where a return path from decompressor to compressor is unavailable or undesirable. Optimistic mode (O) is a bidirectional mode similar to the unidirectional mode, except that a feedback channel is used to send error recovery requests and (option- ally) acknowledgements of significant context updates from the decompressor to compressor. The O-mode aims to maximize compression efficiency and sparse usage of the feedback channel. 3. Reliable mode (R) is a bidirectional mode which differs in many ways from the previ- ous two modes. The most important differences include intensive use of feedback chan- nel and a strict logic at both compressor and decompressor that prevents loss of context synchronization between compressor and decompressor except for very high residual error rates. The U-mode is used when the link is unidirectional or when feedback is not possible. For bidirectional links, O-mode uses positive feedback packets (ACK) and R-mode use positive and negative feedback packets [ACK and NACK]. The ROHC mecha- nism always starts header compression using U-mode even if it is used over a bidi- rectional link and it does not send retransmissions when an error occurs; thus 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 defined as follows [16]: 1. Initialization and refresh (IR), where the compressor has been just 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. 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 check- sum to make the other side generate the headers based on prediction and verify the headers o