ent. The static fields are sent initially and then are not sent anymore, and the fields with varying information are always sent. The ROHC mechanism is based on a context, iv which is maintained by both ends, i.e., the compressor and the decompressor (see Figure 5-3). The context encompasses the entire header and ROHC information. Each context has a Context Identifier, which identifies the flows. The ROHC scheme operates in one of the following three operation modes [14-19]: 1. Uni-directional mode (U): in the uni-directional mode of operation, packets are only sent in one direction, from compressor to decompressor. This mode therefore makes ROHC usable over links where a return path from decompressor to compressor is unavailable or undesirable. 2. Optimistic mode (O): the bi-directional optimistic mode is similar to the uni-directional mode, except that a feedback channel is used to send error recovery requests and (optionally) acknowledgments 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): the bi-directional reliable mode differs in many ways from the previous two. The most important differences are a more intensive usage of the feedback channel and a stricter logic at both the compressor and the decompressor that prevents loss of context synchronization between compressor and decompressor except for very high residual bit error rates. The U-mode is used when the link is uni-directional or when feedback is not possible. For bi-direc- tional links, O-mode uses positive feedback packets (ACK) and R-mode use positive and negative iv The context of the compressor is the state it uses to compress a header. The context of the decompressor is the state it uses to decompress a header. Either of these, or combinations of the two, is usually referred to as "context." The context contains relevant information from previous headers in the packet stream, such as static fields and possible re