Fundamentals of DWDM : In this article, I will explain about the basic concept of DWDM in detail. You must have read about DWDM on the internet but I will try to explain about DWDM in a very easy and simple way. DWDM is a technology that uses the characteristics of a single mode optical fiber like-bandwidth and low attenuation , multiple wavelengths adopted as carriers and allows them to be simultaneously transmitted across the fiber.
DWDM Most commonly used equipment in the telecom sector due to its characteristics. It has many advantages such as simple capacity expansion and reliable performance. In particular, it can use a wide variety of services and this gives it a bright prospect of application. All the points of the DWDM will be easily explained which has been confusing till now, but for this you will have to read the entire article.
Now I am explaining why DWDM is needed more in telecom sector. DWDM is quite popular in the telecom sector due to some special features that have been written about below-
(a) Overcome the lack of fiber exhaust / fiber availability problems (better utilization of available fiber).
(b) Space & Power savings at intermediate stations.
(c) Easier capacity expansion.
(d) Cost effective transmission.
(e) No Optical-Electrical-Optical conversion delays.
(f) Wave length leasing instead of Bandwidth leasing.
Space Division Multiplexing (SDM) : Space division multiplexing extends or achieve transmission capacity by adding fibers, and transmission devices are also added linearly.
The SDM method does not adequately use the transmission bandwidth of optical fiber. It is not always possible to add new optical fibers to expand capacity during the construction of communication networks. Indeed, in the initial phase of the project, it is difficult to predict the increasing service demands and plan the number of fibers to lay. Therefore, the SDM method for capacity expansion is quite limited.
Time Division Multiplexing (TDM) : TDM is a very popular and commonly used method for capacity expansion, e.g. Multiplexing of the primary group for the fourth group of conventional PDH, and STM-1, STM-4, STM-16 and STM-64 of the current SDH.
TDM technology can increase the efficiency of optical transmission information in duplication and reduce circuit costs in equipment and line. Furthermore, it is easy to extract specific digital signals from the data stream through this multiplexing method. This is particularly suitable for networks requiring a self-healing rings security strategy.
However, the TDM method has two disadvantages. First, it affects services. Upgrading to a completely higher rate level requires completely replacing network interfaces and equipment. Thus equipment of operation should be interrupted during the upgrade process.
Second, the rate upgrade lacks flexibility. Let's take SDH as an example, when a system with a line rate of 155Mbit / s needs to provide two 155Mbit / s channels, the only way is to upgrade the system to 622Mbit / s, even if two 155Mbit / s. s be unused.
Broadcast multiple channels using WDM systems with 8, 16 or 32 channels (multiplexing of 2.5 Gbit / s signals). At the termination, we feed four 2.5 Gbit/s signals to four optical transmission modules. The optical output signals are converted if necessary to defined wavelengths in the 1550 nm window using wavelength transponders.
This makes it possible to use existing standard transmission modules with wavelengths in the 1310 nm or 1550 nm band. Using an optical WDM coupler, the four optical signals are bunched together and forwarded to an optical fiber amplifier (OFA). Depending on the path length, one or more fiber amplifiers boost the optical signal, which is attenuated due to the fiber loss. In many cases, a booster is also used after the WDM coupler.
At the the receiving end, it is common to amplify (using preamplifiers) the optical signals and then separate them using optical fibers and convert them to electrical signals in the receiver modules. This entire arrangement must be duplicated in the opposite direction to carry the signals in that direction.
Channel spacing > 200GHz is called CWDM
Channel spacing > 100 GHz is called WDM
Channel spacing < 100GHz is called DWDM
Channel spacing < 25GHz is called UDWDM
I would like to mention here that 100 GHz is equal to 0.8 nm
The infrared spectrum is shown in a very easy way, which will not be a problem to understand.
C-Band (1530 – 1562nm)
L-Band (1574 – 1608nm)
Channel central frequencies are allocated in a frequency spacing equal to 100 GHz or 0.1 THz. All channels are anchored to central frequencies 193.1 THz Reference. As per given image the channel central wavelength corresponding to the reference frequency is 1552.52 nm.
