**3. Architecture of the smart natural gas system**

The main purpose of the gas system is to supply gas to consumers with the required safety and quality characteristics at a reasonable cost. In order to fulfill this target in a cost-effective manner, different agents are necessary; each of them will be assigned some specific role.

The gas transmission network is the physical medium through which consumers can obtain the amount of gas they need. Depending on the type of network (characterized, among other parameters, by the pressure level and its capacity), there are transmission and distribution networks. Distribution networks are structures connected to the transmission network and carry the gas to the final consumers. The owners of the network, which are in charge of maintenance and development of those infrastructures, will therefore be transmitters and distributors.

Distribution (low and medium pressure network) is the final link of the chain in delivering natural gas to customers. While some large industrial or commercial customers, as well as power generators fueled by gas, receive natural gas directly from high-capacity interstate and intrastate pipelines, most users receive natural gas from their local gas utility, also called local distribution company (LDC).

Unlike other energy sources, the gas can be stored in large amounts at different points of the grid. The network operator, whose mission is to ensure that the gas system remains balanced and stable so that energy transactions can be performed safely and reliably, can use these gas stores.

Depending on the type of network they manage, these operators may be transmission or distribution operators. The operation of the network is an activity that could be developed by agents different from the owners of the infrastructures, so that these operators would be different from the transmitters and distributors previously defined.

Other agents that should be established according to size and characteristics of the gas system, as well as the configurations that the network can adopt, are the aggregators, whose mission is grouping small distributed demand resources into larger packages that can provide significant value to the system as a whole.

Finally, the figure of the gas retailer appears as an intermediary of the retail market between the final consumers (small amounts of energy) and the mechanisms of the wholesale market and gas production (large amounts of energy). The different agents and the activities they would carry out within the new framework that smart grids offer to the natural gas market are further discussed subsequently.
