Tag Archive: wavelength

Continued… DWDM Part XI

It’s nice to be back online again after a long Eid ul-Fitr break and quite busy at the office.

Frost & Sullivan published its Market Insight on “WDM PON: How Long Is It Going To Take?” on 16 Nov 2009 and the extract of the report was posted in the previous post (DWDM Part X), among others stated that “…. The idea is right with WDM-PON but the technology still needs 4-5 years to become practical. ….”. Thus, Adeel Najam (the author) anticipated that WDM-PON will become practical in 2013-2014.

In Malaysia, deployment of DWDM has taken place. Thanks to the bold Telcos/Carriers such as Fiberail and TIME dotCom.

TIME dotCom has a fiber optic network that covers a large portion of Malaysia and features 5 redundant routes totaling 6,000 kilometers. Upgrading from SDH to DWDM in 2009 gives TIME dotCom a new competitive edge and increased market share.

The new DWDM network has been operational  since December 2009 and TIME dotCom decided to make the move to DWDM because their concentration is in the wholesale market where the customers now want more bandwidth, and they needed to have a very effective network that could provide 99.999% reliability. SDH networks on the other hand are subject to congestion and did not provide the efficiencies the company wanted for its customers.

The maximum connection speed that the SDH network could support was 10 Gbps. TIME’s new DWDM network can support as many as 88 wavelengths per fiber, with each wavelength providing 10 Gbps connectivity. And in the future, the DWDM system can be easily upgraded to support 40 Gbps or even higher data rates per wavelength.

TIME’s DWDM network with 88 wavelengths per fiber where the capacity is virtually unlimited plus the GMPLS-based mesh protection at the wavelength level that offers automatic reroute protection even in the event of multiple fiber cuts. This scheme enables the TIME to offer 99.999% reliability that it can provide using its 5 fiber routes and therefore, gives TIME an important differentiator.

The DWDM network provides substantially higher bandwidth with fewer network elements and should be less expensive to operate, particularly when costs are calculated on a per-bit basis and the carrier can expect a significant OPEX reduction.

With new DWDM+GMPLS-based mesh protection, TIME’s Cross Peninsular Cable System (CPCS) network is claimed to be the most robust transborder terrestrial system ever built. Designed as a fully meshed network over 5 diverse fiber routes running along both
coasts, alongside major highways and via utility corridors, CPCS traverses more than 6,000 km with dedicated fiber optics connecting Thailand and Singapore.

Below is TIME’s DWDM network diagram:

TIME dotCom's DWDM Network

TIME dotCom's DWDM Network

Source:  Tellabs Insight Magazine, 3rd Quarter 2010. Click  here to download.

Tellabs Insight Magazine, 3rd Quarter 2010

Tellabs Insight Magazine, 3rd Quarter 2010


GMPLS:  Generalized Multi Protocol Label Switching

OPEX:  Operating Expenses/Expenditures

PON:  Passive Optical Network

SDH:  Synchronous Digital Hierarchy

WDM:  Wavelength Division Multiplexing

To be continued… DWDM Part XII


Continued… DWDM Part IX


Passive optical networks (PONs) have evolved to provide much higher bandwidth in the access network. A PON is a point-to-multipoint optical network, where an optical line terminal (OLT) at the CO is connected to many optical network units (ONUs) at remote nodes through one or multiple 1:N optical splitters. The network between the OLT and the ONU is passive, i.e., it does not require any power supply.

PONs use a single wavelength in each of the two directions—downstream (CO to end users) and upstream (end users to CO)—and the wavelengths are multiplexed on the same fiber through coarse WDM (CWDM). For example, the Ethernet PON (EPON) uses 1490 nm wavelength for downstream traffic and the 1310 nm wavelength for upstream traffic. Thus, the bandwidth available in a single wavelength is shared amongst all end users. Such a solution was envisaged primarily to keep the cost of the access network low and economically feasible for subscribers. Various blends of the PON have emerged such as the Ethernet PON (EPON) of IEEE 802.3ah [1], the broadband PON (BPON) of ITU-T G.983, and the generic framing procedure based PON (GFP PON) of ITU-T G.984.

