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Category: WDM & Optical Access

Time for optimizing your DWDM network with FMT

Time for optimizing your DWDM network with FMT

DWDM technology has been widely applied during these years and will continue to provide the bandwidth for large amounts of data. In fact, the capacity of systems will grow as technologies advance that allow closer spacing, and therefore higher numbers, of wavelengths. But DWDM is also moving beyond transport to become the basis of all-optical networking. How to deploy DWDM network with the lowest cost has always been a hot topic. At FS, we remain committed to being customer-oriented, and now we have launched a brand-new FMT ( FS.COM multi-service transport) platform to help you optimize your DWDM network performance. So what is FMT platform and what can FMT do for you?

About FMT

FMT platform

Developed by FS.COM this year, FMT is a multiservice transport system featured by a high density design and low insertion loss. It aims at optimizing the performance of DWDM network and delivering better services for customers. It can satisfy the requirements of both CWDM transmission and DWDM transmission, especially for DWDM long haul transmissions. Compared with the old generation of DWDM network components, the products of FMT series have been enhanced in every aspects, thus it can provide our customer higher networking performance with better management and lower power consumption. Next I will introduce you what is exactly included in our FMT platform.

All-in-One FMT Series DWDM Networking System

Designed to provide the best service at the lowest price, our FMT platform makes devices like EDFA, OEO, DCM, OLP and VOA into small plug-in cards and provides standard rack units as well as free software to achieve better management and monitoring. You may ask why you need to add these devices and how they help your DWDM network. Next I will introduce them to you respectively:

  • OEO. DWDM transceiver can produce wavelength directly. However, it has higher power consumption. Add an OEO converter between switch can decrease fault risks caused by high temperature and power consumption at switches.


  • EDFA. Light loss occurs at many place: the device, the optical fibers, fiber connectors, fusion splicing points etc. To overcome light loss in DWDM network and ensure long distance transmission of high quality, EDFA is usually add in the network.


  • DCM. In DWDM network, there would be compounded dispersion in optical fiber. The dispersion compensation in DWDM network can be optimized by DCM to finite dispersion. The using of DCM differs from different situations. It is usually suggested to add DCM in your network if the transmission distance is longer than 40km.


  • OLP. Optical line protection system is vital in DWDM network. Using vacant optical fiber to build a backup is what many providers are doing to ensure high secure optical transmission network. 1:1 OLP, 1+1 OLP and OBP are all available.


  • VOA. Our variable optical attenuator provides accurate attention and easy operation for optical power adjustment. You can control the optical power with a more flexible and reliable manner without affecting the network on running.


  • Red/Blue Splitter. Red/blue spitter or filter can combine the red transmit channel and the blue receive channel onto a single fiber. This product is commonly used with simplex DWDM network which uses a single fiber for both transmitting and receiving at the same time.

FMT Red/blue Splitter


FMT platform is definitely an ideal choice for you to optimize your DWDM network. And except from ensuring the good DWDM network transmission quality over long distance, we also offer customized one-stop solution including both products and network designing for your DWDM network. For more details please visit FS.COM.

Migration From CWDM to DWDM

Migration From CWDM to DWDM

CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing) have their own advantages and their combination can greatly enhance transmission performance while reducing OPEX and CAPEX spending. Hybrid CWDM/DWDM solution is a flexible and scalable solution which offers many benefits to network carriers, especially in network expansion and upgrade. Today, I will introduce a simple, plug-and-play option for creating hybrid systems of DWDM channels interleaved with existing CWDM channel plans.

Understanding the Principle of Hybrid CWDM/DWDM

The difference between CWDM and DWDM lies in the channel spacing between neighbored wavelengths, for CWDM 20 nm and for DWDM 0.8/0.4 nm (using 100 GHz/50 GHz grid). A typical CWDM mux has a window of +-6.5nm per channel. Thereby several DWDM channels can be transmitted simultaneously in only one CWDM channel. Typically, 1530nm or 1550nm CWDM channels are commonly used for achieving hybrid CWDM/DWDM.

