 in the system or even causing loss if the queue becomes full. If source 1. Address field defining the virtual channel to which the cell is activity is low, such that the queue becomes empty, then 'idle cells' assigned, divided into two parts, a virtual path identifier (VPI), are inserted on the link to maintain the contiguous cell flow. and a virtual channel identifier (VCI). The sequence of cells from a particular user or source all carry the 2. Payload type identifier (PTI). same value in the header address field and this cell sequence (Sent first) GFC * header 3 Byte GFC field only exists at the NNI payload Used for part of VPI field at UNI Figure 41.2 The ATM cell: (a) cell structure; (b) cell header structure ATM basic principles 41/5 Source A Source B Source C Queue Idle cell insertion Cellstream Transmission Link Figure 41.3 ATM multiplexing constitutes a 'virtual channel': the rate of transmission of cells Digital Hierarchy (SDH) based interface where the cell stream is within the virtual channel can be variable, reflecting the source mapped into the C-4 container and then packed into the VC-4 virtual activity and resource availability and it is in this sense that the container along with the VC-4 Path Overhead (POH). The ATM cell transfer mode is said to be asynchronous. boundaries are aligned with the STM-1 octet boundaries and the H-4 pointer indicates the next occurrence of a cell boundary, offer- ing a supplement to the HEC Cell delineation mechanism (Figure 41.3.2.2 Physical layer 41.4). An ATM multiplex is capable of being supported on a range of 41.3.3 ATM switching possible physical media and mapped into a number of existing bit-stream structures. For B-ISDN two possibilities are envisaged at 41.3.3.1 Switch function the User-Network interface (UNI): a cell-based interface where timing is derived from the cell-stream and OAM messages are The cell header address field is sub-divided into two parts: the carried in special OAM cells with a unique header; a Synchronous virtual path identifier (VPI) 