Serial interfaces have been used for digital communications almost as long as digital logic has been in existence. By far the largest majority of these serial interfaces operate at what are today considered to be relatively slow speeds. This would include computer interfaces like RS-232C/V.24 and RS-422/V.35, and telecommunications interfaces like ISDN and T1/E1 digital telephone lines. Whenever more than one of these types of interfaces must be connected between two points, it usually requires a separate electrical or optical cable for each serial link.