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Wavelength-division multiplexing is the process of multiplexing wavelengths of different frequencies onto a single fiber. This operation creates many virtual fibers, each capable of carrying a different signal. Figure 4-1 shows a schematic of a bidirectional WDM system. This system has n service interfaces and n wavelengths transmitted in either direction over a single fiber. Each wavelength operates at a different frequency.
The above model can be defined as there are multiple transmitting nodes named as Tx1, Tx2….up to Txn. They are all sending there traffic in the form of wavelengths through the one block named as MUX. After all wavelengths are merged to a single wavelength MUX generates a single output placed on the single fiber. Now as we know that the generated signal is of light nature, so there are no overhead bytes can be added, as WDM is Transparent system so system will not process the data at any intermediate node, therefore OSC is separately added to the WDM signal so it will perform the supervision of the signal throughout the journey. After covering some length it is natural that signal will be weakened and need some amplification so to meet this situation OA is added, it will regenerate and amplify the received signal. The point to note is there total four OSCs are shown in the DWDM typical model. It is because OSC signal is not part of the message signal and it will be added when optical channel starts and will be discarded from original signal when signal needs amplification, but again it will be added after the signal is amplified, and once again and it will be discarded when signal is received at DEMUX point. DEMUX will separate the different wavelengths and then each wavelength is received by its concerned Rx1, Rx2….Rxn.
WDM uses wavelengths to transmit data parallel-by-bit or serial-by-character, which increases the capacity of the fiber by assigning incoming optical signals to specific frequencies (wavelengths) within a designated frequency band and then multiplexing the resulting signals out onto one fiber. Each signal can be carried at a different rate (OC-3/STM-1, OC-48/STM-16, and so on) and in a different format (SONET/SDH, ATM, data, and so on). This can increase the capacity of existing networks without the need for expensive recabling and tremendously reduce the cost of network infrastructure upgrades. WDM supports point-to-point, ring, and mesh topologies. Existing fiber in a SONET/SDH fiber plant can be easily migrated to WDM. Most WDM systems support standard SONET/SDH short-reach optical interfaces to which any SONET/SDH-compliant client device can attach. Long-haul WDM topologies are typically point to point. Perhaps the biggest reason for implementing WDM is the deployment speed of bandwidth service delivery. It is much easier to add a wavelength than to trench and add new fiber.
Four kinds of WDM systems are available:
Metro WDM (<200 km)
Long-haul or regional WDM (200 km to 800 km)
Extended long-haul WDM (800 km to 2000 km)
Ultra-long-haul WDM (>2000 km)
2.2 WDM FUNDAMENTALS.
WDM can be considered a form of frequency-division multiplexing (FDM) coupled with timed-division multiplexing (TDM), as depicted in Figure 2.2. The exact relationship between a WDM wavelength and frequency is determined from the equation c = ¬ * f; where c is the speed of light in a vacuum (3 * 108 m/s), ¬ is the wavelength measured in a vacuum; and f is the frequency. Frequency is standardized (rather than wavelength) because it is independent of the transmission medium, such as fiber or air. In WDM systems, the wavelength is measured in nanometers (nm) and the frequency is measured in gigahertz (GHz). Light travels considerably slower in a denser medium. The speed of light in glass is approximately 2 * 108 m/s.
Fig. 2.2: WDM Depicted as a Combination of FDM and TDM
Various frequencies of light can travel down a single fiber, and each frequency can constitute a channel. However, transmitting light of various frequencies without some kind of clocking and pulsing isn't of much use in a digital communications system. Breaking down each lambda into time slots with framing and proper clocking mechanisms (similar to SONET/SDH) enables us to transmit information in the form of timed and framed pulses over a wavelength. Now, imagine a single wavelength capable of carrying an OC-192/STM-64 or roughly 10 Gbps worth of information. If we inject 80 lambdas over the same fiber, its bandwidth potential increases by a factor of 80, and the fiber will be able to carry up to 800 Gbps worth of information over a single fiber. In full-duplex mode, the resulting bandwidth would be 1.60 Tbps.
A few constraints are imposed by the laws of physics, in terms of injecting so many wavelengths into a fiber core. The diameter of the fiber core (8 m for SMF), laser spectral width, channel spacing, the spectrum of light that can be used (C, L, or S bands), and linear and nonlinear impairments. The power of WDM resides in its potential to carry massive quantities of data, up to terabit levels. By varying the TDM signaling and framing, WDM can be customized to carry various kinds of application traffic from simple SONET/SDH to Gigabit Ethernet as well as SAN protocols, such as ESCON, FICON, and Fiber channel. The optical frequency bands used with various WDM systems are as follows:
O-band (original)- A range from 1260 nm to 1360 nm
E-band (extended)- A range from 1360 nm to 1460 nm
S-band (short wavelength)- A range from 1460 nm to 1530 nm
C-band (conventional)- A range from 1530 nm to 1565 nm
L-band (long wavelength)- A range from 1565 nm to 1625 nm
U-band (ultra-long wavelength)- A range from 1625 nm to 1675 nm
Standard SMF (ITU G.652) is recommended for use with O-band WDM systems. Low-water-peak fiber (ITU G.652.C) is recommended for use with E-band WDM systems, and nonzero dispersion-shifted fiber (ITU G.655) is recommended for use with S-, C-, and L-band WDM systems.
