15. 1.3.3 TDM versus WDM Network Evolution 16 x STM-1 16 x STM-1 N channels Colored signals with specific wavelengths 16 x STM-1 16 x STM-1 N terminals WDM STM-16 terminal STM-16 terminal STM-16 terminal STM-16 terminal 1R 1R 1R 1R N terminals 16 x STM-1 3R 3R 3R REGENERATOR REGENERATOR REGENERATOR 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 16 x STM-1 STM-16 terminal STM-16 terminal N terminals N terminals
16. 1.3.3 TDM versus WDM Unlimited Transmission Capacity WDM + TDM Year N x 2.5Gbps TDM Single Channel 128 x 2.5Gbps 64 x 2.5Gbps 32 x 2.5Gbps 16 x 25Gbps 8 x 25Gbps 4 x 25Gbps 2 x 25Gbps 8 x 10Gbps 16 x 10Gbps 40 x 10Gbps N x 10Gbps 200Gbps 100Gbps 10Gbps 140Mbps 565Mbps 2.5Gbps 10Gbps 40Gbps Global capacity per fiber (Gbps) 0.1 1 10 100 1000 1984 1986 1988 1990 1992 1994 1996 1998 2000 2001 2004
18. 2 Varieties of WDM 2.1 WDM Evolution 64 channels 50GHz spacing 64 channels 100-200GHz spacing Dense WDM, integrated systems with network management, add-drop functions 2-8 channels Narrowband WDM 200-400Ghz spacing Passive WDM components 2 channels Wideband WDM 1310nm, 1550nm Evolution of WDM Late 1990’s 1996 Early 1990’s 1980’s
19. 2 Varieties of WDM 2.2 Conventional WDM Definition Transmit Receive End system End system End system End system ( 1310nm + 1550nm ) ( 1310nm + 1550nm )
20. 2.3 DWDM Definition Absorption Spectrum of the Optical Fiber Wavelength (nm) Loss (dB/km) 800 1000 1200 1400 1600 1800 1 0.5 10 5 S C L
30. 3.4 Optical Amplifier Types RFA TDFA EDFA GS-TDFA EDTFA GS-EDFA L Band C Band 1575 1620 1560 1530 S Band S+ Band 1480 1420 Wavelength (nm)
31. 3.4 Optical Amplifier Erbium-Doped Fiber Amplifier Active fiber doped with Erbium ions Ground state Excited state Energy supply (pumping) Spontaneous emission Stimulated emission electron Noise generation Optical amplification photon Signal in Signal in Signal out Pump coupler Pump laser diode ERBIUM-DOPED FIBER AMPLIFIER
36. Answer the Questions (1/8) Is the following statement true or false? WDM systems cost more than installing more fibers. Select the correct answer. True False
37. Answer the Questions (2/8) What is the multiplexing concept used for SDH? Select the correct answer. TDM SDM WDM
38. Answer the Questions (3/8) Is the following statement true or false? WDM systems are input format independent. Select the correct answer. True False
39. Answer the Questions (4/8) Which band is in the third optical window? Select the correct answer. A C B S L
40. Answer the Questions (5/8) Associate each wavelength spacing to the correct multiplexing technology. To do this, draw a line between each wavelength spacing from the list on the left and the appropriate technology from the list on the right. 100GHz 50GHz 3000GHz DWDM WDM 400GHz CWDM
41.