An enhancement of the PON supports an additional downstream wavelength, which may be used to carry video and CATV services separately. Many telecom operators are considering to deploy PONs using a ?ber-to-the-x (FTTx) model (where x = building (B), curb (C), home (H), premises (P), etc.) to support converged Internet protocol (IP) video, voice, and data services—de?ned as “triple play”—at a cheaper subscription cost than the cumulative of the above services deployed separately.

Although the PON provides higher bandwidth than traditional copper-based access networks, there exists the need for further increasing the bandwidth of the PON by employing wavelength-division multiplexing (WDM) so that multiple wavelengths may be supported in either or both upstream and downstream directions. Such a PON is known as a WDM-PON.

Interestingly, architectures for WDM-PONs have been proposed as early as the mid-1990s. However, these ideas have not been commercialized yet for many reasons:- lack of an available market requiring high bandwidth, immature device technologies, and a lack of suitable network protocols and software to support the architecture. It is believed that many of the above factors have been mitigated over the years.

Below is the abstract of Frost & Sullivan Market Insight on “WDM PON: How Long Is It Going To Take?”, which was authored by Adeel Najam and published on 16 Nov 2009 ~

Although wavelength multiplexing seems like the ideal path for PON technology evolution with advantages like scalability and capacity boost, WDM PON technology needs to mature before it can become widespread. Shooting multiple waves in a fiber is the logical way to maximize the utilization of fiber and maximize the investment in the network. The idea is right with WDM-PON but the technology still needs 4-5 years to become practical.  …….

However WDM-PON has significant potential when the third wave of fiber deployments comes in 4 to 5 years. The third wave of fiber deployments will be the largest one and will come when fiber local loops are deployed in the urban areas of emerging markets. With the growth in wireless broadband, wireless providers will also start to connect their bay-stations with fiber. By that time WDM PON is expected to have matured to be used as an attractive PON technology.”


Wavelength-division-multiplexed passive optical network (WDM-PON) technologies for broadband access by Amitabha Banerjee and others, Optical Society of America.

WDM PON: How Long Is It Going To Take? by Adeel Najam, Frost & Sullivan Market Insight

To be continued… DWDM Part XI


Continued… DWDM Part IX

For the past eights parts, I’ve shared more on introduction of DWDM (DWDM 101) and in previous DWDM Part VII, I’ve also shared on DWDM as 3rd option for service providers to provide an economical solution in resolving fiberoptic capacity crisis. Having so much said on DWDM 101, DWDM Part IX onward will focus more on its industrial application and today I’ll start on sharing the info on WDM-PON technology for broadband access that utilizes CWDM or DWDM.


The passive optical network (PON) is an optical fiber based network architecture, which can provide much higher bandwidth in the access network compared to traditional copper-based networks. Incorporating wavelength-division multiplexing (WDM) in a PON allows one to support much higher bandwidth compared to the standard PON, which operates in the “single-wavelength mode” where one wavelength is used for upstream transmission and a separate one is used for downstream transmission. The WDM-PON is becoming a revolutionary and scalable broadband access technology  that  will  provide  high  bandwidth  to  end  users.

The access network, also known as the “first-mile network,” connects the service provider central offices (COs) to businesses and residential subscribers. This network is also commonly referred as the subscriber access network, or the local loop. As we all know that the bandwidth demand in the access network has been increasing rapidly over the past several years. Residential subscribers demand first-mile access solutions that have high bandwidth and offer
media-rich services. Similarly, corporate users demand broadband infrastructure through which they can connect their local-area networks to the Internet backbone.

The predominant broadband access solutions deployed today are the digital subscriber line (DSL) and community antenna television (CATV) (cable TV) based networks. However, both of these technologies have limitations because they are based on infrastructure that was originally built for carrying voice and analog TV signals, respectively; but their retrofitted versions to carry data are not optimal. Currently deployed blends of asymmetric DSL (ADSL) technologies provide 1.5 Mbits/s of downstream bandwidth and 128 Kbits/s of upstream bandwidth at best.

