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WDM Hybrid transmission scheme

CWDM system is widely used in the DWDM system. Because of that the advantages of CWDM technology is it uses a relatively low cost without cooling of distributed feedback lasers and inexpensive passive filters.

Moreover, if use CWDM technology, it is possible to use cheaper compact transceiver. However, due to the relatively large CWDM channel spacing, so the number of wavelengths available to the system will be reduced, this also limits the transmission capacity of the system.

Under the current ITU G.694.2, with 20nm intervals, then can accommodate up to 18 CWDM wavelengths. For many applications, the general standard single mode fiber (SSF) in which the eight wavelengths loss will be very large. Therefore, based on the G.694.2 CWDM technology can only use eight wavelengths in the SSF, they are 1470nm,1490nm,1510nm,1530nm,1550nm,1570nm,1590nm and 1610nm. So far, as long as the customer's WDM networks require more channels, you have to be converted to the use of DWDM. Because DWDM wavelength small intervals, allow a large number of channels increase (generally have 32,64,128 channels), and the channel interval can reach 200, 100, or even 50GHz, but the cost per channel is significantly increased. Therefore, customers must assess their volume of business in the future development of the situation, to determine at a lower initial cost of installation flexibility relatively poor CWDM system, or to the higher initial cost of installation flexibility better DWDM systems.

In consideration of the following circumstances, "DWDM" refers specifically to a channel spacing of 100GHz DWDM systems. In addition, the cost difference between CWDM and DWDM systems is generally within 20% to 40% of.

Shown in Fig. 1 mentioned above is used widely distributed CWDM wavelengths, the channel spacing is 20nm. When using SFF transmission, the channel outside 1470-1610nm, light attenuation will be increased dramatically. Therefore, in order to achieve the proper transmission performance, CWDM hac only a maximum of eight wavelengths. In contrast, DWDM in the C-band and L-band even in a much narrower spectral range can still use a smaller channel spacing. Example of a 100GHz DWDM, its two intervals between adjacent channels is generally about 0.8nm, then at least you can have 64 channels - there are 32 in the C-band channels, plus 32 L-band channels ( There are some systems in the L-band can have more channels).


Single-stage CWDM system is upgraded to DWDM system 

Several WDM equipment manufacturers can provide a transitional product between CWDM and DWDM method, what they used is when all of the installed capacity has been CWDM system needs to be expanded, to expand on the use of DWDM filter for each channel CWDM ports . Shown in Figure 1, you can have up to eight interval of 100GHz DWDM channels corresponding to a CWDM channel. Therefore, the principle of a CWDM channel up to the equivalent of eight DWDM channels. The biggest drawback of this method is that, on the one hand not all CWDM channels can overlap in the spectrum with the corresponding DWDM channel, the other about 50% of DWDM channels as with the guard band edge and / or CWDM filter (red arrow ) overlap and can not be used. Eight-channel CWDM system are shown in Table 1 to step DWDM system upgrade.


We assume that the individual specifications of active and passive devices choice is appropriate, then by a simple calculation of the spectral overlap situations we get the figures in Table 1. In this scheme, the maximum number of channels that can be achieved is 32. It is noteworthy that, in such a CWDM filter structure, each step will be to upgrade the transmission system outages, because the active devices need to be replaced for CWDM DWDM wavelength during the upgrade process. In other cases, the use of two filter CWDM transmission structure. This approach allows the user to upgrade the service in progress DWDM wavelengths, while compared with the single-stage method can achieve relatively high channel flexibility.

CWDM system band structure

Two-stage filter structure based on wavelength bandwidth are generally used in DWDM systems. The main use of this method in the technical reasons for the intermediate wavelength channel group, also known as channel bandwidth to achieve high optical isolation. Because multi-node network total optical power is very different, it must be done in order to support optical isolation signal error-free transmission in a multi-node network. But also provide a filter module for each wavelength bandwidth advantage deeper modular system is increased, which can reduce investment, simplify upgrades wavelength.

