Category Archives: WDM Optical Network

Introduction to WDM Transponders

With the development of wavelength-division multiplexing (WDM) technology, the network traffic volume is increasing and the demand for more network bandwidth is also on the rise. By converting the operating wavelength of the incoming bitstream to an ITU-compliant wavelength, WDM transponder serves as a key component in WDM system. As an important technology in the fiber optical network, WDM is moving beyond transport to become the basis of all-optical networking. And how to optimize WDM network has always been a hot topic. The transponder is a device to optimize the performance of WDM network, which plays an important in the whole system of WDM network. This article will introduce you the information on WDM transponders.

What Is a WDM Transponder?

Also called as an OEO (optical-electrical-optical) transponder, a WDM transponder is an optical-electrical-optical wavelength converter, which has been widely adopted in a variety of networks and applications. The picture below shows us how a bidirectional transponder works. In this picture, the transponder is located between a client device and a DWDM system. And we can see clearly that, from left to right, the transponder receives an optical bitstream operating at one particular wavelength (1310 nm), and then 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.

WDM transponder

The Application of a WDM Transponder

According to its function, the application of a WDM transponders can be classified into the following types.

  • Wavelength Conversion. It is known to us that when a CWDM Mux/Demux or DWDM Mux/Demux is added into a WDM network, there is a requirement to convert optical wavelengths like 850nm, 1310nm and 1550nm to CWDM or DWDM wavelengths. Then the OEO transponder comes to assist. The OEO transponder receives, amplifies and re-transmits the signal on a different wavelength without changing the signal content.
  • Fiber Mode Conversion. Multimode fiber optic cables (MMF) are often used in short distance transmission, while single-mode fiber optic cables (SMF) are applied in long optical transmission. Therefore, in some network deployment, considering the transmission distances, MMF to SMF or SMF to MMF conversions are needed. WDM transponders can convert both multimode fiber to single-mode fiber and dual fiber to single fiber.
  • Signal Repeating. In long haul fiber optic transmission, WDM transponder also can work as repeaters to extend network distance by converting wavelengths (1310nm to 1550nm) and amplifying optical power. The OEO converters convert the weak optical signals from the fiber into electrical signals, and regenerate or amplify, then recover them into strong optical signals for continuous transmission.
WDM Transponder and FMT Solution

At FS, OEO transponders are made into small plug-in cards to be used on the FMT platform. FMT platform makes devices like EDFA, OEO, DCM, OLP and VOA into plug-in cards and provides standard rack units as well as free software to achieve better management and monitoring. In addition, FMT series products like OEO, DCM and OLP also have higher performance than that of old ones. FMT series OEO transponder can convert optical signals into DWDM wavelengths, reducing the fault risk caused by high power consumption of DWDM fiber optic transceiver. Since the OEO transponder is made into small plug-in card in the FMT platform, it only occupies one slot in the special designed chassis when installed, thus saving a lot of space. In addition, all these FMT plug-in cards, including OEO, in a rack unit share the same power source and support hot plug & play operation. And they can be inserted or removed flexibly in the racks for DWDM networking.

FMT

Conclusion

Since the OEO transponder plays an important role in WDM network, such as receiving, amplifying and re-transmitting the signal on a different wavelength, adding an OEO transponder into the WDM network is very essential. The OEO transponders in our FMT series are made into small plug-in cards with high quality to ensure good transmission performance. For more information on our FMT system, please visit www.fs.com.

Brief Introduction to EDFA

In fiber optic communication systems, problems arise from the fact that no fiber material is perfectly transparent. The visible-light or infrared beams carried by a fiber are attenuated as they travel through the material. This necessitates the use of optical amplifiers. And EDFA (Erbium Doped Fiber Amplifier) is a representative one in the optical amplifier. There is one saying that EDFA is the most popular optical amplifier in optical network communications. Next, we will begin with the definition of EDFA.

The Definition of EDFA

An EDFA, also called optical amplifier or an erbium-doped fiber amplifier or erbium amplifier, is an optical or IR (Infrared Radiation) repeater that amplifies a modulated laser beam directly, without opto-electronic and electro-optical conversion. The device uses a short length of optical fiber doped with the rare-earth element erbium. When the signal-carrying laser beams pass through this fiber, external energy is applied, usually at IR wavelengths. This so-called pumping excites the atoms in the erbium-doped section of optical fiber, increasing the intensity of the laser beams passing through. The beams emerging from the EDFA retain all of their original modulation characteristics, but are brighter than the input beams.

