MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI - 621213 DEPARTMENT : ECE SUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS SUBJECT CODE : EC 2402 1. Define SONET/SDH. [AUC NOV 2007] UNIT V: OPTICAL NETWORKS PART -A (2 Marks) Synchronous optical network(sonet) is the transport technology commonly used in backbone and customer access networks. SONET is a set of standards defining the rates and formats for optical networks. A similar standard SDH is established in Europe 2. List out the SONET layer? [AUC NOV 2012] Path layer Line layer Section layer Physical or photonic layer 3. Write the line rate for SONET STS-1 frame? [AUC NOV 2013] The SONET STS-1 frame has an actual line rate of 51.84 Mbps that can be calculated as: Line rate = 90 columns * 9 rows * 8 bits/byte * 8000 frames/sec = 51.84Mbps. 4. What are the SONET topologies? [AUC MAY 2010] Point-to-point configuration Hubbed configuration Point-to-multi p[oint configuration Ring configuration 5. Define solitons? [AUC MAY 2013] Solitons are narrow pulses with high peak powers and special shapes. The most commonly used soliton pulses are called fundamental solitons.
6. Define optical CDMA? [AUC NOV 2012] Optical code division multiplexing (OCDMA) is a process by which each. Communication channel is distinguished by a specific optical code rather than a wavelength, as in WDM, or a time-slot, as in TDM. 7. Write the path over head? [AUC MAY 2012] Its main function is map the signals in to a format required by a line layer. Their function includes reading modifying the POH and interpreting. 8. Define WDM. [AUC NOV 2012] WDM is wavelength division multiplexing. The optical beam consists of different wavelengths and several channel information is transmitted over a single channel. 9. Define broadcast and select network. [AUC MAY 2013] A broadcast and select network is second generation network in which each transmitter sends a message at a fixed frequency to the central star coupler through fiber path. 10. What are the SONET network management functions? [AUC April 2004, MAY2010] Performance management Fault management Configuration management Security management Accounting management 11. Difference between SONET and SDH? [AUC MAY2012] SONET It can be abbreviated as synchronous optical networking It was deployed in north America Its bit rate is 51.84 Mbps SDH It can be abbreviated as synchronous digital hierarchy It was deployed in Europe Its bit rate is 155.2 Mbps 12. What are the basic performances of WDM? [AUC MAY 2009] Insertion loss Channel width Cross talk
13. What are the techniques to reduce optical feedback? [AUC NOV 2008] Fiber end faces with a curved surface to the laser emitting facet. Index matching oil or gel at air glass interfaces. PC connectors Optical isolators within the transmitter module.. PART (B) 1. Explain the architecture of SONET? [AUC NOV 2010] The SONET standard as ultimately developed by ANSI defines a digital hierarchy with a base rate of 51.840 Mbit s 1, as shown in Table 15.1. The OC notation refers to the optical carrier level signal. Hence the base rate signal is OC-1. The STS level in brackets refers to a corresponding synchronous transport signal from which the optical carrier signal is obtained after scrambling (to avoid a long string of ones or zeros and hence enable clock recovery at receivers) and electrical to optical conversion.* Thus STS-1 is the basic building block of the SONET signal hierarchy. Higher level signals in the hierarchy are Optical network transmission modes, layers and protocols. Figure 15.9 European multiplexing hierarchy: (a) existing pleisochronous structure; (b) synchronous multiplexing obtained by byte interleaving (where a byte is 8 bits) an appropriate number of STS-1 signals in a similar manner to that described for the European standard PCM system.
