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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


Today, Wednesday, August 11, 2010 is a first day of Ramadan in Malaysia (Ramadan 1, 1431H). I would like to take this opportunity to wish Blog readers and visitors a blessed Ramadan. May the light that we celebrate at Ramadan show us the way and lead us together on the path of peace and social harmony.

Have a wonderful Eid Mubarak

Have a wonderful Eid Mubarak

Wish you a very Happy Ramadan Mubarak.


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


My apology for being “off mode” for 2 weeks since my last post on July 22. Last week Fri-Sun, my colleagues and I attended our company’s corporate team building event at Jeram Besu, Pahang (Malaysia). It was an enjoyable event with the main activities of white water rafting and 4WD off-road to Jerembun waterfall (not forget to mention that we indulged ourselves with eating durian, king of fruits).

I was also quite busy lately as I was on DWDM assignment. I take this opportunity to explore DWDM and would like to share the info on this blog. This new topic and other future topics on interesting technology or solution will be posted under the new category of “Next Generation Network (NGN)“.

What on earth is DWDM?

DWDM is a short or an acronym for Dense Wavelength Division Multiplexing. DWDM is a fiber-optic transmission technique. It involves the process of multiplexing many different wavelength signals onto a single fiber. So each fiber have a set of parallel optical channels each using slightly different light wavelengths. It employs light wavelengths to transmit data parallel-by-bit or serial-by-character.

In short, DWDM is a technology that uses fiber-optics transmission techniques that employ light wavelengths to transmit data as shown below.
How DWDM works

How DWDM works

Figure above shows a diagram that depicts how DWDM works.  As shown, four incoming sources are:
1. Multiplexed onto one single fiber
2. Transmitted
3. Demultiplexed onto four outgoing fibers (incoming signals are retrieved)

What so special about DWDM?

DWDM is a very crucial component of optical networks that will allow the transmission of data: voice, video-IP, ATM and SONET/SDH respectively, over the optical layer.

This allows service providers to offer the following Triple Play services as IP data over ATM or voice over SONET (or SDH):

  • Video
  • Multimedia
  • E-mail
Development of DWDM Technology
Let’s backtrack a little bit to explore the history of Wavelength Division Multiplexing (WDM) development. 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. Figure below 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 and the one most found in DWDM systems.
Example of simple form of WDM with 2 channels

Example of simple form of WDM with 2 channels


  • DWDM: Dense Wavelength Division Multiplexing
  • WDM: Wavelength Division Multiplexing
  • NGN: Next Generation Network
  • ATM: Asynchronous transfer mode
  • SONET: Synchronous data transmission on optical media (American National Standards)
  • SDH: Synchronous Digital Hierarchy (international equivalent of SONET)
  • IP: Internet Protocol

Dense Wavelength Division Multiplexing (DWDM) by Luc Pelletier & Miguel Pinard
Introduction to DWDM for Metropolitan Networks

To be continued… DWDM Part II


Continued… Part VIII – International Submarine Cable facts and figures.

I’d like to share another global submarine cable routes map for 2010, which is quite similar with the 2009 version (posted yesterday) but with additional new cable systems (if any).

Global Submarine Cable Map 2010

Global Submarine Cable Map 2010 -----Courtesy: TeleGeography

>> Follow the following link to download free a larger image size (1600×1200):

(File Name:

The global/international submarine cable map depicts in-service cables with a minimum capacity of 5 Gbps after full upgrades. Note that the cable routes on the map are stylized and do not reflect physical cable location.

The top-5 longest submarine cable system:

SeaMeWe-3 is the longest submarine cable built to date (please correct me if I’m wrong). The system spans 39,000 kilometers from Norden in Germany to Keoje in South Korea. Its 39 landing points connect 32 different countries.