1530 nm C52-C61
1550 nm C27-C42


Deploy Hybrid CWDM/DWDM

Deployment of hybrid CWDM/DWDM is very easy by using a simple, plug-and-play solution offered by FS.COM—FMU 2-slot 1U Rack Mount with matching CWDM & DWDM plug-in modules. As shown in the following picture:


The CWDM plug-in module is a high-band 8 channels module of 1470nm-1610nm wavelengths, with expansion port. And the DWDM module can be 8 channels C54-C61 or 8 channels C34 -C41. In addition, by using EXP port, the further expansion is available.


Hybrid CWDM/DWDM solution offers many advantages for users, especially in cost savings and flexibility of network expansion. The 2-slot plug-and-play solution is a very cost-effective to achieve hybrid CWDM/DWDM in the limited 1U rack. In addition, it’s very flexible and scalable that users can easily deploy the CWDM, DWDM or hybrid CWDM/DWDM. If you are interested in the FMU 2-slot plug-and-play solution and want to know more details, please contact or visit here.

Related article: How much do you know about the ports on WDM MUX/DEMUX?

PLC Splitters Deployment Strategy—Cascaded or Centralized?

PLC Splitters Deployment Strategy—Cascaded or Centralized?

PLC splitters is commonly used fiber optic splitters of fiber optic networks such as FTTx (e.g. FTTH, Fiber to the Home) and PON (Passive Optical Network). In a PON system, the PLC splitter functions to share both cost and bandwidth of the OLT (Optical Line Terminal) among multiple ONTs (Optical Network Terminations) as well as reducing the fiber lines required in the OSP (outside plant). Depending on the customer distribution, there are two common deployment strategies of PLC splitters—Cascade and Centralized.

Cascaded Splitting

In the cascaded splitting model, the PLC splitters are located in the FDH (Fiber Distribution Hub) and at OSP locations, as shown in the picture below. Splitters in OSP, namely cascaded splitters, help minimize the amount of fiber that requires to be deployed to provide service, reducing distribution cable material costs. However, there are some disadvantages of this strategy. Because the cascaded splitters create inefficient use of OLT PON ports and increase the testing and turn-up time of customers.

PLC splitters -cascaded splitting

Advantage & Disadvantages Overview
Advantages Disadvantages
• Least Expensive Passive System to Design
— Minimal up-front network CAPEX requirements
— Uses Fiber-lean Feeder and Distribution System
• Limitations on Bandwidth and Adaptability
— No Single Splitter Configuration or Adaptation Point
— High splitting ratio may limit future network scalability and electronics
• Can be efficient
— Where Take Rate is High and Fairly Stable
— Where Growth is Not an Issue
• Inefficient where Take Rate is Low (<50%)
• Complex to Grow/Scale


Centralized Splitting

In the centralized splitting model, as its name suggested, all PLC splitters are centrally located in FDH locations (see the picture below). Using this deployment strategy, the OLT utilization can be maximized which provide a single point of access for troubleshooting. In addition to provide best OLT utilization & flexibility in limited take rate builds, the centralized splitting also provides easy craft access for testing and turn-up, as well as allowing for ease in transitioning to other PON technologies. But nothing is perfect. The cost of the centralized splitting is an increasing cost of distribution cable material.

PLC splitters -cascaded splitting

Advantage & Disadvantages Overview
Advantages Disadvantages
• LCP Consolidates Local Subscriber Configuration
—Ability to service 32 or more (typ. 64-500) Subscribers per PON
—Provides for a central turn-up location pointing to the OLT and ONT
• Requires Truck-Roll to FDH for Splitter Connection
—May increase cost of incremental subscriber turn-up
• Balances network scalability with up-front CAPEX
—Fiber-lean Feeder & Fiber-rich Distribution System
—Provides dedicated Optical Path from FDH to Subscribers
—Easily adaptable for future WDM PON and split ratio changes
• Supports Efficient Growth Strategies


Cascaded or Centralized?

