Bunz, Carl-Martin (2002) Meeting report: 2nd Iranian Unicode Meetinghttp://www.unicode.org/L2/L2002/02009-iranian.pdf "For the Sogdian script (as well as the Uyghur script), two possible encoding strategies were discussed. While the soundest solution would consist in providing a code block of its own, a mapping onto the existing Unicode block of Mongolian (U+1800-18af) would be historically also adequate, given that the latter script developed from it."
