The FAS, otherwise known as user control information has two purposes. One purpose is to indicate framing information to allow for optimisations at a lower level for example when blocking cells together or when interrupting a host. There are two bits of this framing information. Bit 7 of the second octet is defined to be set on the first cell of a block. Bit 6 of the second octet is defined to be set on the last cell of a block. These two bits are denoted ``fs'' and ``fe'' respectively in table 1. The other six bits in the second octet and all of the first octet are available for use as sequencing information.