Now we are able to talk about wavelength allocation for DWDM. C-Band (1530 – 1562nm) It is also called conventional band or 1550 band.
L-Band (1574 – 1608nm) It is also called Long wavelength band or 1580nm band.
Channel central frequencies are allocated in a frequency spacing equal to 100 GHz or 0.1 THz. All channel central frequencies are anchored for the 193 THz reference. In given image the channel central wavelength corresponding to the reference frequency is 1552.52 nm.
Note 1: In DWDM system Optical carriers spacing are allocated 100 GHz (0.1 THz) grid in Rec. G. 692.
2: As mentioned in above image Tone channel is dedicated for operation & maintenance support.
3. C13 is the Center Wavelength.
L-Band (1574 – 1608nm): L-Band is known as Long wavelength band or 1580nm band. As per given below image the channel central frequencies are allocated in equal frequency spacing of 100 GHz or 0.1 THz.
In the 2R mode, the transponders are providing Re-amplification and Re-sampling. In the 3R mode, the transponders provide Re-amplification, Re-sampling and Re-timing.
2R transponders are bit rate independent and can operate continuously from 155 Mbit/s to 1.25 Gbit/s, whereas 3R transponders can only operate at 622 Mbit/s and 2.488 Gbit/s.
Jitter and wander are terminated in transponders operating at 3R mode. In the 2R mode the transponder will not terminate the jitter of the incoming signal but will instead add jitter to the signal.
The amount of added jitter depends on the bit rate and the input power levels. For 2R operation specifies the added jitter to be 75 ps peak-to-peak at maximum.
There two type of modes: 2R/3R operation:
2R is known as Re-amplification, Re-sampling
PDH 140 Mbit/s, 565 Mb/s
SDH STM-16, STM-4, STM-1
SONET,ATM,IP,Digital CATV)
Output signal quality depends on input quality
3R is known as Re-amplification, Re-sampling, Re-timing
SDH STM-16
Eliminating periodic time jitter (output is a standard SDH signal)
Perform Wavelength conversion via OEO process:
Typically used to convert from legacy equipment Wavelength (e.g;1310 nm) to wavelength for DWDM. Includes electrical signal regeneration. Taken an optical signal by a device and performs electrical. That device 3R regeneration & re-transmits the signal in optical form. In to Wavelength grid as per G.192. It is concept of transponder that for every input Wavelength one transponder is required. In fact this is very useful for Wavelength leasing, as customer can Send any wavelength.
Emphasize the fact that we are flexible company which can make custom design system according to customer requirements.
3 types of transmitters:
V - Direct modulation low dispersion 1800ps*nm
B - Direct modulation high dispersion 3000ps*nm
VL - Externally modulated super high dispersion 12000ps*nm.
Various transponder outputs (wavelengths) will be provided to Mux as inputs. Each input is equipped with a selective filter of some wavelength. The output of these filters is coupled to a single mode fiber. At the receiver end, these wavelengths are again separated by a demux and directing them to individual transponders. Mux and Demux are both identical components, the only difference being that they operate in the opposite direction.
At the receiver end, these wavelengths are again separated by a demux and directing them to the individual, Transponder device that enables sending multiple data streams to a single physical line. Both Mux and Demux are similar components, with the only difference being that they operate in the opposite direction.
It is also a function by which a connection to an (ISO) layer is used to support more than one. Relation to the next higher layer. (2) A device for combining several channels carried by a line or fiber.
In DWDM data transmission process, a function that allows two or more data sources to share a common transmission medium such that each data source has its own channel.
(a) Booster/Post Amp- In DWDM booster boosts the signal at Transmitter end to compensate relatively low output power of laser transmitters.
(b) Line Amp- In DWDM Line Amplifier used at regular intervals to compensate fiber transmission loss.
(c) Pre Amp- In DWDM Pre-Amplifier boosts signal prior to Optical detectors to increase the Rx sensitivity.
