Simple DWDM system. Run at the booth
For a few days, the Modultech MT-EW-2U compact DWDM system arrived at our office and I managed to drive it a bit on the stand.
This platform is based on the “all-in-one” principle: in a single case with a height of 2U, a DWDM node can be fully mounted for 16 channels up to 10 Gbit / s each. It can be equipped with active transponders with error correction or the use of “color” signals from DWDM transceivers.
In our configuration, we used simplified configuration without transponders and with external fiber-type dispersion compensators, although the platform allows the use of built-in compact compensators based on Bragg gratings.
The dual-unit MT-EW platform can be used both for the terminal node and for the transit node.
Structurally, the platform consists of two independent optical subsystems of eight channels each; mode of operation is determined by the choice of component modules and switching circuit.
In our test, we will launch nine 10Gbit / s channels through a two-fiber path 120km long. We will check the quality of communication with a bit-error rate tester (BERT).
For the 120km track, the following scheme will be used:
The Eltex MES-3124F Ethernet switch with 24 SFP 1GbE ports and four SFP + 10GbE ports equipped with optical DWDM SFP + transceivers for 80km Modultech MT-DP-xx192-08CD will be used as the terminal equipment.
The requirements for optical signals in such a system will be determined by the characteristics of the optical transceivers and the conditions for minimizing nonlinear effects in the optical fiber. Their summary is given in the table below:
When setting up a multichannel system, the following factors should be considered:
- The attenuation of the signal on the optical multiplexer (the section of the optical path from the output from the transceiver to the input to the power amplifier) ​​can be up to 7 dB and strongly depends on the purity of the optical connections
- The gain of the power amplifier is regulated in the range of 12-17 dB, the preamplifier - in the range of 20-30 dB.
- To increase the signal-to-noise ratio, it is desirable to launch the maximum possible signal level in one DWDM channel into the path. At the same time, it is necessary to adhere to the limitation of + 8 dBm / channel, otherwise the probability of non-linear signal distortion is high. If necessary, reduce the level of the signal at the entrance to the line should use an attenuator.
- By adjustments on the receiving side, the signal-to-noise ratio improves extremely slightly compared to adjustments on the transmitting side.
- Optical amplifiers have a saturation mode at about + 17dBm of the total power of the output group signal. This affects the signal levels in one DWDM channel as the number of simultaneously transmitted channels increases. In fact, in the amplifier saturation mode, the gain depends on the number of transmitted channels and is less than the nominally set.
To test the system, we will install a test bench:
The following components will be used in the test:
Build a system does not cause unnecessary questions. All optical ports of the DWDM system are terminated into LC / UPC sockets, dispersion compensators have SC / UPC ports. The main part of the installation is done with duplex patch cords, the optimal length is 30 cm. Marking "TX", "RX", "IN", OUT "on the platform indicates the direction of signal transmission relative to the system itself: through the ports" RX "or" IN "the optical signal enters the platform, through the ports" TX "or" OUT "optical the signal goes out of the platform.
Optical connections for multicast signals are performed according to the following table:
Data channels are connected to service I / O cards. In this configuration, simple optical adapter cards are used, which are simply pins of the optical channels of the multiplexer for connecting external “color” signals.
Each multiplexer in the platform serves up to eight channels paired to service card slots in pairs, each service card serves up to two customer communication channels. If you need to connect more channels, the multiplexers are cascaded through dedicated upgrade ports.
The distribution of frequencies (wavelengths) of individual communication channels by service card slots is static and is completely determined by the optical scheme of the installed multiplexer.
It is better to connect client channels in two stages: first connect the transmitting ports, equalize the levels of optical signals according to the criteria described in the first section and only after that connect the receiving ports of the transceivers. Such a sequence will protect the optical receivers from overload and facilitate system setup.
After connecting the transmitters, it is necessary to check the uniformity of levels in the group signal and their compliance with the general conditions using a spectrum analyzer. During initial installation, the spectrum analyzer is connected directly to the line, the common output of the multiplexer or amplifier. When setting up a working system, it is necessary to use the monitoring ports (“TAP”) of optical amplifiers.
In the presented system, the signal to the monitoring port is transmitted with attenuation by 13.6 dB (5%).
