ng messag messagee (or data teleg telegram ram)) from from the the data data highway, and process one outgoing command telegram. telegram. The cable conveying messages to and from the central interlocking is a screened twisted pair carrying relatively high signal levels. Cribbens discusses in detail the performance requirements for this vital component of the system: the minimum refresh rate for the TFMs TFMs,, the the neces necessi sity ty of real real-t -tim imee encod encodin ing g and and decod decodin ing g of tran transm smit itte ted d data data,, the the geographic extent of the interlocking area and the need for an acceptable range without the need for repeaters (circa 15 km), are all factors that contribute to the design. A data rate of 20k bits per second has been adopted, and a cyclic polling strategy implemented to ensure early detection of communications breakdown at either end of the link. The data path is duplicated and TFMs and central interlocking are designed to tolerate single faults on the line---detected through missed or corrupted messages. In each addressing cycle 25 bits bits of mess messag agee data data are are padd padded ed with with five five parit parity y bits bits to form form a trun truncat cated ed (31,2 (31,26) 6)
Hamming code which is transmitted in Manchester encoded biphase form. TFMs are configured to reply immediately upon receipt of a message from the central interlocking. Cribbens argues convincingly that the SSI transmission system is highly secure.
GENERIC SSI SOFTWARE
SSI has been designed to be data-driven with a generic program operating on rules held in a `geographic' database. These data configure each SSI installation differently, and define the specific specific interlocking interlocking functions functions (although the more primitive primitive functions functions are directly supported by the software). The relationship between generic program and the data is one in which the former acts as an interpreter for the latter---f latter---for or this reason we
usuall usually y refer refer to the generic generic softwa software re as the control inte interpreter rpreter in the the sequ sequel el.. The The Motorola 6800 microprocessors used in SSI have a 16-bit address space: 60---80k bytes are EPROM which hold the generic program (about 20k bytes), and the Geographic Data; 2k bytes are RAM, and the rest is used for input and output devices. The modest RAM is used, mainly, to hold the system's record of the state of the railway---generally referred referred to railway, or the internal state in the sequel. as the image of the railway,
All SSI software is organized on a cyclic basis with the major cycle determining the rate at which track-side track-side equipment equipment receive fresh commands, and the rate at which the image of the railway is updated. During one minor cycle the generic program: performs all redunda redundancy ncy manage managemen ment, t, self-t self-test est and error error recove recovery ry procedu procedures res;; updates updates system system (software) timers and exchanges data with external devices such as panel processors; decodes one incoming data telegram and processes an associated block of Geographic Data; and processes the data associated with one outgoing command telegram. The latter phase is the most computational intensive part of the standard minor cycle because it is through these data that the Interlocking calculates the correct signal aspects.
The SSI minor cycle has a minimum duration of 9.5 ms, and a minimum major cycle time of 608 ms. However, SSI can operate reliably with a major cycle of up to 1,000 ms, with an individual minor cycle extensible to 30 ms. This flexibility is needed for handling panel requests. If the required minor cycle processes mentioned above can be completed in under the minimum minor cycle time, the control interpreter will process one of any pending panel requests (which are stored in a ring buffer). The data associated with a panel request must not require more than a further 20 ms of processing time---the data are structured such that accurate timing predictions can be made at compile time. If the minor cycle cycle is too long long the tracktrack-sid sidee functi functiona onall module moduless will will interp interpret ret the gaps gaps betwee between n
messages as data link faults, and will w ill drive the equipment to the safe state in error.
The initialization software compares the internal state of each of the three interlocking processors to determine the required start up procedure. When power is first applied a `mod `m odee 1' st star artu tup p is nece necess ssar ary: y: this this sets sets the the inte intern rnal al stat statee to a (des (desig igna nate ted) d) safe safe configuration, forces all output telegrams to drive the track-side equipment to the safe state and disables processing of panel requests; after a suitable delay so that TFM inputs can bring the internal state up to date, the Interlocking can be enabled under supervision from the technician's technician's console. After a short power failure failure much of the contents contents of RAM
will have been preserved and a `mode 2' or `mode 3' start up is appropriate. A `mode 2' start up resets the internal state to the safe configuration but preserves any restrictions that had been applied through the technician's console---the system is disabled for a period long enough for all trains to come to a halt, and allowed to restart normal operation automatically. A `mode 3' start up involves a similar reset but the status of routes is also preserved, and the system restarts immediately.
