Introduction of the Transients in Optical WDM Networks


A systems analysis continues to be completed to consider dynamical transient effects in the physical layer of an Optical WDM Network. The physical layer dynamics include effects on different time scales. Dynamics from the transmission signal impulses possess a scale of picoseconds. The timing recovery loops in the receivers be employed in the nanoseconds time scale. Optical packet switching in the future networks will have microsecond time scale. Growth and development of such optical networks is yet continuing. Most of the advanced development work in optical WDM networks is presently focused on circuit switching networks, where lightpath change events (for example wavelength add/drop or cross-connect configuration changes) happen on the time scale of seconds.
It is focused on the dynamics from the average transmission power associated with the gain dynamics in Optical Line Amplifiers (OLA). These dynamics may be triggered by the circuit switching events and have millisecond time scale primarily defined by the Amplified Spontaneous Emission (ASE) kinetics in Erbium-Doped Fiber Amplifiers (EDFAs). The transmission power dynamics will also be influenced by other active components of optical network, for example automatically tunable Optical Attenuators, spectral power equalizers, or other light processing components. When it comes to these dynamics, a typical power of the lightpath transmission signal is recognized as. High bandwidth modulation from the signal, which actually consists of separate information carrying pulses, is mostly ignored.
Ring WDM networks implementing communication between two fixed points are very well established technology, in particular, for carrying SONET over the WDM. Such simple networks with fixed WDM lighpaths happen to be analyzed in many detail. Fairly detailed first principle models for transmission power dynamics exist for such networks. These models are implemented in industrial software allowing engineering design calculations and dynamical simulation of these networks. Such models could possibly have very high fidelity, but their setup, tuning (model parameter identification) and exhaustive simulations covering a variety of transmission regimes are potentially very labor intensive. Adding description of new network components to such model could need a major effort.
The problems with detailed first principle models is going to be greatly exacerbated for future Mesh WDM networks. The near future core optical networks will be transparent to wavelength signals on a physical layer. In such network, each wavelength signal travels through the optical core between electronic IP routers around the optical network edge using the information contents unchanged. The signal power is attenuated in the passive network elements and boosted by the optical amplifiers. The lightpaths is going to be dynamically provisioned by Optical Cross-Connects (OXCs), routers, or switches independently on the underlying protocol for data transmission. Such network is basically a circuit switched network. It might experience complex transient processes of the average transmission power for every wavelength signal at the event of the lightpath add, drop, or re-routing. A mix of the signal propagation delay and channel cross-coupling might result in the transmission power disturbances propagating across the network in closed loops and causing stamina oscillations. Such oscillations were observed experimentally. Additionally, the transmission power and amplifier gain transients could be excited by changes in the average signal power because of the network traffic burstliness. If for some period of time the wavelength channel bandwidth is not fully utilized, this could result in a loss of the average power (average temporal density of the transmitted information pulses).
First circuit switched optical networks are already being designed and deployed. Fraxel treatments develops rapidly for metro area and long term networks. Engineering design of circuit switched networks is complicated because performance has to be guaranteed for all possible combinations of the lightpaths. Further, as such networks develop and grow, they potentially need to combine heterogenous equipment from a variety of vendors. A system integrator (e.g., fiber-mart) of such network might be different from subsystems or component manufacturer. This creates a necessity of developing adequate means of transmission power dynamics calculations which are suitable for the circuit switched network business. Ideally, these methods should be modular, independent on the network complexity, and use specifications on the component/subsystem level.
fiber-mart has technical approach to systems analysis that’s to linearize the nonlinear system around a fixed regime, describe the nonlinearity like a model uncertainty, and apply robust analysis that guarantees stability and gratifaction conditions within the presence of the uncertainty. For a user of the approach, there is no need to understand the derivation and system analysis technicalities. The obtained results are very simple and relate performance to basic specifications of the network components. These specifications are somewhat not the same as those widely used in the industry, but could be defined from simple experimentation using the components and subsystems. The obtained specification requirements may be used in growth and development of optical amplifiers, equalizers, optical attenuators, other transmission signal conditioning devices, OADMs, OXCs, and any other optical network devices and subsystems influencing the transmission power.

