ference values for compression and decompression. Moreover, additional information describing the packet stream is also part of the context, for example information about how the IP identifier field changes and the typical inter-packet increase in sequence numbers or timestamps [9]. CHAPTER 5 The IEEE 802. 16m Convergence Sub-Layer feedback packets (ACK and NACK). ROHC always starts header compression using U-mode even if it is used in a bi-directional link. ROHC does not attempt re-transmission when an error occurs and the erroneous packet is dropped. The ROHC feedback is used only to indicate to the compressor side that there was an error and probably the context is damaged. After receiving a negative feedback, the compressor always reduces its compression level. The ROHC compressor has three compression states as follows [10]: 1. Initialization and Refresh (IR), where the compressor has just been created or reset and full packet headers are sent; 2. First Order (FO) where the compressor has detected and stored the static fields such as IP addresses and port numbers on both sides of the connection; 3. Second Order (SO) where the compressor is suppressing all dynamic fields such as RTP sequence numbers, and sending only a logical sequence number and partial checksum to cause the other side to generate based on prediction, and verify the headers of the next expected packet. Each compression state uses a different header format in order to send the header information. 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 Timestamp (TS), forming the minimal size packets (see Figure 5-4). 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 upper compression states. It only