DWDM Topology Design: How to Make it Right?

Network expansion spurs the demand for faster data transmission and higher capacity over the network. In this case, DWDM emerges as a cost-effective solution to handle these issues, working efficiently to combine multiple wavelengths together and sent them over one single fiber. With the ability to carry up to 140 channels theoretically, higher capacity can be achieved by DWDM technology. This article guides you through some basics of DWDM topology.

Common DWDM Topology Overview

DWDM networks are grouped into four major topological configurations: DWDM point-to-point with or without add-drop multiplexing network, fully connected mesh network, star network, and DWDM ring network with OADM nodes and a hub. The requirements of each DWDM topology differ, and based on various application, it may involve different optical components. Besides these four common DWDM topology, there also exists hybrid network topology, consisting of stars and/or rings that are interconnected with point-to-point links.

Configurations of DWDM Topology

This section illustrates the four basic DWDM topology configurations, help to understand the major differences and applications of them.

Point-To-Point Topology

Point-to-point topology is typically found in long-haul transport, which demands for ultra high speed (10-40Gb/s), ultra high aggregate bandwidth, high signal integrity, great reliability, and fast path restoration capability. The transmitter and receiver within this DWDM topology can be several hundred kilometers away, and the number of amplifiers between the two end points is generally less than 10. Together with add-drop multiplexing, point-to-point DWDM topology enables the system to drop and add channels along its path. A DWDM point-to-point system includes lasers, an optical multiplexer and demultiplexer, fibers, optical amplifiers, and an optical add-drop multiplexer.

Ring-Configuration Mesh and Star Networks

Basically, a DWDM ring network includes a fiber in a ring configuration that fully interconnects nodes. Two fiber rings are even presented in some systems for network protection. This ring DWDM topology is commonly adopted in a local or a metropolitan area which can span a few tens of kilometers. Many wavelength channels and nodes may be involved in DWDM ring system. One of the nodes in the ring is a hub station where all wavelengths are sourced, terminated, and managed, connectivity with other networks takes place at this hub station. Each node and the hub have optical add-drop multiplexers (OADM) to drop off and add one or more designated wavelength channels. As the number of OADMs increases, signal loss occurs and optical amplifier is needed here.

In the ring DWDM topology, a hub station works to manage channel assignment so that a fully connected network of nodes with OADM is accomplished. The hub also makes it possible to connect other networks. A DWDM mux/demux can be connected to an OADM node to multiplex several data sources. The following picture demonstrates a simple DWDM ring topology with a hub and two nodes (A and B).

Transmit and Receive Directions of DWDM Hub

In the previous part, we’ve mentioned DWDM hub, which serves as a very essential parts in a DWDM system. Here we further explain the transmit and receive direction of a DWDM hub, proving system solutions for your reference.

Transmit Direction

A DWDM hub accepts various electrical payloads, such as communications transport protoco/Internet Protocol (TCP/IP), asynchronous transfer mode (ATM), STM, and high-speed Ethernet (l Gb/s, 10 Gb/s). Each traffic type (channel) is sent to its corresponding physical interface, where a wavelength is assigned and is modulated at the electrical-to-optical converter. The optically modulated signals from each source are then optically multiplexed and launched into the fiber.

Receive Direction

When a hub receives a WDM signal, it optically demultiplexes it to its component wavelengths (channels) and converts each optically modulated signal to a digital electrical signal. Each digital signal then is routed to its corresponding electrical interface: TCPIIP, ATM, STM, and so on However, that each channel requires its own clock recovery circuitry because all channels may be at different bit rates.

Conclusion

The network topology of your DWDM system depends on various factors, including the number of nodes, maximum traffic capacity, scalability, number of fiber links between nodes and so on. Attentions also should be attached to the network components involved in the DWDM system. Hope this article could help to get more understanding towards DWDM technology.

