The service identifier is a feature ofISDN that prevents connections
between incompatible terminals. The service identifier istransmitted when
so that the receiving device can recognize whether it can communicate with the corresponding device. The service identifier is a hexadecimal three-digit number with which the services telephone, fax, terminal adapter, screen text, data transmission, teletex, telecontrol, remote signalling and video tele phony are identified. It prevents, for example, the connection between a telephone and a personal computer (PC) or a fax machine.