Three Major Applications for Optical AmplifierThree Major Applications for Optical Amplifier

The above picture illustrates the three major applications for optical fiber amplifiers: booster, in-line amplifier, and pre-amplifier. These applications are described in more details below:

Booster Amplifier

Booster amplifiers are placed directly after the optical transmitter. In this application, booster amplifier is adopted to compensate for the losses of optical elements between the laser and optical fibers so that the increased transmitter power can be used to go further in the link.

In-line Amplifier

In-line amplifiers or in-line repeaters are placed along the transmission link to compensate for the losses incurred during propagation of optical signal. They take a small input signal and boost it for re-transmission down the fiber. Here it should also be pointed out that to control the signal performance and the noise added by the EDFA is important, because noise added by amplifier will limit the system length.

Pre-amplifier

Pre-amplifiers are placed just before the receiver to increase the signal level before the photodetection takes place in an ultra-long haul system so as to improve receiver sensitivity. By placing a pre-amplifier, a much larger signal can be presented to the receiver, thus easing the demands of the receiver design.

Top EDFA Products Overview

By now, you should have a basic idea of what an EDFA is and what it is used for, next I will introduce you some truly excellent EDFA products on the market.

Type
Description
22dBm Output Booster DWDM EDFA C-band 24dB Gain, 1U Rack Mount
16dBm Output Mid-stage DWDM EDFA C-band 26dB Gain, Plug-in Card for FMT Multi-Service Transport System
17dBm Output Mid-stage DWDM EDFA C-band 17dB Gain, Plug-in Card for FMT Multi-Service Transport System
Conclusion

Of the various technologies available for optical amplifiers, EDFA technology is the most advanced, and consequently the vast majority of optical amplifiers are designed based on this technology. In addition, the combination of reliable performance and relatively low cost allows EDFA to be widely deployed in modern optical networks.

Optical Facility Protection for WDM Network

Wavelength-division multiplexing (WDM) is nothing new to us. It is a technology that multiplexes multiple optical signal on a single optical fiber by using different wavelengths of laser light. The multiple transmission paths involved in WDM network effectively relieve fiber exhaustion and extend link capacity, but they also make facility protection more essential than ever, because solid facility protection is the key to the availability of the link and the data being transmitted. This article introduces two methodologies that proven to be valid for optical link protection: electrical switching and optical switching.

Why Facility Protection is Essential to WDM Network?

With the explosion of information, the demand for extremely high-capacity data transmission began to soar. Enterprises and companies were asked to deliver greater volumes of traffic at much higher rates. Which spurs the need to store data in different facilities and to transport these data over different paths, so that if any network failure or downtime occurs, they can soon recover and keep the business running. In a properly protected WDM network, customers will have two or more sites that are connected to each other by diver paths, ensuring the availability and reliability of the network all the time. But fiber may break for many reasons including damage from the physical environment and human faults. Thus facility protection becomes vitally important.

Effective Facility Protection Methods for WDM Network

There are basically two methods for optical facility protection: one is electrical switching which adopts a cross connect to duplicate and select the working or protecting path, with two independent optics involved per each path and two Mux/Demux. And the other is optical switching, unlike electrical switching, it typically uses an optical switch to select the working or protected path.

Electrical Switching

In electrical switching, each service is simultaneously transmitted and received from two dark fibers. The signal from the device on the left side is transmitted to both working and protecting fiber, then it is delivered to the end device on the right side.

facility protection with electrical switching

So how the cross connect duplicates the Tx signals and selects the working and protecting path (Rx) for the receiving signal? In fact, the Tx signal is sent through the cross connect and duplicated through both transponders. On the Rx direction, the cross connect switches the signal to the receiving optical power of the transponder.

electrical switching details

Optical Switching

An optical switch is involved in this method to duplicate the data to the working and protecting fiber with an optical splitter, and selecting the operating fiber according to the optical power signals of all the services. One of the distinct differences between optical switching and electrical switching is that it simply offers no protection for the WDM optic.

facility protection with optical switching

Electrical Switching vs. Optical Switching: How to Choose?