Cascaded splitting strategy is an ideal option particularly when high take rates are certain or in extremely rural areas where fiber costs become more of a factor. Centralized splitting strategy that has its benefits including flexibility, ease of testing, and overall cost efficiency in many applications should be also considered. In a word, the best deployment strategy is the one that meets the requirements and expectations of the provider by reducing CAPEX, optimizing long-term OPEX, and making a future-proof network that can cope with new technologies without dramatic changes. This is why there are sometimes advantages to mixing both of them, creating a hybrid that leverages the advantages of each others.

Application Schemes of WDM Transponder (O-E-O)

Application Schemes of WDM Transponder (O-E-O)

The WDM (Wavelength Division Multiplexing) transponders, also called O-E-O (optical-electrical-optical) wavelength converters are widely deployed in a variety of networks and applications nowadays, especially the in WDM networking system. In the previous article “What’s the Difference Between Transceiver & Transponder?“, we have learn that the basic concept about the transponder and make clear the difference with transceiver. Today, I am going to talk something about the applications of WDM transponders through several practical cases.

Reviewing the Concept of WDM Transponder

WDM transponder is an optical-electrical-optical (O-E-O) wavelength converters which is designed to performs an O-E-O operation to convert wavelengths of light. Figure 1 shows bidirectional transponder operation (the transponder is located between a client device and a DWDM system). From left to right, the transponder receives an optical bit stream operating at one particular wavelength (1310 nm). And then it converts the operating wavelength of the incoming bitstream to an ITU-compliant wavelength and transmits its output into a DWDM system. On the receive side (right to left), the process is reversed. The transponder receives an ITU-compliant bit stream and converts the signals back to the wavelength used by the client device.


Figure 1. WDM Transponder Working Principle

Application Schemes of WDM Transponder

As the above mentioned, the WDM transponder is widely used in many networks and applications. Here are three classical application schemes of the WDM transponders.

1. Convert Multimode to Single-Mode Fiber

As we know, multimode fiber (MMF) is used for short-distance transmission while the single-mode fiber (SMF) is used for the longer-distance transmission. Mode conversion is required in the network since the network distance requires to exceed the limit of MMF or in the case that equipment is designed with multi-mode port but connectivity is required to single-mode equipment. As the Figure 2 shown, two switches are connected by the WDM transponders which convert the MMF to SMF, enabling the network connectivity across the distance between the switches.

Multimode to Single-Mode Fiber Conversion

Figure 2. Multimode to Single-Mode Fiber Conversion

In addition, the WDM transponder can also be used between a 10G SFP+ DAC (Direct Attach Copper) cable to a SMF. As the Figure 3 shown, a 10m 10G SFP+ DAC cable is used to connect the 10G switch port to the transponder (in the left location); A pair of multimode SFP+ transceivers provide the connectivity between the transponder and the 10G switches (in the right location). This solution is ideal for the application which requires to connect switches exceeds the distance limitation (10 meter) of 10G DAC cable.

10G DAC to Single-Mode Fiber to Multi-mode Fiber Conversion

Figure 3. 10G DAC to Single-Mode Fiber to Multi-mode Fiber Conversion

2. Covert Dual Fiber to Single Fiber

Dual fiber transmission and single fiber transmission are two transmission modes applied in the network. Depending on the type of equipment and the fiber installed facility, dual fiber to single fiber conversion is required sometimes. Here are two situations:

a. Dual Fiber to Single Fiber Conversion

In this case, two dual fiber switches are connected with a single fiber via two transponders. The single fiber is single-mode fiber (1310/1550 nm) and operates with BiDi (bi-directional) wavelengths. See Figure 4.

Dual Fiber to Single Fiber Conversion

Figure 4. Dual Fiber to Single Fiber Conversion

b. Double Fiber Capacity With Dual Fiber to Single Fiber Conversion

Dual fiber uses the same wavelength over two different strands of fiber—one strand as Transmit (Tx) and the other as Receiver (Rx). See Figure 5.

 Traditional Dual Fiber Connection

Figure 5. Traditional Dual Fiber Connection

By converting the dual fiber to single fiber, the network can double the capacity of the exisiting infrastructure, as the Figure 6 and 7 shown.