Optical Amplifier Used in CATV Transmission Network


CATV technology has matured steadily over the past several years, and has expanded into diverse applications. However, as the quick expansion in technology and services, it’s important to improve CATV network component performance for higher visual and audio signals transmission. Optical amplifier for CATV application is the key element in such transmission. This post intends to give a clear introduction of optical CATV amplifier and its application in CATV transmission.
Introduction to CATV Amplifier
CATV amplifier is also a type of EDFA (Erbium Doped Fiber Amplifier) amplifier which is the most popular optical amplifier in optical network communications. It is mainly used to amplify damped TV signals (compensation for loss) for improved signal quality before sending them to each subscriber. Moreover, CATV amplifiers not only amplify the signal, but also amplify the noise on the line, and bring some return loss. That’s why a quality CATV amplifier price is a little high, because it can provide better performance for the whole network transmission.
Why CATV Amplifier Is Needed?
As we all know, CATV network is a multi-channel TV system to transmit high quality video and sound signal from a large number of digital or analog broadcast television and radio channel via fiber optic cable or coaxial cable. CATV amplifier often acts as booster optical amplifier in this system to get satisfying transmission effect. The following picture illustrates a basic long haul CATV transmission system using EDFA amplifier.
In most cases, the satellite providers deliver high quality digital video and audio to users’ home depending on the users’ equipment. However, the signal incoming cable feed is connected to more than one equipment with use of optical splitters. And if the incoming signal gets fragmented and rerouted, the overall speed and quality will be worse. Under this condition, an optical amplifier can be used to boost the signal power and help users get better services.
CATV Amplifier in Long-Haul CATV Transmission System
As have mentioned above, a basic long-haul CATV communication link consists of head end, transmitter, receiver, optical amplifier, and sometimes fiber splitter is also needed in this type of transmission network. The head end receives TV signals off the air or from satellite feeds, and supplies them to the transmission system. The optical splitters are often utilized in a poin-to-multipoint configuration. Here are two CATV fiber network cases using CATV booster amplifier.
Case one
This is a point-to-multipoint medium size private CATV network. In the head end, the transmitter receives signals from the RF combiner on the 1310nm or 1550nm wavelength. Then the signals split into several parts and are received by the CATV receiver. Finally, all the signals are amplified by the CATV amplifier and sent to the subscriber.
Case two
In the above application case, the optical amplifier lies behind the CATV receiver, but in this case, it’s a little different.
As we can see from the graph, the CATV amplifier lies in the front of the receiver to boost the transmission distance longer. Except for that, this transmission network also deploys two DWDM Mux/Demux to multiply the eight different wavelengths into one fiber for better transmitting. Please note that this graph just illustrates part of the long-haul CATV system.


Wavelength Division Multiplexing (WDM) Increases Network Capacity

WDM is a method of separating or combining multiple wavelengths out of or into a single fiber strand with each wavelength carrying a different signal. Using optical filters lets a certain range of wavelengths pass through, while another range is allowed. Thin-film filter technology (TFF) is often used to achieve this effect. Multiple thin layers are stacked and interference effects are created by sequential reflections on the interface between the layers. This lets light reflect for certain wavelengths and pass through for others.
The capacity of a network can be increased cost effectively by using WDM. Two types of WDM are commonly used:
Dense Wave Division Multiplexing (DWDM) devices are mainly used when more wavelengths are required between sites and when the network extends over a very long distance. Forty wavelength channels from 1530 nm to 1570 nm are distributed in the C-band. To increase capacity, DWDM can be overlaid on a CWDM infrastructure.
Coarse Wave Division Multiplexing (CWDM) has 18 different wavelength channels standard, spaced 20 nanometers (nm) apart between 1270 nm and 1610 nm. Most systems only use the top eight channels between from 1470 nm and 1610 nm. CWDM systems have the advantage that they can always be upgraded at a later stage. This limits the initial installation costs. The requirements on the lasers is not severe due to the wide channel spacing, allowing less expensive lasers without any temperature control to be used.
The insertion loss of DWDM and CWDM is typically lower than that of optical splitters. This increases the reach of a network from a centralized office substantially. As every customer has wavelength(s) assigned to them, this provides better security and makes eavesdropping virtually impossible.
WDMs Can Be Utilized In Different Ways:
Add/Drop Vs Mux/Demux.
A multiplexer, also known as a mux, combines several wavelength channels on one fiber, while a de-multiplexer (demux) separates them at the other side. A mux/demux configuration is very useful to increase a fiber’s end-to-end capacity. A mux is normally located at a central office, while demuxes are placed in either a splice closure or cabinet. From there the fibers are routed in a star-shaped topology to their ultimate destination.
An alternative to separating the wavelengths at one side, individual wavelengths can be added or dropped at various points across the line. This process does not affect other wavelengths. This is often preferable when the distance between sites is long or they are grouped in a circular structure.
One Or Two Fibers?
An alternative to sending signals at different wavelengths through the same fiber is to use two different fibers. Many CWDM systems use two fibers where one is used for upstream signals and the other for downstream. In this configuration, each customer uses two fibers and one wavelength. Each customer will have two wavelengths if they use a single fiber.

