Asynchronous Serial Interface, or ASI, is a method of carrying an MPEG Transport Stream (MPEG-TS) over 75-ohm copper coaxial cable or optical fiber. It is popular in the television industry as a means of transporting broadcast programs from the studio to the final transmission equipment before it reaches viewers sitting at home.
The ASI standard is maintained by CENELEC, the European Committee for Electrotechnical Standardization, and is part of the collection of standards known as Digital Video Broadcast, or DVB.
ASI carries MPEG data serially as a continuous stream with a constant rate at or less than 270 megabits per second, depending on the application. It cannot run faster than this, which is the same rate as SDI and also the rate of a DS4 telecommunications circuit which is typically used to transport the stream over commercial telephone/telecommunications digital circuits (Telco). The MPEG data bits are encoded using a technique called 8B/10B which stands for 8-bit bytes mapped to 10-bit character codes. Encoding maintains DC balance and makes it possible for the receiving end to stay synchronized. When on 75-ohm coaxial cable, ASI is terminated with BNC male connectors on each end. Electrically, the coaxial standard specifies an output voltage of 800 millivolts peak-to-peak, while the receiver must be able to operate from a voltage anywhere from 200 mV to 880 mV. ASI is electrically identical to and has the same bit rate as standard definition SDI. When ASI is on optical fiber, it is multimode fiber.
There are two data transmission packet sizes commonly seen by the ASI interface and the cable carrying it: the 188 byte packet and the 204 byte packet, the fundamental building blocks of the MPEG Transport Stream. The 188 byte format is by far the most common packet size, used by the vast majority of transmissions. When optional Reed–Solomon error correction data are included, a format primarily developed by Cable Television industry, the packet grows an extra 16 bytes to 204 bytes total. [note 1]