Note: In Malaysia, the biggest ADSL service is known as “streamyx” provided by the incumbent service provider TM (Telekom Malaysia).

Moreover, the distance of any DSL subscriber to a CO must be less than 5,486 meters (18000 ft) because of signal distortions. Although variations of DSL such as very-high-bit-rate DSL (VDSL), which can support up to 50 Mbits/s of downstream bandwidth, are gradually emerging, these technologies have much more severe distance limitations. For example, the maximum distance over which VDSL can be supported is limited to 457 m (1500 ft). CATV networks provide Internet services by dedicating some radio frequency (RF) channels in a coaxial cable for data. However, CATV networks are mainly built for delivering broadcast services, so they don’t fit well for the bidirectional communication model of a data network. At high load, the network’s performance is usually frustrating to end users.

Source:  Wavelength-division-multiplexed passive optical network (WDM-PON) technologies for broadband access by Amitabha Banerjee and others, Optical Society of America.

To be continued… DWDM Part IX


Continued… DWDM Part VIII

As mentioned in last post,  today I’ll share again on the “highway analogy” but using different diagram, which I believe is more appealing for clearer understanding on TDM-DWDM relationship.

Consider a highway analogy where one fiber can be thought of as a multilane highway. Traditional TDM systems use a single lane of this highway and increase capacity by moving faster on this single lane. In optical networking, utilizing DWDM is analogous to accessing the unused lanes on the highway (increasing the number of wavelengths on the embedded fiber base) to gain access to an incredible amount of untapped capacity in the fiber. An additional benefit of optical networking is that the highway is blind to the type of traffic that travels on it. Consequently, the vehicles on the highway can carry ATM packets, SONET, and IP.

By beginning with DWDM, service providers can establish a grow-as-you-go infrastructure, which allows them to add current and next-generation TDM systems for virtually endless capacity expansion as shown in the following diagram that illustrates the capacity expansion potential of DWDM.

Capacity Expansion Evolution

Capacity Expansion Evolution

DWDM also gives service providers the flexibility to expand capacity in any portion of their networks – an advantage no other technology can offer. Carriers can address specific problem areas that are congested because of high capacity demands. This is especially helpful where multiple rings intersect between two nodes, resulting in fiber exhaust.

Service providers searching for new and creative ways to generate revenue while fully meeting the varying needs of their customers can benefit from a DWDM infrastructure as well. By partitioning and maintaining different dedicated wavelengths for different customers, for example, service providers can lease individual wavelengths-as opposed to an entire fiber-to their high-use business customers.

Compared with repeater-based applications, a DWDM infrastructure also increases the distances between network elements-a huge benefit for long-distance service providers looking to reduce their initial network investments significantly. The fiber-optic amplifier component of the DWDM system enables a service provider to save costs by taking in and amplifying optical signals without converting them to electrical signals. Moreover, DWDM allows service providers to do it on a broad range of wavelengths in the 1.55-µm region. For example, with a DWDM system multiplexing up to 16 wavelengths on a single fiber, carriers can decrease the number of amplifiers by a factor of 16 at each regenerator site. Using fewer regenerators in long-distance networks results in fewer interruptions and improved efficiency.

Source: Dense Wavelength Division Multiplexing (DWDM) by The International Engineering Consortium (IEC)

To be continued… DWDM Part IX


Continued… DWDM Part VII

Yesterday, I shared on 2 options for the service providers to provide an economical solution in resolving fiberoptic capacity crisis.  Today, I’ll further share on the 3rd option, DWDM.

Option #3

The third choice for service providers is dense wavelength division multiplexing (DWDM), which increases the capacity of embedded fiber by first assigning incoming optical signals to specific frequencies (wavelength, lambda) within a designated frequency band and then multiplexing the resulting signals out onto one fiber. Because incoming signals are never terminated in the optical layer, the interface can be bit-rate and format independent, allowing the service provider to integrate the DWDM technology easily with existing equipment in the network while gaining access to the untapped capacity in the embedded fiber.

DWDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber to increase capacity as shown in the following diagram.