This differs from the bit interleaving approach utilized in the existing North American digital hierarchy (see Table 12.1). The STS-1 frame structure shown in Figure 15.10 is precisely 125 μs and hence there are 8000 frames per second. This structure enables digital voice signal transport at 64 kbit s 1 (1 byte per 125 μs) and the North American DS1-24 channel (1.544 Mbit s 1), as well as the European 30- channel (2.048 Mbit s 1) signals (see Table 12.1) to be accommodated. Other signals in the two hierarchies can also be accommodated. The basic STS-1 frame structure illustrated in Figure 15.10 comprises nine rows, each of 90 bytes, which therefore provide a total of 810 bytes or 6480 bits per 125 μs frame. This results in the 51.840 Mbit s 1 base rate mentioned above. The first 3 bytes in each row of the STS-1 frame contain transport overhead bytes, leaving the remaining 783 bytes to be designated as the synchronous payload envelope (SPE). Apart from the first column (9 bytes) which is used for the path overhead, the remaining 774 bytes in the SPE constitute the SONET data payload. The transport overhead bytes are utilized for functions such as framing, scrambling, error monitoring, synchronization and multiplexing while the path overhead within the SPE is used to provide end-to-end communication between systems carrying digital voice, video and other signals which are Figure 15.10 STS-1 frame structure
to be multiplexed onto the STS-1 signal. In the latter case a path is defined to end at a point at which the STS-1 signal is created or taken apart (i.e. demultiplexed) into its lower bit rate signals. The STS-1 SPE does not have to be contained within a single frame; it may commence in one frame and end in another. A payload pointer within the transport overhead is employed to designate the beginning of the SPE within that frame. This provides the flexibility required in order to accommodate different bit rates and a variety of services. Moreover, to accommodate sub-sts-1 signal rates a virtual tributary (VT) structure is defined comprising four rates: 1.728 Mbit s 1 (VT 1.5); 2.304 Mbit s 1 (VT 2); 3.456 Mbit s 1 (VT 3); and 6.912 Mbit s 1 (VT 6). For example, it may be observed that the 1.544 Mbit s 1 and 2.048 Mbit s 1 signal streams can each be mapped into a VT 1.5 and VT 2 respectively. Finally, the higher order multiplexing of a number of STS-1 signals is obviously important in order to achieve the higher bit rates required for wideband services. The format of the STS-N signal frame is shown in Figure 15.11 which, as mentioned previously, is obtained by byte interleaving N STS-1 signals. In this case the transport overhead bytes of each STS-1 (i.e. the first three single-byte columns of each STS-1 signal shown in Figure 15.10)
are frame aligned to create the 3N bytes of transport overhead, which is illustrated in Figure 15.11. However, the SPEs do not require alignment since the service payload pointers within the associated transport overhead bytes provide the location for the appropriate SPEs. The synchronous digital hierarchy, as defined by the ITU-T [Refs 18 20], operates in the same manner as described above but differs in some of its terminology [Ref. 22]. In this case the 125 μs frame structure is referred to as a synchronous transport module (STM) and the base rate STM-1 is 155.520 Mbit s 1 which corresponds to OC-3 (STS-3), as may be observed from Table 15.2. Hence the European 140 (i.e. 139.264) Mbit s 1 pleisochronous signal can be mapped within an STM-1 signal when including a suitable overhead. 2. Write short notes on wavelength routed networks. [AUC NOV 2011] The optical layer is based on wavelength-dependent concepts when it lies directly above the physical layer. Hence the entire physical interconnected network provides wavelength signal service among the nodes using either single or multihop. This situation is illustrated in Figure 15.18. Three network nodes are interconnected using two wavelength channels (i.e. λ1 and λ2) where the solid line connecting the nodes represents the available wavelength channel and the dashed line identifies that the wavelength channel is in use.
If the network node 1 is required to connect with node 3 then as indicated there is no single wavelength channel available to establish a lightpath between them. When a light path cannot be established on a link using a single wavelength channel it is referred to as a wavelength continuity constraint. A methodology to reduce this wavelength continuity constraint is to switch the wavelength channel at node 2 by converting the incoming wavelength λ2 to λ1 (which is available between nodes 2 and 3) to enable a link between node 2 and 3 to be established. This process is shown in Figure 15.18(b). Wavelength conversion (see Section 10.5) is required to convert from λ2 to a compliant wavelength (i.e. λ1) at the output port of network node 2 (which functions as an intermediate node) in order to provide a path. Hence the newly set up path uses two wavelength stages (i.e. two hops) to interconnect nodes 1 and 3. Such networks which employ wavelength conversion devices (or switches) are known as wavelength convertible networks. Several network architectures can be employed to implement wavelength convertible networks. Three different WDM network architectures employing the wavelength conversion function are shown in Figure 15.19. Full wavelength conversion, where each network link utilizes a dedicated wavelength converter, is depicted in Figure 15.19(a). All the wavelength channels at the output port of the optical switch will be converted into their compliant wavelength channel by the appropriate wavelength converter (WC). For example, the topmost wavelength converter changes incoming λ1 into λ2 which is then connected to a multiplexer. There is no need, however, for wavelength conversion of the local add/drop channels. It is not always required to provide the wavelength conversion function within every network node and it is more cost effective to implement networks with fewer and hence shared wavelength converters. So-called sparse wavelength convertible network architectures employing a number of wavelength converters as a wavelength converter bank (WCB) functioning on a shared basis per link and per node are shown in Figure 15.19(b) and (c), respectively.
