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DWDM Network Protection

DWDM Network Protection : In this article, network protection of DWDM will be discussed. This topic also has a very special importance in itself. Before knowing about the network protection, we will also discuss a little about the network construction of DWDM. So to be satisfied with the information given, you all have to read the entire article carefully.

Since the load of DWDM systems is large, reliability is especially important. In DWDM system there are two major protection modes for point-to-point line protection. One, based on individual wavelength, is 1+1 or 1:N protection implemented on the SDH layer. The other is based on optical multiplex section protection and protects the multiplexed signals simultaneously in the optical path. This kind of protection is also called optical multiplex section protection (OMSP). In addition, the other protections based on ring networks.
Protection system in DWDM
Protection system in DWDM

Basic network modes of DWDM 

In this article, I am specifically discussing the network protection of DWDM so I will not discuss DWDM network constitution in detail here.

The basic network modes of a DWDM system are point-to-point, chain, and ring. Other complex network forms can be combined using these three modes. When applied together with STM-16 devices, they can form very complex optical transmission networks.
Basic network modes of DWDM system
DWDM network mode

From the given image, you can understand the basic network mode of DWDM but I am mentioning here some important points of the ring network.

Local area networks DWDM optical add / drop multiplexes, especially in metropolitan network applications, can be used to create ring networks according to user demand. Typically in ring networks, path protection ring and multiplex section protection is provided by the SDH appliance itself, so the DWDM tool is not required to provide other security methods. But according to user requirements wavelength protection can be provided. 

Protection Based on Individual Wavelength

1+1 protection implemented on the SDH layer
1+1 protection implemented on the SDH layer

1 + 1 protection based on individual wavelengths and applied to the SDH layer. This protection system mechanism is similar to the 1 + 1 MSP of the SDH system. All system equipment requires backup, such as SDH terminals, multiplex / de-multiplex, optical line amplifiers, and optical fiber cable lines.

SDH signals are designed for permanent operation and protection system. At the end of the receiver, the position of the SDH signals received by the two DWDM systems is monitored and a more appropriate signal is selected. Now we can say that this method has high reliability, but its cost is relatively high.

In DWDM systems, the switching of each SDH channel is unrelated to other channels, that is, when Tx1 of DWDM System 1 fails and switches to DWDM System 2, Tx2 can still operate in the DWDM system 1. Switching must be completed within 50ms once a switching condition is detected.

Other type protection is 1:n protection based on individual wavelength and implemented on SDH layer. The DWDM system can apply to 1: N protection and SDH layer based on individual wavelengths. According to the figure below, Tx11, Tx21 and Txn1 share a protection section and, together with Txp1, form a 1: n protection relationship. Tx12, Tx22 and Txn2 share a protection section and, together with Txp2, form a 1: n protection relationship.

And similarly, Tx1m, Tx2m and Txnm share a protection section and, together with Txpm, form a 1: n protection relationship. SDH Multiplex Section Protection (MSP) monitors and measures the status of received signals and conducts pulls and selection for appropriate SDH signals from the protection section.
1:n protection based on individual wavelength and implemented on SDH layer
1:n protection based on individual wavelength and implemented on SDH layer

In the DWDM system, the switching of each SDH channel is unrelated to the other channels, that is, even when the Tx11 of DWDM system 1 fails and switches to DWDM protection system 1, Tx12, Tx13 Tx1m, the DWDM can still function in the system.

Once a switching condition is detected, protection switching must be completed within 50ms. 1: n protection based on individual wavelengths within the same DWDM system. Consider a DWDM line that can carry multiple SDH channels. Passive wavelengths in the same DWDM system can act as protection channels.
1:n protection based on individual wavelength within the same DWDM system
1:n protection based on individual wavelength within the same DWDM system
As per given image DWDM system of n+1 wavelength channels with n working wavelengths and 1 protective wavelength as protection system. However, in practical systems, the reliability of optical fibers and optical fiber cables is worse than that of the equipment.

So practically, it does not mean much to provide protection only for the system instead of the line. Once a switching condition is detected, the protection switching must be completed within 50ms.

Optical Multiplex Section Protection (OMSP)

This technique provides 1 + 1 protection only in the optical path instead of the terminal line. At the transmitted end and receiving end, 1 2 optical splitters and switches are used, respectively. 

Or other approaches are adopted (such as glowing status which refers to a case that the optical amplifier stays at low bias current and the pumping source at low output and that the launched signal is small and can merely detected for monitoring to determine whether it is in normal operating state).

At the transmit end, the multiplexed optical signals are  separated while at the receive end they are routed. The features of the optical switch are low insertion loss, transparent to the wavelength amplification region of optical fiber, fast in speed. Moreover, it can be highly integrated or miniaturized.
Optical multiplex section protection (OMSP)
Optical multiplex section protection (OMSP)
As per given image optical multiplex section protection scheme that employs optical splitters and optical switches. In this security system, only optical fiber cable and DWDM line systems are backed up, and SDH terminals and multiplexes are not supported in DWDM system terminal stations.

In practical systems, an N: 2 coupler can be used to replace multiplexers and 1: 2 splitters. Compared to 1 + 1 protection, this reduces costs. OMSP practically means only two independent optical fiber cables.

Applications in Ring Networks

DWDM systems can also be used to create ring networks. One application is to connect point-to-point DWDM systems based on individual wavelengths to form a ring, as shown in the image. At the SDH layer, 1: n security is implemented. The SDH system must adopt ADM equipment.
A ring formed by point-to-point DWDM systems
Protection in DWDM System

In the protection system shown in the figure below, the path protection ring and MSP protection ring of the SDH system can be implemented. The DWDM system only provides "virtual" optical fiber. The protection of each wavelength at the SDH layer is independent of the protection mode for the other wavelengths. This ring can be two-fiber or 4-fiber.
A ring formed by OADMs details
Protection system formed by OADM
Another application mode of DWDM technology in ring networks is to employ OADM with add / drop multiplexing capability to create rings. Currently, the ring network formed by OADM can be classified into two modes. Wavelength path protection is based on individual wavelength protection, that is, 1 + 1 individual wavelength protection which is similar to the path protection of the SDH system.

The second line is the protection ring that protects the multiplexed wavelength signals. When a fiber is cut, the "loop back" function near the fiber cutoff can be implemented in two nodes. Thus all services are safe. This is similar to SDH's MSP. From the aspect of specific forms, the line protection ring can be divided into a two-fiber bi-directional ring and a two-fiber unidirectional ring, and a four-fiber bi-directional ring can also be formed. In a two-fiber bi-directional ring, half of the wavelength functions as the wavelength and the other acts as the protection wavelength.


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