## 4 Communication Infrastructure  
The management of DERs in a VPP requires the monitoring of information from several DER installations, and from sensors monitoring the environmental conditions. Such functionality requires an ICT infrastructure able to support the aggregation of data and the virtualization of DERs. A possible architecture of the communication infrastructure among different VPPs and the distribution network is shown in Figure 6.  
With reference to the schematic of Figure 6, DERs represent physical devices installed on the local power grid, and they are part of the plant-level communication layer. PV inverters or Battery Energy Storage Systems (BESSs) are examples of DER units. Each of these devices is responsible for the monitoring and transmission of data measured at the plant level. DERs should be connected with the DER-AC by means of a field-bus communication to allow the DER-AC to implement control functionalities. In addition, the DER-AC is responsible for the first level of aggregation (data virtualization) with respect to the DSO-DMS or to the IO-DMS. The DER-AC aggregates data provided by DER units, and estimates the total average power and the total energy produced by the supervised DER units over a predefined interval as well as the total available energy in storage systems (i.e., considering that more BESSs can be installed at the same plant level). In normal operation mode, the frequency of data transmission is limited to a sample every few minutes. A typical value is in the range of 5 to 15 min [11]. The total amount of data transmitted per each node per each transmission is typically less than a hundred bytes. Thus, the communication bandwidth required by the plant-level communication is well within 100 kbps. The DSO-DMS (or IO-DMS) agent defined in Figure 5 can be expanded into two parts, as highlighted in the ICT architecture: the Virtual Power Plant Monitor (VPPM) and the Distribution Management System (DMS). The VPPM monitors DERs fed by the same secondary substation or  
Figure 6: Architecture of the Information and Communication Technology (ICT) infrastructure supporting the proposed VPP approach. VPPM: Virtual Power Plant Monitor. by a section of the distribution grid, which can be assumed to be homogenous from the DSO/IO point of view. The VPPM agent is typically located locally, while the DMS agent supervises the entire distribution grid. The VPPM is responsible for the second level of aggregation, which works both temporally (considering an interval larger than the previous one) as well as spatially (aggregating data from several customers). The communication infrastructure among the VPPM and DER-ACs forms the local level. Finally, the DSO/IO aggregates data coming from several VPPMs in its DMS to improve the state of estimation of the grid and to optimize the energy flows, and forms the supervisory communication level. In both the cases, the total amount of data exchanged is well below 1 Mbps, and no particular latency requirements are needed. The VPP ICT architecture defined in Figure 6 requires a communication infrastructure with a proper structure. As mentioned in the previous paragraph, the communication system is organized in three hierarchical levels: the plant level, the local level, and the supervisory level. Each communication level has its specific requirements in term of bandwidth, latency, time synchronization, and cyber-security. The plant level connects all the devices installed on the field (such as inverters, storage systems, energy meters, and sensors) to the DER-AC, which performs the monitoring and the control functionalities. Typically, this level does not require communication technologies able to provide large bandwidth, while the real-time behavior (i.e., deterministic latency) is a strict requirement due to the control functionalities performed at this level.