The Payload Type data field is used in various protocols to identify the user signal. For example, in ATM, DQDB, B-ISDN, in the Session Announcement Protocol (SAP), in the RTP protocol and in Optical Transport Networks (OTN).
In ATM, the payload type data field (PT) comprises 3 bits in the header of the ATM cell. It is used for payload identification. A distinction is made between payload information and management information. The first bit of the PT data field indicates whether the ATM cell contains user data or control data. If the ATM cell contains user data, then the second bit is used to indicate congestion. And the third bit is used to indicate whether the ATM cell is the last of an AAL5 frame.
The PT data field in the DQDB header is identical to that in the ATM header.