ixels by 96 vertical pixels. Although this resolution is too low for use in many applications, it suffices for communicating over the PSTN with V.90 modems. SQCIF seems to be most useful for providing a brief sample of a more extensive video segment such as a movie or so-called infomercial. H.324 also allows customers to dynamically choose between two low bit-rate audio codecs, Multi-Pulse Maximum Likelihood Quantization (MP-MLQ) or Algebraic Codebook Excited Linear Prediction (ACELP) and, therefore, to choose audio quality. The two standards feature 6.3 Kbps or 5.3 Kbps audio, respectively. The standard leverages T-84 for still-frame graphics. Finally, H.263 codecs can be configured for a lower data rate or for better error recovery because parts of the data stream's hierarchical structure are optional. The H.263 standard incorporates four negotiable options to improve performance. One is P-B frames, which, much like MPEG frames, provide forward and backward frame prediction. The other three options are Unrestricted Motion Vectors, Syntax-based Arithmetic Coding, and Advance Prediction. P-B framing allows two images to be coded as a single unit. The term "P-B" is derived from MPEG (see Appendix B). A P-B frame consists of one P (predictive) frame, which is created by using a past frame as a model and one B (bi-directional) frame that is interpolated (created through comparison) from a past P frame and the P-frame currently being computed. Computationally demanding, P-B framing can really compress a video stream down to a low bit rate. The technique works best for simple sequences; it starts to show stress when there is significant frame-to-frame motion. When an H.263 codec is in the Unrestricted Motion Vectors mode, it can project motion outside the frame and use those vectors (associated with non-existent pixels) to predict movement along the edge of an image (or sequence of images). This mode also allows an extension of the H.261 motion vector range, and allows larger motion Standards for Videoconferencing vec