For multimedia applications on existing communication networks, the International Telecommunication Union (ITU-T) has developed H.32x multimedia communication series protocol, the following are the main standards to do a simple description:
H.320, the standard for multimedia communication on the narrowband video telephone system and terminal (N-ISDN);
H.321, the standard for multimedia communication on the B-ISDN;
H.322, the standard of multimedia communication on the LAN guaranteed by QoS;
H.323, a standard for multimedia communication on a packet-switched network without QoS guarantee;
H.324, a standard for multimedia communication on low bit rate communication terminals (PSTN and wireless network).
Among the above standards, H.323 Standard defines the network is the most widely used, such as Ethernet, token, FDDI, etc.because of H. The application of 323 standard has naturally become a hot spot in the market, so we will focus on H.323。H.323 Four main components are defined in the proposal: terminal, gateway, gateway management software (also known as gateway or gate) and multi-point control unit.
1、Terminal (Terminal)
All terminals must support voice communication, and the video and data communication capabilities are optional.all H.323 The terminal must also support the H.245 Standard, H. The 245 standard is used to control the channel usage and the channel performance.H.323 The main parameters of voice codec in voice communication are specified as follows: ITU recommended voice bandwidth / KHz transmission bit rate / Kb/s compression algorithm annotation G.711 3.4 56,64 PCM simple compression, applied to the PSTN in G.728 3.4 16 LD-CELP voice quality is the same as in G.711, which is applied to the low-bit-rate transmission G.722,748,56,64 ADPCM Voice quality higher than G.711, as applied to the high-bit-rate transmission G.723.1G.723.0 3.4 6.35.3 LP-MLQ Voice quality is acceptable, G.723.1 For the VOIP forum using G.729G.729A 3.4 8 CS-ACELP time delay is lower than the G.723.1, the voice quality is higher than the G.723.1。
2、Gateway (Gateway)
This is H. An option for the 323 system. The gateway can transform the protocols, audio and video coding algorithms and control signals used by different systems to accommodate the communication of system terminals. As as PSTN based H.324 System and narrowband ISDN based H.320 System and H.323 System communication, you need to configure the gateway;
3. Guan Shou (Gatekeeper)
This is H.323 An optional component of the system is the software to complete the management function. It has two main functions: the first is to H.323 Management of application; the second is the management of terminal through gateway communication (such as call establishment, dismantling, etc.). The administrator can perform address conversion, bandwidth control, call authentication, call record, user registration, communication domain management and other functions through customs keeping.one H. The 323 communication domain can have multiple gateways, but only one gateway can work.
4. Multipoint control unit (Multipoint Control Unit)
The MCU enables the multi-point communication on the IP network, and the point-to-point communication is not required. Form the whole system into a star topology through the MCU. The MCU contains two main components: multipoint controller MC and multipoint processor MP, or without MP. H between MC processing terminals.245 Control information to establish a minimum public naming for audio and video processing. MC does not directly process any media information stream, but leaves it to MP. The MP mixes, switches, and processes the audio, video, or data information.
There are two parallel architecture of IP phone in the industry, one is the ITU-TH introduced above.323 Protocol, the other is the SIP protocol (RFC2543) proposed by the Internet Engineering Task Group (IETF), and the SIP protocol is more suitable for intelligent terminals.
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