➤For boosting power before splitting into many branches in CATV
➤Pre-Amplifiers for increasing receiver sensitivity
➤In line amplifiers for periodic compensation of loss
➤For overcoming losses at cross connect, add/drop etc.
DWDM Most commonly used equipment in the telecom sector due to its characteristics. It has many advantages such as simple capacity expansion and reliable performance. In particular, it can use a wide variety of services and this gives it a bright prospect of application. All the points of the DWDM will be easily explained which has been confusing till now, but for this you will have to read the entire article.
Why DWDM?
Anyone can ask the question, why DWDM itself? Why use DWDM? What is the main reason for this? Before answering this question, I would like to explain about the principle of transmission in WDM. WDM – called Wavelength Division Multiplexing. The ability of WDM is to use different wavelengths in a single fiber, to split and to combine them. Look at the image below, you will understand the whole thing very easily. |
| wavelength split and combine over a single fiber |
(a) Overcome the lack of fiber exhaust / fiber availability problems (better utilization of available fiber).
(b) Space & Power savings at intermediate stations.
(c) Easier capacity expansion.
(d) Cost effective transmission.
(e) No Optical-Electrical-Optical conversion delays.
(f) Wave length leasing instead of Bandwidth leasing.
From the below given image you can understand very easily why DWDM is used in telecom network or why DWDM is needed.
 |
| Compare between DWDM transmission and TDM transmission |
In recent years, fiber optics has developed almost exclusively towards higher bit rates, with each fiber transporting a signal from a single laser. Wavelengths of 1310 nm and 1550 nm are used in telecommunications applications, corresponding to the second and third optical window. It would be nice to discuss here about DWDM technology background as well.
DWDM Technology Background
With the dramatic growth of voice services and the emergence of various new services, especially the rapid change of IP technology, network capacity will inevitably face a significant challenge. Space Division Multiplexing (SDM) or Time Division Multiplexing (TDM) these two traditional methods for transmission network capacity expansion adopted.Space Division Multiplexing (SDM) : Space division multiplexing extends or achieve transmission capacity by adding fibers, and transmission devices are also added linearly.
The SDM method does not adequately use the transmission bandwidth of optical fiber. It is not always possible to add new optical fibers to expand capacity during the construction of communication networks. Indeed, in the initial phase of the project, it is difficult to predict the increasing service demands and plan the number of fibers to lay. Therefore, the SDM method for capacity expansion is quite limited.
Time Division Multiplexing (TDM) : TDM is a very popular and commonly used method for capacity expansion, e.g. Multiplexing of the primary group for the fourth group of conventional PDH, and STM-1, STM-4, STM-16 and STM-64 of the current SDH.
TDM technology can increase the efficiency of optical transmission information in duplication and reduce circuit costs in equipment and line. Furthermore, it is easy to extract specific digital signals from the data stream through this multiplexing method. This is particularly suitable for networks requiring a self-healing rings security strategy.
However, the TDM method has two disadvantages. First, it affects services. Upgrading to a completely higher rate level requires completely replacing network interfaces and equipment. Thus equipment of operation should be interrupted during the upgrade process.
Second, the rate upgrade lacks flexibility. Let's take SDH as an example, when a system with a line rate of 155Mbit / s needs to provide two 155Mbit / s channels, the only way is to upgrade the system to 622Mbit / s, even if two 155Mbit / s. s be unused.
Basics of optical DWDM systems
 |
| Basics of DWDM |
Broadcast multiple channels using WDM systems with 8, 16 or 32 channels (multiplexing of 2.5 Gbit / s signals). At the termination, we feed four 2.5 Gbit/s signals to four optical transmission modules. The optical output signals are converted if necessary to defined wavelengths in the 1550 nm window using wavelength transponders.
This makes it possible to use existing standard transmission modules with wavelengths in the 1310 nm or 1550 nm band. Using an optical WDM coupler, the four optical signals are bunched together and forwarded to an optical fiber amplifier (OFA). Depending on the path length, one or more fiber amplifiers boost the optical signal, which is attenuated due to the fiber loss. In many cases, a booster is also used after the WDM coupler.
