DWDM Solutions for Metropolitan Area Network

by Fiber-MART.COM

Today’s services like voice, video and data networks are getting complex and growing tremendous, as well as requiring more bandwidth and faster transmission rates over farther distances. All these cause ferocious competition in the service provider market. And it’s impossible to charge more for the ever-increasing access speed, which resulted in the deployment of DWDM (dense wavelength-division multiplexing) technology in the long-haul transoceanic and terrestrial networks. As DWDM equipment becomes more economical and cost-effective, to deploy them in the metropolitan area network (MAN) is necessary.

 

Why DWDM is Necessary for MANs?

When it comes to DWDM for MANs, many people may ask “Why is DWDM suddenly so popular for MAN applications?” Considering both the DWDM technology and MAN, this question can be discussed from two aspects.

 

The first one is the situation of MAN at present. The practical demands for bandwidth and speed of users affect service providers in the MANs. They have to face problems such as service diversity, bandwidth scalability and fiber exhaustion. Bandwidth scalability deals with the capability of the fiber to carry traffic at exponentially greater speeds. Only by that, the bandwidth provided by service managers may meet the requirements. As for fiber exhaustion, adding more fiber cables may be helpful, but it will cost more and need a long time to finish.

 

Another one is the advantages of deployment DWDM technology. DWDM technology was first deployed on long-haul routes in a time of fiber scarcity. And it finds applications mainly in ultra-high bandwidth long haul as well as in ultra-high-speed metropolitan or inner-city networks, and at the edge of other networks like SONET, SDH and IP. As the ubiquitous deployment of DWDM network and the passive or active DWDM equipment, DWDM technology is expected to be the low-cost solution in MANs and many access-type networks like FTTH and FTTD.

 

Essential DWDM Equipment Used in MAN

As we know, in DWDM networks, multiple wavelengths of light are multiplexed into one single fiber. Therefore, DWDW equipment should be capable of combing or removing signals. Here are three main types of DWDM equipment used in MAN.

 

DWDM Multiplexer and Demultiplexer (DWDM Mux/Demux)

DWDM Mux/Demux modules deliver the benefits of DWDM technology in a fully passive solution. They provide a good solution for long-distance links where wavelengths are packed tightly together over the C-band range of wavelengths, up to 48 wavelengths in 100GHz grid (0.8nm) and 96 wavelengths in 50GHz grid (0.4nm). Usually the common configuration of DWDM Mux/Demux is from 8 channels to 96 channels.

 

DWDM Optical Add/Drop Multiplexer (DWDM OADM)

In order to connect different sites via individual wavelengths, a DWDM system needs to have the capability to add and drop services at distinct locations. That’s why DWDM OADM is needed. OADM can pass-through, remove or add the channels as required. It’s a pass type of DWDM equipment. And the optical losses are different for the add, drop, and pass-through channels.

 

DWDM Erbium-Doped Fiber Amplifier (DWDM EDFA)

Since DWDM network is designed for long runs transmission, the optical power of signals will attenuate with the distances increase. To ensure the quality of signals, DWDM EDFA are used according to the power budget between the transmitter and receiver. Usually based on the used places, there are three common types of fiber amplifiers: booster, in-line amplifier and pre-amplifier. And the gain also can be customized based on specific needs.

 

fiber-mart.COM DWDM Solution for Metropolitan Area Network

Today’s explosion in data traffic, fueled in large part by the growth of the Internet, has caused rapid depletion of available fiber bandwidth in the metropolitan area. The bandwidth bottleneck is now that of the MANs, where capacity has not increased as greatly as the long-haul networks. To meet the demand for long runs transmission in MANs, fiber-mart.COM provides FMT multi-service transport platform for flexible and high density DWDM networking solution. All DWDM equipment like EDFA, OEO, DCM and OLP (optical line protection) come in small plug-in cards, enabling easy installation and management during network deployment. For more details about this DWDM solution, please read this article: Extend DWDM Network Transmission Distance With Multi-Service Transport Platform

How to Troubleshoot a Faulty PON With an OTDR

It is easy to trouble shoot the failure which occurs on a point-to-point FTTx network by using an optical time domain reflectometer (OTDR) test. However, troubleshooting a faulty point-to-multipoint network (i.e. PON network) differs significantly and are more complex than a point to point network. This post will introduce the potential faults which may occur in a PON, and explain how to troubleshoot them with an OTDR.