Double Fiber Capacity With Dual Fiber to Single Fiber Conversion (two links)

Figure 6. Double Fiber Capacity With Dual Fiber to Single Fiber Conversion (two links)

In this application, two other switches are added to each link. The transponder doubles the capacity of the dual fiber link by converting each strand from a dual fiber link to a BiDi single fiber link.

Double Fiber Capacity With Dual Fiber to Single Fiber Conversion (single link)

Figure 7. Double Fiber Capacity With Dual Fiber to Single Fiber Conversion (single link)

In this case, the transponder doubles the capacity of two dual fiber links by converting each strand of the dual fiber to two BiDi single fiber link, providing redundancy protection between the two switches.

3. Wavelengths Conversion

Wavelengths conversion is the most common application of a WDM transponder. Fiber network equipment with fixed fiber interfaces (ST, SC, LC FC, etc.) operating over legacy wavelengths (850 nm, 1310 nm, 1550 nm) must be converted to CWDM (Coarse Wavelength Division Multiplexing) or DWDM (Dense Wavelength Division Multiplexing) wavelengths via a WDM transponder which is to automatically receives, amplifies, and then re-transmits a signal on a different wavelength without altering the data/signal content. As the Figure 8 shown, a 10G switch with signal output at 1310 nm is required to link to a CWDM Mux/DeMux channel port (1610nm wavelength). A transponder configured with a standard SMF SFP+ and a 1610nm CWDM SFP+ is used between the switches and CWDM Mux/DeMux, achieving the wavelength conversion.

Wavelenth Conversion in CWDM System

Figure 8. Wavelenth Conversion in CWDM System

Tips on Using WDM Transponders

After leaning the above contents, we know that WDM transponders can be used in mode conversion, dual to single fiber conversion as well as the wavelength conversion. When using the WDM transponders, you should note the following point:

  • It is not necessary to use the transponder for single wavelength transmission;
  • The number of transponders used in the network depends on your actual network plan;
  • You should choose the corresponding transceivers and linecard to configure the transponder according to your requirement.
FTTX Installation Verification Testing & Maintenance Troubleshooting

FTTX Installation Verification Testing & Maintenance Troubleshooting

FTTH-installation-testing-troubleshootingWith the increasing demands for bandwidth, FTTH (Fiber to the Home) is considered as a next-generation technology for delivering more bandwidth, reliability, flexibility and security to end-users. PON (Passive Optical Network) is the main structure of FTTH network which can provide optical fiber bandwidth advantages at a lower cost than P2P (Point to Point) architecture affords. However, PON presents unique test challenges when installing and maintaining the FTTH network. Furthermore, optical testing is typically performed at various points in a network’s lifetime. Thus, it is very necessary to know which tests should be done and when and where to do it.

In general, optical testing in FTTH can be divided into two part—installation verification testing and maintenance troubleshooting. This is a very general classification according to the state of the network. Installation verification testing occurs as the network is being constructed or after network installation is complete, but before the network is activated. While maintenance troubleshooting is performed when service outages occur, and typically requires rapid response to restore service as quickly as possible.

In installation verification testing, the most complete testing is performed, including insertion and return loss testing as well as OTDR (Optical Time Domain Reflectometer) testing. Pass/fail criteria may be applied to end-to-end length, loss and optical return loss results, as well as to individual event loss and reflectance measurements for splices, connectors, and splitters. Formal reports may be generated, including all of the measurements, OTDR traces, pass/fail criteria and pass/fail results.

Maintenance troubleshooting is done through reroute restoration and/or fault location, repair, and verification before restoring active service. Troubleshooting may also require non-disruptive fiber identification to ensure in-service fibers are not disconnected. Maintenance personnel may require a visual fault locator (VFL) to precisely pinpoint the location of breaks or macro-bends in splice or access enclosures.

Actually, the installation verification testing and maintenance troubleshooting is not as simple as description above. They usually include various test according to different test location and purpose. And for different tests, the performed time and test equipment, as well as the test wavelengths are not fully consistent. To understand this better, here I use a table to summarize.

Click the table to view large version.