The arrangement of wavelength converters organized in a WCB is illustrated in the inset to Figure 15.19(b). This figure depicts a WCB servicing the optical fiber links where only the required wavelength channels are switched through the WCB (i.e. in Figure 15.19(b) the wavelength channel λ2 is converted to wavelength channel λ3). By contrast two optical switches are required to construct the shared per node wavelength convertible network architecture indicated in Figure 15.19(c). Optical switch 2 switches the converted wavelength channels to their designated nodes. channel λ2 is converted to wavelength channel λ3 via the shared WCB and is then switched through optical switch 2 to provide connection to the multiplexer). A large number of wavelength channels on the network links, however, increase the complexity of switching nodes in ordinary OXCs which switch traffic only at the wavelength level. Moreover, the complexity worsens when multigranular OXCs (MG-OXCs) are used where the traffic is required to be accessed at multiple levels (i.e. at granularities such as the fiber, wavelength, digital cross-connects, etc.). In these cases the MG-OXC output traffic does not simply either terminate at or transparently pass through a node, but may also be required to transport from one layer to another via multiplexers/demultiplexers. This complexity can be reduced if more wavelength channels are grouped into one single waveband to be switched as a unique channel. Such waveband switching (WBS)
networks have been proposed as a possible solution to ease the complexity of numerous wavelength-driven channels, especially in optical core networks.
3. Explain the concept of WDM. [AUC NVO 2010] WDM is wavelength division multiplexing. The optical beam consists of different wavelengths and several channel information is transmitted over a single channel. At the transmitting end, a multiplexer is needed to combine several optical outputs into a serial spectrum of closely spaced wavelength signals and couple them onto a single fiber. At the receiving end, a demultiplexer is required to separate the optical signals into appropriate detection channels for signal processing. At the transmitting end, the basic design challenge is to have the multiplexer provide a low-loss path from each optical source to the multiplexer output. The optical signals that are combined generally do not emit significant amount of optical power outside the designated channel spectral width, inter channel crosstalk factors therefore are relatively unimportant at the transmitting end. To prevent spurious signals from entering a receiving channel (i.e., to give good channel isolation of the different wavelengths being used), the demultiplexer must exhibit narrow spectral operation, or very stable optical filters with sharp wavelength cutoffs must be used. In general, a -10 db level is not satisfactory, whereas a level of -30 db is acceptable.
4. Write short notes on optical CDMA. [AUC NOV 2011] CDMA used extensively in radio frequency communication systems, especially in 2G and 3G cellular telephone networks. Basic Advantage is the way it handles a finite BW among a large number of users (more users can transmit the same data over the same Bandwidth) TDMA and WDMA schemes present significant drawbacks in Local Area Systems when large number of users must be considered. TDMA: one user tx at a time System capacity = users * tx rate WDMA: Four wave mixing as discussed (next slide) Optical CDMA does not need time and frequency management because all the users transmit using the whole BW at the same time! It can also operate asynchronously (as in wireless applications) without packet collisions. Slot allocation requirements are not needed here in contradiction to TDMA and WDMA
Simple implementation, using existing fiber networks Reduce the cost in every aspect: Equipment, outside plant Facilities, Operational Support systems SECURITY Eliminate many of intermediate time-division multiplexing steps required by SONET The principle is the same as in wireless application. Each user is assigned a unique code (spreading length -L-) which is multiplied by each bit. This code is only known to the receiver in order to demodulate the data. The most important part for correct detection is the code. This code must be uncorrelated from other user s codes and be orthogonal. O-CDMA divides the fiber spectrum into individual codes, all derived from a single broadband optical source (WDM divides the spectrum into narrow optical wavelengths) It is a simple 3 step process: Source Filter Modulator Filter: Spatial Filter can be thought an optical Bar code (fixed or programmable)
