C field set to all zeros (0000). In order to avoid unwanted interactions between this mode and the controlled access mode, where hosts are expected to modify their transmissions according to the activity of the GFC field, it is required that all CPE (customer premise equipment) and public network equipment monitor the GFC field to ensure that attached equipment is operating in uncontrolled mode. A count of the number of nonzero GFC fields should be measured for nonoverlapping intervals of 30,000 10,000 cell times. If ten or more nonzero values are received within this interval, an error is indicated to layer management (Ref. 3). 20.4.1.2 Routing Field (VPI/VCI). Twenty four bits are available for routing a cell. There are 8 bits for virtual path identifier (VPI) and 16 bits for virtual channel iden- tifier (VCI). Preassigned combinations of VPI and VCI are given in Table 20.1. Other preassigned values of VPI and VCI are for further study, according to the ITU-T Orga- nization. The VCI value of zero is not available for user virtual channel identification. The bits within the VPI and VCI fields are used for routing and are allocated using the following rules: The allocated bits of the VPI field are contiguous. The allocated bits of the VPI field are the least significant bits of the VPI field, beginning at bit 5 of octet 2. The allocated bits of the VCI field are contiguous. The allocated bits of the VCI field are the least significant bits of the VCI field, beginning at bit 5 of octet 4. Table 20.1 Combination of Preassigned VPI, VCI and CLP Values at the UNI Meta-signaling (refer to Rec. I.311) XXXXXXXX X 00000000 00000001e General broadcast signaling (refer to Rec. 1.311) XXXXXXXX ² 00000000 00000010 e Point-to-point signaling (refer to Rec. 1.311) XXXXXXXX X 00000000 000001016 Segment OAM F4 flow cell (refer to Rec. 1.610) YYYYYYYYb 00000000 00000011° End-to-end OAM F4 flow cell (refer to Rec. 1.610) YYYYYYYYb 00000000 00000100 Segment OAM F5 flow cell (refer to Rec. 1.610) YYYYYYYYb ZZZZZZZZ ZZZZZZZZc End-to-