In our case, at the general output of the multiplexer, the following spectrogram was obtained:
It is recommended to save in the spectrogram archives obtained at the control points of the system both when the system is started up and when the configuration changes (adding, deleting channels, expanding the system, etc.).
After all the necessary connections have been made and the basic optical path has been configured, it is necessary to check the quality of the traffic transmission.
To do this, it is best to use special BER-testers, allowing you to quickly assess the reliability of transmission of simple bit sequences.
System tuning is best to start with a configuration in which all amplifiers are adjusted to the lowest possible gain levels and there is a margin of several decibels before saturation of the amplifiers. If necessary, this margin can be made by switching on the attenuator between the multiplexer output and the input of the power amplifier.
Consistently connecting the bit tester to different channels, you need to make sure that there are no transmission errors.
If you detect errors on a separate channel, you need to:
- check signal levels and optical spectrum at optical receivers. Errors can result from both too weak and too strong a signal on the photodetector. The signal-to-noise ratio should be better than 25dB.
- clean the optical sockets and connectors of the transmitters and receivers of the failed channel.
If a too weak signal is detected at the input of the optical receiver and there is no result after cleaning the connections, it is necessary to resort to adjustments of the optical gain. It should be borne in mind that the adjustment of the amplifiers directly affect all optical channels, after adjustment, you must check the performance of other communication channels.
If errors are detected on most of the included channels, attention should be paid to possible non-linear effects in the optical fiber and the sufficiency of the signal-to-noise ratio in the channels. It must be remembered that the adjustment on the transmitting side is more effective than on the receiving side.
One of the possible causes of unstable communication for 10 Gbit / s lines is uncompensated chromatic dispersion. In this configuration, dispersion compensators are used for 60km of standard G.652 fiber and transceivers with resistance up to 80km, so with a line length of 120km at the input of the transceivers, the level of uncompensated dispersion will be about 60km. Despite a sufficient margin of 20 km, dispersion can lead to unstable coupling. In this case, it is recommended to enable a dispersion compensator with a large nominal value.
When setting up this system and turning on nine simultaneous channels, a positive result (that is, the absence of errors on all channels) was achieved after eliminating attenuators and minimum gain values ​​in the settings:
As follows from the report on the state of the power amplifier, it operates in the saturation mode (output power - + 18 dBm), which will lead to a decrease in the signal levels in each channel when additional channels are turned on. This should not lead to degradation of the service, since the signal level at the output from the system is about -9 dBm in all channels, which with a large margin overlaps the sensitivity of the SFP + module.
When operating the system, it is necessary to continuously monitor with the help of monitoring systems the quality of signal transmission (the presence and number of errors on transceivers) together with the levels of the optical signal on amplifiers and transceivers. The results of such monitoring will greatly facilitate the resolution of incidents, tracking the quality of the services provided and further expanding the system.
The description of the management and monitoring system is the topic of a separate article, for which I hope it will not arise.
This platform is based on the “all-in-one” principle: in a single case with a height of 2U, a DWDM node can be fully mounted for 16 channels up to 10 Gbit / s each. It can be equipped with active transponders with error correction or the use of “color” signals from DWDM transceivers.
In our configuration, we used simplified configuration without transponders and with external fiber-type dispersion compensators, although the platform allows the use of built-in compact compensators based on Bragg gratings.
The dual-unit MT-EW platform can be used both for the terminal node and for the transit node.
Structurally, the platform consists of two independent optical subsystems of eight channels each; mode of operation is determined by the choice of component modules and switching circuit.
In our test, we will launch nine 10Gbit / s channels through a two-fiber path 120km long. We will check the quality of communication with a bit-error rate tester (BERT).
Description of general conditions.
For the 120km track, the following scheme will be used:
The Eltex MES-3124F Ethernet switch with 24 SFP 1GbE ports and four SFP + 10GbE ports equipped with optical DWDM SFP + transceivers for 80km Modultech MT-DP-xx192-08CD will be used as the terminal equipment.