Optical CDMA is a broadcast technology, with all information going to all parts of the network. When a receiver is placed anywhere on the network with a bar code that matches a transmitter, that signal alone is decoded and extracted from the network. The second requirement for an all-optical network, the ability to economically add users. A simple tap and insert coupler is installed in the lateral fiber run to multiple users, and a receiver is installed at each terminating location
5. Write short notes on solitons? [AUC MAY 2012] Solitons are narrow pulses with high peak powers and special shapes. The most commonly used soliton pulses are called fundamental solitons. The shape of these However, the soliton pulses take advantage of nonlinear effects in silica, specifically self-phase modulation discussed in to overcome the pulse-broadening effects of group velocity dispersion. Thus these pulses can propagate for long distances with no change in shape. As mentioned in Section 2.4, and discussed in greater detail. A pulse propagates with the group velocity 1/β1 along the fiber, and in general, because of the effects of group velocity dispersion, the pulse progressively broadens as it propagates. If β2 = 0, all pulse shapes propagate without broadening, but if β2 _= 0, is there any pulse shape that propagates without broadening? The key to the answer lies in the one exception to this pulse-broadening effect that we already encountered in Section 2.4, namely, that if the chirp parameter of the pulse has the right sign (opposite to that of β2), the pulse initially undergoes compression. But we have seen that even in the pulse subsequently broadens. This happens in all cases where the chirp is independent of the pulse envelope. However, when the chirp is induced by SPM, the degree of chirp depends on the pulse envelope. If the relative effects of SPM and GVD are controlled just right, and the appropriate pulse shape is chosen, the pulse compression effect undergone by chirped pulses can exactly offset the pulse-broadening effect of dispersion. The pulse shapes for which this balance between pulse compression and broadening.
A pulse either undergoes no change in shape or undergoes periodic changes in shape only are called solitons. The family of pulses that undergo no change in shape are called fundamental solitons, and those that undergo periodic changes in shape are called higherorder solitons. A brief quantitative discussion of soliton propagation in optical fiber appear. The significance of solitons for optical communication is that they overcome the detrimental effects of chromatic dispersion completely.optical amplifiers can be used at periodic intervals along the fiber so that the attenuation undergone by the pulses is not significant, and the higher powers and the consequent soliton properties of the pulses are maintained. Solitons and optical amplifiers, when used together, offer the promise of very high-bit-rate, repeater less data transmission over very large distances. By the combined use of solitons and erbium-doped fiber amplifiers, repeater less data transmission at a bit rate of 80 Gb/s over a distance of 10,000 km has been demonstrated in the laboratory [NSK99].
6. Explain about the nonlinear effects. [AUC MAY 2013]. There are two categories of nonlinear effects. The first arises due to the interaction of light waves with phonons (molecular vibrations) in the silica medium one of several types of scattering effects, of which we have already met one, namely,rayleigh scattering. The two main effects in this category are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). The second set of nonlinear effects arises due to the dependence of the refractive index on the intensity of the applied electric field, which in turn is proportional to the square of the field amplitude. The most important nonlinear effects in this category are self-phase modulation (SPM) and four-wave mixing (FWM). In scattering effects, energy gets transferred from one light wave to another wave at a longer wavelength (or lower energy). The lost energy is absorbed by the molecular vibrations, or phonons, in the medium. (The type of phonon involved is different for SBS and SRS.) This second wave is called the Stokes wave. The first wave can be thought of as being a pump wave that causes amplification of the Stokes wave. As the pump propagates in the fiber, it loses power and the Stokes wave gains power. In the case of SBS, the pump wave is the signal wave, and the Stokes wave is the unwanted wave that is generated due to the scattering process. In the case of SRS, the pump wave is a high-power wave, and the Stokes wave is the signal wave that gets amplified at the expense of the pump wave. In general, scattering effects are characterized by a gain coefficient g, measured in meters per watt, and spectral width _f over which the gain is present. The gain coefficient is a measure of the strength of the nonlinear effect. In the case of selfphase modulation, the transmitted pulses undergo chirping. This induced chirp factor becomes significant at high power levels. We have already