At the the receiving end, it is common to amplify (using preamplifiers) the optical signals and then separate them using optical fibers and convert them to electrical signals in the receiver modules. This entire arrangement must be duplicated in the opposite direction to carry the signals in that direction.
Classification of WDM
Classification of WDM is done on other parameters as well, but especially classification of WDM based on channel spacing. Classification of WDM is based on the Channel spacing between 2 Wave lengths-Channel spacing > 200GHz is called CWDM
Channel spacing > 100 GHz is called WDM
Channel spacing < 100GHz is called DWDM
Channel spacing < 25GHz is called UDWDM
I would like to mention here that 100 GHz is equal to 0.8 nm
Infrared Spectrum
C-Band (1530 – 1562nm)
L-Band (1574 – 1608nm)
 |
| DWDM Channel Spacing |
Now we are able to talk about wavelength allocation for DWDM. C-Band (1530 – 1562nm) It is also called conventional band or 1550 band.
L-Band (1574 – 1608nm) It is also called Long wavelength band or 1580nm band.
Channel central frequencies are allocated in a frequency spacing equal to 100 GHz or 0.1 THz. All channel central frequencies are anchored for the 193 THz reference. In given image the channel central wavelength corresponding to the reference frequency is 1552.52 nm.
Allocation of Wavelength in C-Band
All channel central frequencies are anchored for the 193 THz reference. It is very clear from given image the channel central wavelength corresponding to the reference frequency is 1552.52 nm. |
| Wavelength in C-Band |
2: As mentioned in above image Tone channel is dedicated for operation & maintenance support.
3. C13 is the Center Wavelength.
Wavelength allocation in L-Band
 |
| L-Band |
Note 1: According to the given image Optical carriers spacing are allocated 100 GHz (0.1 THz) grid in Rec. G. 692.
2: As per given image Tone channel is dedicated for operation & maintenance support.
In DWDM systems, DWDM components are classified into two types: multiplex and D-multiplex. The main function of multiplex is to combine multiple signal wavelengths into a single optical fiber for transmission.
The main function of D-multiplex is to seperate the multiple signal wavelengths transmitted in the optical fiber. The key to the performance of the DWDM system is the DWDM component which requires adequate multiplexing channels, low insertion loss, large crosstalk attenuation, wide pass-band, etc.
DWDM also has some special components with MUX and DE-MUX, so we can say that the main components of DWDM are-
(1) Transponder
(2) Mux/Demux
(3) Optical Amplifier
(4) OADM
(5) Regenerator
A device that carries an optical signal performs electrically 3R regeneration and transmits the signal in optical form, in the wavelength grid according to G.192. This allows any wavelength as input to the DWDM. For each input wavelength, a transponder is required. This is very useful for wavelength leasing, as the customer can send any wavelength.2: As per given image Tone channel is dedicated for operation & maintenance support.
Comparison of CWDM and DWDM Technologies
 |
| CWDM and DWDM Technology |
DWDM Components
 |
| More components of DWDM |
The main function of D-multiplex is to seperate the multiple signal wavelengths transmitted in the optical fiber. The key to the performance of the DWDM system is the DWDM component which requires adequate multiplexing channels, low insertion loss, large crosstalk attenuation, wide pass-band, etc.
DWDM also has some special components with MUX and DE-MUX, so we can say that the main components of DWDM are-
(1) Transponder
(2) Mux/Demux
(3) Optical Amplifier
(4) OADM
(5) Regenerator
I do not want a long theory to be written and you get bored after reading and you do not understand the whole thing. I do not want a long theory to be written and you get bored reading it and you don't understand the whole thing, so I try to make technical points better by image. Now I am going to tell the typical DWDM network architecture by an image,Which will make you understand the network architecture of DWDM completely.
 |
| DWDM Network Architecture |
Transponder
2R/3R TX Transponders
2R’ and ‘3R’. modesIn the 2R mode, the transponders are providing Re-amplification and Re-sampling. In the 3R mode, the transponders provide Re-amplification, Re-sampling and Re-timing.
