The best-known medieval Chinese name for Tibet is Tubo (Chinese: also written as or ; pinyin: Tubo or Tufan). This name first appears in Chinese characters as in the 7th century (Li Tai) and as in the 10th-century (Old Book of Tang describing 608609 emissaries from Tibetan King Namri Songtsen to Emperor Yang of Sui). In the Middle Chinese spoken during that period, as reconstructed by William H. Baxter, was pronounced thux-phjon and was pronounced thux-pjon (with the x representing tone).
Try to answer this question if possible (otherwise reply "unanswerable"): Who else in addition to Emperor Yang Songsten does the Old Book of Tang describe?
unanswerable