MODULE 2
OPTICAL FIBER COMMUNICATION
INTRODUCTION The The dem demand for high-c h-capac apaciity long ong-haul haul telec elecom omm muni unicat cation system stemss is increasing at a steady rate, and is expected to accelerate in the next decade. At the same time, communication networks which cover long distances and serve large areas with a large information capacity are also in increasing demand. To satisfy satisfy the requirements requirements on long distances, the communication communication channel must have a very low loss. On the other hand, a large information capacity can only be achieved with a wide system bandwidth which can support a high data bit rate (> Gbit/s) [3]. Reducing the loss whilst increasing the bandwidth of the communication channels is therefore essential for future telecommunications systems.
Of the many different communication channel available optical fiber proved to the most promising due to its low attenuation, low losses and various other advantages over twisted cables and other means of transmission.
Communication between stations and signalmen is done through telephone. In some places, IR still uses twisted pair cables and elderly Stronger exchanges. This is currently being upgraded to optical fiber and microwave communications. The main impetus for this change came from the Department of Telecommunications, who no longer had the expertise to maintain a large network of heritage technology. Drivers and guards were equipped with VHF radio systems in 1999 to communicate with each other and with station masters.
OPTICAL FIBER COMMUNICATION SYSTEM A thin glass strand designed for light transmission. A single hair-thin fiber is capable of transmitting trillions of bits per second. In addition to their huge transmission capacity, optical fibers offer many advantages over electricity and copp er wire. Light pulses are not affected by random radiation in the environment, and their error rate is significantly lower. Fibers allow longer distances to be spanned before the signal has to be regenerated by expensive "repeaters." Fibers are more secure, because taps in the line can be detected, and lastly, fiber installation is streamlined due to their dramatically lower weight and smaller size compared to copper cables.
Optical fiber v/s copper cables
The optical optical fiber fiber acts as a low loss, loss, wide bandwi bandwidth dth transmi transmissi ssion on channel. channel. A light light source is required to emit light signals, which are modulated by the signal data. To enhance enhance the performanc performancee of the system, system, a spectra spectrally lly pure light light source source is require required. d. Advances in semiconductor laser technology, especially after the invention of double hete hetero rost stru ruct ctur ures es (DH) (DH),, resu result lted ed in stab stable le,, effi effici cien ent, t, smal smalll-si size zed d and and comp compac actt semiconductor laser diodes (SLDs). Using such coherent light sources increases the bandwidth of the signal which can be transmitted in a simple intensity modulated (IM) syst system em [13]. [13]. Other Other modul modulat ation ion metho methods, ds, such such as phase phase shift shift keyin keying g (PSK) (PSK) and frequency-shift keying (FSK), can also be used. These can be achieved either by directly modulating the injection current to the SLD or by using an external electro or acoustooptic modulator
ORIGIN AND CHARACTERISTICS OF OPTICAL FIBER
In the late 1970s and early 1980s, telephone companies began to use fibers extensively to rebuild their communications infrastructure. According to KMI Corporation, specialists in fiber optic market research, by the end of 1990 there were approximately eight million miles of fiber laid in the U.S. (this is miles of fiber, not miles of cable which can contain many fibers). By the end of 2000, there were 80 million miles in the U.S. and 225 million worldwide. worldwide. Copper cable is increasingly increasingly being replaced replaced with fibers for LAN backbones backbones as well, and this usage is expected to increase substantially.
Pure Glass An optical fiber is constructed of a transparent co re made of nearly pure silicon dioxide (SiO2), through which the light travels. The core is surrounded by a cladding layer that reflects light, guiding the light along the core. A plastic coating covers the cladding c ladding to protect the glass surface. Cables also include fibers of Kevlar and/or steel wires for strength and an outer sheath of plastic or Teflon for protection.
Enormous Bandwidth For glass fibers, there are two "optical windows" where the fiber is most transparent and effi effici cien ent. t. The The cent center erss of thes thesee wind window owss are are 1300 1300 nm and and 1550 1550 nm, nm, prov provid idin ing g approximately 18,000GHz and 12,000GHz respectively, for a total of 30,000GHz. This enormous bandwidth is potentially usable in one fiber. Plastic is also used for shortdistance fiber runs, and their transparent windows are typically 650 nm and in the 750900 nm range.
Singlemode and Multimode There are two primary types of fiber. For intercity cabling and highest speed, singlemode fiber with a core diameter of less than 10 microns is used. Multimode fiber is very common for short distances and has a core diameter from 50 to 100 microns. See laser, WDM, fiber optics glossary and cable categories.