2.3 UNIDIRECTIONAL WDM
Unidirectional WDM systems multiplex a number of wavelengths for transmission in one direction on a single fiber. For example, signals at various wavelengths in the C-band are multiplexed together for transmission over a single fiber. The receiver receives multiplexed wavelengths on a separate fiber. The end-WDM device is responsible for demultiplexing the wavelengths and feeding them to the appropriate receiver. Figure 2.3 shows unidirectional WDM. Unidirectional WDM systems are very common with cable providers who transmit multicast traffic to downstream receiving stations.
Fig. 2.3: Unidirectional WDM
2.4 BIDIRECTIONAL WDM
A bidirectional WDM system transmits and receives multiple wavelengths over the same fiber. For example, signals at various wavelengths in the 1550-nm band are multiplexed together for transmission over a single fiber. At the same time, separate wavelengths in the 1550-nm band are also received over the same fiber. The end-WDM device is responsible for multiplexing and demultiplexing the wavelengths from and to their respective transmitters and receivers. Figure 2.4 shows a bidirectional WDM system.
Fig. 2.4: Bidirectional WDM
2.4.1 BIDIRECTIONAL WDM TECHNIQUES.
Various techniques are used to achieve full-duplex bidirectional transmission over a single fiber. Basically, the counter-propagating signals on the same fiber have to be separated by using suitable devices. Figure 2.5 shows three methods used to achieve bidirectional transmission.
Fig. 2.5: Bidirectional WDM Techniques
220.127.116.11 Band separation method.
In this method, the transmitted channels are divided in two or four groups known as sub-bands, traveling in opposite directions. Sub-bands are separated and combined by optical interleavers inserted in line along the transmission medium. To prevent the adjacent bands from interfering with each other along the transmission fiber and to allow for easier band separation, a spectral gap known as a guard-band is left between them. The need for the guard-band leads to inefficient utilization of the available spectral bandwidth in bidirectional WDM systems and fundamentally limits the number of transmissible channels. Typically, the number of wavelengths supported by the band-separation method is 32.
18.104.22.168 Interleaving filter method.
The interleaving technique uses wavelength-interleaving filters at each end of the span. As shown in Figure 4-5, interleaved channels are used in both directions of transmission. Even channels travel east to west, whereas odd channels travel west to east. As a consequence, channel spacing for wavelengths traveling in the same direction has to be doubled. However, the interleaving filters have a high insertion loss that contributes to higher system losses.
22.214.171.124 Circulator method
In this technique, the same wavelengths are transmitted in both directions of propagation. To separate transmit and receive direction at any node, optical circulators are used. A circulator is a multiport device that allows signals to propagate in certain directions based on the port that the signal came from. The circulator essentially acts as an isolator that allows only unidirectional propagation.
CLASSIFICATION OF WDM SYSTEMS.
2.5.1Open WDM system
The system supports optical interface conversion in WDM terminal equipment and can interconnect with SDH equipment from any vendor.
2.5.2 Integrated WDM system
The system does not support optical interface conversion in WDM terminal equipment. The performance of optical transponder units (OTUs) in SDH equipment must meet the following requirements of the WDM system:
Wavelength accuracy, spectral characteristics, transmit optical power and so on.
2.5.3 Semi-open WDM system
The system supports optical interface conversion at the TX end in WDM terminal equipment and can interconnect with SDH equipment from any vendor.
2.6 CHANNEL SPACING
The minimum wavelength separation between two different channels multiplexed on a fiber is known as channel spacing. Channel spacing ensures that neighboring channels do not overlap, causing power coupling between one channel and its neighbor. Channel spacing is a function of the precision of a laser. The more precise the tuning, the lower the channel spacing required. For example, a 100-GHz-spaced laser typically has one-half the preciseness of a 50-GHz-spaced laser. The precision of the laser has a linear relationship with the cost of the laser. The spacing that can be used is affected by the existing fiber characteristics. Fiber characteristics fall in the linear and nonlinear domains. Examples of linear fiber impairments would be that of attenuation and dispersion, whereas examples of nonlinear fiber impairments include four-wave mixing, cross-phase modulation, SRS, and SBS.
Another factor that affects channel spacing is the optical amplifier's capability to amplify the channel range. The closer the wavelengths are placed, the more important it is to ensure that the centers are identifiable from other signals on the same fiber. Channel-spacing values range from 200 GHz (1.6 nm), 100 GHz (0.8 nm), 50 GHz (0.4 nm), 25 GHz (0.2 nm), up to 12.5 GHz (0.1 nm). The ITU has published a wavelength grid as part of International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.694.1 and G.694.2 to provide interoperable standards for companies to work from.