Increased Network Capacity - WDM

Increased Network Capacity - WDM

As shown above and still referring to the highway analogy, DWDM, on the other hand, relates to the accessing the unused lanes on the highway. Another way to increase auto throughput is to add more lanes that is equivalent to wavelength multiplexing. Eeach signal carried can be at a different rate (OC-3, OC-12, OC-24, etc.) and in a different format (SONET, ATM, data, etc.) For example, a DWDM network with a mix of SONET signals operating at OC–48 (2.5 Gbps) and OC–192 (10 Gbps) over a DWDM infrastructure can achieve capacities of over 40 Gbps. A system with DWDM can achieve all this gracefully while maintaining the same degree of system performance, reliability, and robustness as current transport systems – or even surpassing it.  Future DWDM terminals will carry up to 80 wavelengths of OC–48, a total of 200 Gbps, or up to 40 wavelengths of OC–192, a total of 400 Gbps—which is enough capacity to transmit 90,000 volumes of an encyclopedia in one second. Wow!

How is this possible?

The technology that allows this high-speed, high-volume transmission is in the optical amplifier. Optical amplifiers operate in a specific band of the frequency spectrum and are optimized for operation with existing fiber, making it possible to boost lightwave signals and thereby extend their reach without converting them back to electrical form. Demonstrations have been made of ultrawideband optical-fiber amplifiers that can boost lightwave signals carrying over 100 channels (or wavelengths) of light. A network using such an amplifier could easily handle a terabit of information. At that rate, it would be possible to transmit all the world’s TV channels at once or about half a million movies at the same time.  Wow! Wow!

In next post, I’ll share again on the “highway analogy” but using different diagram that I believe is more appealing for clearer understanding on TDM-DWDM relationship.

Source: Dense Wavelength Division Multiplexing (DWDM) by The International Engineering Consortium (IEC)

To be continued… DWDM Part VIII


Continued… DWDM Part VI

For today’s post (Part VI) and next post (Part VII), I”ll share on the options to resolve fiberoptic capacity challenges that include graphical illustration on TDM-DWDM analogy for a better understanding as promised in DWDM Part IV post.

At present, I believe many carriers are nearing 100% capacity utilization across significant portions of of their networks. Another problem for carriers is the challenge of deploying and integrating diverse technologies in one physical infrastructure. Customer demands and competitive pressures mandate that carriers offer diverse services economically and deploy them over the embedded network. DWDM provides service providers an answer to that demand. As mentioned previously, use of DWDM allows service providers to offer services such as e-mail, video, and multimedia carried as Internet protocol (IP) data over asynchronous transfer mode (ATM) and voice carried over SONET/SDH. Despite the fact that these formats—IP, ATM, and SONET/SDH—provide unique bandwidth management capabilities, all three can be transported over the optical layer using DWDM. This unifying capability allows the service provider the flexibility to respond to customer demands over one network.

A platform that is able to unify and interface with these technologies and position the carrier with the ability to integrate current and next-generation technologies is critical for a carrier’s success.

Resolving the Capacity Crisis

Faced with the multifaceted challenges of increased service needs, fiber exhaust, and layered bandwidth management, service providers need options to provide an economical solution as follows:

Option #1

One way to alleviate fiber exhaust is to lay more fiber,  and, for those networks where the cost of laying new fiber is minimal, this will prove the most economical solution. However, laying new fiber will not necessarily enable the service provider to provide new services or utilize the bandwidth management capability of a unifying optical layer.

Option #2

A second option is to increase the bit rate using time division multiplexing (TDM), where TDM increases the capacity of a fiber by slicing time into smaller intervals so that more bits (data) can be transmitted per second as shown in the following diagram.

TDM Increased Network Capacity

TDM Increased Network Capacity

The figure above is a highway analogy, where one fiber can be considered as a multi-lane highway, can be used to explain the difference between the two (TDM and DWDM). TDM relates to traffic flow on single lane of the highway. To increase the throughput of autos, one can increase their speed that is equivalent to time multiplexing.

