Name of Course : E1-E2 CFA Chapter 15 Topic : DWDM Date of Creation : 28.03.2011
DWDM 1.0 Introduction The emergence of DWDM is one of the most recent and important phenomena in the development of fiber optic transmission technology. Dense wavelength-division multiplexing (DWDM) revolutionized transmission technology by increasing the capacity signal of embedded fiber. One of the major issues in the networking industry today is tremendous demand for more and more bandwidth. Before the introduction of optical networks, the reduced availability of fibers became a big problem for the network providers. However, with the development of optical networks and the use of Dense Wavelength Division Multiplexing (DWDM) technology, a new and probably, a very crucial milestone is being reached in network evolution. The existing SONET/SDH network architecture is best suited for voice traffic rather than today s high-speed data traffic. To upgrade the system to handle this kind of traffic is very expensive and hence the need for the development of an intelligent all-optical network. Such a network will bring intelligence and scalability to the optical domain by combining the intelligence and functional capability of SONET/SDH, the tremendous bandwidth of DWDM and innovative networking software to spawn a variety of optical transport, switching and management related products. 2.0 Development of DWDM Technology 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 early 1990s saw a second generation of WDM, sometimes called narrowband WDM, in which two to eight channels were used. These channels were now spaced at an interval of about 400 GHz in the 1550-nm window. By the mid-1990s, dense WDM (DWDM) systems were emerging with 16 to 40 channels and spacing from 100 to 200 GHz. By the late 1990s DWDM systems had evolved to the point where they were capable of 64 to 160 parallel channels, densely packed at 50 or even 25 GHz intervals. As fig. 1 shows, the progression of the technology can be seen as an increase in the number of wavelengths accompanied by a decrease in the spacing of the wavelengths. Along with increased density of wavelengths, systems also advanced in their flexibility of configuration, through add-drop functions, and management capabilities. BSNL, India For Internal Circulation Only Page:1
3.0 Varieties of WDM Figure 1 Evolution of DWDM 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. 3.1 WDM Traditional, passive WDM systems are widespread with 2, 4, 8, 12, and 16 channel counts being the normal deployments. This technique usually has a distance limitation of less than 100 km. 3.2 CWDM Today, coarse WDM (CWDM) typically uses 20-nm spacing (3000 GHz) of up to 18 channels. The CWDM Recommendation ITU-T G.694.2 provides a grid of wavelengths for target distances up to about 50 km 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 1270 nm to 1610 nm spaced by 20 nm. BSNL, India For Internal Circulation Only Page:2
3.3 DWDM Dense WDM common spacing may be 200, 100, 50, or 25 GHz with channel count reaching up to 128 or more channels at distances of several thousand kilometers with amplification and regeneration along such a route. 4.0 DWDM System Function DWDM stands for Dense Wavelength Division Multiplexing, an optical technology used to increase Bandwidth over existing fiber optic backbones. Dense wavelength division multiplexing systems allow many discrete transports channels by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers. So, if you were to multiplex 32 STM-16 signals into one fiber, you would increase the carrying capacity of that fiber from 2.5 Gb/s to 80 Gb/s. Currently, because of DWDM, single fibers have been able to transmit data at speeds up to 400Gb/s. A key advantage to DWDM is that it's protocol and bit rate-independent. DWDMbased networks can transmit data in SDH, IP, ATM and Ethernet etc. Therefore, DWDMbased networks can carry different types of traffic at different speeds over an optical channel. DWDM is a core technology in an optical transport network. Dense WDM common spacing may be 200, 100, 50, or 25 GHz with channel count reaching up to 128 or more channels at distances of several thousand kilometers with amplification and regeneration along such a route. 1 2 : 32 1 2 32.. Fig. 2 Block Diagram of a DWDM System BSNL, India For Internal Circulation Only Page:3
The concepts of optical fiber transmission, loss control, packet switching, network topology and synchronization play a major role in deciding the throughput of the network. 5.0 Transmission Windows Today, usually the second transmission window (around 1300 nm) and the third and fourth transmission windows from 1530 to 1565 nm (also called conventional band) and from 1565 to 1625 nm (also called Long Band) are used. Technological reasons limit DWDM applications at the moment to the third and fourth window. The losses caused by the physical effects on the signal due by the type of materials used to produce fibers limit the usable wavelengths to between 1280 nm and 1650 nm. Within this usable range the techniques used to produce the fibers can cause particular wavelengths to have more loss so we avoid the use of these wavelengths as well. 6.0 DWDM System Components Figure 3 shows an optical network using DWDM techniques that consists of five main components: 1. Transmitter (transmit transponder): - Changes electrical bits to optical pulses - Is frequency specific - Uses a narrowband laser to generate the optical pulse 2. Multiplexer/ demultiplexer: - Combines/separates discrete wavelengths 3. Amplifier: - Pre-amplifier boosts signal pulses at the receive side - Post-amplifier boosts signal pulses at the transmit side (post amplifier) and on the receive side (preamplifier) - In line amplifiers (ILA) are placed at different distances from the source to provide recovery of the signal before it is degraded by loss. - EDFA (Eribium Doped Fiber Amplifier) is the most popular amplifier. 4. Optical fiber (media): - Transmission media to carry optical pulses - Many different kinds of fiber are used - Often deployed in sheaths of 144 256 fibers 5. Receiver (receive transponder) - Changes optical pulses back to electrical bits - Uses wideband laser to provide the optical pulse BSNL, India For Internal Circulation Only Page:4
Figure 3: DWDM System Components 5.0 Benefits of DWDM Increases bandwidth (speed and distance) Does not require replacement or upgrade their existing legacy systems Provides "next generation" technologies to meet growing data needs Less costly in the long run because increased fiber capacity is automatically available; don't have to upgrade all the time 6.0 Conclusion DWDM promises to solve the "fiber exhaust" problem and is expected to be the central technology in the all-optical networks of the future. This increase means that the incoming optical signals are assigned to specific wavelengths within a designated frequency band, and then multiplexed onto one fiber. This process allows for multiple video, audios, and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems. This technology responds to the growing need for efficient and capable data transmission by working with different formats, such as SONET/SDH, while increasing bandwidth. xxxx BSNL, India For Internal Circulation Only Page:5