OPERATION OF OPTICAL FIBER
In an optical fiber, a refracted ray is one that is refracted from the core into the cladding. Specifically a ray having direction such that where r is r is the radial distance from the fiber r on the transverse plane, θ(r axis, φ(r φ(r )) is the azimuthal angle of projection of the ray at r on θ(r )) is the angle the ray makes with the fiber axis, n (r ) r ) is the refractive index at r , n (a ) is the refractive index at the core radius, a . Refracted rays correspond to radiation modes in the terminology of mode descriptors. For the fiber to guide the optical signal, the refractive index of the core must be slightly higher than that of the cladding. In different types of fibers, the core and core-cladding boundary function slightly differently in guiding the signal. Especially in single-mode fibers, a significant fraction of the energy in the b ound mode travels in the cladding. c ladding.
Diagram of total internal reflection in an optical fiber
The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances. distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some Some prem premiu ium m optic optical al fibe fibers rs show show much much less less sign signal al degr degrada adati tion on -- less less than than 10 percent/km at 1,550 nm
A FIBER-OPT IBER -OPTIC IC RELAY REL AY SYSTEM SY STEM
To understand how optical fibers are used in communications systems, let's look at an example from a World War II movie or documentary where two naval ships in a fleet need to communicate with each other while maintaining radio silence or on stormy seas. One ship pulls up alongside the other. The captain of one ship sends a message to a sailor on deck. The sailor translates the message into Morse code (dots and dashes) and uses a signal light (floodlight with a Venetian blind type shutter on it) to send the message to the other ship. A sailor on the deck of the other ship sees the Morse code message, decodes it into English and sends the message up to the captain.
Now, imagine doing this when the ships are on either side of the ocean separated by thousands of miles and you have a fiber-optic communication system in place between the two ships. Fiber-optic relay systems consist of the following: Transmitter - Produces and encodes the light signals Optical fiber - Conducts the light signals over a distance Optical regenerator - May be necessary to boost the light signal (for long distances) Optical receiver - Receives and decodes the light signals
Transmitter The transmitter is like the sailor on the deck of the sending ship. It receives and directs the optical device to turn the light "on" and "off" in the correct sequence, thereby generating a light signal. The transmitter is physically close to the optical fiber and may even have a lens to focus the light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).
Optical Regenerator As mentioned above, some signal loss occurs when the light is transmitted through the fiber, especially over long distances (more than a half mile, or about 1 k m) such as with undersea cables. Therefore, one or more optical regenerators is spliced along the cable to boost the degraded light signals.
An optical regenerator consists of optical fibers with a special coating (doping). The doped portion is "pumped" with a laser. When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal with the same characteristics as the incoming weak light signal. Basically, the regenerator is a laser amplifier for the incoming signal. See Photonics.com: Fiber Amplifiers for more details.
Optical Receiver The optical receiver is like the sailor on the deck of the receiving ship. It takes the incoming digital light signals, decodes them and sends the electrical signal to the other user's computer, TV or telephone (receiving ship's captain). The receiver uses a photocell or photodiode to detect the light.
USES OF OPTICAL OPTI CAL FIBER
The optica opticall fiber fiber can be used used as a medium medium for telecomm telecommuni unicat cation ion and networ networkin king g because it is flexible and can be bundled as cables. Although fibers can be made out of either transparent plastic or glass, the fibers used in long-distance telecommunications appli applicat catio ions ns are are alwa always ys glas glass, s, becau because se of the the lowe lowerr opti optica call abso absorp rpti tion on.. The The light light transm transmit itted ted through through the fiber fiber is confine confined d due to total total intern internal al reflec reflectio tion n within within the material. This is an important property that eliminates signal crosstalk between fibers with within in the the cabl cablee and and allo allows ws the the rout routin ing g of the the cabl cablee with with twis twists ts and and turn turns. s. In telecommunications applications, the light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use.
Parts of a single optical fiber
Core - Thin glass center of the fiber where the light travels Cladding-Outer optical material surrounding the core that reflects the light back into the core Buffer coating - Plastic coating that protects the fiber from damage and moisture
Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction, however bidirectional communications is possible over one strand by using two different wavelengths (colors) and appropriate coupling/splitting devices.