The top-5 longest submarine cable system in the world are as follows:

  1. 39,000 km >> SeaMeWe-3
  2. 30,500 km >> Southern Cross
  3. 30,476 km >> China-USA
  4. 28,000 km >> FLAG Europe-Asia
  5. 25,000 km >> South America-1

Global Lit Submarine Cable Capacity

The first intercontinental telephony submarine cable system, TAT-1, connected North America to Europe in 1956 and had an initial capacity of only 640 Kbps. Since then, total trans-Atlantic cable capacity has soared to well over 7 Tbps in 2007. In recent years, the annual growth of lit trans-Atlantic capacity has slowed, while the growth rates on the intra-Asian and Europe-Asia routes have increased rapidly.

Source: TeleGeography


Continued… Part VII – Strategic Importance of Submarine Cables, Coastal Cable Routes and International Cable Routes.

The last 2 posts were the updates of Part V where I have posted description and additional diagrams that I hope will give clearer picture of typical submarine cable system.

Today I’ll discuss on why the submarine telecommunications cables are so important to us and how these submarine cables look like on global telecommunication map.

Strategic Importance of Submarine Cables

  • Submarine cables are the backbone of the international telecommunications network
  • Almost 100% of transoceanic Internet traffic is sent via submarine cable
  • The submarine cable network is designed to be resilient, however faults can disrupt activities we take for granted – banking, airline bookings, internet shopping, education, health, defense, and of course, our communication with one another
  • Many Governments now recognize the strategic value of submarine cables and are taking stronger measures to help protect them

Coastal Cable Routes

Some key notes on coastal cables are as follows:

  • Near-shore, cables need protection from shipping, fishing & other activities
  • To reduce risk, cables are identified on nautical charts and may be placed within a “protection zone”
  • A zone is a legal entity where activities harmful to cables are banned
  • Cable burial in water depths to 1500m is also a key protective measure

The following coastal cable route map shows the “protection zone” for Southern Cross cable terminal, New Zealand.

Protection zone for Southern Cross cable terminal, New Zealand

Protection zone for Southern Cross cable terminal, New Zealand. ---------Courtesy: Telecom New Zealand

The following map shows the new protection zones (yellow) proposed for Australia where value of submarine cables to economy is assessed at A$5 billion. Wow!

New protection zones (yellow) for Australia

New protection zones (yellow) for Australia ------Courtesy: Australian Communications & Media Authority

International Submarine Cable Routes

Global communications depend on a network of submarine telecommunications cables which connect the land-based networks of many countries. The following international submarine cable routes maps demonstrate how the world’s oceans are wired.

International Cable Routes

International Submarine Cable Routes ----Courtesy: Global Marine Systems Ltd

>> Follow the following link to download free a larger image size (1034×576):

(File Name:

Below is the International Submarine Cable Routes map for 2009.

Submarine Cable Map 2009

Submarine Cable Map 2009 -----Courtesy: TeleGeography

>> Follow the following link to download free a larger image size (1600×1200):

(File Name:

Source: International Cable Protection Committee Ltd, TeleGeography

To be continued… Part VIII – International Submarine Cable facts and figures.


Yesterday, I’ve discussed the description of typical submarine cable system in reference to the “Typical Submarine Cable System” diagram. Under “Terminal Equipment” description, the terminal equipment such as PFE, SLTE, SIE, SSE and NME have been mentioned (please refer yesterday’s post for description of these acronyms). The terminal equipment are typically housed in the cable landing station premises.

For your better understanding, today I’ll post another version of “Typical Submarine/Undersea Cable System” diagram that include terminal equipment components. I have 2 similar diagrams that I would like to share as follows:

Typical Undersea Cable System

Four major components form an undersea cable system

Note: Mux in the diagram is an acronym for Multiplexer.

The second diagram below demarcates the submarine cable system into two parts namely “Dry Plant” and “Wet Plant”.

Submarine Cable System Diagram

Submarine Cable System Diagram

Note: PoP in the diagram is an acronyms for Point of Presence

Source: TeleGeography


In my previous post Submarine Cable Network, Malaysia Outlook (Part V) on July 9, 2010, I’ve included a diagram of “Typical Submarine Cable System” (refer below) but unfortunately I forgot to describe the diagram. Pardon moi!

So today, I’m gonna make it up by giving the description of the diagram as follows:

Typical Submarine Cable System

Typical Submarine Cable System

Cable Landing Station

The landing station is the first point at which the submarine cable is terminated/connected to the landing country. The landing station will be used as international gateway for internet traffic by ISP.

