ommunication." Within H.324, H.223 defines the multiplexing of the individual audio and video streams into a single synchronized data stream that suits the bandwidth constraints of POTS. To transmit digital data on analog telephone lines, synchronous modems transmit a High-level Data Link Control (HDLC) protocol that adds a framing protocol layer. Using a standard asynchronous interface, videoconferencing would add another framing layer. Because the H.263 codec performs framing, any additional framing the modem performs is redundant. The V.80 interface mediates by letting the host define the frame boundaries for the multiplexed video and audio signal, but allowing the modem to complete the bit- oriented framing. H.263 The formal name for H.263 is "Video Coding for Low Bit Rate Communication." H.263 is one of two alternatives for video compression under H.324. The other is H.261, which we have already covered. Although early H.263 drafts specified data rates of less than 64 Kbps, the final Video Communications: The Whole Picture Recommendation included no such limitation. The coding algorithm of H.263 adds many performance and error recovery improvements to H.261. For instance, it incorporates half pixel precision for motion compensation; H.261 is limited to full pixel precision. H.263 codecs do not require a loop filter, which is required by H.261 codecs. Like H.261, H.263 makes QCIF picture formatting mandatory. It provides support (as options) for additional frame sizes. One is the CIF, also used in H.261 codecs. Another is Sub Quarter Common Intermediate Format (SQCIF), which is used for very low-resolution images. H.263 compliant codecs compete with higher bit rate video coding standards (such as MPEG) because of the standard's support for two other formats, 4CIF and 16CIF. 4CIF is four times the resolution of CIF, and 16CIF is 16 times the resolution of CIF. H.324 implementations will most often use SQCIF because it is helpful for low bandwidth video communication. SQCIF resolves an image at 128 horizontal p