ndicate user cells. Within these four, 2 and 3 (010 and 0111) are used to indicate congestion has been experienced. The fifth and sixth code groups (100 and 101) are used for virtual channel connection (VCC) level management functions. Any congested network element, upon receiving a user data cell, may modify the PTI as follows: Cells received with PTI = 000 or PTI = 010 are transmitted with PTI = 010. Cells received with PTI = 001 or PTI = 011 are transmitted with PTI = 011. Noncon- gested network elements should not change the PTI. 20.4.1.4 Cell Loss Priority (CLP) Field. Depending on network conditions, cells where the CLP is set (i.e., CLP value is 1) are subject to discard prior to cells where the CLP is not set (i.e., CLP value is 0). The concept here is identical with that of frame relay and the DE (discard eligibility) bit. ATM switches may tag CLP = 0 cells detected by the UPC (usage parameter control) to be in violation of the traffic contract by changing the CLP bit from 0 to 1. 20.4.1.5 Header Error Control (HEC) Field. The HEC is an 8-bit field that covers the entire cell header. The code used for this function is capable of either single-bit error correction or multiple-bit error detection. Briefly, the transmitting side computes the HEC field value. The receiver has two modes of operation as shown in Figure 20.6. In the default mode there is the capability of single-bit error correction. Each cell header is examined and, if an error is detected, one of two actions takes place. The action taken depends on the state of the receiver. In the correction mode, only single-bit errors can be corrected and the receiver switches to the detection mode. In the detection mode, all cells with detected header errors are discarded. When a header is examined and found not to be in error, the receiver switches to the correction mode. The term no action in Figure 20.6 means no correction is performed and no cell is discarded. It should be noted that there is no error protection for the payload of a cell. If an error 