42. Answer the Questions (7/8) Is the following statement true or false? The main function of the transponder is to convert the B&W wavelength to a colored wavelength. Select the correct answer. True False
43. Answer the Questions (8/8) What is the amplification range for an EDFA? Select the correct answer. 1530-1610nm 1490-1530nm 1530-1565nm
It is the nature of modern communications networks to be in a state of ongoing evolution. Factors such as new applications, changing patterns of usage and redistribution of content make the definition of networks a work in progress. Nevertheless, we can broadly define the larger entities that make up the global network based on variables such as transport technology, distance, applications and so on. One way of describing the Metropolitan Area Network (MAN) would be to say that it is neither the long-haul nor the access parts of the network, but the area that lies between those two. Long-Haul Networks Long-haul networks are at the core of the global network. Dominated by a small group of large transnational and global carriers, long-haul networks connect the MANs. Their application is transport, so their primary concern is capacity. In many cases these networks, which have traditionally been based on Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) technology, are experiencing fiber exhaust as a result of high bandwidth demand. Access Networks At the other end of the spectrum are the access networks. These networks are the closest to the end users, at the edge of the MAN. They are characterized by diverse protocols and infrastructures and they span a broad spectrum of rates. Customers range from residential Internet users to large corporations and institutions. The predominance of IP traffic, with its inherently bursty, asymmetric and unpredictable nature, presents many challenges, especially with new real-time applications. At the same time, these networks are required to continue to support legacy traffic and protocols, such as IBM’s Enterprise Systems CONnection (ESCON). Metropolitan Area Networks (MANs) Between these two large and different networking domains lie the MANs. These networks channel traffic within the metropolitan domain (among businesses, offices and metropolitan areas) and between large long-haul Points Of Presence (POPs). The MANs have many of the same characteristics as the access networks, such as diverse networking protocols and channel speeds. Like access networks, MANs have been traditionally SONET/SDH based, using point-to-point or ring topologies with Add/Drop Multiplexers (ADMs).
The explosion in demand for network bandwidth is largely due to the growth in data traffic, specifically Internet Protocol (IP). Leading service providers report bandwidths doubling on their backbones about every six to nine months. This is largely in response to the 300 percent growth per year in Internet traffic, while traditional voice traffic grows at a compound annual rate of only about 13 percent. At the same time that network traffic volume is increasing, the nature of the traffic itself is becoming more complex. Traffic carried on a backbone can originate as circuit based (TDM voice and fax), packet based (IP) or cell based (ATM and Frame Relay). In addition, there is an increasing proportion of delay sensitive data, such as voice over IP and streaming video. In response to this explosive growth in bandwidth demand, along with the emergence of IP as the common foundation for all services, long-haul service providers are moving away from TDM-based systems, which were optimized for voice but now prove to be costly and inefficient. Meanwhile, metropolitan networks are also experiencing the impact of growing congestion, as well as rapidly changing requirements that call for simpler and faster provisioning than is possible with older equipment and technologies. Of key importance in the metropolitan area is the growth in Storage Area Networks (SANs).
Faced with the challenge of dramatically increasing capacity while constraining costs, carriers have two options: either install new fibers or increase the effective bandwidth of existing fibers. Laying new fibers is the traditional means used by carriers to expand their networks. Deploying new fibers, however, is a costly proposition. Laying new fibers may make sense only when it is desirable to expand the embedded base. Increasing the effective capacity of existing fibers can be accomplished in two ways: Increase the bit rate of existing systems. Using TDM, data is now routinely transmitted at 2.5Gbps (STM-16) and, increasingly, at 10Gbps (STM-64). Recent advances have resulted in speeds of 40Gbps (STM-256). The electronic circuitry that makes this possible, however, is complex and costly, both to purchase and to maintain. In addition, there are significant technical issues that may restrict the applicability of this approach. Transmission at STM-64 over Single-Mode (SM) fiber, for example, is 16 times more affected by Chromatic Dispersion (CD) than the next lower aggregate speed, STM-16. The greater transmission power required by the higher bit rates also introduces nonlinear effects that can affect waveform quality. Finally, Polarization Mode Dispersion (PMD), another effect that limits the distance a light pulse can travel without degradation, is also an issue. These characteristics of light in fiber are discussed in the Optical Fiber Transmission – Fundamentals training. Increase the number of wavelengths on a fiber. In this approach, many wavelengths are combined onto a single fiber. Using Wavelength Division Multiplexing (WDM) technology, several wavelengths, or light colors, can simultaneously multiplex signals of 2.5 to 40Gbps each over a strand of fiber. Without having to lay new fibers, the effective capacity of existing fiber plant can routinely be increased by a factor of 16 or 32. Systems with 128 and 160 wavelengths are in operation today, with higher density on the horizon. The specific limits of this technology are not yet known.