Fibers, like waveguides, can have various transmission modes. The fibers used for longdistance communication are known as single mode fibers, as they have only one strong propagation propagation mode. This results in superior superior performance performance compared compared to other, multi-mode multi-mode fibers, where light transmitted in the different modes arrives at different times, resulting in dispersion of the transmitted signal. Typical single mode fiber optic cables can sustain transmission distances of 80 to 140 km between regenerations of the signal, whereas most multi-mode fiber has a maximum transmission distance of 300 to 500 meters. Note that
single mode equipment is generally more expensive than multi-mode equipment. Fibers used in telecommunications typically have a diameter of 125 µm. The transmission core of single-mode fibers most commonly has a diameter of 9 µm, while multi-mode cores are available with 50 µm or 62.5 µm diameters.
Because of the remarkably low loss and excellent linearity and dispersion behavior of single-mode optical fiber, data rates of up to 40 gigabits per second are possible in realworld use on a single wavelength. Wavelength division multiplexing can then be used to allow many wavelengths to be used at once on a single fiber, allowing a single fiber to bear an aggregate bandwidth measured in terabits per second.
Moder Modern n fibe fiberr cabl cables es can can cont contai ain n up to a thous thousand and fibers fibers in a singl singlee cable cable,, so the the perf perfor orma manc ncee of optic optical al netw networ orks ks easi easily ly accom accommo moda date te even even toda today' y'ss dema demand ndss for for bandwidth on a point-to-point basis. However, unused point-to-point potential bandwidth does not translate to operating profits, and it is estimated that no more than 1% of the optical fiber buried in recent years is actually 'lit'.
Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, installation in conduit, lashing to aerial telephone poles, submarine installation, or insertion in paved streets. In recent years the cost of small fiber-count pole mounted cables has greatly decreased due to the high Japanese and South Korean demand for Fiber to the Home (FTTH) installations.
Recent advances in fiber technology have reduced losses so far that no amplification of the optical signal is needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical op tical networking, particularly over undersea spans where the cost reliability of amplifiers is one of the key factors determining the performance of the whole cable system. In the past few years several manufacturers of submarine cable line terminal equipment have introduced upgrades that promise to quadruple the capacity of older submarine systems installed in the early to mid 1990s.
APPLICATIONS OF OPTICAL FIBER
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Fibers can be used as light guides in medical and other applications where bright light needs to be brought to bear on a target without a clear line-of-sight path.
Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters.
Bundles of fibers are used along with lenses for long, thin imaging devices called endoscopes, which are used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
In some high-tech buildings, optical fibers are used to route sunlight from the roof to other parts of the building.
Optical fibers have many decorative applications, including signs an d art, artificial Christmas trees, and lighting.
ADVANTAGES OF OPTICAL FIBER
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Low loss, so repeater-less transmission over long distances is possible
Large data-carrying capacity (thousands of times greater, reaching speeds of up to 3TB/s)
Immunity to electromagnetic interference, including nuclear electromagnetic pulses (but can be damaged by alpha and beta radiation)
No electromagnetic radiation; difficult to eavesdrop
High electrical resistance, so safe to use near high-voltage equipment or between areas with different earth potentials p otentials
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Low weight
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Signals contain very little power
DISADVANTAGES OF OPTICAL FIBER
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Higher cost
Need for more expensive optical transmitters and receivers
More difficult and expensive to splice than wires
At higher optical powers, is susceptible to "fiber fuse" wherein a bit too much light meeting with an imperfection can destroy several meters per second . A "Fiber fuse" protection device at the transmitter can break the circuit to prevent damage, if the extreme conditions for this are deemed possible.
Cannot carry electrical power to operate terminal devices. However, current telecommunication trends greatly reduce this concern: availability of cell phones and wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack of real interest in hybrid metal-fiber cables; and increased use of fiber-based intermediate systems).
MODULE 3
MICROWAVE COMMUNICATION
INTRODUCTION
The international telecommunications system relies on microwave and satellite links for long-distance international calls. Cable links are increasingly made of optical fibers. The capacit capacity y of these these links links is enormou enormous. s. The TDRS-C TDRS-C (track (tracking ing and data-r data-rela elay y satell satellite ite communications) satellite, the world’s largest and most complex satellite can transmit in a single single second the contents of a 20-volume encyclopedia, encyclopedia, with each volume containing containing 1,200 pages of 2,000 words. A bundle of optical fibers, no thicker than a finger, can carry 10,000 phone calls – more than a copper wire as thick as an arm.
Microwave image of 3C353 3C353 galaxy at 8.4 GHz (36 mm). mm). The overall linear linear size of the radio structure structure is 120 kpc.