 

Brief Introduction of PON

A PON (passive optical network) is a telecommunications network that uses point-to-multipoint fiber to the premises in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises. A basic PON (see Figure 1) consists of an optical line terminal (OLT) at the service provider’s central office and a number of optical network termination (ONT) or optical network units (ONUs) near end users. Sometimes, a second splitter can be connected in cascade to the first splitter to dispatch services to buildings or residential areas (see Figure 2). The International Telecommunications Union (ITU-T) and Institute of Electrical and Electronic Engineers (IEEE) have created several standards for optical access systems based on PON architecture (G.982, G.983 or G.984 for ITU and 802.3ah or 802.3av for IEEE).

 

Figure 1. Simple PON Topology

 

Figure 2. Cascaded PON Topology

Due to its architecture, operators can easily determine which subscribers are affected, and can also identify possible fault elements such as how many customers are affected and whether the PON is cascaded by using the network monitoring system at the Network Operation Center (NOC).

Possible Scenarios & Potential Faults of a PON

 

In general, we divide the faulty case of a PON as three scenarios. One case is that only one customer is affected. And the other case is occured in the cascaded PON and all affected customers are connected to the same splitter. The last case is all customers dependant on the same OLT are affected whether the PON is cascaded or not. In the first case, there are three probable potential faults. Fault may appear in the distribution fiber between the cutsomer and the closest splitter, or in the ONT equipment, or even appear in the customer’s home wiring. See Figure 3 (a) & (b).

 

Figure 3 (a). PON Case 1—Possible Faults When Only One Subscriber is Affected

 

Figure 3 (b). PON Case 1—Possible Faults When Only One Subscriber is Affected

When all customers connected to the same splitter cannot receive service, but others connected to the same OLT can, namely the second case, the cause may be that there is a fault at the last splitter or in the fiber link between the cascaded splitters. See Figure 4.

 

Figure 4. PON Case 2—Cascaded PON with Affected Subscribers Connected to Last Splitter

To the thrid case described above, if all customers are affected, the fault may occur in the splitter closest to the OLT, or in the feeder fiber/cable of the network, or directly in the OLT equipment, as the Figure 5 shown.

 

Figure 5. PON Case 3—All Subscribers are Affected (All Connected to the First Splitter)

In addition, we should know that if connectors are available at the splitters, terminals, or drops, isolating part of the faulty network will become easier. Inspecting connectors and taking OTDR measurements using 1310/1550 nm wavelengths are often performed on network sections that are out of service.

 

Why Use The Specific In-service Portable OTDR Device?

In order to troubleshoot PON networks in service, two dedicated tools are available — PON power meter and In-service 1625 or 1650 nm OTDR. As we know, a PON power meter is normally employed to verify that the signal is transmitted correctly to and from the ONT. A PON meter measures the power levels of all the signals and can then discriminate whether the issue comes from the customer’s ONT or from the network. However, you might be very confused with that why use In-service OTDR. The use of a classical OTDR with 1310 or 1550 nm test wavelengths would interfere with the traffic signals and disturb the traffic. At the same time, the traffic signals could also disturb the receiver of the OTDR, making it difficult to interpret OTDR traces. Due to mutual disturbances, classical OTDRs cannot be used, and specific in-service OTDRs are required.

The in-service OTDR was designed specifically for testing live fiber networks. This dedicated device uses an out-of band wavelength (test wavelength far away from traffic wavelength) to enable OTDR testing without disturbing either the network transmitters or the receivers. In the case of a PON network, WDM is no longer needed, except for monitoring purposes (using a remote fiber test system). The PON network is a point-to-multipoint configuration and the troubleshooting test is performed directly from an accessible element (ONT or splitter). The operator can disconnect the element because service is already off downstream toward the customer. First, the in-service OTDR must not disturb the other customers while shooting the OTDR test wavelength upstream toward the OLT, which is most likely the case, as OLTs reject signals above 1625 nm, based on ITU-T recommendations. Second, the traffic signals that the OTDR receives will be rejected to obtain accurate OTDR traces. The specific long-pass filter used to protect the OTDR diode can be added either via a jumper between the OTDR and the network or built into the OTDR.

