August 1, 1997

by Alan Zeichick

Real-Time Voice and Video

Familiar and unfamiliar terms relating to voice and video over the network.

CIF With resolution of 352 horizontal pixels by 288 vertical pixels, the Common Intermediate Format is a popular size for video conferencing images. For lower bandwidth applications, video systems often use either QCIF (Quarter CIF), which displays images at 176 pixel by 144 pixel resolution, or SQCIF (Sub-Quarter CIF), which is actually one-ninth of CIF's resolution, at 128 pixels by 96 pixels. High-bandwidth video can be described as 4CIF (704 pixels by 576 pixels) or 16CIF (1,408 pixels by 1,152 pixels). See H.261.

CTI Computer-Telephony Integration relates to the implementation of traditional telephone-based audio (and sometimes video) services over a data network. CTI may be implemented over systems that guarantee bandwidth, such as ATM, or over frame-based networks like Ethernet or frame relay. For more CTI-related terms, see the Computer Telephony Glossary.

G.711 One of the major ITU-T codec (coder-decoder) standards for audio (voice and music), G.711 can be incorporated into broader multimedia standards, such as H.320 and H.323, or used on its own for computer telephony. G.711 specifies an audio signal with a 3.4KHz bandwidth (that is, an ordinary analog voice signal) over a 64Kbit/sec data path. Related standards include G.722, which defines a 64kbit/sec 7.0KHz bandwidth audio stream, and the lower-bandwidth G.728 standard, which defines a 16Kbit/sec 3.4KHz audio stream. For computer-based audio over narrow-band phone lines, there's G.723, which supports a compressed 3.4KHz signal over a POTS (Plain Old Telephone System) line and is used in the H.324 multimedia standard. See H.320, H.323, H.324.

H.233 The ITU-T's data-encryption standard for real-time multimedia, H.233 is supported across a wide range of standard services, including H.320, H.323, and H.324. A related standard is H.234, which specifies how encryption keys are handled. See H.320, H.323, H.324.

H.261 An ITU-T video-compression codec (coder-decoder) standard, H.261, which is supported by H.320, H.323, and H.324, supports CIF and QCIF images. H.261 was designed for use with ISDN and assumes data rates in multiples of 64Kbits/sec. A newer standard, H.263 (supported by H.324), improves H.261's efficiency and adds support for SQCIF-, 4CIF-, and 16CIF-sized images. See CIF, H.320, H.323, H.324.

H.320 One of the major ITU-T standards for real-time multimedia, H.320, is the standard for video conferencing over narrow-band circuit-switched WAN services such as ISDN. This standard includes specifications for T.120-based data, G.711- and G.728-based voice, H.261-based video, and H.233- and H.234-based encryption. The basic H.320 standard has been enhanced in H.323 to include basic packet-switched networks. Related standards for real-time multimedia that are not broken out separately in this glossary are H.321, for broadband ISDN and ATM; H.322, for guaranteed-bandwidth packet-switched networks; and H.310, for higher-resolution multimedia over ATM. See G.711, H.233, H.261, H.323, T.120.

H.323 An extension of the older H.320 standard, the ITU-T's H.323, covers video conferencing not only over narrow-band WAN services, but also on packet-switched networks such as corporate LANs and the Internet. H.323 is based on the Internet Engineering Task Force's (IETF's) Real-Time Protocol (RTP), H.261-based and H.263-based video, and T.120. See H.261, H.320, RTP, T.120.

H.324 The ITU-T's standard for real-time multimedia over standard POTS (Plain Old Telephone System) lines using 28.8Kbit/sec V.34 modems or better, H.324, like H.320, incorporates the T.120 standard for data sharing, H.261-based video compression, and the H.233 and H.234 encryption standards. Unlike H.320, H.324 uses the G.723 audio standard. See G.711, H.233, H.261, H.320, T.120.

MMX According to Intel, the acronym "MMX" has no particular meaning, but it's generally inferred to mean "Multimedia Extensions." Specifically, MMX is implemented as a set of new microprocessor instructions for Intel's MMX-enhanced Pentium CPUs and its new Pentium II CPUs. MMX is not specific just to Intel, however, because Advanced Micro Devices, a competing chip manufacturer based in Sunnyvale, CA, has MMX-enabled its new K6 processor family. Many new Windows-based multimedia products for computer telephony or video conferencing are written to take advantage of the new MMX instructions.

QOS Quality of Service is a catchphrase for a network that can transport data without losing cells, with predictable end-to-end delay, and with real-time delivery of data once the connection is completed. High-quality multimedia over a network, whether in real-time or merely playing audio or video files from a server, requires a network that can deliver QOS. Protocols such as ATM (Asynchronous Transfer Mode) are designed to deliver multiple levels of QOS. Attempting to deliver QOS using IP requires additional services such as RSVP (Resource Reservation Protocol), which allows bandwidth to be reserved and to be supported on intermediate devices such as routers.

RTP The Real-Time Transport Protocol is the IETF's standard for transporting real-time data, such as voice or video, over a packet-based network that doesn't guarantee Quality of Service (QOS). A related standard is RTCP, or the Real-Time Transport Control Protocol, which provides feedback between two units (point-to-point) or a larger group (known as multicast or multipoint). The ITU-T's non-QOS multimedia standards such as H.323 and H.324 are based on RTP/RTCP. See H.323, H.324, QOS.

T.120 This ITU-T standard covers the data-sharing component of real-time multimedia over the network and is the basis for such applications as whiteboard sharing and file collaboration during a point-to-point or multipoint session. T.120 is included in video conferencing standards as H.320, H.323, and H.324. See H.320, H.323, H.324.