To understand the effects of channel spacing on bandwidth capacity, consider a sample operating frequency band between 1505 nm and 1625 nm. This band provides a 120-nm spectrum. In theory, a fiber should be able to carry 150 wavelengths with 100-GHz spacing between wavelengths, 300 wavelengths with 50-GHz spacing, 600 wavelengths with 25-GHz spacing, or 1200 wavelengths with 12.5-GHz spacing. Assuming that each wavelength operates at 40 Gbps, it provides a theoretical maximum of 6 Tbps with 100-GHz spacing, 12 Tbps with 50-GHz spacing, 24 Tbps with 25-GHz spacing, or 48 Tbps with 12.5-GHz spacing.
Experimental systems have achieved in excess of 10-Tbps bandwidth capacity for a single fiber over a 100-km distance.
2.6 COARSE WAVELENGTH-DIVISION MULTIPLEXING
Coarse wavelength-division multiplexing (CWDM) systems are suited for the short-haul transport of data, voice, video, storage, and multimedia services. CWDM systems are ideally suited for fiber infrastructures with fiber spans that are 50 km or less and that don't need signal regeneration or the presence of optical amplifiers. The WDM laser bit rate directly determines the capacity of the wavelength and is responsible for converting the incoming electrical data signal into a wavelength.
CWDM systems use lasers that have a bit rate of up to 2.5 Gbps (OC-48/STM-16) and can multiplex up to 18 wavelengths. This provides a maximum of 45 Gbps over a single fiber. The transmitting laser and receiving detector are typically integrated into a single assembly called a transceiver.
Figure 2.6 shows a CWDM schematic. CWDM systems are characterized by a channel spacing of 20 nm or 2500 GHz as specified by the ITU standard G.694.2. The CWDM grid is defined in terms of wavelength separation. This grid is made up of 18 wavelengths defined within the range 1270 nm to 1610 nm. CWDM transceivers support the use of lower-cost distributed feedback (DFB) lasers that don't need any external cooling. These lasers are characterized with a drift of about 6 nm over a temperature range of 0 to 70 degrees Celsius. This, coupled with laser variations of up to +/-3 nm, yields a total wavelength variation of approximately 12 nm. The 20-nm channel spacing or guard-band between wavelengths provides adequate tolerance for the +/-12-nm wavelength drift. CWDM transceivers typically use highly sensitive avalanche photodiodes (APD) as receivers. CWDM gigabit interface converters (GBICs) support ITU standard G.694.2 wavelengths are increasingly being used with CWDM systems as a lower-cost alternative to conventional CWDM transceivers.
F:\Users\ALI\Pictures\untitled6.bmpFig. 2.6: Coarse WDM
2.7 DENSE WAVELENGTH-DIVISION MULTIPLEXING
Dense wavelength-division multiplexing (DWDM) systems are suited for the short-haul and the long-haul transport of data, voice, video, storage, and multimedia services. DWDM systems are ideally suited in the metro or long-haul core where capacity demands are extremely high. These higher-capacity demands result from the aggregation of services received from multiple customers at the enterprise edge. In such a case, the service provider is faced with the option of obtaining permits, retrenching, and installing new fiber versus obtaining DWDM equipment and lighting up wavelengths. If more than 18 wavelengths are required during the planned life cycle of the equipment to meet the future capacity expectations, a DWDM system should be considered versus a CWDM system.
Typical DWDM systems use lasers that have a bit rate of up to 10 Gbps (OC-192/STM-64) and can multiplex up to 240 wavelengths. This provides a maximum of 2.4 Tbps over a single fiber. Newer DWDM systems will be able to support 40-Gbps wavelengths with up to 300 channels, resulting in 12 Tbps of bandwidth over a single fiber. DWDM transceivers consume more power and dissipate much more heat than CWDM transceivers. This creates a requirement for DWDM cooling subsystems.
Figure 2.7 shows a DWDM schematic. Metro DWDM systems deployed today typically use 100-GHz or 200-GHz frequency spacing. DWDM common spacing can be 200, 100, 50, 25, or 12.5 GHz with a channel count reaching up to 300 or more channels at distances of several thousand kilometers with amplification and regeneration along such a route. As specified by the ITU standard G.694.1, DWDM systems are characterized by channel spacing of 50 or 100 GHz. The ITU DWDM frequency grid is anchored to 193.1 THz. DWDM systems have a significantly finer granularity between wavelengths (100-GHz typical spacing) versus their CWDM counterparts. ITU grid DWDM products operate in the C-band between 1530 and 1565 nm or L-band between 1565 and 1625 nm. It must be noted that not all fiber plants deployed in the past can be used for DWDM transmission, because most DWDM equipment currently uses the C-band or L-band window. Legacy fiber was optimized for transmission in the O-band (1310-nm band) window. All fiber should be characterized and tested before deploying a DWDM infrastructure.
Fig. 2.7: Dense WDM
2.8 WAVELENGTH ALLOCATION.
Thus far, the ITU-T standards recommends 81 channels in the C band with a constant spacing of 50 GHz anchored at 193.1 THz. This range can be extended to the L band (191.4 to 185.9 THz) where sources and amplifiers become available now. This will add 111 channels at 50-GHz spacing (see Table 2.1).
Table 2.1: Frequency Standards with Corresponding Wavelengths
Wavelength (Vacuum) (nm)