Q. 1. What is wavelength division Multiplexing (WDM) and whatare the advantages of WDM transmissionAns. WDM—The .technology of combining a number of wavelengths ontothe same Fiber is known WDM.WDM is the basic technology of optical networking. It is a technique forusing a fiber to carry many separate and independent channels. In WDMmultiple optical carriers at different wavelengths are modulated by usingindependent electrical bit streams and are then transmitted over thesame fiber. It increases the information carrying capacity of a fiber.Advantages —(i) It has greater transmission capacity(ii) Duplex transmission(iii) Simultaneous transmission of various signals(iv) Easy system expansion.(v) Lower cost(vi) Faster access to new channels.Q. 2. Explain key features of WDM system.Ans. The key system features of WDM are(1) Capacity upgrade — WDM system can increase the capacity of a fibernetwork.(ii) Transparency — In WDM, each optical channel can carry anytransmission format with WDM, any type of information, analog or digitalcan be sent simultaneously over the same fiber.(iii) Wavelength routing—In addition to using multiple wavelengths toincreaselink capacity; the use of wavelengths sensitive optical routing devicesmakes it possible to use wavelength as another dimension in designingcommunication networks and switches.Q. 3. What are the different types of WDM MUXS and DEMUXS.Ans. Depending on the wavelength spacing, WDM, MUXS and DEMUXSare classified into three types.
(i) Broad band WDMS (B WDM)—they combine and separate 1310 and1510 nm channels or even 850nm and l3lOnm channels.(ii) Narrow band WDMs (NWDM) — They combine and separatewavelength channels with centre to centre spacing greater than 200 GHz.(iii) Dense WDMs (DWDMs)—They combine and separate wavelengthchannels not more than 200 GHz.Q. 4. Explain in detail WDM systems with suitable diagrams.Ans. In fiber optic communications, wavelength division multiplexing(WDM) is a technology which multiplexes multiple optical carrier signalson a single optical fiber by using different wavelengths of laser light tocarry different signals. This allows for a multiplication in capacity inaddition to enabling bidirectional communications over one strand of fiber.WDM system uses a multiplexer at the transmitter to join the signalstogether and demultiplexer at the receiver to split them apart.A block diagram of WDM system is show in Figure.Fig. 6.1 a shows WDM system with multiplication in capacity of system. InWDM, several baseband modulated channels are transmitted along asingle fiber with each channel located at different wavelength. Each of Ndifferent wavelength channels are operating at slower speeds, but theaggregate system is transmitting at N times the individual channel speed,providing a significant capacity enhancement. In this WDM system,numbers of electrical signals are efficiently combined and transmittedover a single fiber. The signals can be voice, video or data which may beeither in analog or multiplexer format. A WDM multiplexer couples lightfrom different sources to the transmitting fiber. At the receiver WDMdemultiplexer separates the different carriers before the photo detectionof the individual signals. The WDM channels are separated in wavelengthto avoid cross-talk when they are demultiplixed by a non ideal opticalfiber.