Utilizing the WDM – Increase Fiber Capacity Without Construction

Imagine turning a dirt road into a multilane highway without having to perform any new construction. That is what Wave Division Multiplexing (WDM) allows with an existing fiber network. This technology can greatly reduce the cost of increasing network capacity without having to move a single shovelful of dirt or hang a single new fiber.
It’s no secret that outside-plant (OSP) fiber construction is expensive. Construction costs vary, but they are always hefty, and they increase greatly if cable is buried. In addition to construction, the costs of permitting, zoning, raw materials and splicing are significant. Thus, avoiding installing new fiber is best whenever possible.
Many communications providers are experiencing fiber exhaust in their networks. This means that the cable counts initially deployed are not able to handle today’s needs. Now, emerging technologies in cell backhaul, business class services and others are creating a need for yet more fibers. However, in most cases, ever-increasing labor and material prices make new fiber construction too costly to consider for many projects.
WDM allows operators to place new equipment at either end of a fiber strand and combine multiple wavelength channels on a single fiber strand. Many existing systems use only a small amount of the spectrum available on single piece of glass. Using either coarse wave-division multiplexing (CWDM) or dense wave-division multiplexing (DWDM), operators can combine many different services on a single fiber by assigning a different color, or wavelength, to each service. Multiplexers are used to combine all these wavelengths onto a single fiber, and demultiplexers are used to separate the colors farther on in the network.
Mobile devices, cloud computing, over-the-top video, DOCSIS 3.1 with IPTV, and online gaming are just a few of the drivers for increased bandwidth demand. As demand continues to rise, service providers will need long-term strategies to develop a bigger pipe.
Cellular backhaul, FTTx and commercial business services are also creating a need for more fiber capacity. 3G and 4G cellular services require more bandwidth than cellular services needed in years past and therefore require a fiber link to each cell site. A provider may own a fiber sheath that runs right past a cell tower, but all its fibers may currently be used to maximum capacity. Providing lit services or dark fiber to cell towers can be very profitable but not if it requires plowing or hanging new fiber to these cell sites.
Business-class services are becoming popular revenue sources for communications companies. Businesses are often willing to sign long-term contracts and pay more than residential customers. In some cases, businesses require fiber to meet their bandwidth needs. The same issue arises here: How is it possible to serve these new customers without having to install new OSP fiber to those sites?
Most legacy fiber networks use a single wavelength, or color, on each fiber. Think of it as two people on different mountaintops using white-lens flashlights to communicate via Morse code – not very sophisticated, but it works.
All of a sudden, two more people want to start communicating between those two mountaintops. What is the solution? Use different colored lenses on the flashlights to communicate. Senders and receivers will recognize and send only their own colors of light and ignore the others.
This is basically what a WDM network does. It uses multiple colors of light over the same medium (fiber). Transmitters tuned to specific wavelengths send light into a passive combiner called a mux (short for multiplexer). All the wavelengths travel down the common fiber and are separated using a passive demultiplexer (also called a demux). Now each receiver at the other end will be able to receive just its own discrete signal.
In other words, WDM maps multiple optical signals to individual wavelengths and multiplexes the wavelengths over a single fiber. WDM can carry multiple protocols without having to convert them to a common signal format. A single fiber is able to do virtually anything that’s needed.
There are two main types of WDMs. The advantage of CWDM technology is that it is relatively inexpensive compared with DWDM. The transmitters used in CWDM are less expensive, as they do not need to be tuned as precisely as DWDM transmitters. However, CWDM has drawbacks, too: Only 18 channels are available, and fiber amplifiers cannot be used with them. Thus, they are not the ideal choice for long-haul networks.
CWDM channels each consume 20 nm of space and together use up most of the single-mode operating range. The wavelengths most commonly used are the eight channels in the 1470 to 1610 nm range. Any transceiver used in CWDM applications operates within one of these channels.
DWDM allows many more wavelengths to be combined onto one fiber. It also leverages the capabilities of fiber amplifiers, which can amplify the 1550 nm or C band commonly used in DWDM applications. This makes it ideal for use in long haul and areas of greater customer density. Instead of the 20 nm spacing in CWDM (equivalent to about 15 million GHz), DWDM uses either 50, 100 or 200 GHz spacing in the C and sometimes the L bands. This allows many more wavelengths to be packed onto the same fiber.
The downside of DWDM is that the lasers need to be much more accurate and require precise temperature ranges to operate. This makes DWDM applications much more expensive than CWDMs. The introduction of the ITU-T G.694.1 grid in 2002 made integrating DWDM technology easier. It created an industry standard for DWDM.
Before deploying any WDM equipment, it is necessary to ensure that the glass in place will support all the required wavelengths. Low-water-peak or zero-water-peak fiber is more suitable for WDM applications, and older glass types may have water peak issues. If the glass is too old, it may be necessary to bite the bullet and install some new fiber.
Assuming the glass is appropriate for WDM, should you use CWDM or DWDM technology to solve fiber exhaust problems? As previously noted, CWDM can support a maximum of 18 channels and is not ideal for long haul. So CWDM would typically be best for applications that do not require the signal to travel great distances and in locations where not many channels are required. The availability of SFP transceivers may also be a limiting factor.
For applications that require a high number of channels or for long-haul applications, DWDM is the ideal solution. Though the electronics and passives are not cheap, they are considerably more cost-effective than putting in new fiber.
It’s important to ensure that the CWDM and DWDM passives will operate properly in the environment where they will be placed. This becomes especially important when putting CWDM passives in the outside plant. Before buying a mux or demux for use in an unconditioned cabinet or splice case, verify that the operating temperature will fit the application. Many vendors specify the storage temperature but not the operating temperature.
The operating temperature of an optical component is the actual temperature range in which the component will work. Usually, a component must remain within a specified temperature range to perform at a specified optical performance level.
The storage temperature of an optical component is the temperature at which an optical component can be stored without causing any degradation or component failure when it is used in the component’s specified operating temperature limits. Some storage temperatures can exceed the actual operating temperature of the components. When sourcing WDM filters, ensure that they will be able to operate within the temperatures in which they will be deployed.
Another design consideration with any WDM network is insertion loss. Though WDM creates a huge increase in capacity, it also creates insertion loss in a network. Using the maximum insertion loss values in the link budget is a good idea; keep in mind that some manufacturers do not include the connector loss if the device is terminated.
Calculate the loss for both the mux and demux components. The maximum insertion loss on a typical eight-channel CWDM is 3 dB, so for a mux/demux solution, add 6 dB of insertion loss.
WDM filters can be designed to drop individual colors at a specific location and keep sending the rest down the fiber path. In some applications, combining several wavelengths at a certain location and then dropping individual channels to customers along the same route may be desirable. This is the most common type of design used in fiber-to-the business and cell tower applications.
WDM technology is a very effective method for overcoming fiber exhaust. Placing passive filters and WDM transceivers at each end of a fiber optic network can greatly increase bandwidth without having to spend capital on new fiber construction projects. Most current fiber technologies use only a small sliver of the available bandwidth capacity of single-mode glass, so a properly designed WDM network can unlock a floodgate of available power in a network. Using many channels on the same piece of optical fiber enables operators to serve businesses, cell towers and residential customers with the same fiber. Fiber counts are no longer a constraint.