Traditionally, this has been the industry method of choice (DS-1, DS-2, DS-3, etc.). However, when service providers use this approach exclusively, they must make the leap to the higher bit rate in one jump, having purchased more capacity than they initially need. Based on the SONET hierarchy, the next incremental step from 10 Gbps TDM is 40 Gbps—a quantum leap that many believe will be a big challenge (if not be possible) for TDM technology in the near future. This method has also been used with transport networks that are based on either the synchronous* optical network (SONET) standard for North America or the synchronous* digital network (SDH) standard for international networks.

The telecommunications industry adopted the SONET or SDH standard to provide a standard synchronous* optical hierarchy with sufficient flexibility to accommodate current and future digital signals. SONET or SDH accomplishes this by defining standard rates and formats and optical interfaces. For example, multiple electrical and optical signals are brought into a SONET terminal where they are terminated and multiplexed electrically before becoming part of the payload of an STS–1, the building block frame structure of the SONET hierarchy.  The STS–1 payloads are then multiplexed to be sent out on the single fiber at a single rate: OC–3 to OC–12 to OC–48 and eventually to OC–192. SDH has a similar structure with STM-n building block resulting in signal rates of STS–1 through STM–64.

SONET and SDH, two closely related standards, provided the foundation to transform the transport networks as we know them today. They govern interface parameters; rates, formats, and multiplexing methods; and operations, administration, maintenance, and provisioning (OAM&P) for high-speed transmission of bits of information in flashing laser-light streams.

*Note: A synchronous mode of transmission means that the laser signals flowing through a fiber-optic system have been synchronized to an external clock. The resulting benefit is that data streams transmitting voice, data, and images through the fiber system flow in a steady, regulated manner so that each stream of light can readily be identified and easily extracted for delivery or routing.

Source: Dense Wavelength Division Multiplexing (DWDM)  by The International Engineering Consortium (IEC)

To be continued… DWDM Part VII


Continued… DWDM Part V – Capabilities & Merits and Limitations

Today, I’ll continue sharing the info on other four aspects of DWDM capabilities and merits (brief info). Yesterday, I’ve shared on “Extendibles” aspect.

  • Reliable:

Tunable lasers, used in DWDM technology, eliminate adjacent channel interference and optimize DWDM performance.

  • Flexible:

Tunable lasers, used in DWDM technology, also allow service providers to adapt to capacity changes by automatically adding or dropping channels or wavelengths in response to application requirements. Service providers can benefit from a DWDM infrastructure by partitioning and maintaining different dedicated wavelengths for different customers. For example, service providers can lease individual wavelength as opposed to an entire fiber to their high-use business customers.

  • Scalable:

The number wavelengths and bit rate can be upgraded for all DWDM systems, however planning for this is critical. If service providers put together their networks in a specific way and then want to upgrade, one of two things must happen:- they need either more power or additional signal-to-noise margin.

  • Available:

DWDM technology supports variety of optical interfaces; works with many other technologies such as SONET/SDH, ATM, IP, WDM, and Gigabit Ethernet.

I’ve shared above the capabilities and merits of DWDM, which look promising.  How about its limitations? Any technology typically has its limitations. Check it out below!


  • DWDM standards are ‘may ‘not yet fully specified. As a result, DWDM networks generally use proprietary management and multiplexing procedures.
  • DWDM service is adversely affected by sudden temperature changes in optical-cross connects and manufacturing imperfections in the fiber optic plant.
  • Signal attenuation, optical signal-to-noise ratio (OSNR), chromatic dispersion, and crosstalk at the Optical Layer also disrupt DWDM network performance.
  • DWDM-based equipment such as precision filters, optical amplifiers, cooled lasers, and routers are new, cutting-edge patented, which can make DWDM too expensive for some small to mid-sized companies. – In the initial phase of implementation, DWDM systems typically work with existing SONET/SDH-based equipment, SONET/SDH applications, and employ the in-place fiber optic plant for long-haul optical network transmission.
  • DWDM transmits multiple wavelengths or channels over a single fiber. As more channels are being multiplexed, data rates on a single channel and the complexity also increase. – More research is needed to understand how to utilize the huge bandwidth capacity of a DWDM system quickly and cost-effectively – how many DWDM channels and how high a data rate of each channel can be utilized.

