Committed to connecting the world. ITU-T work programme. ITU-T A. Justification for the specific reference: This reference explains the concept of Robust Header Compression almost mandatory for VoLTE and how it is used in practice. RoHC has a direct impact on call set up performance. Relationship with other existing or emerging documents: References within the referenced RFC are listed under item 8.
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Such large overheads may be tolerable in local wired links where capacity is often not an issue, but are excessive for wide area networks and wireless systems where bandwidth is scarce. ROHC compresses these 40 bytes or 60 bytes of overhead typically into only one or three bytes, by placing a compressor before the link that has limited capacity, and a decompressor after that link. The compressor converts the large overhead to only a few bytes, while the decompressor does the opposite. Redundant information is transmitted in the first packets only.
The next packets contain variable information, e. These fields are transmitted in a compressed form to save more bits. For better performance, the packets are classified into streams before being compressed. This classification takes advantage of inter-packet redundancy. The classification algorithm is not defined by the ROHC protocol itself but left to the equipment vendor's implementation. Once a stream of packets is classified, it is compressed according to the compression profile that fits best.
A compression profile defines the way to compress the different fields in the network headers. Several compression profiles are available, including the following:. Both the compressor and the decompressor start in U-mode. They may then transition to O-mode if a usable return link is available, and the decompressor sends a positive acknowledgement, with O-mode specified, to the compressor.
The transition to R-mode is achieved in the same way. In the Unidirectional 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. In order to handle potential decompression errors, the compressor sends periodic refreshes of the stream context to the decompressor.
The Bidirectional Optimistic mode is similar to the Unidirectional 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 aims for sparse usage of the feedback channel. The Bidirectional Reliable mode differs in many ways from the previous two modes.
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. Whatever the mode is, both the compressor and the decompressor work in one of their three states. They are basically finite state machines. Every state refers to a defined behaviour and compression level. The ROHC algorithm is similar to video compression, in that a base frame and then several difference frames are sent to represent an IP packet flow.
This has the advantage of allowing ROHC to survive many packet losses in its highest compression state, as long as the base frames are not lost. In Initialization and Refresh IR state, the compressor has just been created or reset, and full packet headers are sent. In First-Order FO state, the compressor has detected and stored the static fields such as IP addresses and port numbers on both sides of the connection. The compressor is also sending dynamic packet field differences in FO state.
Thus, FO state is essentially static and pseudo-dynamic compression. In Second-Order SO state, 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 predictively generate and verify the headers of the next expected packet.
In general, FO state compresses all static fields and most dynamic fields. SO state is compressing all dynamic fields predictively using a sequence number and checksum.
VoIP header. In this state, the 8-bit ROHC header contains three fields:. The size of the sequence number SN field governs the number of packets that ROHC can lose before the compressor must be reset to continue. The RFC defines a generic compression mechanism. It may be extended by defining new compression profiles dedicated to specific protocol headers.
New RFCs were published to compress new protocols:. From Wikipedia, the free encyclopedia. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
ITU-T work programme
Robust Header Compression