Tag Archive: multiplex


Continued… DWDM Part IX

WDM-PON

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.”

Sources:

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


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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

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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!

Limitations

  • 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

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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

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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
ways:

  • 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.

Sources:

Introduction to DWDM for Metropolitan Networks

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

To be continued… DWDM Part IV

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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

Acronyms:

  • 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
Source:

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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