One of the main indicators for dimension and size of a telephone network is the number of fixed main lines in the network. A main line is a telephone line that connects a subscriber's terminal equipment to the Public Switched Telephone Network (PSTN). It could be an exclusive exchange line or a shared line. However, the number of main lines in a network does not include subscribers' extension lines or private branch exchange lines, and therefore it is not affected by the number of subscribers' operating stations. This is what differentiates between the number of main lines and the number of access lines or the number of subscribers in a telephone network.