| 摘要 |
An ATM network element receives a stream of data associated with a particular user. This data stream is further partitioned into frames and ATM cells. A predefined traffic set is associated with the user. The ATM network element evaluates the received data stream for conformance to the user's traffic set at every frame boundary. The Payload Type Indicator (PTI) of an ATM cell header is used to indicate whether that cell belongs to a non-conforming frame or a conforming frame. In particular, if a frame is determined to be non-conforming, the ATM network element sets the PTI field of at least the end-of-frame cell (i.e., the cell with the UUI bit set to one in AAL5), to a predefined code representative of non-conforming (hereafter referred to as an NCF code). As a result, downstream network elements are able to determine which of the currently buffered cells belong to a non-conforming frame by looking at both the PTI field and the frame boundary marker (UUI bit). For instance, if a downstream network element determines that the end-of-frame cell belongs to a non-conforming frame, any of the cells from that frame may be discarded from a buffer irrespective of the PTI field status of the other user data cells for that frame.
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