# ARMONIA: A Unified Access and Metro Network Architecture
## 2 ARMONIA Architecture
### ARMONIA Metropolitan Network  
A metropolitan network is the main part of the ARMONIA's overall network infrastructure. Metropolitan and core optical networks in recent years are usually based on elastic optical networks (EONs) [18]. EONs are an evolution of WDM (wavelength division multiplexing) technology, providing resource (spectrum) flexibility and efficiency. ARMONIA considers a general-purpose elastic optical network, equipped with coherent receivers. In order to serve more efficiently the high access demands, the metropolitan network is organized into hierarchical layers (Figure 4). The exact number of these layers may depend on several parameters such as the coverage area, the number and distribution of users, the traffic characteristics, the capabilities of the networking devices, and other factors.  
The nodes of the lower layer, the metro access nodes, are typically co-located with the OLT nodes inside the operators' central offices and are organized in ring and mesh topologies. In the intermediate metro layers, the supported transmission rates increase as more powerful transceivers are employed to transfer the aggregated traffic. The higher layers of the metro infrastructure consist of fewer nodes, namely the metro core nodes, and are organized in mesh topologies. Internet traffic from access networks (wired-PON or wireless-4G/5G) is concentrated on IP routers connected to metropolitan access nodes. The traffic is then transferred to the main intersections of the metropolitan network in order to be routed to the final destination. Traffic can also be forwarded through an IP router directly to a key node of the metropolitan network.  
The use of coherent receivers at the nodes of the ARMONIA network provides real-time monitoring capabilities. Potentially, each receiver in the network can be extended almost for free (through software) to act as an optical performance monitor (soft-OPM). Therefore, a rich amount of information can be leveraged to make dynamic optimization decisions. These monitors can be accessed by the SDN controller through the NETCONF protocol following the network disaggregation model. In disaggregated optical networks, which are recently gaining consensus, networking hardware can be provided by multiple vendors, avoiding vendor lock-in. In addition, open and standardized interfaces(OpenConfig [19] and ONF Transport API [20]) based on Yet Another Next Generation (YANG) modeling enable vendor-neutral provisioning, monitoring, and resource management operations.  
An increasing number of telecommunications providers are gradually introducing datacenters into their network infrastructure and especially in the metropolitan area. Generally, the nodes near the edge of the network provide lower layer computation and storage resources while the metro core nodes support higher layer ones. Hence, a hierarchical computing infrastructure is created that provides the necessary capacity for the creation of virtual network infrastructure (NFVI) services. This infrastructure is interconnected with high-speed optical connections through the metropolitan network.