Time Division Multiplexing (TDM) was invented as a way of maximizing the amount of voice traffic that could be carried over a medium. In the telephone network before multiplexing was invented, each telephone call required its own physical link. This proved to be an expensive and unscalable solution. Using multiplexing, more than one telephone call could be put on a single link. TDM can be explained by an analogy to highway traffic. To transport all the traffic from four tributaries to another city, you can send all the traffic on one lane, providing the feeding tributaries are fairly serviced and the traffic is synchronized. So, if each of the four feeds puts a car onto the trunk highway every four seconds, then the trunk highway would get a car at the rate of one each second. As long as the speed of all the cars is synchronized, there would be no collision. At the destination, the cars can be taken off the highway and fed to the local tributaries by the same synchronous mechanism, in reverse. This is the principle used in synchronous TDM when sending bits over a link. TDM increases the capacity of the transmission link by slicing time into smaller intervals so that the bits from multiple input sources can be carried on the link, effectively increasing the number of bits transmitted per second. With TDM, input sources are serviced in round-robin fashion. Though fair, this method results in inefficiency, because each time slot is reserved even when there is no data to send. This problem is mitigated by the statistical multiplexing used in Asynchronous Transfer Mode (ATM). Although ATM offers better bandwidth utilization, there are practical limits to the speed that can be achieved due to the electronics required for Segmentation And Reassembly (SAR) of ATM cells that carry packet data.
The telecommunications industry adopted the Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely related standards that specify interface parameters, rates, framing formats, multiplexing methods and management for synchronous TDM over fiber. SONET/SDH takes n bit streams, multiplexes them and optically modulates the signal, sending it out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) x n. Thus traffic arriving at the SONET/SDH multiplexer from four places at 2.5Gbps will go out as a single stream at 4 x 2.5Gbps, or 10Gbps. This principle is illustrated in the above figure, which shows an increase in the bit rate by a factor of four in time slot T.
WDM increases the carrying capacity of the physical medium (fiber) using a completely different method from TDM. WDM assigns incoming optical signals to specific frequencies of light (wavelengths or lambdas) within a certain frequency band. This multiplexing closely resembles the way radio stations broadcast on different wavelengths without interfering with each other. Because each channel is transmitted at a different frequency, we can select from them using a tuner. Another way to think about WDM is that each channel is a different color of light. Several channels then make up a "rainbow". In a WDM system, each of the wavelengths is launched into the fiber and the signals are demultiplexed at the receiving end. Like TDM, the resulting capacity is an aggregate of the input signals, but WDM carries each input signal independently of the others. This means that each channel has its own dedicated bandwidth. All signals arrive at the same time, rather than being broken up and carried in time slots.
A fundamental difference between SDH TDM and WDM is that WDM can carry multiple protocols without a common signal format, while SDH cannot.
SDH TDM takes synchronous and asynchronous signals and multiplexes them to a single higher bit rate for transmission at a single wavelength over fiber. Source signals may have to be converted from electrical to optical, or from optical to electrical and back to optical before being multiplexed. WDM takes multiple optical signals, maps them to individual wavelengths and multiplexes the wavelengths over a single fiber. From both technical and economic perspectives, the ability to provide potentially unlimited transmission capacity is the most obvious advantage of WDM technology. The current investment in fiber plant cannot only be preserved, but optimized by a factor of at least 32. As demands change, more capacity can be added, either by simple equipment upgrades or by increasing the number of lambdas (wavelengths) on the fiber, without expensive upgrades. Capacity can be obtained for the cost of the equipment whereas existing fiber plant investment is retained. Bandwidth aside, the most compelling technical advantages of WDM can be summarized as follows: Transparency: Because WDM is a physical layer architecture, it can transparently support both TDM and data formats such as ATM, Gigabit Ethernet, ESCON and Fiber Channel with open interfaces over a common physical layer. Scalability: WDM can have leverage with the abundance of dark fiber in many metropolitan area and enterprise networks to quickly meet the demand for capacity on point-to-point links and on spans of existing SONET/SDH rings. Dynamic provisioning: Fast, simple and dynamic provisioning of network connections give providers the ability to provide high-bandwidth services in days rather than months.