WDM technology can be a reliable, cost-effective method of solving fiber exhaust problems and expanding bandwidth across campuses, municipalities, school districts, and other networks. In our first installment, we covered the basics behind the technology and how it works. In this installment, we will discuss how to begin deciding which WDM is right for you, as well as addressing some common misconceptions about this incredibly valuable technology.
Deciding between CWDM and DWDM is a complex issue, with many network- and application-specific variables that need to be considered. While we recommend a consultation with an expert to get a definitive answer, here are some preliminary considerations:
Common Misconception 1: WDM is extremely expensive to install.
For many network operators, the concept of “WDM” is inextricably linked with large, complex active line systems that cost hundreds of thousands of dollars. For most applications, this is a case of upselling by their OEMs. In fact, you can reap many of the benefits much more cost-effectively with a passive filter system. Passive CWDM and DWDM systems can be monitored via a tap port on the faceplate of most mux/demuxes.
Common Misconception 2: WDM can only cover long distances.
Network operators are often discouraged from adopting passive WDM system because the rated distances of the transceivers are much longer than required. For example, the shortest rated distance for CWDM transceivers is 40km, and 80km for DWDM transceivers. Is it still possible to use these optics if your campus is only, say, 8km, or even 100m apart?
The answer is yes! With the proper level of attenuation on the transmitting side of your transceivers, you can still deploy a passive WDM solution to add services and conserve fiber.
Further Reading
For more information about different WDM strategies and how to use them, you can check out our coverage on our ZS line of standard passives, this application note on some simple passive architectures, and an overview WDM strategies using a single strand of common fiber.
You can also schedule a consultation with our experts, who will walk you through your options step by step and find the perfect solution for your network.