Source: http://www.fiber-optic-solutions.com/dwdm-topology-design-make-right.html

How Does Erbium Doped Fiber Amplifier (EDFA) Benefit WDM Systems

Optical network that involves WDM (wavelength division multiplexing) currently gains in much popularity in existing telecom infrastructure. Which is expected to play a significant role in next generation networks to support various services with very different requirement. WDM technology, together with EDFA (Erbium Doped Fiber Amplifier), allowing the transmission of multiple channels over the same fiber, that makes it possible to transmit many terabits of data over distances from a few hundred kilometers to transoceanic distances, which satisfy the data capacity required for current and future communication networks. This article explains how can WDM system benefit from this technology.

Basics of EDFA

The key feature of EDFA technology is the Erbium Doped Fiber (EDF), which is a conventional silica fiber doped with erbium. Basically, EDFA consists of a length of EDF, a pump laser, and a WDM combiner. The WDM combiner is for combining the signal and pump wavelength, so that they can propagate simultaneously through the EDF. EDFA can be designed that pump energy propagates in the same direction as the signal (forward pumping), the opposite direction to the signal (backward pumping), or both direction together. The pump energy may either by 980nm pump energy or 1480nm pump energy, or a combination of both. The most common configuration is the forward pumping configuration using 980nm pump energy. Because this configuration takes advantage of the 980nm semiconductor pump laser diodes, which feature effective cost, reliability and low power consumption. Thus providing the best overall design in regard to performance and cost trade-offs.

Why EDFA Is Essential to WDM Systems?

We know that when transmitting over long distance, the signal is highly attenuated. Therefore it is essential to implement an optical signal amplification to restore the optical power budget. This is what EDFA commonly used for: it is designed to directly amplify any input optical signal, which hence eliminates the need to firstly transform it to an electronic signal. It simply can amplify all WDM channels together. Nowadays, EDFA rises as a preferable option for signal amplification method for WDM systems, owing to its low-noise and insensitive to signal polarization. Besides, EDFA deployment is relatively easier to realize compared with other signal amplification methods.

4-Channel WDM System With or Without EDFA: What Is the Difference?

Two basic configurations of WDM systems come in two forms: WDM system with or without EDFA. Let’s first see the configuration of WDM system without using it. At the transmitter end, channels are combined in an optical combiner. And these combined multiple channels are transmitted over a single fiber. Then splitters are used to split the signal into two parts, one passes through the optical spectrum analyzer for signal’s analysis. And other passes through the photo detector to convert the optical signal into electrical. Then filter and electrical scope is used to observe the characteristics of signal. In this configuration signals at long distance get attenuated. While this problem can be overcome by using erbium doped fiber amplifier.

As for WDM system which uses EDFA, things are a little bit different. Although the configuration is almost the same as WDM system without it, some additional components are used. These components are EDFAs which are used as a booster and pre-amplifier, and another additional component is optical filter. With the adoption of optical amplifier, this system doesn’t suffer from losses and attenuation. Hence, it is possible to build broadband WDM EDFA which offer flat gain over a large dynamic gain range, low noise, high saturation output power and stable operation with excellent transient suppression. The combination provides reliable performance and relatively low cost, which makes EDFAs preferable in most applications of modern optical networks.

Conclusion

Among the various technologies available for optical amplifiers, EDFA technology proves to be the most advanced one that holds the dominate position in the market. In future, the WDM system integrated with high performance EDFA, as well as the demand for more bandwidth at lower costs have made optical networking an attractive solution for advanced networks.

Source: http://www.fiber-optic-solutions.com/fiber-amplifier-edfa-wdm-systems.html

How to Use WDM for Fiber Capacity Expansion?

Imagine turning a cottage into a majestic skyscraper without having to deliver any innovation or construction. This is what wavelength division multiplexing (WDM) allows with your existing fiber optic network. The hunger for bandwidth propels service providers to make a substantial investment in upgrading fiber cabling infrastructure. This can be a challenge both economically and practically. However, the WDM technology offers an alternative to increase capacity on the fiber links that are already in place. Without deploying additional optical fiber, WDM greatly reduces the cost of network expansion.