 

Most equipment providers enable the use of the 1625 nm wavelength for safe testing. Some countries, such as Japan, are nevertheless pushing the 1650 nm wavelength as reflected in the ITU-T L.41 recommendation, which provides maintenance wavelengths on fiber-carrying signals. The 1650 nm wavelength is preferred based on the design of the filters and also because it is further away from the traffic signals (current and future PON technologies).

Case of PON Troubleshooting with OTDR

 

In order to make the whole troubleshooting or testing work smothly, it is essential to select the right OTDR tool, the correct pulse width, and the best location to start troubleshooting. OTDR configuration should be set according to the equipment being qualified and the distance to cover.

 

In response to the possible scenarios and potential faults of a PON described above, here are some solutions with OTDR to be introduced in the following. To avoid complexity, this document only analyzes the cases where connectors are only available at the ONT/OLTs.

 

Solution 1: Troubleshooting of the Distribution Fiber

Simple PON—Only one subscriber affected. Consider that no connectors are available at the splitter.(see Figure 6)

Case Test Location OTDR Direction What must be Seen Comment Pulse Width to be use Specific OTDR

Case 1 one customer down Customer’s home Disconnect the ONT Upstream Distribution fiber up to the closest splitter Testing through the splitter is not required, as the issue is only on the disrtibution fiber side. Short pluse 3 to 30 ns In-service OTDR

 

Figure 6. OTDR is Shot Upstream and Trace only Matters up to the Splitter

Solution 2: Troubleshooting of the Distribution Fiber and the Fiber between the Two Splitters in case of a Cascaded Network

All customers linked to the second splitter are down. Let’s consider the case where no connectors are available at the splitter.(See Figure 7.)

Case Test Location OTDR Direction What must be Seen Comment Pulse Width to be use Specific OTDR

Case 2 all the customers are down after the second splitter Customer’s home Disconnect the ONT Upstream Distribution fiber up to the two splitters Testing through the closest splitter is required. Medium pluse 100 to 300 ns In-service OTDR – short dead zone

This case requires viewing the signal after the splitter. The OTDR used must be optimized for this application and have the shortest possible dead zone as the splitter typically provides 7 to 10 dB loss.

 

Figure 7. OTDR is Shot Upstream and Trace should Display the Traffic through the Last Splitter up to the First One

Solution 3: Troubleshooting of the Feeder

Information received at the NOC shows that all customers are down. As the problem likely comes from the feeder side, the most common way to test the faulty network is to shoot an OTDR downstream from the OLT.(See Figure 8.)

Case Test Location OTDR Direction What must be Seen Comment Pulse Width to be use Specific OTDR

Case 3 all customers are down OLT Downstream Feeder Testing through the splitter is unnecessary. Short pluse 3 to 30 ns Unnecessary

 

Figure 8. OTDR is Shot Downstream and Trace should Display the Traffic Down to the First Splitter

Note: OTDR testing directly from the OLT is certainly the preferred choice when a faulty feeder is suspected (Solution 3), but this method is not recommended in the other cases.

Multimode simplex SC to SC fiber optic patch cord

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The proper way to store cables

We’ve all experienced it. The crushing dread of pulling out a cable that you need to not only to find that it is a tangled mess but then discover that it no longer works. This can be the nightmare of anyone that works in an industry that relies on wires and cables to help keep equipment working. No matter if you work in audio, video or simply just want cables that you can rely on in order to enjoy media on your devices, having nicely kept cables not only looks great, but can prolong the life of this much needed resource.

One of the big problems with cables is that they can so easily get tangled. Long lengths of wire can be very unwieldy and can be hard to take the time to pack up and secure for future use. Many people who depend on these cords usually do not have the time to devote to standing in one place for long periods of time, wrapping a cord up in their hands. However, the importance of proper cable storage and maintenance should not be underestimated. A properly wrapped cable has the ability to last longer than one that is abused and hastily shoved into a drawer, box or case.

 

Many people are not aware of techniques that can help them save their cables. If a cable is stored improperly or wound the wrong way, it can fray on the inside and can cause damage, leading to the cable malfunctioning. Luckily, learning the proper way to wind and store a cable is very easy. One of the best ways to get started is to simply make a simple loop with the cord in your hand. Then make a loop around in the opposite direction, creating another circle, and coming out in the same place over the fingers again, but inverted. This will reduce the fraying on the inside of the wire and will help prolong the life of the wire. An added benefit is that the cable will not get tangled and all that is needed to unravel it is just to simply start unwinding. The wire will then unravel straight.