HISTORY OF TELEGRAPHIC SIGNALS Telegraph operators in a ‘cable room’ during the late 1950s or early 1960s. At this time, telegrams were encoded as perforations on tape. The tape was fed into a machine that read the perforations and sent them as signals down a land line. A receiver at the far end reproc reprocess essed ed the messag messagee back back onto onto tape. tape. A teleph telephone one operato operatorr would would then then ring ring the intended recipient and read out the message.
A telegraph receiver invented by the British physicist Charles Wheatstone in about 1840. In addition to the telegraph, Wheatstone also invented the rheostat (variable electrical resistor), and carried out experiments in underwater telegraphy. He also invented the concertina and the symphonium, a chromatic mouth organ.
Communications over a distance, generally by electronic means. Long-distance voice communication was pioneered in 1876 by Scottish scientist Alexander Graham Bell when he invented the telephone. The telegraph, radio, and television followed. Today it is possible to communicate internationally by telephone cable or by satellite or microwave link, with over 100,000 simultaneous conversations and several television channels being carried by the latest satellites.
ORIGIN OF MICROWAVE SIGNALS
The first mechanical telecommunications systems were semaphore and the heliograph (using flashes of sunlight), invented in the mid-19th century, but the forerunner of the prese present nt telecom telecommun munica icatio tions ns age was the electr electric ic telegr telegraph. aph. The earlie earliest st practi practicab cable le telegraph instrument was invented by William Cooke and Charles Wheatstone in Britain in 1837 and used by railway companies. In the USA, Samuel Morse invented a signalling code, Morse code, which is still used, and a recording telegraph, first used commercially between England and France in 1851.
Following German physicist Heinrich Hertz’s discovery of electromagnetic waves, Italian inventor inventor Guglielmo Guglielmo Marconi pioneered a ‘wireless ‘wireless’’ telegraph, telegraph, ancestor of the radio. He established wireless communication between England and France in 1899 and across the Atlantic in 1901.
The moder dern telegr egraph uses teleprinte nters to send coded messages along ong telecommunications lines. Telegraphs are keyboard-operated machines that transmit a five-un five-unit it Baudot Baudot code (see (see baud). baud). The receiv receiving ing telepr teleprint inter er automa automatic ticall ally y prints prints the received message. The modern version of the telegraph is e-mail in which text messages are sent electronically from computer to computer via network connections such as the Internet.
MICROWAVE COMMUNICATION SATTELITES
The The chie chieff meth method od of rela relayi ying ng long long-d -dis ista tanc ncee call callss on land land is micr microw owav avee radi radio o transmission. The drawback to long-distance voice communication via microwave radio transmission is that the transmissions follow a straight line from tower to tower, so that over the sea the system becomes impracticable. A solution was put forward in 1945 by the the sci science ence ficti ction writer iter Arthur thur C Clar Clarke ke,, when when he prop propos osed ed a sys system tem of communications satellites in an orbit 35,900 km/22,300 mi above the Equator, where they would circle the Earth in exactly 24 hours, and thus appear fixed in the sky. Such a syst system em is now now in opera operati tion on inte intern rnat atio ional nally ly,, by Inte Intels lsat at.. The The sate satell llit ites es are are call called ed geostationary satellites (syncoms). The first to be successfully launched, by Delta rocket from Cape Canaveral, was Syncoms 2 in July 1963. Many such satellites are now in use, concentrated over heavy traffic areas such as the Atlantic, Indian, and Pacific oceans. Telegraphy, telephony, and television transmissions are carried simultaneously by highfrequency radio waves. They are beamed to the satellites from large dish antennae or Earth stations, which connect with international networks.