WDM Technology Explanation

Let’s begin with the most fundamental question: What is WDM technology? Short for wavelength division multiplexing, WDM is a way of transmitting multiple simultaneous data streams over the same fiber. Since this happens simultaneously, WDM does not impact transmission speed, latency or bandwidth. WDM functions as multiplexing multiple optical signals on a single fiber by using different wavelengths, or colors, of laser light to carry different signals. Network managers can thus realize a multiplication effect in their available fiber’s capacity with WDM.

To implement WDM to the infrastructure is rather simple, WDM setup generally consists of the following:

• WDM transmit devices, each operating at a different wavelength
• Multiplexer, a passive device that combines the different light sources into a blended one
• Fiber infrastructure
• De- Multiplexer, a passive device that splits the blended light source into separate ones
• WDM receive devices

What Capacity Increase Can We Expect?

There are two variants of WDM: CWDM (coarse wave-division multiplexing) and DWDM (dense wave-division multiplexing). The only difference between them is the band in which they operate, and the spacing of the wavelengths and thus the number of wavelength or channels that can be used.

When using WDM on existing fiber cabling, you should also consider the fiber type (single-mode or multimode) and loss level. For CWDM, 8 to 18 devices may be possible, whereas for DWDM, up to 40 channels are the most common case, but it is possible to reach up to 160 channels.

Choose the Right Type of WDM

We’ve known that both CWDM and DWDM are available to optimize network capacity. Then, here comes another question: should I choose CWDM or DWDM technology? Let’s make a comparison of them.

Coarse Wave Division Multiplexing (CWDM)

CWDM increases fiber capacity in either 4, 8, or 18 channel increments. By increasing the channel spacing between wavelengths on the fiber, CWDM allows for a simple and affordable method of carrying up to 18 channels on a single fiber. CWDM channels each consume 20 nm of space and together use up most of the single-mode operating range.

Benefits of CWDM:

• Passive equipment that uses no electrical power
• No configuration is necessary, much lower cost per channel than DWDM
• Scalability to grow the fiber capacity as needed
• With little or no increased cost
• Protocol transparent and ease of use

Drawbacks of CWDM:

• 18 channels may not be enough, and fiber amplifier cannot be used with them
• Passive equipment that has no management capabilities
• Not the ideal choice for long-haul networks

Dense Wave Division Multiplexing (DWDM)

DWDM allows many more wavelengths to be combined onto one fiber. DWDM comes in two different versions: an active solution and a passive solution. An active solution requires wavelength management and is well-suited for applications involving more than 32 links over the same fiber. In most cases, passive DWDM is regarded as a more realistic alternative to active DWDM.

Benefits of DWDM:

• Ideal for use in long-haul and areas of greater customer density
• Up to 32 channels can be done passively
• Up to 160 channels with an active solution
• Active solutions involve EDFA optical amplifiers to achieve longer distances

Drawbacks of DWDM:

• DWDM solutions are quite expensive
• Active DWDM solutions require a lot of set-up and maintenance expense
• Very little scalability for deployments under 32 channels, much unnecessary cost is incurred per channel

To sum it up, CWDM can be typically used for applications that do not require the signal to travel great distances and in locations where not many channels are required. While for applications that demand for a high number of channels or for long-haul applications, DWDM is the ideal solution.

Considerations for Deploying WDM

Making sure that the CWDM and DWDM will perform properly is critical, so one should account for the following aspects for when deploying.

1.Before buying a mux or demux for use in an unconditioned cabinet or splice case, verify that the operating temperature will fit the application. And ensure that the CWDM or DWDM will be able to operate within the temperatures in which they will be placed.