Varieties of WDM Early WDM systems transported two or four wavelengths that were widely spaced. WDM and the “follow-on” technologies of CWDM and DWDM have evolved well beyond this early limitation. Conventional WDM Traditional, passive WDM systems are wide-spread with 2, 4, 8, 12 and 16 channel counts being the normal deployments. This technique usually has a distance limitation of under 100km. Coarse WDM (CWDM) Today, CWDM typically uses 20-nm spacing (3000GHz) of up to 18 channels. The CWDM Recommendation ITU-T G.694.2 provides a grid of wavelengths for target distances up to about 50km on single-mode fibers as specified in ITU-T Recommendations G.652, G.653 and G.655. The CWDM grid is made up of 18 wavelengths defined within the range 1270nm to 1610nm spaced by 20nm. Dense WDM (DWDM) Dense WDM common spacing may be 200, 100, 50 or 25GHz with channel count reaching up to 128 or more channels at distances of several thousand kilometers with amplification and regeneration along such a route.
Early WDM began in the late 1980s using the two widely spaced wavelengths in the 1310-nm and 1550-nm (or 850-nm and 1310-nm) regions, sometimes called wideband WDM. The above figure shows an example of this simple form of WDM. Notice that one of the fiber pair is used to transmit and one is used to receive. This is the most efficient arrangement. The early 1990s saw a second generation of WDM, sometimes called narrowband WDM, in which two to eight channels were used. These channels were spaced at an interval of about 400GHz in the 1550-nm window.
Based on optical power loss of fibers, spectrum ranges have been characterized for compatibility purposes with light sources, receivers and optical components, including the optical fiber. Three optical windows have been used in optical transmission: the first window at 850nm, the second window at 1300nm, the third window at 1550nm. According to the broad absorption minimum, the third window is best suited for DWDM technology. A fourth window has been defined at 1600nm. For DWDM transmission systems, three optical bands are defined: the S (Short) band: 1460 to 1530nm. the C (Conventional) band: 1530 to 1565nm (third window). the L (Long) band: 1565 to 1625nm (fourth window).
The ITU approved DWDM band extends from 1528.77nm to 1563.86nm and divides into the red band and the blue band. The red band encompasses the longer wavelengths of 1546.12nm and higher. The blue band wavelengths fall below 1546.12nm. This division has a practical value because useful gain region of the lowest cast Erbium-Doped Fiber Amplifiers (EDFAs) corresponds to the red band wavelengths. Thus, if a system only requires a limited number of DWDM wavelengths using the red band, wavelength yields the lowest overall system cost.
Dense WDM common spacing may be 200, 100, 50 or 25GHz with channel count reaching up to 128 or more channels at distances of several thousand kilometers with amplification and regeneration along such a route.
Today, Coarse WDM (CWDM) typically uses 20-nm spacing (3000GHz) of up to 18 channels. The CWDM Recommendation ITU-T G.694.2 provides a grid of wavelengths for target distances up to about 50km on single-mode fibers as specified in ITU-T Recommendations G.652, G.653 and G.655. The CWDM grid is made up of 18 wavelengths defined within the range 1270nm to 1610nm spaced by 20nm. Originally, the term "Coarse Wavelength Division Multiplexing" was fairly generic and meant a number of different things. In general, these things shared the fact that the choice of channel spacing and frequency stability was such that Erbium-Doped Fiber Amplifiers (EDFAs) could not be used. Prior to the relatively recent ITU standardization of the term, one common meaning for Coarse WDM meant two (or possibly more) signals multiplexed onto a single fiber, where one signal was in the 1550-nm band and the other in the 1310-nm band. Recently the ITU has standardized a 20-nanometer channel spacing grid for use with CWDM, using the wavelengths between 1310nm and 1610nm. Many CWDM wavelengths below 1470nm are considered "unusable" on older G.652 spec fibers, due to the increased attenuation in the 1310-1470nm bands. The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs. This therefore limits the total CWDM optical span to somewhere near 60km for a 2.5Gbps signal, which is suitable for use in metropolitan applications. The relaxed optical frequency stabilization requirements allow the associated costs of CWDM to approach those of non-WDM optical components. CWDM is also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310nm while the upstream signal is at 1550nm.