The requirements for optical signals in such a system will be determined by the characteristics of the optical transceivers and the conditions for minimizing nonlinear effects in the optical fiber. Their summary is given in the table below:
| Condition | Criterion |
| Optical transceiver output level | From 0 to + 5dBm, not adjustable. |
| The signal level at the input of the optical transceiver | From -20 to -8dBm. |
| Signal level in one DWDM channel after power amplifier | No more than + 8dBm |
| Signal to noise ratio at the input of the optical transceiver | Better than 25db |
| The total power of the group signal after the power amplifier | No more than + 17dBm |
When setting up a multichannel system, the following factors should be considered:
- The attenuation of the signal on the optical multiplexer (the section of the optical path from the output from the transceiver to the input to the power amplifier) ​​can be up to 7 dB and strongly depends on the purity of the optical connections
- The gain of the power amplifier is regulated in the range of 12-17 dB, the preamplifier - in the range of 20-30 dB.
- To increase the signal-to-noise ratio, it is desirable to launch the maximum possible signal level in one DWDM channel into the path. At the same time, it is necessary to adhere to the limitation of + 8 dBm / channel, otherwise the probability of non-linear signal distortion is high. If necessary, reduce the level of the signal at the entrance to the line should use an attenuator.
- By adjustments on the receiving side, the signal-to-noise ratio improves extremely slightly compared to adjustments on the transmitting side.
- Optical amplifiers have a saturation mode at about + 17dBm of the total power of the output group signal. This affects the signal levels in one DWDM channel as the number of simultaneously transmitted channels increases. In fact, in the amplifier saturation mode, the gain depends on the number of transmitted channels and is less than the nominally set.
Stand installation
To test the system, we will install a test bench:
The following components will be used in the test:
| Name | amount |
| MT-EW-2U compact DWDM platform chassis with two 220V AC power supplies | 2 |
| The module of eight-channel two-fiber optical multiplexer DWDM for compact platform MT-EW-D8 | 2 |
| Optical power amplifier module (booster EDFA) for the compact platform MT-EW-OAB17 | 2 |
| Optical preamp module (pre-EDFA) for the compact platform MT-EW-OAP17 | 2 |
| Optical Client Adapter Module for MT-EW-MOA Compact Platform | eight |
| Dispersion compensator 60km, MT-DCM-60 | 2 |
| Optical DWDM Transceiver 10G SFP + Modultech MT-DP-xx192-08CD , channels 20-28 | 18 |
| G.652 fiber optic in coils of 20km | 12 |
| Optical power meter | one |
| Optical spectrum analyzer DWDM-range | one |
Build a system does not cause unnecessary questions. All optical ports of the DWDM system are terminated into LC / UPC sockets, dispersion compensators have SC / UPC ports. The main part of the installation is done with duplex patch cords, the optimal length is 30 cm. Marking "TX", "RX", "IN", OUT "on the platform indicates the direction of signal transmission relative to the system itself: through the ports" RX "or" IN "the optical signal enters the platform, through the ports" TX "or" OUT "optical the signal goes out of the platform.
Optical connections for multicast signals are performed according to the following table:
| From where | Where | Note |
| MUX / DMUX OUT | Booster EDFA IN | The common output of the multiplexer is connected to the input of the power amplifier. |
| Booster edfa out | Line entry | |
| MUX / DMUX OSC OUT | Booster EDFA OSC | Optical service channel output |
| Line output | Pre-EDFA IN | |
| Pre-EDFA OUT | DCM IN | The amplified receive signal is transmitted to the dispersion compensator. |
| Pre-EDFA OSC | MUX / DMUX OSC IN | Optical service channel input |
| DCM OUT | MUX / DMUX IN | The signal after the dispersion compensator is transmitted to the demultiplexer. |
Data channels are connected to service I / O cards. In this configuration, simple optical adapter cards are used, which are simply pins of the optical channels of the multiplexer for connecting external “color” signals.
Each multiplexer in the platform serves up to eight channels paired to service card slots in pairs, each service card serves up to two customer communication channels. If you need to connect more channels, the multiplexers are cascaded through dedicated upgrade ports.
The distribution of frequencies (wavelengths) of individual communication channels by service card slots is static and is completely determined by the optical scheme of the installed multiplexer.
It is better to connect client channels in two stages: first connect the transmitting ports, equalize the levels of optical signals according to the criteria described in the first section and only after that connect the receiving ports of the transceivers. Such a sequence will protect the optical receivers from overload and facilitate system setup.
