sequence number (ATM) (SN)

In ATM, the sequence number( SN) is a 4 octet long data field in the RM (Resource Management) cell. It is recommendation I.371 from the International Telecommunication Union( ITU-T) for the sequencing of cells.

The sequence number is not used by the ATM Forum for Available Bit Rate( ABR). An ATM switch keeps this field reserved or sets it in accordance with the ITU recommendation.

An SN cell is sent out periodically on all lines of an ATM Inverse Multiplexing( AIMUX) and indicates how many cells have been sent in the time between the last SN cell and the one currently being transmitted. The SN cell is used to restore the order of the payload cells at the receiving location.

In the Realtime Transport Protocol( RTP), the Sequence Number (SN) is a 1- byte data field.