The original SAR1 is shown in table 2, where ``st'' was set on the first cell of a block and ``pf'' indicates that the cell was partly full, which only happened on the last cell of a block.
This was the UDL SAR which had been previously used on Unison. The cell sequence number counted down from a maximum value in the first cell of a block, to 1 in the last cell. Thus the sequence number in the first cell of a block was the expected cell count, and indicated the block size.
The assemble field started at zero when a circuit was opened and was incremented (modulo 64) after each block is transmitted.
Unfortunately this was not compatible with FAS.
EDL networks use a variant of SAR1 where the part-full bit should never be set. EDL does not impinge on the standard protocols.
Note also that on EDL networks the two octets of the SAR are sent in reverse order. This is for the historical reason that the network does not have hardware imposed endian swap on the header.