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An optical network using DWDM techniques consists of five main components: A transmitter (transmit transponder) that: Changes electrical bits into optical pulses. Is frequency specific. Uses a narrowband laser to generate the optical pulse. A multiplexer and a demultiplexer that combines/separates discrete wavelengths on each transmit and receive side. Amplifiers: A pre-amplifier boosts signal pulses at the receive side. A post-amplifier , also called booster, boosts signal pulses at the transmit side (post-amplifier) and on the receive side (pre-amplifier). In-line amplifiers (ILAs) are placed at different distances from the source to provide recovery of the signal before it is degraded by loss. An optical fiber (media): It is the transmission media to carry optical pulses. Many different kinds of fiber are used. A receiver (receive transponder) that: Changes optical pulses back into electrical bits. Uses wideband laser to provide the optical pulse.
B&W Client Interface The function of the client interface is: to receive the optical signal incoming into the DWDM Network Element (NE). perform optical to electrical conversion and data recovery. to provide an outgoing optical signal from the DWDM NE, suitable for the client equipment. DWDM Interface The function of the DWDM interface is: to receive the optical signal incoming on the DWDM line. to perform optical to electrical conversion. to provide an outgoing optical signal from the DWDM NE, suitable with the ITU-T DWDM grid. If you want more details about the electrical to optical conversion, refer to the Optical Fiber Transmission – Fundamentals training.
Because DWDM systems send signals from several sources over a single fiber, they must include some means to combine the incoming signals. This is done with a multiplexer, which takes optical wavelengths from multiple fibers and converges them into one beam. At the receiving end, the system must be able to separate out the components of the light so that they can be discreetly detected. Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them to individual fibers. Demultiplexing must be done before the light is detected, because photodetectors are inherently broadband devices that cannot selectively detect a single wavelength. In a unidirectional system, there is a multiplexer at the sending end and a demultiplexer at the receiving end. Two systems would be required at each end for bidirectional communication and two separate fibers would be needed. In a bidirectional system, there is a multiplexer-demultiplexer at each end. Communication is over a single fiber, with different wavelengths used for each direction. Multiplexers and demultiplexers can be either passive or active in design. Passive designs are based on prisms, diffraction gratings or filters, while active designs combine passive devices with tunable filters. The primary challenges in these devices is to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength.
Post-Amplifiers Post-amplifiers are placed directly after the optical transmitter. This application requires the EDFA to take a large signal input and provide the maximum output level. Small signal response is not as important because the direct transmitter output is usually -10dBm or higher. The noise added by the amplifier at this point is also not as critical because the incoming signal has a large Signal-to-Noise Ratio (SNR). Post-amplifiers are also referred to as: power amplifiers, power boosters, boosters, booster amplifiers. In-Line Amplifiers (ILAs) In-line amplifiers, also called repeaters , modify a small input signal and boost it for retransmission down the fiber. Controlling the small signal performance and noise added by the EDFA reduces the risk of limiting a system’s length due to the noise produced by the amplifying components. Pre-Amplifiers Past receiver sensitivity of -30dBm at 622Mbps was acceptable. However, presently, the demands require sensitivity of -40dBm or -45dBm. This performance can be achieved by placing an Optical Amplifier (OA) prior to the receiver. Boosting the signal at this point presents a much larger signal into the receiver, thus easing the demands of the receiver design. This application requires careful attention to the noise added by the EDFA. The noise added by the amplifier must be minimal to maximize the received SNR.