a general microwave setup
GENERATION AND FREQUENCY BANDS OF MICROWAVE SIGNALS Micr Microwa owave vess can can be gener generat ated ed by a vari variet ety y of mean means, s, gener general ally ly divi divide ded d into into two two categories: solid state devices and vacuum-tube based devices. Solid state microwave devices are based on semiconductors such as silicon or gallium arsenide, and include field-effe field-effect ct transistor transistorss (FET's), (FET's), bipolar bipolar junction junction transistor transistorss (BJT's), (BJT's), Gunn diodes, diodes, and IMPATT diodes. Specialized versions of standard transistors have been developed for higher speeds which are commonly used in microwave applications. Microwave variants of BJT's include the heterojunction bipolar transistor (HBT), and microwave variants of FET's include the MESFET, the HEMT (also known as HFET), and LDMOS transistor. Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, traveling wave tube (TWT), and gyrotron. The microwave spectrum is usually defined as electromagnetic energy ranging from appro approxi xima mate tely ly 1 GHz GHz to 1000 1000 GHz in freq freque uency ncy,, but but olde olderr usag usagee incl include udess lower lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands are defined in the table below: Microwave frequency bands Designa Designation tion Frequen Frequency cy range range
L band
1 to 2 GHz
S band
2 to 4 GHz
C band
4 to 8 GHz
X band
8 to 12 GHz
K u band
12 to 18 GHz
K band
18 to 26 GHz
K a band
26 to 40 GHz
Q band
30 to 50 GHz
U band
40 to 60 GHz
V band
50 to 75 GHz
E band
60 to 90 GHz
W band
75 to 110 GHz
F band
90 to 140 GHz
D band
110 to 170 GHz
MICROWAVE AND WAVEGUIDES
Waveguide, device that controls the propagation of an electromagnetic wave so that the wave wave is forc forced ed to foll follow ow a path path defin defined ed by the the phys physic ical al stru struct ctur uree of the the guide. guide. Waveguides, which are useful chiefly at microwave frequencies in such applications as connecting the output amplifier of a radar set to its antenna, typically take the form of rectan rectangul gular ar hollow hollow metal metal tubes tubes but have have also also been been built built into into integr integrate ated d circui circuits. ts. A waveguide waveguide of a given dimension dimension will not propagate electromagnetic electromagnetic waves lower than a certain frequency (the cutoff frequency). Generally speaking, the electric and magnetic fields of an electromagnetic wave have a number of possible arrangements when the wave is traveling through a waveguide. Each of these arrangement arrangementss is known as a mode of propagation. Waveguides also have some use at optical frequencies. In physics, optics, and telecommunication, a w aveguide is an inhomogeneous (structured) material medium that confines and guides a propagating electromagnetic wave. In the microwave region of the electromagnetic spectrum, a waveguide no rmally consists of a hollow metallic conductor, usually rectangular, e lliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material. In the optical region, a waveguide used as a long transmission line consists of a solid dielectric filament (optical fiber), usually circular in cross section. In integrated optical circuits an optical waveguide may consist co nsist of a thin dielectric film. In the radio frequency region, ionized layers of the stratosphere and refractive surfaces of the troposphere may also act as an atmospheric waveguide. In digital computing, the term waveguide can also be used for data buffers used as delay lines that simulate physical waveguide behavior, such as in digital waveguide synthesis.
propagation in rectangular and circular waveguides
Waveguide propagation modes depend on the operating wavelength and polarization and the shape and size of the guide. In hollow metallic waveguides, the fundamental modes are the transverse transverse electric electric TE1,0 mode for rectangular and TE1,1 for circular waveguides, seen here in cross-section: A dielectric waveguide is a waveguide that consists of a dielectric material surrounded by another dielectric material, such as air, glass, or plastic, with a lower refractive index. An example of a dielectric waveguide is an optical fiber. Paradoxically, a metallic waveguide not a dielectric waveguide. filled with a dielectric material is not a A closed waveguide is an electromagnetic waveguide (a) that is tubular, usually with a circular or rectangular cross section, (b) that has electrically conducting walls, (c) that may be hollow or filled with a dielectric material, (d) that can support a large number of discrete propagating modes, though only a few may be practical, (e) in which each discrete mode defines the propagation constant for that mode, (f) in which the field at any point is describable in terms of the supported modes, (g) in which there is no radiation field, and (h) in which discontinuities and bends cause mode conversion but not radiation. A slotted waveguide is generally used for radar and other similar applications.
USES OF MICROWAVE SIGNALS
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A microwave oven uses a magnetron microwave generator to produce microwaves at a frequency of approximately 2.45 GHz for the purpose of cooking food. Microwaves cook food by causing molecules of water and other compounds to vibrate. The vibration creates heat which warms the food. Since organic matter is made up primarily of water, food is easily cooked by this method.
Microwaves are used in communication satellite transmissions because microwaves pass easily through the earth's atmosphere with less interference than longer wavelengths. There is also much more bandwidth in the microwave spectrum than in the rest of the radio spectrum.
Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.
Wireless LAN protocols, such as Bluetooth and the IEEE 802.11g and b specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses an ISM band in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can be found in many c ountries (but not the USA) in the 3.5–4.0 GHz range.
Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire Gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)
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Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some cell phone networks also use the lower microwave frequencies.
Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using Solar Power Satellite (SPS) (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves.
A maser is a device similar to a laser, except that it works at microwave frequencies.