In the second case, as shown in fig. 6.1 b, we can achieve full duplexcommunication using a single fiber cable. In this the information flow ineach of the two directions is discriminated by the use of two differentwavelengths. it involves sending information in one direction at awavelength from left to right simultaneously by sending informationfrom right to left at wavelength . To achieve bidirectionalcommunication we use MDM which performs the function of Multiplixerand Demultiplexer.Q. 5. Explain the classification of WUM systems depending onchannel resoultion and number of channels.Ans.Depending on channel resolution and number of channels, there aretwo types of WDM sytems.(i) Conventional or coarse WDM(ii) Dense WDMConventional WDM systems provide upto 16 channels in the 3rdtransmission window (c-band) of silica fibers around 1550 nrn. DWDMuses the same transmission window but with denser channel spacing.Channels plans vary but typical sytem would use 40 channels at 100 GHzspacing or 80 channels with 50 GHz spacing. Some technologies arecapable of 25 GHz spacing called ultra dense WDM.Q. 6. Explain the classification of WDM system depending onsytsem architecture.Ans. Depending on system architecture there are three types of WDMsytems(i) High capacity point to point links
(ii) Wide Area and metro area networks(iii) Multiple Access WDM networks.(1) High capcity point to point links—WDM increases the total bit rate inoptical communication system.Fig. 6.2 shows point to point high capacity WDM link. The output ofdifferent transmitters, each hving its own carrier wavelength arecombined in multiplexer. The multiplexed signal is launched into theoptical fiber for transmission to receiver side. At receiver end,demultiplexer separates each carrier and sends each channel to its ownreceiver. Suppose that each channel has bit rate B1 B2…… BN. Thesechannels are transmitted simultaneoulsy over fiber of length L.then total bit rate distance product BL (B1 + B2 + BN) L.If all channels have equal bit rates then the capacity of system will beincreasedby a factor of N.The capacity of WDM fiber links depends on how closely channels can bepacked in the wavelength domain. The channel spacing should exceed213 at a bit rate B but this requirement wastes Band width. The numberof channels that can be transmitted over system are limited by number offactors which include degrdation of signal during transmission and interchannel cross talk during demultiplexirig.(ii) Wide Area and Metro area Netowork—There are three types of opticalnetworks (i) LAN (ii) MAN (iii) WAN depending on the area they cover.Hub, star or ring topology are used for designing optical networks ForMANs and WANS, ring topology is used and for LANs, star topology isused. Both star and ring toplogies are shown in fig. 6.3.
Each node in the star has a transmitter and a receiver, with thetransmitter connected to one of the central passive stars inputs and thereceiver connected to one of the stars output. WDM networks can also usering topology because rings are easy to implement for any networkgeographical configuration. As shown in Fig. 6.3b each node in theunidirectional ring can transmit on a specific signature wavelength andeach node can recover any other node wavelength signal by means of awavelength tunable receiver. In both the star and the ring topology, eachnode has a signature wavelength and any two nodes can communicatewith each other by transmitting on that wavelength. This implies that werequire N wavelengths to connect N nodes. It has an advantage that datatransfer occurs with art uninterrupted optical path between the origin andthe destination known as a single hop network. The optical data start atthe originating node and reach the destination node without stopping atany other intermediate node. A disadvantage of a single hop WDMnetwork is that the network and all its components must accommodate Nwavelengths which may be difficult to achieve in a large network.(iii) Multiple Access WDM networks—Multiple access networks allows abidirectional access to each subscriber. At all time, each user can transmitand receive information from any other user of the network. WDMnetworks can be classified into two categories (1) single hop opticalnetworks (ii) Multiple hop optical networks. In single hop network, allnodes are connected to each other while in multiple hop networks, anoptical signal sent by one node passes through number of intermediatenodes before reaching its destination node. Consider single hop WDMnetwork called Lambda net while is an example of broadcast and selectnetwork.
In Lambdanet, each node has one transmitter emitting a particularwavelength and number of receivers operating at N different wavelengths.The output of all transmitters is joined ii a passive star and equallydistributed to all receivers. Desired channel is selected with the help oftunable optical filter. In place of a tunable filter, each node uses a numberof receivers. In Lambdanet, the capacity and connectivity can be changeddepending on the application. Data can be transmitted at different bitrates with different modulation formats in this network. This network hasa disadvantage that the number of users is limited by the number ofavailable wavelengths. Each node requires number of receivers. Thedisadvantages of Lambdanet can be compensated in rainbow networkwith the help of tunable receiver. Rainbow network use central passivestar together with the high performance parallel interface. The maindrawback of rainbow network is that tuning of receivers is a slow processdue to which packet switching can’t be used.The disadvantage of single hop network to accommodate N wavelength byeach component can be compensated in multichip network in which twonodes can communicate with each other by sending through a third node,with many such intermediate hops possible, A dual bus multihop eightnode WDM network is shown in Fig. 6.5. For which each node cantransmit on two wavelengths and receive on two other wavelengths.
