ses the header. The header compression is a hop-to-hop process and is not applied in end-to-end connection. At each hop in the IP network, becomes necessary to decompress the packet to be able to perform the operations such as routing, QoS negotiation, and parameter adjustment, etc. Header compression is best suited for specific links in the network characterized by relatively low bandwidth, high bit error rates, and long round trip times. The performance of a header compression scheme can be described with three parameters [9]: compression efficiency; robustness; and compression transparency. The compression efficiency is determined by how much the header sizes are reduced by the compression scheme. The compression transparency is a measure of the extent to which the scheme ensures that the decompressed headers are semantically identical to the original headers. If all decompressed headers are semantically identical to the corre- sponding original headers, the transparency is considered to be 100%. Compression transparency is high when damage propagation is low. When the context of the decompressor is not consistent with the context of the compressor, decompression may fail to reproduce the original header. This condition can 5.1 Header compression Voice Datagrams with IPv6/UDP/RTP Headers Voice Datagrams with ROHC Headers Voice Datagrams with IPv6/UDP/RTP Headers Compressor Radio Link Decompressor Reference Header Reference Header FIGURE 5-3 An example of ROHC compression/decompression of IP/UDP/RTP headers for communication over a radio link occur when the context of the decompressor has not been initialized properly, or when packets have been lost or damaged between compressor and decompressor. 5.1.1 Robust Header Compression The RObust Header Compression (ROHC) scheme reduces the size of the transmitted IP/UDP/RTP header by removing redundancies. This mechanism starts by classifying header fields into different classes according to their variation pattern. The fields that are classified as inferred are not s