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In Mölndal case the aim was to build a cost effective and future proof network with high availability able to support their growth.
Mölndal City is a municipality in south-west of Sweden with 60 000 inhabitants, interconnected with the whole public institutions of the town.
Mölndal City decided to build out their existing fiber net. The existing fiber net was a 2-fiber ring running on 100mb/s, and connected the local networks from different municipality premises such as schools, library, health care centers and official buildings. All connected to the head node, located in the city hall.
The drawback in the existing network was the vulnerability. Power failures and other disruptions made the head node lose contact with the other nodes in the network. The existing network also needed to be expanded from about 30 nodes to more than 90 nodes.
The top priorities the municipality had when designing the new fiber infrastructure was: Redundancy, Scalability, Cost.
The solution was to use the CWDM technology. This enables several nodes to use the same fiber and thereby saving cost. To have scalability and growing possibilities, the network was designed as a 10G backbone ring and from this ring expands several CWDM rings at the 3 major sites: Mölndal, Kållered and Lindome.
Each CWDM ring has not more than 12 nodes. By having two nodes in each ring using the same wavelength, expected infrastructure is reach in a logical level. The picture below shows a sketch of a ring with 9 nodes, using 5 wavelengths. The same wavelength are repeated maximum twice in the ring, this creates good redundancy as well as it regenerates the optical signal.
This network was achieved by using standard switches. The switches have been configured to handle the redundancy and the monitoring of the network. The switch turn over time is for the network is 50ms. As for the monitoring, the alarms are sent as SMS to the employees of the municipality IT department.
SFP, Small Form-factor Pluggable, is the optical transceiver type used in most switches today. For this network standard 1Gb/s CWDM SFPs from Skylane optics®. To take for account the attenuation in the add-drop nodes the 80km version with a link budget of 32dB was used, even though the diameter of the rings was less than 80km.
The CWDM technology is enabled by using a multiplexing unit, Mux/Demux, combining all wavelengths at the head node and a one channel Optical Add Drop Module, OADM, at each node. This was supplied by Skylane Optics® The filter units are totally passive devices using no electricity, so they are not affected by power failure.
The experience of Mölndal is that the proposed CWDM solution has been working extremely well. They had some interruptions in the network in the first month after deployment but as they had redundancy built into the network this never affected the end customer. They are very satisfied to now have full control over their own network, not being depending of a service provider. The high capacity of the network enables the IT department to upgrade or re-install the computers in the network remotely, giving an efficient IT support. As a next step Mölndal will now build out their network with 30 more nodes.