Optical amplifiers include rare-earth elements to make rare-earth-doped fibers such as: Erbium. Tellurite. Thulium. Most amplifiers are still experimental and include: EDFA: Erbium-Doped Fiber Amplifier (1530–1565nm). GS-EDFA: Gain-Shifted EDFA (1570–1610 nm). EDTFA: Erbium-Doped Tellurite-based Fiber Amplifier (1530–1610nm). GS-TDFA: Gain-Shifted Thulium-Doped Fiber Amplifier (1490–1530nm). TDFA: Thulium-Doped Fluoride-based Fiber Amplifier (1450–1490nm). The Raman amplification principle is different from previous amplifiers and is mainly used in DWDM unrepeatered links. The amplifier used is called Raman Fiber Amplifier (RFA) (1420–1620nm or more).
Erbium-Doped Fiber Amplifiers (EDFAs) provide the gain mechanism for optical amplification. Optical systems use erbium amplifiers because they work well and are very efficient as amplifiers in the 1500-nm range. Only a few parts per billion of erbium are needed. Erbium is a rare-earth element that, when excited, emits light around 1.54 micrometers (the low-loss wavelength for optical fibers). A weak signal enters the erbium-doped fiber, into which light at 980nm or 1480nm is injected using a pump laser. This injected light stimulates the erbium atoms to release their stored energy as additional 1550-nm light. As this process continues down the fiber, the signal grows stronger. The spontaneous emissions in the EDFA also add noise to the signal. This determines the noise figure of an EDFA.
A Raman amplifier uses intrinsic properties of silica fibers to obtain signal amplification. This means that transmission fibers can be used as a medium for amplification, and hence that the intrinsic attenuation of data signals transmitted over the fiber can be combated within the fiber. An amplifier working on the basis of this principle is commonly known as a Distributed Raman Amplifier (DRA) . The physical property behind DRAs is called Stimulated Raman Scattering (SRS) . This occurs when a sufficiently large pump wave is co-launched at a lower wavelength than the signal to be amplified. The Raman gain depends strongly on the pump power and the frequency offset between pump and signal. Amplification occurs when the pump photon gives up its energy to create a new photon at the signal wavelength, plus some residual energy, which is absorbed as phonons (vibration energy). As there is a wide range of vibration states above the ground state, a broad range of possible transitions are providing gain. Generally, Raman gain increases almost linearly with wavelength offset between signal and pump peaking at about 100nm and then dropping rapidly with increased offset. The position of the gain bandwidth within the wavelength domain can be adjusted simply by tuning the pump wavelength. Thus, Raman amplification potentially can be achieved in every region of the transmission window of the optical transmission fiber. It only depends on the availability of powerful pump sources at the required wavelengths. The disadvantage of Raman amplification is the need for high pump powers to provide a reasonable gain. This opens a new range of possible applications. It is possible, for instance, to partially compensate fiber attenuation using the Raman effect and, thus, to increase the EDFA spacing. The Raman pump wave can be conveniently placed at the EDFA locations. This saves costs as less EDFAs are needed on the link and the number of sites to be maintained is reduced.
Between multiplexing and demultiplexing points in a DWDM system, there is an area in which multiple wavelengths exist. It is often desirable to be able to remove or insert one or more wavelengths at some point along this span. An Optical Add-Drop Multiplexer (OADM) performs this function. Rather than combining or separating all wavelengths, the OADM can remove some while passing others on. OADMs are a key part of moving toward the goal of all-optical networks. OADMs are similar in many respects to SDH Add-Drop Multiplexers (ADMs), except that only optical wavelengths are added and dropped, and no conversion of the signal from optical to electrical takes place. The above figure is a schematic representation of the add-drop process. This example includes both pre- and post-amplifiers. These components that may or may not be present in an OADM, depending upon its design. There are two general types of OADMs. The first generation is a fixed device that is physically configured to drop specific predetermined wavelengths while adding others. The second generation is reconfigurable and capable of dynamically selecting which wavelengths are added and dropped. Thin-film filters have emerged as the technology of choice for OADMs in current metropolitan DWDM systems because of their low cost and stability. For the emerging second generation of OADMs, other technologies, such as tunable fiber gratings and circulators, will come into prominence.