Details of the differences between fiber-optic network cards and HBA cards


In the early SAN storage system, the server and the switch data transmission is through the fiber, because the server is the SCSI instruction to the storage device, can not take the ordinary LAN network IP protocol, so the need to use FC transmission, so this SAN is called FC-SAN, and later appeared in the IP protocol package SAN, can take the ordinary LAN network, so called IP-SAN, the most typical of which is now popular ISCSI.
Details of the differences between fiber-optic network cards and HBA cards
These two methods need to be heavy on the data block package unpacking operation, so the high-performance SAN system is the need to install a dedicated server on the unpacking work to reduce the burden on the processor card, this card we called It can provide a fiber interface (if it is an iSCSI HBA card is to provide a common RJ45 interface) for the corresponding switch connection; In addition, the HBA physical you can put it as NIC or PCI-E slot, so the use of this device is very a network card, many people also put it with ordinary network card or ordinary fiber-optic network card confused. Of course, some iSCSI HBA card can be used as a normal network card, but from the price to consider this is very extravagant.
The general definition of the HBA is the I / O adapter that connects the host I / O bus and the computer’s memory system. According to this definition, like the video card is connected to the video bus and memory, the network card is connected to the network bus and memory, SCSI-FC card is connected to SCSI or FC bus and memory, they should be regarded as HBA. HBA cards have FC-HBA and iSCSI HBA in the future there are other HBA cards, but HBA is usually used in SCSI. Adapter and NIC for FC; NICs are also used for Ethernet and Token Ring networks.
In fact, the network card is often referred to as a type of equipment in general, refers to the installation in the host, through the network cable (twisted pair, fiber optic cable, coaxial cable, etc.) and network switches (Ethernet switches, FC switches , ISCSI switches, etc.), or with other network devices (storage devices, servers, workstations, etc.) to form a network of hardware devices.
So, what is the name of the fiber-optic network card in the end refers to the fiber port HBA card?
In fact, we often say that the fiber-optic network card refers to the fiber channel network HBA card.
Due to the different transmission protocols, the card can be divided into three, one Ethernet card, the second is the FC card, the third is the iSCSI card.
• Ethernet card: The name of the Ethernet Adapter, the transmission protocol for the IP protocol, generally through the fiber optic cable or twisted pair and Ethernet switch connection. The interface type is divided into optical port and electrical port. Optical interface is generally through the fiber optic cable for data transmission, the interface module is generally SFP (transmission rate 2Gb / s) and GBIC (1Gb / s), the corresponding interface for the SC, ST and LC. The current interface type is RJ45, used to connect with the twisted pair, but also with the coaxial cable connection interface, but now has been used less.
• FC card: generally also called fiber optic card, scientific name Fibre Channel HBA. The transport protocol is a Fiber Channel protocol and is typically connected to a Fiber Channel switch through a fiber optic cable. The interface type is divided into optical port and electrical port. Optical interface is generally through the fiber optic cable for data transmission, the interface module is generally SFP (transmission rate 2Gb / s) and GBIC (1Gb / s), the corresponding interface for the SC and LC. The interface type of the electrical interface is generally DB9 pin or HSSDC.
• ISCSI NIC: The ISCSI HBA, which transmits the ISCSI protocol, has the same interface type as the Ethernet card.
Fiber-optic network card” generally refers to the FC HBA card, plug in the server, external storage with the fiber switch; and optical Ethernet card is generally called “fiber Ethernet card” is also inserted in the server, but it is an external Optical Ethernet switch.
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