Track data hub annotations are stored at the remote site as compressed binary indexed files. When a hub track is displayed in the Genome Browser, only the relevant data needed to support the view of the current genomic region are transmitted to UCSC, rather than the entire file. The transmitted data are cached on a UCSC server to expedite future access. This on-demand transfer mechanism eliminates the need to transmit large datasets across the Internet, thereby minimizing upload time into the browser.
