ROADM : The Core of Agile Optical Networks

Sept. 27, 2011 - PRLog -- As the bandwidth requirements grow rapidly, optical network operators face continued challenges of modifying the current optical networks to meet this growing demand. In WDM networks, this modification mainly means adding new WLs, and sometimes changing the WL distribution within the network. The WDM solutions have scaled up in recent years from typically 8-16 Wavelengths to 44-88 Wavelengths solution, with different routes per each wavelengths and mesh connectivity between sites across the ring.

Network managers face the challenge to keep track of all the wavelengths, monitor their correct operation and deploy new wavelengths easily while reducing the operation cost of the network. The Reconfigurable Optical Add Drop Multiplexer (ROADM) adds the required flexibility so that adding WLs or changing the WL destination becomes easy and remotely managed process providing full control and monitoring over the entire high capacity infrastructure. Compared to a non ROADM network, if not already pre-assigned, adding new services requires redesign or major network change which takes time, resources and poses risks of traffic disruptions. This white paper explores the evolution from passive based infrastructure and its limitations compared to ROADM based solutions.

Passive based Solution
In passive optical network infrastructure, the building blocks of the network are based on passive filters with fixed optical wavelength modules such as Mux/Demux, OADM or a combination of Band Pass Filters together with the Mux/Demux. This requires careful design prior to installation in order to meet estimated future network growth and connectivity matrix.
Figure 1 is a drawing of simple passive Mux/DeMux based ring infrastructure.  Each Node across the network is built from 2 passive Mux/DeMux's, one facing the east and one facing the west. The number of supported WL of the solution is defined by the number of channels/filters supported by the Mux/DeMux. Following are a few scenarios which illustrate the limitation and complexity of operation of such passive based networks.
Scenario 1: changing distribution of the “Blue” WL.
In figure 1, the "Blue" WL passes the Node-3 through the "Blue" filter of both Mux/DeMux. However, let's assume that this service is required at Node-3, this will require manual configuration of the network which can be time consuming and costly. To perform this operation, there is a need to re-patch the “Blue” WL at Node-3 which means sending a technical person to the site. In addition, the manual patches need to be documented and maintained. With too many changes, the network administrator might become disoriented with regards to the original network scheme. Also, scaling this type of solution to 88 WL is practically impossible.

Scenario 2: Connecting “Yellow” WL between Nodes-1 to Node-4.
In order to drop the "Yellow" wavelength (service) from Node 1 to Node 4, it must pass through Node-2 and Node-3 and be manually patched in the Mux/DeMux.  However since the “Yellow” wavelength is not available in the Mux/DeMux, it cannot be passed through. This problem is due to a prior design with fixed passive infrastructure; there are not enough wavelengths to pass this new service between the Node-1-Node-4. Therefore, in order to add WL, the network designer needs to replace the Mux/DeMux in Node-2 and Node-3 to support higher number of wavelengths which is not only costly but will most likely also cause traffic disruption. This issue will not occur in ROADM based infrastructure since the ROADM based network supports full C-band expansion (50/100GHz) from day-1.


 Figure 1: Adding new WL

Power Balancing
Another major issue that usually requires manual handling in non-ROADM networks is the power balancing of the bypass wavelengths as opposed to locally added wavelengths on any particular node ( see figure 2). It is necessary to balance the optical power of these WLs at the input point to an EDFA (Erbium-Doped  Fiber Amplifier)  Otherwise, in a link with many EDFAs and multiple channels; some channels might be lost due to a lack of optical power if not balanced along the path.
In non-ROADM networks, this is done by reducing the optical power of the added services at the local node and installing fixed attenuators right after the transceiver. This manual operation involves test equipment and calibration of each new service independently which can be cumbersome and time consuming.  Since the distances between sites differ from site to site, the power levels of the pass through channels change as well, resulting in the need to place different attenuators at each node and documenting them accordingly.

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PacketLight Networks Ltd develops a complete set of CWDM, DWDM and dark fiber solutions. PacketLight customers are carriers, service providers,