When applied for optical facility protection, both methods have their benefits and drawbacks. For electrical switching, the WDM optic is better protected since it uses two uplink transponders per service – one for working and the other for protecting. Since protection is delivered per service, once a single service needs to be switched, the other service won’t be disturbed. Moreover, electrical switching is suited for any network topologies, and no power budget loss is associated with this method. However, electrical switching generally adopted more WDM optics and an additional Mux/Demux, hence fewer services are available through each unit, and it inevitably increases total costs.

While for optical switching which does not offer protection for WDM optic, more ports are available to transport services on each unit. Besides, no additional Mux/Demux is required in this method, so the overall cost of the solution can be decreased. The drawbacks of this method are that the optical switch lowers the optical power budget of the link. And optical switching is not suited for ring topologies for the fact that add and drop functionality is not available per wavelength.

Conclusion

Optical facility protection impacts the link availability, performance and reliability to a large extent. Your choice on facility protection method should always base on your specific needs, and taking power budget, network topology and cost into consideration. I hope this article would be helpful for you to make an informed decision.

Dual-Fiber or Single-Fiber CWDM Mux Demux for Higher Capacity Need?

What would you do if your network capacity can not meet your requirement? Will you put more fibers or update your system? In fact, these two methods are not very recommendable. Why? As your fiber cabling infrastructure is limited for adding fibers and high cost is required for upgrading system, these two methods are unworkable or too expensive. Under this condition, using a pair of CWDM Mux Demux to build a CWDM system with higher capacity is highly recommended. The CWDM Mux Demux is regarded as a key component for a CWDM system, as shown below. It can be simply divided into two types, dual-fiber and single-fiber CWDM Mux Demux. To meet the higher capacity need of your system, this post will mainly introduce the basic knowledge of the dual-fiber and single-fiber CWDM Mux Demux and guide you find a suitable fiber optic Mux Demux for building your CWDM system.

CWDM system

Dual-Fiber CWDM Mux Demux

Dual-Fiber CWDM Mux Demux is a passive device multiplexing and demultiplexing the wavelengths for expanding network capacity, which must work in pairs for bidirectional transmission over dual fiber. It enables up to 18 channels for transmitting and receiving 18 kinds of signals, with the wavelengths from 1270 nm to 1610 nm. The CWDM transceiver inserted into the fiber optic Mux port should have the same wavelength as that of Mux port to finish the signal transmission. For instance, the two reliable 4 channel CWDM Mux Demux showed below use four wavelengths, 1510 nm, 1530 nm, 1550 nm and 1570 nm, their corresponding CWDM transceivers also features the same wavelengths.

Dual Fiber CWDM Mux Demux

When the connection above works, the left 4 channel dual-fiber CWDM Mux Demux uses 1510 nm, 1530 nm, 1550 nm and 1570 nm for transmitting 4 kinds of signals through the first fiber, while the right 4 channel dual-fiber CWDM Mux Demux features 1510 nm, 1530 nm, 1550 nm and 1570 nm for receiving the signals. On the other hand, the transmission from the right to left use the same wavelengths to carry another 4 signals through the second fiber, finally achieving the bidirectional signal transmission.

Single-Fiber CWDM Mux Demux

Single-fiber CWDM Mux Demux should be also used in pairs. One multiplexes the several signals, transmits them through a single fiber together, while another one at the opposite side of the fiber demultiplexes the integrated signals. Considering that the single-fiber CWDM Mux Demux transmitting and receiving the integrated signals through the same fiber, the wavelengths for RX and TX of the same port on the Single-fiber CWDM Mux Demux should be different. Hence, if the 4 channel single-fiber CWDM Mux Demux is used for CWDM system, 8 wavelengths are required, the twice time as that of the dual-fiber one.

Single Fiber CWDM Mux Demux

The working principle of single-fiber CWDM Mux Demux is more complicated, compared to the dual-fiber one. As shown in the figure above, the transmission from the left to right uses 1470 nm, 1510 nm, 1550 nm and 1590 nm to multiplex the signals, transmit them through the single fiber, and using the same four wavelengths to demultiplex the signals, while the opposite transmission carries signals with 1490 nm, 1530 nm, 1570 nm and 1610 nm over the same fiber. As for the wavelength of the transceiver, it should use the same wavelength as TX of the port on the CWDM Mux Demux. For example, when the port of a single-fiber CWDM Mux Demux has 1470 nm for TX and 1490 nm for RX, then a 1470nm CWDM transceiver should be inserted.

