Hello, everyone!
Today, I would like to share with you an article about Wavelength Division Multiplexing Optical Access.
Wavelength Division Multiplexing (WDM) is a high capacity and efficient optical signal transmission technology that is prevalent in long-haul backbone applications, but is now emerging in Metropolitan Area Networks (MAN). WDM uses multiple wavelengths of light, each wavelength corresponding to a distinct optical channel (also known as lightpath or lamda), to transmit information over a single fiber optic cable simultaneously.
Current backbone commercial WDM systems have been increased up to 40 (100GHs spacing), 80 (50GHs spacing) in C-band or 160 wavelengths in C+L-band on a single fiber. It is an economical alternative to installing more fibers and a means to dramatically provide higher capacity.
Current demand of bandwidth is nearly approaching the limit of transmission capacity of copper-based technologies like Digital Subscriber Line (DSL) or cable modem. Although based on aforementioned TDMA PON has the ability to provide up to 1-2.5Gb/s, burst mode reception at OLT and the clock synchronisation of different ONUs have already been the main barrier to limit the TDMA mechanism up to higher signal rates. Therefore, WDM has been considered as a transition path from the current access technologies to the ultimate access solution.
WDM itself inherits many advantages from the WDM technology of backbone or metro area such as large capacity, data transparent, multi-service, easy management, network security, and upgradability . And also combined the merits of PON network, WDM PON has been considered as future-proof ideal solution.
Regarding the wavelength assignment for WDM PON, there are two choices:
Coarse WDM (CWDM)
Dense WDM (DWDM)
CWDM utilises 18 wavelengths from 1270nm to 1610nm covering O, E, S, C, L-band with a wide channel spacing 20nm, therefore, athermal AWG and uncooled laser sources are good enough. Low cost is the most attractive advantage for CWDM. However, the elimination of strong absorption at the water peak in E-band and the need for effective all band amplification become the two critical issues that must be addressed in order to longer reach.
On the other hand, DWDM achieves greater spectral efficiency using 50/100GHs channel spacing and with commercially available fiber-based EDFA, can be scalable in distance and number of users, which makes it a better upgrade option in the long-term future.
WDM optical access is a future-proof last mile technology with enough flexibility to support new, unforeseen applications. WDM switching can dynamically offer each end user a unique optical wavelength for data transmission as well as the possibility of wavelength reuse and aggregation, thereby ensuring scalability in bandwidth assignment.
Fig. a. Typical Passive Optical Access Network
For instance, heavy users (e.g., corporate users) may be assigned a single wavelength whereas light users (e.g., residential users) may share a single wavelength (Above Fig. a.), all on a single fiber. Based on wavelength switched scheme, in a WDM PON network, the OLT contains a multi- wavelength source used to send signals across different wavelengths.
Fig. b. The detailed architecture between OLT and ONU
In the remote node, an optical switch (MUX/DEMUX) selects out one or more associated wavelengths and transmits them to the subscriber ONU as shown in the above Fig. b. in detail. We are also witnessing the exciting convergence of WDM and Ethernet, the most notable example being the National LamdaRail or NRL, which is a high-speed, experimental 40-wavelength DWDM optical testbed developed to rival the scale of research provided by the Arpanet in the 1960s. NLR is the first wide-area use of 10 Gbit/s switched Ethernet and is based on a routed IP network.
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