In order to implement multimedia applications on the existing communication network, the International Telecommunication Union (ITU-T) has developed the H.32x multimedia communication protocol series. The following is a brief description of the main standards
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, 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 the application ofH. 323 standard has naturally become a hot spot in the market, so we will focus on H.323。The H.323 recommendation defines four main components: terminals, gateways, gateway management software (also known as gatekeepers or gateways), and multipoint control units.
1、Terminal
All terminals must support voice communication, and the video and data communication capabilities are optional.All H.323 terminals must also support the H.245 standard, which controls channel usage and performance.H.323 specifies the main parameters of the voice codec in voice communication as follows: ITU recommended voice bandwidth /KHz transmission bit rate /Kb/s Compression algorithm annotation G.711 3.456,64 PCM simple compression, applied to the PSTN G.728 3.416 LD-CELP speech quality as G.711, When applied to low bit rate transmission G.722 7 48,56,64 ADPCM speech quality is higher than G.711, when applied to high bit rate transmission G.723.1G.723.0 3.4 6.35.3 LP-MLQ speech quality is acceptable G.723.1 uses G.729 for VOIP forum. G.729a 3.48CS-ACELP delay is lower than G.723.1, voice quality is higher than G.723.1.
2、Gateway
This is optional for H.323 systems. 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, it is necessary to configure the gateway.
3. Gatekeeper
This is an optional component of the H.323 system that is managed by software. It has two main functions: one is to manage H.323 applications; The second is the management of terminal communication through the gateway, such as call establishment, dismantling, etc. The administrator can perform address translation, bandwidth control, call authentication, call record, user registration, communication domain management and other functions through Gatekeeper.An H.323 communication domain can have multiple gateways, but only one gateway can work.
4. Multipoint control unit
MCU implements multi-point communication on IP network, point-to-point communication is not needed. 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. The MC processes H.245 control information between terminals, establishing a minimal common namer 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 architectures of IP phone in industry, one is ITU-TH.323 protocol introduced above, the other is SIP protocol (RFC2543) proposed by IETF, SIP protocol is more suitable for intelligent terminal.
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