Tag Archive: SONET

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 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


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

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