end OAM F5 flow cell (refer to Rec. 1.610) YYYYYYYYb ZZZZZZZZ ZZZZZZZZC Resource management cell (refer to Rec. 1.371) YYYYYYYYb ZZZZZZZZ ZZZZZZZZc Unassigned cell 00000000 00000000 00000000 The GFC field is available for use with all of these combinations. A Indicates that the bit may be 0 or 1 and is available for use by the appropriate ATM layer function. B Indicates the bit as a "don't care" bit. C Indicates the originating signaling entity shall set the CLP bit to 0. The value may be changed by the network. axxxxxxxx: Any VPI value. For VPI value equal to 0, the specific VCI value specified is reserved for user signaling with the local exchange. For VPI values other than 0, the specified VCI value is reserved for signaling with other signaling entities (e.g., other users or remote networks). bYYYYYYYY: Any VPI value. ZZZZZZZZ ZZZZZZZZ; Any VIC value other than 0. dTransparency is not guaranteed for the OAM F4 flows in a user-to-user VP. The VIC values are preassigned in every VPC at the UNI. The usage of these values depends on the actual signaling configurations. (See ITU-T Rec. 1.311.) Source: ITU-T Rec. 1.361, Table 2/1.361, p. 3 (Ref. 9). 20.4 THE ATM CELL: KEY TO OPERATION Table 20.2 PTI Coding PTI Coding Interpretation User data cell, congestion not experienced. ATM-user-to-ATM-user indication = 0 User data cell, congestion not experienced. ATM-user-to-ATM-user indication = 1 User data cell, congestion experienced. ATM-user-to-ATM-user indication = 0 User data cell, congestion experienced. ATM-user-to-ATM-user indication = 1 OAM F5 segment associated cell OAM F5 end-to-end associated cell Resource management cell Reserved for future functions Source: ITU-T Rec. 1.361, p. 4, para. 2.2.4 (Ref. 9). 20.4.1.3 Payload-Type (PT) Field. Three bits are available for PT identification. Table 20.2 gives payload type identifier (PTI) coding. The principal purpose of the PTI is to discriminate between user cells (i.e., cells carrying information) and nonuser cells. The first four code groups (000-011) are used to i