subject: H.264-based Ip Network, Key Technology And Its Applications-ip Network, H.264-communications [print this page] ITU-T H.264 is the latest video coding standard, known as ISO/IEC14496-10 or MPEG-4AVC, by the Moving Picture Experts Group (MPEG) and ITU Video Coding Experts Group to develop new products.
H.264 sub-two structures, including the video coding layer and the network adaptation layer. Video coding layer processing is a block, macroblock and slice the data, and as far as possible with the network layer independence, which is the core video encoding, which contains many of the tools to achieve error recovery; network adaptation layer film structure is more than dealing with The data so that H.264 can be based on RTP / UDP / IP, H.323 / M, MPEG-2 transport and H.320 protocol networks.
2, IP network restrictions on video compression
1. H.264 applications where IP-based H.264 in the discussion before it is necessary to elaborate on the H.264 IP network and applications related to its transmission and codec requirements. Here the dialogue application, download services and streaming media applications of three occasions.
Dialogue applications such as video telephony and video conferencing, there are strict delay constraints, demands end to end delay is less than 1s, preferably less than 100ms. Codecs, adjust the parameters in real time, error recovery mechanism to change based on actual network changes. Codec complexity can not be very high, such as bi-directional prediction mode can not be used.
Download service, you can use a reliable transport protocol such as FTP and HTTP to transfer all the data. Because the application of non-real time, the encoder can be optimized for efficient coding, and error recovery mechanisms of the delay and did not ask.
Streaming media service application, on the delay between requests between the above, the initialization delay is less than 10s. And real-time encoding to lower than on the delay, the encoder can be optimized for efficient coding (such as bi-directional prediction). Used streaming media services, however, is not usually a reliable transport protocol, so when for error control coding and the channel error correction coding. This article focuses on dialogue
applications and streaming media applications, both IP-based network applications. IP networks can be divided into three types: non-controlled IP network (such as Internet), controlled IP network (WAN) and wireless IP networks (such as 3G networks). These three networks have different IP maximum transmission unit size (MTUsize), bit error probability and TCP use the mark. Maximum Transmission Unit size is the biggest network layer packet length, H.264 encoding to make film length is less than MTU size, so the network layer to avoid a further segmentation of the data. MTU between two IP nodes size is dynamic, often assumed wired IP network MTU size is 1.5 kilobytes, the wireless network MTU size is 100 bytes. Can be seen to apply to the wireless network must use H.264 data partitioning technology enables film length is less than MTU size. TCP transmission control protocol to solve network congestion problems caused by packet loss, and in wireless networks, packet loss is due to errors caused by link layer, TCP is not a good solution to use error control protocol.
2. H.264 protocol used in the environment
Dialogue applications and streaming media applications using the same protocol, the following discussion.
Network layer protocol: use of IP (Internet Protocol). Each IP packet separate from the sender, goes through a series of routers to reach debit. IP MTU size will be larger than the data packet segmentation, restructuring. Each packet transmission time varies. 20 bytes from the IP header checksum to ensure, but data is not protected. IP packet a maximum of 64 kilobytes, but because of MTU size restrictions, not so much.
Transport layer protocols: There are two main protocols, TCP and UDP. TCP provides reliable byte-oriented transport service to and timeout retransmission error control mechanisms such as the foundation. Because of the delay is unpredictable, does not apply to real-time communication transmission. UDP provides unreliable datagram transport service. UDP header contains the check number (8 bytes) can be found and removed the packet containing the bit errors. UDP allows packet loss occurs during transmission, copy, change sequencing. When using the UDP protocol, high-level error recovery protocol must be used.
