| Today, Mobile wireless systems are being designed to support a wide range of services besides the traditional telecommunication services, such as images, videos, multimedia, and so on. These flexible services speed up the development of the IP technology and the process of Full-IP network. When real time services, such as audio or video streams, are carried by IP technology, Real Time Protocol (RTP) is often used to ensure real-time. So a typical audio packet header is composed of 20 bytes of an IPv4 header (or 40 bytes of an IPv6 header), 8 bytes of a UDP header and 12 bytes of a RTP header. Totally the packet header length will reach to 40 bytes (or 60 bytes for IPv6), but there is often only 15-20 bytes of payload data. Obviously, this will largely waste the most precious wireless bandwidth resource. So it is necessary to compress the packet header to improve the resource utilization. Recently introduced as a new standard by IETF, RObust Header Compression (ROHC) has been proposed to compress the packet headers for packet transmission over a wireless link.In this paper, we introduce the algorithms used in ROHC, the base architecture of ROHC system and the main process of header compression in detail. ROHC has an extensible architecture. It can compress not only the IP/UDP/RTP packets but also the IP/UDP and IP packets. And we call them RTP profile, UDP profile and IP profile respectively. We also introduce the compress process and packet format of all these profiles in this paper. At the last of this paper, we present some schemes based on ROHC architecture, such as reusing UDP Checksum field,effective establishment of the TS_STRIDE and TS_OFFSET,re-calculating SN value when continuous packet-dropping happens, to improve the compression effectiveness and the reliability of ROHC system. We also present a new video stream system combining the video encoding method with the ROHC system to improve the reliability of video stream system with no influence on compression effectiveness. |
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