Dual-Fiber vs. Single-Fiber CWDM Mux Demux

We always consider whether an item is worth buying according to its performance and cost. In view of the performance, the single-fiber CWDM Mux Demux can carry signals through only one fiber supporting fast speed transmission and saving the fiber resource, while the dual-fiber one requires two fibers for transmission with a higher reliability. Besides, using single-fiber CWDM Mux Demux can be easier to install. In view of the cost, the single-fiber CWDM Mux Demux is much more expensive than the dual-fiber. And the simplex fiber cable also costs higher than duplex fiber cable. Thereby, the whole cost for building single-fiber CWDM system must be much more higher. Like the two sides of the same coin, both the dual-fiber and single-fiber CWDM Mux Demux have their own advantages and disadvantages. Which one you should choose just depends on your system needs and your budget for building the CWDM system.

Embedded CWDM Solution for Fiber Channel SAN Extension

CWDM, as an established optical transport technology, is universally employed in optical network for transmission distance extension and fiber exhaust reduction. This technology has evolved for years and now is available for Fiber Channel applications with the rate up to 4.25Gb/s. Moreover, when compared with traditional transmission approach via multiple fibers, embedded CWDM technology also makes economic sense while used in 4G Fiber Channels, and that’s what we are going to address in this article.

Fiber Channel Overview

The 4G Fiber Channel effectively improves storage area networks (SANs) performance by doubling speed and offering backward compatibility with 2G and 1G systems. With the proliferation of bandwidth-extensive applications, fiber capacity is on the edge of exhaustion. However, the demand for extremely high-capacity data transmissions began to soar. In this case, it is critical for IT technicians to enhance Fiber Channel SAN capacity without increasing costs.

Economical Fiber Channel Solution: Embedded CWDM Technology

CWDM technology expands fiber capacity by multiplexing optical light signals of different wavelength on a single optical fiber. In a CWDM network, components like CWDM Mux/Demux and CWDM transceivers are indispensable. With CWDM, IP data (Gigabit Ethernet or 10G Ethernet) and storage data (4G/2G/1G Fiber Channel) can be transported over a single fiber infrastructure, eliminating the need for complex protocol conversion.

Until now, standalone CWDM solutions are the commonly used methods to transport LAN connections and SAN connections simultaneously between main and recovery sites. Despite that it generates high equipment cost and reduces system reliability. In this case, embedded CWDM emerges as an ideal alternative for use in Fiber Channel applications. Embedded CWDM integrates CWDM optics (like CWDM SFP transceivers) directly into the Fiber Channel switch or Ethernet router, offering better reliability and simplicity. Thus instead of laying more fibers and equipment, users can extend system capacity only by adding new CWDM SFPs, which greatly reduces human labor and overall expenditure.

embedded CWDM solution with CWDM SFPs

Embedded CWDM for Distance Extension in 4G Fiber Channel

While used for SAN distance extension, CWDM functions to reduce the amount of required WAN fibers. Here we take the example of a SAN extension between a primary site and secondary site. With solution A, the implementation requires several WAN fibers to get the capacity required.

multiple fiber in SAN

As for solution B, CWDM is adopted to multiplex several signals onto a single fiber pair. CWDM SFPs are directly plugged into the ports from the Fiber Channel switch to generate signals of specific CWDM wavelength. The CWDM Mux combines wavelengths onto a fiber pair, while the CWDM Demux splits these wavelengths into several fiber on the receiver site. Thus the number of required WAN fibers is reduced by the number of wavelengths used.

embedded CWDM in SAN extension

Benefits of Embedded CWDM Solution

Embedded CWDM system is easier to operate, which requires no additional network management or training. It introduces more reliability, flexibility and simplicity due to fewer components involved in the system. And its advantages become more evident when it comes to cost: Embedded CWDM solution simply offers lower investment expenditure and operation cost, since it removes the need for adding new fibers and equipment, which can be cost-prohibitive. Even that CWDM SFPs and CWDM Mux/Demux should be involved in CWDM system, the overall cost is just a fraction of multiple fiber transmission.

Conclusion

CWDM solution allows IT managers to achieve network capacity expansion in a more cost-effective, simplified and flexible way. Besides, it also provides enhanced performance and reliability for current need and future growth. For more CWDM solutions and information, visit www.fs.com or contact us via sales@fs.com.