For example, if node 1 wants to communicate with node 5, it transmits onwavelength A1 and on.ly a single hop is required, However if node I wantsto communicate with node 2, if first must transmit to node 5, which thentransmit to node 2, including two hops. Any extra hops increase thetransmit time between two communicating nodes and decrease thethrough put. However a multiple hop networks reduce the requirednumber of wavelengths and the wavelength tenability range of thecomponents.Q. 7. With the help of a block diagram, explain the basic conceptof subcarrier multiplexing.Ans. Subcarrier Multiplexing allows multiple broadband signals to betransmitted over single mode fiber. In sub carrier multiplexing, instead ofdirectly modulating terahertz optical carrier wave with—lOOs Mbpsbaseband data, the baseband data are impressed on a gigahertzsubcarrier wave that is subsequently impressed on the THz optical carrier.In SCM, microwave subcarriers are used for multiplexing rather thanoptical carrier. SCM is similar to commercial radio, in which many stationsare placed at different radio frequencies such that a radio receiver cantune its filter to the appropriate subcarrier radio frequency. Themultiplexing and demultiplexing of the SCM channels is doneelectronically not opticallyIn analog SCM system, analog format is used to modulate each microsubcarrier. The output of all subcarriers is added by microwave powercombiner. The output of microwave power combiner is used to modulatethe intensity of a optical transmitter i.e. laser. Optical signal is thentransmitted through optical fiber. At the receiver photo detector detectsthe optical signal. Microwave receiver detects and selects all the channels.If the communication channel is perfectly linear, the received power willbe in the same form as transmitted power. But no communication channelis perfect so distortion occurs in received power which is called
intermediation distortion (IMD). IMD occurs due to various non linearmechanisms. The performance of SCM system depends on the CNR ofdemodulated signal. CNR is the ratio of RMS value of carrier power toRMS value of noise power at the receiver.Advantages/Disadvantages—Advantages(i) Several channels can share the same expensive optical components.(ii) Electrical components are less expensive than optical componentsused in SCM systems.DisadvantageSCM is limited in maximum subcarrier frequencies and data rates by theavailable bandwidth of the electrical and optical components. SCM mustbe used in conjunction with WDM to utilize any significant fraction of thefiber band width.DIIM SCM systemIn digital SCM system multilevel QAM format is used to modulate the lightintensity of laser. By using QAM to modulate the light intensity of laser,no coherent detection is required at receiver. It also requires lower CNRas compared to AM-VSB systems. The capacity of as SCM system can beincreased by employing hybrid techniques that mix analog and digitalformats. Hybrid SCM systems can transmit a large number of channelsover the same fiber simultaneously. Hybrid SCM systems can transport upto 80 analog and 30 digital channels using a single optical transmitter.Such systems are affected by non linear mechanisms such as SPM andSBS, and clipping noise.Q. 8. Explain in detail code division multiplexing will necessarydiagrams.Ans. Code Division Multiplexing (CDM) is a technique in which eachchannel transmits its bits as a coded channel specific sequence of pulse.This coded transmission is accomplished by transmitting a unique timedependent series of short pulses, which are placed within chip timeswithin the larger bit time. CMD allows signals from a series ofindependent sources to be transmitted at the same time over the samefrequency band. This is accomplished by using orthogonal codes to spreadeach signal over a large common frequency band. At the receiver, theappropriate orthogonal code is than used again to recover the particularsignal intended for a particular user.Direct Sequence Encoding