However, in order to keep the wire in this form and in order to store it, first leave enough length at the end of the wire in order to thread it through the circle. Then, continue to wrap it around the outside of the circle and through again to the inside, creating a spiral covering the outside of the circle and keeping all of the parts in place.

 

Another great way to wind and store a cable is to start like the above example, but instead of creating a spiral at the end, create a circle at the start and then start a spiral until you run out of wire. This also has the added benefit of shortening the cord so that it takes up less room when in use. Both of the above examples are great ways you can use to help prolong the life of your cables and store them in a quick and easy way.

Understanding Loss in Fiber Optic

Fiber optic transmission has various advantages over other transmission methods like copper or radio transmission. Fiber optic which is lighter, smaller and more flexible than copper can transmit signals with faster speed over longer distance. However, many factors can influence the performance of fiber optic. To ensure the nice and stable performance of the fiber optic, many issues are to be considered. Fiber optic loss is a negligible issue among them, and it has been a top priority for many engineers to consider during selecting and handling fiber optic. This article will offer detailed information of fiber optic loss.

light-in-fiber-optic

When a beam of light which carrying signals travels through the core of fiber optic, the strength of the light will become lower. Thus, the signal strength becomes weaker. This loss of light power is generally called fiber optic loss or attenuation. This decrease in power level is described in dB. During the transmission, something be happened and causes the fiber optic loss. To transmit optical signals smoothly and safely, fiber optic loss must be decreased. The cause of fiber optic loss located on two aspects: internal reasons and external causes of fiber optic, which are also known as intrinsic fiber core attenuation and extrinsic fiber attenuation.

 

Intrinsic Fiber Core Attenuation

Internal reasons of fiber optic loss caused by the fiber optic itself, which is also usually called intrinsic attenuation. There are two main causes of intrinsic attenuation. One is light absorption and the other one is scattering.

 

Light absorption is a major cause of fiber optic loss during optical transmission. The light is absorbed in the fiber by the materials of fiber optic. Thus light absorption is also known as material absorption. Actually the light power is absorbed and transferred into other forms of energy like heat, due to molecular resonance and wavelength impurities. Atomic structure is in any pure material and they absorb selective wavelengths of radiation. It is impossible to manufacture materials that are total pure. Thus, fiber optic manufacturers choose to dope germanium and other materials with pure silica to optimize the fiber optic core performance.

 

Scattering is another major cause for fiber optic loss. It refers to the scattering of light caused by molecular level irregularities in the glass structure. When the scattering happens, the light energy is scattered in all direction. Some of them is keeping traveling in the forward direction. And the light not scattered in the forward direction will be lost in the fiber optic link as shown in the following picture. Thus, to reduce fiber optic loss caused by scattering, the imperfections of the fiber optic core should be removed, and the fiber optic coating and extrusion should be carefully controlled.

Intrinsic fiber core attenuation including light absorption and scattering is just one aspect of the cause in fiber optic loss. Extrinsic fiber attenuation is also very important, which are usually caused by improper handling of fiber optic. There are two main types of extrinsic fiber attenuation: bend loss and splicing loss.

 

Bend loss is the common problems that can cause fiber optic loss generated by improper fiber optic handling. Literally, it is caused by fiber optic bend. There are two basic types. One is micro bending, and the other one is macro bending (shown in the above picture). Macro bending refers to a large bend in the fiber (with more than a 2 mm radius). To reduce fiber optic loss, the following causes of bend loss should be noted:

 

1.Fiber core deviate from the axis;

2.Defects of manufacturing;

3.Mechanical constraints during the fiber laying process;

4.Environmental variations like the change of temperature, humidity or pressure.

 

fiber optic splicing is another main causes of extrinsic fiber attenuation. It is inevitable to connect one fiber optic to another in fiber optic network. The fiber optic loss caused by splicing cannot be avoided, but it can be reduced to minimum with proper handling. Using fiber optic connectors of high quality and fusion splicing can help to reduce the fiber optic loss effectively.

 

The above picture shows the main causes of loss in fiber optic, which come in different types. To reduce the intrinsic fiber core attenuation, selecting the proper fiber optic and optical components is necessary. To decrease extrinsic fiber attenuation to minimum, the proper handling and skills should be applied.