Technology FAQ

Q: What resources do carriers have at their disposal to maximize the existing fiber in their networks?

A: There are three readily-available and simple to install and use wave division multiplexing (WDM) technologies; 2-channel WDM, coarse wavelength division multiplexing (CWDM), and dense wavelength division multiplexing. These technologies can provide a carrier with a single extra wavelength or virtual fiber, 18 additional wavelengths, or up to 160 additional wavelengths. All of these technologies utilize the existing fiber in a carrier’s network.

Q: What is WDM (Wavelength Division Multiplexing)?

A: The technology for adding two or more optical signals of different wavelengths, transmitting simultaneously, over a single fiber cable to be separated by wavelength at the far end. The most common application (2-channel WDM) combines 1310nm and 1550nm wavelengths over a single fiber.

Q: What is CWDM (Coarse Wavelength Division Multiplexing)?

A: The technology for combining up to 18 ITU wavelengths and transmitting them simultaneously onto a single fiber to be separated at the far end. The International Telecommunications Union (ITU) standard for CWDM defines the 18 channels, 20 nanometers spaced, between 1271nm and 1611nm wavelengths.

Q: What is DWDM (Dense Wavelength Division Multiplexing)?

A: The technology for combining up to 160 wavelengths and transmitting them simultaneously onto a single fiber to be separated at the far end. DWDM uses wavelength spacing of as little as 25Ghz and requires the use of lasers with very tight tolerance and stability. The DWDM wavelength band is from roughly 1530nm to 1565nm. This is the band in which the erbium doped fiber amplifier (EDFA) will work.

Q: What are the main differences between WDM, CWDM & DWDM applications?

A: In most cases, WDM is the most cost effective solution to fiber congestion allowing for 2 to 1 or 3 to 1 fiber gains combining 1310nm, 1550nm, & 1490nm wavelengths on a single fiber. In the event that more channels are required to expand the capacity of the existing fiber infrastructure, CWDM allows for a cost effective solution for short fiber spans. With the 20nm spacing, CWDM can be implemented inexpensively gaining up to 18 to 1 capacity on an existing fiber. With the current loss characteristics of fiber optic signals at the 1310nm and 1490nm wavelength windows, WDM and CWDM applications are better suited for shorter fiber spans. Where high capacity or long span lengths are required, the DWDM solution is the preferred method for gaining fiber capacity. With its high-tolerance lasers optimized in the 1550nm window (for lower loss), the DWDM systems are an ideal solution for more demanding networks. DWDM systems can utilize and EDFA to amplify all the wavelengths in the DWDM window and extend the system to lengths of 500km.

Q: What are the advantages of each of these three WDM technologies?

A: Two-channel WDM (and three channel) can be use to quickly and simply add an additional (or two additional) wavelengths. It is very simple to install and turn up and very inexpensive.

CWDM can simply and quickly add up to 18 additional wavelengths on ITU standard frequencies. It is ideal for moderately growing cross-sections of lengths up to 100km. Since the wavelengths are spaced 20nm apart less expensive lasers can be used resulting in a very low-cost, moderate capacity solution.

DWDM offers a very high-capacity, long haul solution for higher-growth cross-sections that may need to span greater distances. A DWDM system can be placed for a relatively low initial first cost and channels (wavelengths) easily added as growth occurs. EDFA Amplification in conjunction with dispersion management can extend the reach of the system up to thousands of km's.

Q: What are the limitations of each of these WDM technologies?

A: Two (or three) channel WDM is limited to the one or two additional channels. The reach of the system is typically limited to the loss of the 1310nm channel.

CWDM systems, although multi-channel do not have any optical amplification mechanism available and are typically limited to the loss of the highest loss of the 18 channels. Additionally, the channels in the region from 1360nm to 1440nm could experience higher loss due (1 to 2dB/km)  to the water vapor peak in this region in some of the older fibers deployed.

DWDM systems are typically limited in reach to 4-5 amplifier spans due to ASE noise buildup in the EDFA's. Modeling tools are available to determine exactly how many EDFA's. can be deployed. In longer systems (> 120km) dispersion can become an issue resulting in the installation of dispersion compensation modules. The DWDM band is limited to the wavelength range of 1530nm to 1565nm due to the amplification range of the EDFA.

Q: What is Reach Extension and how can I use it?

A: Reach extension is the general term for the “boosting” or re-creating the signal in order to allow it to allow it to travel greater distances. Because of the analog nature of transmission, optical signals undergo degradation when transmitted across optical links due to dispersion, loss, crosstalk and nonlinearities associated with fiber and optical components. Two common techniques are used to combat these degenerative effects: Regeneration and Amplification. Regeneration is the re-creation of the signal by converting the optical signal to an electrical signal, manipulating the signal, then converting it back to an optical signal. Amplification is the increasing of the amplitude of the optical signal (db power) of the signal without converting it to an electrical signal.

Q: What is 1R, 2R & 3R regeneration?

A: There are three different levels of optical regeneration that can be employed to enable signals to be transported greater distances.

• 1R- amplification: This regeneration technique adds optical power to the signal without affecting the shape or timing of the signal. The EDFA simply adds photons to the incoming optical signal at the specific wavelength and phase of that signal. It does not reshape or retime (apply synchronization) to the incoming signal. A byproduct of the EDFA is the generation of amplified spontaneous emission (ASE) noise which accumulates with each EDFA and can only be “cleaned up” by converting the optical signal to electrical and then back to optical. Typically the number of cascaded EDFA's. is limited to four or five.
• 2R- amplification and reshaping: This technique reshapes the deteriorated signal along with using basic amplification techniques. The signal shape is recreated to resemble the original signal but does not apply synchronization or timing to the signal. Jitter accumulation due to lack of synchronization will limit the number of 2R regenerators that can be cascaded..
• 3R- regeneration, reshaping and re- timing: Along with amplification and reshaping 3R regeneration also recreates the original duty cycle (timing) of the original signal thus creating the ideal option for extending the life of synchronous and asynchronous signals. An almost infinite number of 3R regenerators can be cascaded.

Q: What is wavelength conversion and why is it important?

A: Wavelength conversion is the changing of one wavelength to another for transport. Due to the loss characteristics of 1310nm & 850 nm signals, sometimes it is necessary to convert these signals to 1550 nm wavelength to transport them over a long fiber span and take advantage of the lower loss at 1550nm. Wavelength conversion is also used in the conversion of wideband optical signals, such as 1310nm or 1550nm, to discrete ITU CWDM or DWDM wavelengths to allow multiple wavelengths to be combined with other wavelengths for transport over a common fiber.

Q: If I convert my 1310nm signal to an xWDM wavelength do I have to convert it back to 1310nm before the far end receiver?

A: No, typically not. Most optical equipment built in the last 10 years is likely to have a wide band receiver that will works over a range of ~1260nm to ~1620nm. This means that a card that transmits at 1310nm will more than likely receive a 15xx.xxnm signal that has been converted for DWDM applications or 1491nm for CWDM applications.