ESS or Electronic Switching System is the technology to facilitate communication between phones in a region and overall the phones across the world by optimizing the number of channels needed for all the phones to communicate.
For example, if only two telephones were involved, only 2 channels (or wires) would be needed. If there were 3 phones involved, then we would need 6 channels and so on. As the number goes up, the number of dedicated channels would go up drastically and that would be a huge mesh of channels or wires needed.
In order to optimize that, the Electronic Switching System or ESS comes in and plays it’s part. Part of the functions that an ESS system achieves is what can be compared to the role of a traffic police or traffic lights in order to manage the traffic.