G.711 - 3.4 kHz bandwidth audio transmitted at 56 or 64 kbps. Uses A-law or u-Law PCM.

G.722 - 7 kHz bandwidth audio transmitted at 48, 56 or 64 kbps. Uses ADPCM.

G.723.1 - 3.4 kHz bandwidth audio at 5.3 or 6.4 kbps. Developed for H.324 POTS standard. Being considered for other uses. Uses CELP processing.

G.728 - 3.4 kHz bandwidth audio transmitted at 16 kbps. Uses CELP processing.

G.729 - 3.4 kHz bandwidth audio transmitted at 8 kbps. Uses CS-ACELP processing.

Gateway - connects different protocols, e.g., H.323 to H.320

Graphics - Transmission of still images, usually from a video source, but in some cases PC-generated.

GSTN - General Switched Telephone Network. Also known as POTS.

Ground Start - Method of supervisory signaling, primarily for analog in-band networks. Customer Premise Equipment (CPE) transmits offhook condition by creating a zero voltage condition. See loop start and wink start for other similar techniques. 

H.221 - Multiplex standard used in H.320. Combines audio, video, data in one bitstream.

H.223 - Multiplex standard for H.324.

H.224 - Standard for data transfer in H.320.

H.225.0 - Multiplex standard for H.323.

H.230 - Standard for control and indication signaling within H.320.

H.231 - Standard for specifying MCU requirements in H.320.

H.233 - The ITU-T's data-encryption standard for real-time multimedia.

H.234 - specifies how encryption keys are handled.

H.242 - Standard for establishing communication in H.320.

H.243 - Standard for multipoint communication in H.320.

H.245 - Standard for multimedia system control in H.323, H.324 and H.310. Includes features such as capability exchange and signaling.

H.261 - Required video compression standard in the H.320, H.323 and H.324 standards.

H.262 - ITU-T name for MPEG-2. H.262 is used within H.310, and is an optional mode of H.320.

H.263 - Video compression standard created and required for H.324. Is also an optional mode of the other H-series standards.

H.281 - Standard for Far End Camera Control.

H.310 - Standard for multimedia conferencing and one-way video applications on B-ISDN.  Not based on H.320.

H.320 - Standard for multimedia conferencing on narrowband switched digital networks. Can be used from 56 kbps to 2 Mbps.

H.321 - Standard which specifies how H.320 terminals can be used on broadband switched digital networks ( B-ISDN ).

H.322 - Standard which specifies how H.320 terminals can be used on isochronous ethernet LANs ( guaranteed bandwidth and quality of service. )

H.323 - Standard for multimedia conferencing on traditional packet-switched LANs.

H.324 - Standard for multimedia conferencing on analog phone lines ( POTS ).

H.331 - Standard which specifies how H.320 terminals can be used in a one-way broadcast mode.

H0 - Switched 384 kbps service.

Half duplex - Method to either transmit or receive without the ability to do both simultaneously.

Half-duplex audio - Audio that permits only one site to speak at a time.

Hub - A device used to concentrate incoming data from multiple nodes onto a common network medium. Also commonly referred to as a concentrator or repeater.

Hybrid - A device that converts a 4-wire voice communication circuit into a 2-wire circuit.

Hyper Cardioid Microphone - A super directional microphone with a narrow heart-shaped pick up pattern used for specialized audio applications.