Asynchronous serial interface

Asynchronous Serial Interface, or ASI, is a physical (connector and electrical) definition for serial data over 75-ohm coaxial cable at rates at or less than 270 megabits per second. Electrically, the signal is typically around 1 volt.

ASI has one purpose only: the transmission of an MPEG Transport Stream (MPEG-TS). ASI is designed for that one purpose: the transport of the MPEG transport stream over coaxial cable, usually as part of its run within a transmission facility before conversion to fiber or wireless carriage.

The MPEG transport stream is the only standard protocol universally used for real-time transport of broadcast audio and video media today. Even when tunneled over IP, the MPEG Transport Stream is the universal lowest-common-denominator of all long-distance audio and video transport. In the US, it can be broadcast to homes as the ATSC Transport Stream; in Europe, it is broadcast to homes as the DVB-T Transport Stream. All broadcast satellite transmissions see it as the DVB-S Transport Stream.

It is usually made up of one or more television channels with accompanying audio, sometimes with additional audio-only or data transmission channels, When that composite data transmission path, asynchronous but formatted data, travels through space as RF, it is usually called DVB-S, DVB-T, or ATSC. But when carried on coaxial cable, unmodulated, it is called an ASI signal.

It is physically carried on 75-ohm coaxial cable, terminated with BNC male connectors on each end; the same data itself was or will probably be transported by wireless or fiber links, but then it will not be called ASI, even though it is precisely the same data.

ASI only refers to the data over coaxial cable. Specifically it refers to the electrical interface, a one-way transmission of serial data, and must be less than 270 megabits per second, the limit of the MPEG Transport Stream, and the rate of the DS4 telecommunications circuit which is typically used to transport the stream over commercial telephone/telecommunications digital circuits (Telco).

It is a one-way transmission, similar to RS-232 asynchronous data—a stream of raw but formatted zeros and ones—designed to primarily travel through coaxial cable at speeds that range from 6-200 megabits per second. Though 270 megabits per second is the rate of the underlying available bandwidth, Transport Streams, and therefore ASI transmissions, usually top out at around 200 megabits per second.

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