After connecting the transmitters, it is necessary to check the uniformity of levels in the group signal and their compliance with the general conditions using a spectrum analyzer. During initial installation, the spectrum analyzer is connected directly to the line, the common output of the multiplexer or amplifier. When setting up a working system, it is necessary to use the monitoring ports (“TAP”) of optical amplifiers.
In the presented system, the signal to the monitoring port is transmitted with attenuation by 13.6 dB (5%).
In our case, at the general output of the multiplexer, the following spectrogram was obtained:
It is recommended to save in the spectrogram archives obtained at the control points of the system both when the system is started up and when the configuration changes (adding, deleting channels, expanding the system, etc.).
System configuration and verification
After all the necessary connections have been made and the basic optical path has been configured, it is necessary to check the quality of the traffic transmission.
To do this, it is best to use special BER-testers, allowing you to quickly assess the reliability of transmission of simple bit sequences.
System tuning is best to start with a configuration in which all amplifiers are adjusted to the lowest possible gain levels and there is a margin of several decibels before saturation of the amplifiers. If necessary, this margin can be made by switching on the attenuator between the multiplexer output and the input of the power amplifier.
Consistently connecting the bit tester to different channels, you need to make sure that there are no transmission errors.
If you detect errors on a separate channel, you need to:
- check signal levels and optical spectrum at optical receivers. Errors can result from both too weak and too strong a signal on the photodetector. The signal-to-noise ratio should be better than 25dB.
- clean the optical sockets and connectors of the transmitters and receivers of the failed channel.
If a too weak signal is detected at the input of the optical receiver and there is no result after cleaning the connections, it is necessary to resort to adjustments of the optical gain. It should be borne in mind that the adjustment of the amplifiers directly affect all optical channels, after adjustment, you must check the performance of other communication channels.
If errors are detected on most of the included channels, attention should be paid to possible non-linear effects in the optical fiber and the sufficiency of the signal-to-noise ratio in the channels. It must be remembered that the adjustment on the transmitting side is more effective than on the receiving side.
One of the possible causes of unstable communication for 10 Gbit / s lines is uncompensated chromatic dispersion. In this configuration, dispersion compensators are used for 60km of standard G.652 fiber and transceivers with resistance up to 80km, so with a line length of 120km at the input of the transceivers, the level of uncompensated dispersion will be about 60km. Despite a sufficient margin of 20 km, dispersion can lead to unstable coupling. In this case, it is recommended to enable a dispersion compensator with a large nominal value.
Results
When setting up this system and turning on nine simultaneous channels, a positive result (that is, the absence of errors on all channels) was achieved after eliminating attenuators and minimum gain values ​​in the settings:
192.168.0.179:> sh int 1/15/1 edfa <Interface EDFA 1/15/1> Admin : Enable Alias Name : Gain : 12.0 Service Status : IS Alarm : NoDetect InputPower : 6.5 dBm OutputPower : 18.6 dBm ModuleTemperature : 31.7 C OpCurrent : 352.1 mA OpPower : 214.2 mW ChipTemperature : 25.0 C CoolingCurrent : 115.5 mA Serial Number : 990647 Type : BA 192.168.0.179:> sh int 1/16/1 edfa <Interface EDFA 1/16/1> Admin : Enable Alias Name : Gain : 20.0 Service Status : IS Alarm : NoDetect InputPower : -4.8 dBm OutputPower : 15.2 dBm ModuleTemperature : 29.0 C OpCurrent : 162.1 mA OpPower : 109.3 mW ChipTemperature : 25.0 C CoolingCurrent : 58.0 mA Serial Number : 990651 Type : PA As follows from the report on the state of the power amplifier, it operates in the saturation mode (output power - + 18 dBm), which will lead to a decrease in the signal levels in each channel when additional channels are turned on. This should not lead to degradation of the service, since the signal level at the output from the system is about -9 dBm in all channels, which with a large margin overlaps the sensitivity of the SFP + module.
When operating the system, it is necessary to continuously monitor with the help of monitoring systems the quality of signal transmission (the presence and number of errors on transceivers) together with the levels of the optical signal on amplifiers and transceivers. The results of such monitoring will greatly facilitate the resolution of incidents, tracking the quality of the services provided and further expanding the system.
The description of the management and monitoring system is the topic of a separate article, for which I hope it will not arise.
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Source: https://habr.com/ru/post/191466/
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