Do fields in FAST have a fixed size?

Fields in FAST do not have a fixed size and do not use a field separator. Instead, there is a notion of a stop bit (the high order bit on each byte of the message acts as a stop bit) signaling the end of the field.

All of the above concepts used together allow the sender to compress a message (sometimes as much as 90%) and the receiver to restore the original message content from the encoded bytes.
The next section looks at a small message example to show encoding and decoding in practice.