Landing stations house terminal equipment, including lasers, multiplexers, and power supply, that takes the optical signal from the submarine cable and passes it on to a terrestrial system.

Terminal Equipment

Terminal equipment, typically housed in the equipment room and landing station room, is what is necessary to light the entire length of the cable and to provide a point of connection for the
submarine cable to the terrestrial infrastructure in the country. They include power feeding equipment (PFE), transmission equipment, Submarine Line Terminal Equipment (SLTE), SONET/SDH Interconnecting Equipment (SIE), System Supervisory Equipment (SSE), and network monitoring equipment.

Network Management System

The landing station would have a state of the art network management system which can monitor/manage the network effectively. Network Management Systems shall provide fault, configuration, performance, and security management at local, and global levels.

Buried Cable Segment

Submarine cables are typically buried as they approach shore. This helps protect submarine cables from trawlers and fishing operations from accidentally breaking the submarine cable along the shore.

Submarine Cables

Submarine cables are laid on the ocean floor and require several layers of amour to to protect the system from damage due to debris, pressure, or shifts along the ocean floor.


Repeaters are placed along the length of the submarine cable system to correct and amplify the signal carried by the system. The distance between repeaters is relative to the overall system bandwidth; higher capacity systems require repeaters to be spaced closer together.

Source: TeleGeography, Fujitsu

Follow this link to view or download the larger image of “Typical Submarine Cable System”.


Continued… Part VI – Cable Size and Comparing Submarine Cables vs Satellite.

Today, I’ll discuss on the submarine telecommunications cable physical size and the comparison between sub-cable and satellite.

  • Modern submarine telecommunications cables are small; deep-ocean types without protective armor are typically 17-20 mm diameter, similar to that of a garden hose.
  • Armored fiber-optic cables on the other hand may reach 50 mm diameter.
  • In contrast, submarine oil/gas pipes reach 900 mm diameter, and fishing trawls typically range over 5,000 – 50,000 mm width.
  • Cable lengths vary; one of the longest is the SEA-ME-WE 4 system at ~20,000 km SEA-ME-WE 3 system at ~39,000 km.

Below is a diagram to better illustrate the size comparison:

Modern fiber-optic cable in hand (for scale) and relative to 300mm diameter subsea pipe.

There is a common misconception that nowadays most international communications are routed via satellites, when in fact well over 95% of this traffic is actually routed via submarine telecommunications fiber-optic cables. Data and voice transfer via these cables is not only cheaper, but also much quicker than via satellite.

In 1988, the first trans-oceanic fiber-optic cable was installed, which marked the transition when submarine telecommunications cables started to outperform satellites in terms of the volume, speed and economics of data and voice communications.

However, despite  the  success  of  submarine  telecommunications, satellite transmission remains a necessary adjunct. Satellites  provide  global  broadcasts  and  communications for  sparsely  populated  regions  not  served  by  cables. They also form a strategic back-up for disaster-prone regions.

Below are the advantages comparison between submarine telecommunications cables and satellites.

Main advantages of sub-cables

  • High reliability, capacity & security.
    • Securely  and  consistently deliver  very  high-capacity  communications  between  population  centers.
  • None of the delays present in satellite traffic
  • Cost-effective on major routes, hence rates cheaper than satellites
    • The advantages  of  low  cost  and  high  bandwidth  are  becoming attractive  to  governments  with  low  population  densities.

Yield: Submarine cables carry >95% of international voice & data traffic.

Almost  all  transoceanic telecommunications  are  now  routed  via  the  submarine cable network instead of satellite.

Main advantages of satellites

  • Suitable for disaster-prone areas
  • Provides wide coverage for mobile subscribers
  • Suitable for linking isolated regions and small island nations into the international telecom network

Yield: Satellites carry <5% of international voice & data traffic

Source: International Cable Protection Committee Ltd

To be continued… Part VII – Strategic Importance of Submarine Cables, Coastal Cable Routes and International Cable Routes.

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