WeekWeek--44

Wavelength Division MultiplexingWavelength Division MultiplexingNeed for wavelength division multiplexingNeed for wavelength division multiplexing

Coarse wavelength division multiplexingCoarse wavelength division multiplexing

Dense wavelength division multiplexingDense wavelength division multiplexing

Need for WavelengthNeed for Wavelength--Division MultiplexingDivision Multiplexing

� The power of the Internet and the World Wide Web resides in its content.

� Retrieval of high-quality content from application servers, such as web servers, video servers, and e-commerce sites, in the shortest possible time has driven the need for speed for individual and corporate end users are same.

� Residential customers, small- and medium-sized businesses, and even large businesses are demanding affordable high-speed access services, such as xDSL and cable modem access.

� Larger enterprise customers continue to push for high-speed, managed multiservice IP virtual private networks (VPNs) with strict quality of service (QoS).

� With increased aggregation at the access layer, the need arises for bandwidth at the distribution and the core of the network.

ContCont’’dd

� This rapidly developing growth has need for extremely scalable high-bandwidth core technologies.

� Technology has seen the limits of bandwidth and transmission speeds over traditional TDM media systems.

� Traditional networks have been built using a combination of circuit-switched TDM technology along with a TDM-capable Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) infrastructure.

� TDM and SONET are essentially serial time-division multiplexed technologies that have finite limits in terms of bandwidth due to constraints, such as frame size, framing rate, clock speed.

ContCont’’dd

� The original driver for the development of WDM technology was the need for sheer (complete) bandwidth.

� This requirement translated into a tangible need to pull additional terrestrial fiber-optic cable.

� The infrastructure and construction costs associated with the deployment of large-scale fiber plants were and continue to be prohibitively high.

� In short, WDM can be applied wherever there is a need for fiber relief.

� WDM technology was initially expensive to engineer, deploy, and manage, which restricted the initial market deployment.

� Many WDM manufacturers have addressed these limitations by providing point-and-click network provisioning tools, network design modeling tools, and various operational enhancements.

WavelengthWavelength--Division MultiplexingDivision Multiplexing� Wavelength-division multiplexing (WDM) 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.

� This system has n service interfaces and n wavelengths transmitted in either direction over a single fiber.

� Each wavelength operates at a different frequency.

ContCont’’dd

� 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 upgrading the existing infrastructure of network.

� 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. � It is much easier to add a wavelength than to add new fiber

ContCont’’dd

� Four kinds of WDM systems are available:

� Metro WDM (2000 km)

ContCont’’dd

� Long-haul WDM systems, most often user service interfaces are OC-48/STM-16 interfaces.

� Other interfaces commonly supported include Ethernet, Fast Ethernet, Gigabit Ethernet, 10 Gigabit Ethernet, ESCON, Sysplex Timer and Sysplex Coupling Facility Links, and Fibre Channel.

� On the client side, there can be SONET/SDH terminals, add/drop multiplexers (ADMs),ATM switches, and routers.

ContCont’’dd

� WDM can be considered a form of frequency-division multiplexing (FDM) coupled with timed-division multiplexing (TDM).

� The exact relationship between a WDM wavelength and frequency is determined from the equation c = λ * f; where c is the speed of light in a vacuum (3 * 108 m/s), λ is the wavelength measured in a vacuum; and f is the frequency.

� In WDM systems, the wavelength is measured in nanometers (nm) and the frequency is measured in gigahertz (GHz).

� The speed of light in glass is approximately 2 * 108 m/s. � Various frequencies of light can travel down a single fiber, and

each frequency can formally appoint a channel. � 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.

ContCont’’dd

Optical Frequency Bands used with Optical Frequency Bands used with various WDM Systemsvarious WDM Systems

� 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.

Unidirectional WDMUnidirectional 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.

� Unidirectional WDM systems are very common with cable providers who transmit multicast traffic to downstream receiving stations.

ContCont’’dd

Bidirectional WDMBidirectional 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.

Bidirectional WDM TechniquesBidirectional WDM Techniques

BandBand--Separation MethodSeparation 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 gap known as a guard-band is left between them.

� Typically, the number of wavelengths supported by the band-separation method is 32.

InterleavingInterleaving--Filter MethodFilter Method� The interleaving technique uses wavelength-interleaving filters

at each end of the span. � Interleaved channels are used in both directions of

transmission.

� Even channels travel east to west, whereas odd channels travel west to east.

� Channel spacing for wavelengths traveling in the same direction has to be doubled.

InterleavingInterleaving--Filter Method Filter Method (Cont(Cont’’d)d)� However, the interleaving filters have a high insertion loss that

contributes to higher system losses.� 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.

Circulator MethodCirculator 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.

WavelengthWavelength patternspatternsCoarseCoarse WavelengthWavelength--DivisionDivision MultiplexingMultiplexing� CWDM systems are suited for the short-haul transport of data,

voice, video, storage, and multimedia services. � Ideally suited for fiber infrastructures with fiber spans that

are 50 km or less and that don't need signal regeneration.� 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.� 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.

ContCont’’dd

Dense WavelengthDense Wavelength--Division MultiplexingDivision 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 ofobtaining permits, reducing cost, and installing new fiber versus obtaining DWDM equipment and lighting up wavelengths.

� If more than 18 wavelengths are required during the planned lifecycle of the equipment to meet the future capacity expectations, a DWDM system should be considered versus a CWDM system.

ContCont’’dd� 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.

ContCont’’dd� 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.

� The ITU standard G.694.1, DWDM systems are characterized by channel spacing of 50 or 100 GHz.

� Current DWDM products operate in the C-band between 1530 and 1565 nm or L-band between 1565 and 1625 nm.

� The old products or equipment work on O-band 1310 nm.� In AUP 1310 nm is used with 1 GHz.