Shown below, is to apply this concept to the case of a bandwidth of 2 CWDM system. In this example, we will eight channels into two bands, A and B, each containing four CWDM wavelengths (A band, 1470,1490,1590,1610nm; B band, 1510,1530,1550,1570nm) . A band of wavelengths in the B band symmetrically distributed on both sides. In practical applications, the use of a band-pass filter can be divided into A and B of the two wavelength bands. Band pass filter passband edge specifications are based on standard CWDM channel filter set. As shown in Figure 2, the most important features of this sub-band scheme that completely covers the B band DWDM C-band (marked with red arrows). Therefore, while the A-band using CWDM and DWDM C-band is feasible. In addition, B-band again uses a set of four CWDM wavelengths, the four wavelengths in optical networks are widely used for many years. It can be said, in general, this symmetry of the sub-band solution supports all passive optical devices appearing on the market, but also allows the use of standard CWDM and DWDM C-band. 


Shown in Figure 3 is similar to the one described above in Scheme asymmetric band structure. In this scheme, the wavelength allocation is A band comprising 1470,1490,1510 and 1610nm; B band includes 1530,1550,1570 and 1590nm. In this case, because the DWDM C-band and L-band B-band completely covered, so the asymmetric band plan to support simultaneous use of CWDM and DWDM C-band and L-band, making the flexibility of the system is greatly improved. However, the first scenario is based on the standard device design, the second scenario is for the band-pass filter and channel filter modules for specific passive components and design.


Two CWDM system is upgraded to DWDM system 

Because of the introduction of the second filter CWDM system, so it greatly improves the flexibility of the entire system architecture. Figure 4 is a WDM system terminals may upgrade steps. CWDM simple structure shown in Figure 4a, 4b and 4c. In Figure 4a, CWDM bandwidth filter itself is only to be used as an independent filter, which is similar to a two-channel WDM system, the system has two wavelengths, respectively, mentioned above may be any of A band and B band wavelength match. Since this is a single-class infrastructure, then insert the second stage filter module, you need to interrupt service.

However, since many current WDM modules are equipped with TDM function, so, even if a second channel WDM terminal can also support applications 4,8,16 or more channels according to the system TDM port density. As shown in Figure 4b and 4c are two steps CWDM upgrade increments of four channels. Compared with the eight-channel module, four channel spacing of this channel filter module between the upgrade process can reduce the initial investment. Figure 4d and 4e shows the CWDM / DWDM hybrid system, in which the band-pass filter port A and port B are connected to CWDM and DWDM part of the system. In general, DWDM part itself had DWDM DWDM channel bandwidth filters and filter (that is, another two filters). However, the structure shown in Fig. 4e requires asymmetric band-pass filter, and in Figure 4d asymmetrical structure can be used both in a stepwise manner to be symmetric in a stepwise manner.

According to Figure 4, the system has two main upgrade is possible: one is within a pure CWDM system (Figure 4 abc) upgrade; the other is the first upgrade to CWDM / DWDM hybrid system (Figure 4 abd), and then further extended to FIG 4e.


Table 2 summarizes the different structural channel corresponding flexibility - steps a, b, d and e to provide a shifted manner, so that the capacity of a hybrid system can be up to 68 channels. In order to achieve uninterrupted service upgrades, according to step a, you should avoid single-stage operation. And because from step c, d and e upgrades need to exchange B band CWDM channels. Therefore, a and c are not able to achieve further upgrades without service interruption.


Compared with the standard single-level system, where spoken against the concept of a two-stage filter CWDM system has two main advantages:

During the upgrade process, due to the use of such a low cost structure 2,4 and 8-channel CWDM system to improve the channel spacing of the filter, reduces the initial investment, implements upgrades without service interruption, but the system also supports all standard compliant with ITU DWDM channel spacing.

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