 transitions to the SO compression state after re-transmitting the updated information and re-establishing the change pattern in the decompressor. In the U-mode, the feedback channel is not used. To increase the compression level, an optimistic approach is used for the compressor to ensure that the context has been correctly established at the decompressor side. This means that the compressor uses the same header format for a number of packets. Since the compressor does not know whether the context is lost, it also uses two timers to be able to return to the FO and IR compression states. The decompressor works at the receiving end of the link and decompresses the headers based on the header fields' information of the context. Both the compressor and the decompressor use a context to store all the information about the header fields. To No Static Repeated Failures Success No Dynamic Success Repeated Failures Success Compressor States Modes of Operation Decompressor States FIGURE 5-4 ROHC state machines [9] 5.2 Service flow classification and identification ensure correct decompression, the context should be always synchronized. The decompressor has three states as follows: (1) No Context (NC), where there is no context synchronization; (2) Static Context (SC), where the dynamic information of the context has been lost; and (3) Full Context (FC), when the decompressor has all the information about header fields. In FC state, the decompressor transitions 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 received packets. If k CRC failures 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 (see Figure 5-4). The values of the ROHC compression parameters that determine the efficiency and robustness are not defined in ROHC specification and are not negotiated initially, but are stated as implementation dependen