These limitations probably have been overcome by the researchers or technologists. So, please accept my apology.

Source: DWDM: Technologies and Initiatives by Khoa Duc Tran

To be continued… DWDM Part VI


Continued… DWDM Part IV

To recap, DWDM technology has been developed to increase the capacity of a single fiber SONET/SDH technology, which transmits information via a single channel or wavelength of light via each fiber optic strand. The term “dense” refers to high-wavelength or high-channel count per fiber. In essence, each wavelength represents a different transmission channel and can transmit data at 10 Gbps.

DWDM can offer potentially unlimited bandwidth at multi-gigabit and multi-terra-bit rates by carrying multiple light waves of different frequencies on a single fiber. A single fiber can carry up to 128 wavelengths and researchers/technologists are working on DWDM technologies that could carry more than 1,000 channels within a single fiber.

Operationally, optical network deployments represent a significant CAPEX (Capital Expenditures) investment. However, the rapidly falling cost of raw fiber will accelerate the adoption of this technology.

For today post, I’ll discuss on the capabilities and merits of DWDM as practical considerations for the service providers to deploy DWDM networks.

  • Extendibles:

DWDM is a more cost-effective alternative to SONET/SDH, which employs Time Division Multiplexing (TDM). A highway analogy, where one fiber can be considered as a multi-lane highway, can be used to explain the difference between the two. TDM relates to traffic flow on single lane of the highway. To increase the throughput of autos, one can increase their speed that is equivalent to time multiplexing.

DWDM, on the other hand, relates to the accessing the unused lanes on the highway. Another way to increase auto throughput is to add more lanes that is equivalent to wavelength multiplexing.  DWDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber to increase capacity. Each signal transmitted can be at a different rate (OC–3/12/24, etc.) and in a different format (SONET/SDH, ATM, IP, WDM, and Gigabit Ethernet, etc.).

[I’ll discuss further on TDM-DWDM analogy with graphical illustrations in future post for a better understanding]

The operations associated with TDM electronic-to-optical and optical-to-electronic somehow rather slow the performance of SONET/SDH networks. These operations convert data signals from the electronic network to optical format, route the signals to their proper destinations within the optical part of the infrastructure, and then convert them back again for their continued journey over the electronic portion of the network.

DWDM technology, on the other hand, employs an Erbium Doped Fiber Optic Amplifier (EDFA) with advanced filtering techniques to amplify optical signals without converting them to electrical signals.DWDM uses Optical Bi-directional Line Switched Ring (OBLSR) topologies to optimize bandwidth capacity of the in-place fiber optic plant and the traffic volumes transported via the Optical Layer. A DWDM infrastructure also increases the distances between network elements – a big benefit for long-distance (long-haul) service providers looking to reduce their initial network investments significantly.

DWDM uses advanced wavelength routing protocols and gigabit routers for provisioning of wavelength or channel capacity. DWDM employs tunable lasers for enabling development of multiple, independent, narrowly spaced transmission channels or wavelengths on a single fiber optic strand.

Source:  DWDM: Technologies and Initiatives by Khoa Duc Tran

To be continued… DWDM Part V – Capabilities and Merits


Continued… DWDM Part III

In my first posting on DWDM (Part I), I briefly discussed how DWDM works and perhaps today I’ll discuss further the way it functions from various angles and examples for clarity purposes.

DWDM System Functions

At its core, DWDM involves a small number of physical-layer functions. These are depicted in figure below, which is similar to figure given in Part I. Figure below shows a DWDM schematic for four channels. Each optical channel occupies its own wavelength (wavelength is expressed (usually in nanometers) as an absolute point on the electromagnetic spectrum. The effective light at a given wavelength is confined narrowly around its central wavelength).

