der fields. In FC state the decompressor tran- sitions to the initial states as soon as it detects corruption of the context. The decompressor uses the "k out of n" rule by looking at the last n packets with CRC failures. If k CRC fail- ures have occurred, it assumes the context has been corrupted and transitions to an initial state (SC or NC). The decompressor also sends feedback according to the operation mode (Fig. 2.16). The values of the ROHC compression parameters that determine the efficiency and robust- ness are not defined in ROHC specification and are not negotiated initially but are stated as implementation dependent. The values of these parameters remain fixed during the com- pression process. The compression parameters are defined as follows [16]: L: In U-mode and O-mode, the ROHC compressor uses a confidence variable L in order to ensure the correct transmission of header information. Timer_1 (IR_TIMEOUT): In U-mode, the compressor uses this timer to return to the IR compression level and periodically resends static information. New Radio Access Layer 2/3 Aspects and System Operation 229 Timer_2 (FO_TIMEOUT): The compressor also uses another timer in U-mode, and this timer is used to go downward to FO compression level, if the compressor is working in SO compression level. Sliding window width (SWW): The compressor, while compressing header fields such as sequence number (SN) and TS, utilizes window-based least significant bit (W_LSB) encoding that uses a sliding window of width equal to SWW. W_LSB encoding is used to compress those header fields whose change pattern is known. When using this encoding, the compressor sends only the least significant bits. The decompressor uses these bits to construct the original value of the encoding fields. and n: The ROHC decompressor uses a "k out of n" failure rule, where k is the num- ber of packets received with an error in the last n transmitted packets. This rule is used in the state machine of the decompressor to assume the damage of context and move downw