The cell format is essentially the same as for B-ISDN, except nothing
is said about the header data format. Thus a cell has a 4 octet
header, followed by a single octet Header Error Check (HEC) and a 48
octet payload. The HEC may be omitted since there is dispute about its
usefulness on a Taxi link. If present the HEC is formed from the CCITT
CRC polynomial 
, initialised to zero and
exclusive-ored with 01010101 before insertion.
The start of cell that includes the HEC is indicated by transmission
of a TAXI TT command, the 53 octets of the cell will follow this. A
cell that does not contain a HEC is started with a Taxi SR character
which is followed by 52 data octets. Thus receiver can determine
whether to check for the HEC based on the start mark, and receivers
which only accept one form can drop cells of the other type. At least
one TAXI Sync character should be inserted between cells
. Note that the TAXIchips cannot be told to
transmit a Sync character, instead a cycle with no transmission should
be used.