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The G Series of ITU recommendations are referred to as "Transmission systems and media, digital systems and networks." Most of the recommendations below G.600 are related to analog telephone equipment and circuits. The G.600 series is related to transmission media (twisted- pair, coaxial, and fiber-optic cable) for the telephone system. The G.700 series recommendations are related to digital transmission systems, in particular, coding of analog signals into digital signals. These standards are listed below.

• G.601 and G.602    Cable general characteristics
• G.611 through G.614    Symmetric cable pairs
• G.621 through G.623    Land coaxial cable
• G.631    Submarine cables
• G.650 through G.655    Optical fiber cable
• G.661 through G.692    Characteristics of optical components and subsystems
• G.701 through G.707    Digital transmission systems vocabulary, bit-rate hierarchy, physical/electrical characteristics, synchronous frame structures, and other
• G.711 and G.712    PCM (pulse code modulation) of voice frequencies and transmission performance characteristics of PCM channels
• G.720 through G.729    Coding of speech, silence compression schemes, channel coding for wireless, ADPCM (Adaptive Differential Pulse Code Modulation)
• G.731 through G.755    Characteristics of multiplexing equipment
• G.761 through G.767    Principal characteristics of transcoder and digital multiplication equipment
• G.772 through G.776    Operations, administration, and maintenance features of transmission equipment
• G.780 through G.785    Principal characteristics of multiplexing equipment for the synchronous digital hierarchy
• G.801 through G.842    General aspects and design objectives of digital networks, internetworking, architectures of transport networks, timing characteristics, error and jitter controls
• G.851 through G.855    Management of transport networks
• G.872    Architecture of optical transport networks
• G.901 through G.958    Digital line systems, parameters of optical fiber cable systems, FDM (frequency division multiplexing) systems
• G.960 through G.967    ISDN access systems
• G.971 through G.976    Optical fiber submarine cable systems
• G.981 through G.983    Optical line systems for local and access networks
• G.991.1    High-bit-rate digital subscriber line (HDSL) transceivers

In particular, the G.711, G.723, G.726, G.728, and G.729 voice-coding techniques are the most important for multimedia networking. These techniques improve bandwidth, voice quality and other factors. G.723.1 defines a technique for compressing speech or audio at very low rates (5.3 and 6.3 Kbits/sec). The techniques are components of the IMTC (International Multimedia Teleconferencing Consortium) standard for Internet telephony. See "Compression Techniques" for more information, as well as "Voice over IP (VoIP)" and "Multimedia."

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