2R transponders are bit rate independent and can operate continuously from 155 Mbit/s to 1.25 Gbit/s, whereas 3R transponders can only operate at 622 Mbit/s and 2.488 Gbit/s.
Jitter and wander are terminated in transponders operating at 3R mode. In the 2R mode the transponder will not terminate the jitter of the incoming signal but will instead add jitter to the signal.
The amount of added jitter depends on the bit rate and the input power levels. For 2R operation specifies the added jitter to be 75 ps peak-to-peak at maximum.
There two type of modes: 2R/3R operation:
2R is known as Re-amplification, Re-sampling
PDH 140 Mbit/s, 565 Mb/s
SDH STM-16, STM-4, STM-1
SONET,ATM,IP,Digital CATV)
Output signal quality depends on input quality
3R is known as Re-amplification, Re-sampling, Re-timing
SDH STM-16
Eliminating periodic time jitter (output is a standard SDH signal)
 |
| Transponders in DWDM |
Typically used to convert from legacy equipment Wavelength (e.g;1310 nm) to wavelength for DWDM. Includes electrical signal regeneration. Taken an optical signal by a device and performs electrical. That device 3R regeneration & re-transmits the signal in optical form. In to Wavelength grid as per G.192. It is concept of transponder that for every input Wavelength one transponder is required. In fact this is very useful for Wavelength leasing, as customer can Send any wavelength.
Mux / Transponders in DWDM
The Transponder is one of the most important card in the optical system - It is the interface between the optical layer to the other:SDH,PDH,ATM…Emphasize the fact that we are flexible company which can make custom design system according to customer requirements.
3 types of transmitters:
V - Direct modulation low dispersion 1800ps*nm
B - Direct modulation high dispersion 3000ps*nm
VL - Externally modulated super high dispersion 12000ps*nm.
 |
| MUX |
Various transponder outputs (wavelengths) will be provided to Mux as inputs. Each input is equipped with a selective filter of some wavelength. The output of these filters is coupled to a single mode fiber. At the receiver end, these wavelengths are again separated by a demux and directing them to individual transponders. Mux and Demux are both identical components, the only difference being that they operate in the opposite direction.
OMUX and ODMUX
Various transponder outputs (wavelengths) will be provided to Mux as inputs. Each input is equipped with a selective filter of some wavelength. The output of these filters is coupled in single mode fiber.At the receiver end, these wavelengths are again separated by a demux and directing them to the individual, Transponder device that enables sending multiple data streams to a single physical line. Both Mux and Demux are similar components, with the only difference being that they operate in the opposite direction.
It is also a function by which a connection to an (ISO) layer is used to support more than one. Relation to the next higher layer. (2) A device for combining several channels carried by a line or fiber.
In DWDM data transmission process, a function that allows two or more data sources to share a common transmission medium such that each data source has its own channel.
 |
| MUX and De-MUX System in DWDM |
Optical Amplifier
Where do we require Optical Amps ? Simple answer optical amplifiers required at-(a) Booster/Post Amp- In DWDM booster boosts the signal at Transmitter end to compensate relatively low output power of laser transmitters.
(b) Line Amp- In DWDM Line Amplifier used at regular intervals to compensate fiber transmission loss.
(c) Pre Amp- In DWDM Pre-Amplifier boosts signal prior to Optical detectors to increase the Rx sensitivity.
Application
➤Booster amplifiers for boosting optical power into the fiber➤For boosting power before splitting into many branches in CATV
➤Pre-Amplifiers for increasing receiver sensitivity
➤In line amplifiers for periodic compensation of loss
➤For overcoming losses at cross connect, add/drop etc.
You can also READ an important Amplifier used in DWDM System-
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ReplyDeleteThere is a correction to the article to be made. CWDM is 18 channels at 20 nm spacing, not Ghz spacing. (IE: 1271 nm, 1291 nm, 1311 nm ...thru... 1611 nm)
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