2.Take the insertion loss of WDM network into account. Using the maximum insertion loss value in the link budget is a good idea. Calculate the loss for both the mux and demux components.

Conclusion

WDM technology provides an ideal solution for fiber exhaust problem that many communication providers are experiencing. It eliminates the need for investing on new fiber construction projects while greatly increases fiber capacity of the existing infrastructure. Hope what presented in the article could help you to choose the right WDM solution.

Source: http://www.fiber-optic-solutions.com/use-wdm-fiber-capacity-expansion.html

Introduction to Erbium Doped Fiber Amplifier (EDFA)

In optical communication network, signal travels through fibers in every large distances without significant attenuation. However, when it comes to the distance up to hundreds of kilometers, to amplify the signal during transit becomes rather essential. In this case, an optical fiber amplifier is required to achieve signal amplification in long distance optical communication. This article aims to give a brief introduction to the most deployed fiber amplifier—Erbium doped fiber amplifier (EDFA).

What Is EDFA?

An EDFA is an optical or IR repeater that amplifies a modulated laser beam directly, without opto-electronic and electro-optical conversion. Generally speaking, EDFA is an optical repeater device that is used to boost the intensity of optical signals being carried through a fiber optic communications system.

Working Principle of EDFA

EDFA serves as a kind of optical amplifier which is doped with the rare earth element erbium so that the glass fiber can absorb light at one frequency and emit light at another frequency. An external semiconductor laser couples light into the fiber at infrared wavelengths of either 980 or 1480 nanometers. This action excites the erbium atoms. Additional optical signals at wavelengths between 1530 and 1620 nanometers enter the fiber and stimulate the excited erbium atoms to emit photons at the same wavelength as the incoming signal. This action amplifies a weak optical signal to a higher power, effecting a boost in the signal strength. The following picture shows 13dBm output C-band 40 channels booster EDFA for DWDM Networks.

The Advantages of EDFA

The EDFA obtains the advantages of high gain, wide bandwidth, high output power, high pumping efficiency, low insertion loss, and it is not sensitive to the polarization state.

• It provides in-line amplification of signal without requiring electronics, and the signal does not need to be converted to electrical signal before amplification. The amplification is entirely optical.
• It provides high power transfer efficiency from pump to signal power.
• The amplification is independent of data rate.
• The gain is relatively flat so that they can be cascaded for long distance use. On the debit side, the devices are large. There is gain saturation and there is also the presence of amplified spontaneous emission (ASE).

The Applications of EDFA

The EDFA was the first successful optical amplifier and a significant factor in the rapid deployment of fiber optic networks during the 1990s. By adopting EDFA in conventional optical digital communication system applications, we can save a certain amount of optical repeaters. Meanwhile, the distance relay could also be increased significantly, which is vital for the long-haul fiber optic cable trunking systems. The EDFA is usually employed in these circumstances:

EDFA can be employed in the high-capacity and high-speed optical communication system. It offers a constructive and ideal solution for handling low sensitivity of receivers and short transmission distances because of a lack of OEO repeater.

In addition, EDFA can be adopted in long-haul optical communication system, such as land trunk optical transmission system and the submarine optical fiber cable transmission system. It helps to lower construction cost dramatically by reducing the quantity of regenerative repeaters.

Moreover, EDFA can also be employed in wavelength-division multiplexing (WDM) system, especially dense wavelength-division multiplexing (DWDM) system. It enables the problems of insertion loss to be solved successfully and reduces the influences of chromatic dispersion.

Conclusion

By far, being the most advanced and popular optical amplifier, EDFA has been widely adopted in the optical fiber communication networks. Featured by flat gain over a large dynamic gain range, low noise, high saturation output power and stable operation with excellent transient suppression, it surely will capture a rather vital and indispensable position in optical communication in the near future.

First published: http://www.fiber-optic-solutions.com/introduction-to-erbium-doped-fiber-amplifier-edfa.html