f the next expected packet. Each compression state uses a different header format in order to send the header informa- tion. The IR compression state establishes the context, which contains static and dynamic header information. The FO compression state provides the change pattern of dynamic fields. The SO compression state sends encoded values of sequence number (SN) and time- stamp (TS), forming the minimal size packets (Figs. 2.6-2.15). Using this header format, all header fields can be generated at the other end of the radio link using the previously established change pattern. When some updates or errors occur, the compressor returns to 228 Chapter 2 No static Repeated failures Success No dynamic Success Repeated failures Success Compressor states Modes of operation Decompressor states Figure 2.16 ROHC state machines [16]. upper compression states. It only transitions to the SO compression state after retransmitting the updated information and reestablishing the change pattern in the decompressor. In the U-mode the feedback channel is not used. To increase the compression level an opti- mistic approach is used for compressor to ensure that the context has been correctly estab- lished at decompressor side. This means that 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 ensure correct decompression the context should be always synchronized. The decompressor has three states as follows: (1) no context (NC) where there is NC 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 hea