In CDM systems, encoder and decoders are used at transmitter andreceiver ends. The encoder spreads the signal spectrum using spreadspectrum technique. The spread spectrum technique ha& followingfeatures(i) Each information bearing signal is transmitted with a band width inexcess of the minimum band width necessary to send the information.(ii) The bandwidth is increased by using a spreading code that isindependent of the information.(iii) The receiver has advance knowledge of the spreading code and usesthis knowledge to recover the information from the received spread outsignal. Spread spectrum has a advantage that the transmitted signal isundetectable by other receivers that do not know the spreading code.There are different methods which can be used for data coding such asdirect sequence encoding, time hopping and frequency hopping. DirectSequence Encoding is a modulation technique. It phase modulates a sinewave pseudo randomly with a continuous string of pseudo noise (PN)code symbols called “chips”, each of which has a much shorter durationthan an information bit. That is each information bit is modulated by asequence of much faster chips. Therefore, the chip rate is much higherthan the information signal bit rate. Direct sequence spread spectrumtransmissions multiply the data being transmitted by a “noise” signal. Thisnoise signal is a pseudorandom sequence of 1 and — 1 value, at afrequency much higher than that of the original signal, thereby spreadingthe energy of the original signal into a much wider band width. This noisesignal is used to reconstruct the original data at the receiving end bymultiplying it by same pseudorandom sequence. This process is known asde-spreading. For despreading to work correctly, the transmit and receivesequences must be synchronized.Spectral Encoding—spreading of spectrum can also be accomplished usingthe technique of frequency hopping. Frequency hopping spread spectrumis a method of transmitting radio signals by rapidly V switching a carrieramong many frequency channels using a pseudorandom sequence knownto both transmitter and receiver frequency hopping technique is differentfrom WDM as fixed frequency is not assigned to a given channel. Allchannels share the entire bandwidth by using different carrier frequenciesat different times according to a code. A spectrally encoded signal can berepresented in the form of a matrix as shown in figure 6.7.
The matrix columns correspond to time slots and matrix rows correspondbe assigned frequencies. The matrix element mix equals I if and only ifthe frequency w1 is transmitted in the interval t. Different users areassigned different frequency-hop patterns (or codes) to ensure that twousers do not transmit at the same frequency during V the same time slot.So orthogonal codes are used. Pseudo-orthogonal codes are used in thecase of asynchronous transmission.The overall bandwidth required for frequency hopping is much wider thanthat required to transmit the same information using only one carrierfrequency. However, because transmission occurs only on a small portionof this bandwidth at any given time, the effective interference bandwidthis really the same. The frequency hopping approach reduce thedegradation caused by narrowband interferes.Q. 9. With the help of block diagram briefly explain optical TDMsystems.Ans. Time division multiplexing is a type of analog or digital multiplexingin which two or more signals or bit streams are transferredsimultaneously as sub channels in one communication channel. The timedomain is divided into several recurrent timeslots of fixed length, one foreach sub channel. The electrical TDM becomes difficult to implement at bitrates above 10 Tb/s. To implement TOM at more than 10 Tb/s, opticalTOM is used. OTDM can increase the bit rate of a signal optical carrier toabove I Tb/s. In OTDM, the number of optical signals at a bit rate B sharethe same carrier frequency and are optically multiplexed to generate a bitstream at the bit rate NB where N is the number of channels.Optical TDM Principles
OTDM system is shown in figure 6.8. In this system four bit streams aremerged into one. Rz coding modulation techniques is used. The systemoperates as follows.Each time slot is subdivided into 4 bit times. Each bit time is furtherdivided into two halves. For a 1 bit the first half of the bit time will beoccupied by an optical pulse. For a 0 bit the whole bit time will be dark. Alaser produces a short pulse at the beginning of each time slot. The lasersignal is split 4 ways. Each signal is then delayed by a fixed amount. Theneach signal is separately modulated to carry its own unique informationstream. The signals.are then recombined to form a single data stream.During all this the original signal has lost a very large amount of power.So whole stream must be amplified to reach strength suitable fortransmission on the link.To demultiplex the OTDM data stream, we must extract each slow speeddata stream based on timing. Data for a single channel is extracted byusing NOLM (Non linear op6c op Minor).NOLM is used as an AND gate to select every nth pulse from the TDMstream.