DWDM Functional Schematic

DWDM Functional Schematic

From the above figure (view it from left to right), the DWDM system performs the following main functions:

  • Generating the signal—The source, a solid-state laser, must provide stable light within a specific, narrow bandwidth that carries the digital data, modulated as an analog signal.
  • Combining the signals—Modern DWDM systems employ multiplexers to combine the signals. There is some inherent loss associated with multiplexing and demultiplexing. This loss is dependent upon the number of channels but can be mitigated with optical amplifiers, which boost all the wavelengths at once without electrical conversion.
  • Transmitting the signals—The effects of crosstalk and optical signal degradation or loss must be reckoned with in fiber optic transmission. These effects can be minimized by controlling variables such as channel spacings, wavelength tolerance, and laser power levels. Over a transmission link, the signal may need to be optically amplified.
  • Separating the received signals—At the receiving end, the multiplexed signals must be separated out. Although this task would appear to be simply the opposite of combining the signals, it is actually more technically difficult.
  • Receiving the signals—The demultiplexed signal is received by a photodetector.

In addition to these functions, a DWDM system must also be equipped with client-side interfaces to receive the input signal. This function is performed by transponders. On the DWDM side are interfaces to the optical fiber that links DWDM systems.

As mentioned earlier above and in Part I, optical networks use Dense Wavelength Multiplexing as the underlying carrier. The most important components of any DWDM system are transmitters, receivers, Erbium-doped fiber Amplifiers (EDFA), DWDM multiplexors (aka Mux) and DWDM demultiplexors (aka Demux). The block diagram below gives the structure of a typical DWDM system with these components (view it from right to left).

Block Diagram of a DWDM System

Block Diagram of a DWDM System

Optical Transmission Principles

Optical fiber transmission plays a major role in deciding the throughput of the DWDM network. The DWDM system has an important photonic layer, which is responsible for transmission of the optical data through the network. The following basic principles are necessary for the proper operation of the system.

Channel Spacing
The minimum frequency separation between two different signals multiplexed is known as the Channel spacing. Since the wavelength of operation is inversely proportional to the frequency, a corresponding difference is introduced in the wavelength of each signal. The factors controlling channel spacing are the optical amplifier’s bandwidth and the capability of the receiver in identifying two close wavelengths sets the lower bound on the channel spacing. Both factors ultimately restrict the number of unique wavelengths passing through the amplifier.

Signal Direction
An optical fiber helps transmit signal in both directions. Based on this feature, a DWDM system can be implemented in two

  • Unidirectional: All wavelengths travel in the same direction within the fiber. It is similar to a simplex case. This calls in for laying one another parallel fiber for supporting transmission on the other side.
  • Bi-directional: The channels in the DWDM fiber are split into two separate bands, one for each direction. This removes the need for the second fiber, but, in turn reduces the capacity or transmission bandwidth.

Signal Trace
Signal Trace is the procedure of detecting if a signal reaches the correct destination at the other end. This helps follow the light signal through the whole network. It can be achieved by plugging in extra information on a wavelength, using an electrical receiver to extract if from the network and inspecting for errors. The receiver then reports the signal trace to the transmitter.

Taking into consideration the above two factors, the international bodies have established a spacing of 100GHz to be the worldwide standard for DWDM. This means that the frequency of each signal is less than the rest by atleast 0.1THz.


Introduction to DWDM for Metropolitan Networks

Optical Networking And Dense Wavelength Division Multiplexing (DWDM) by Muralikrishna Gandluru

To be continued… DWDM Part IV


Continued… DWDM Part II

Continue from last post on DWDM history, the figure below shows the evolution or progression of the WDM technology that 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.

Evolution of DWDM

Evolution of DWDM

  • Early WDM began in the late 1980’s 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 1990’s 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-1990’s, dense WDM (DWDM) systems were emerging with 16 to 40 channels and spacing from 100 to 200 GHz.
  • By the late 1990’s 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.

Increases in channel density resulting from DWDM technology have had a dramatic impact on the carrying capacity of fiber. In 1995, when the first 10 Gbps systems were demonstrated, the rate of increase in capacity went from a linear multiple of four every four years to four every year as shown in figure below.

Growth In Fiber Capacity

Growth In Fiber Capacity

Source:   Introduction to DWDM for Metropolitan Networks

To be continued… DWDM Part III

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