Dear Kashmiri,
How did you think Greeks knew about India (Indica)? Or how do you think Chinese scholars knew about India (Heuang Tsuang)? When people from far can know about a country, how come people from the same part of the sub continent know about it well?
Culturally entire subcontinent (including today's Pakistan) was very very inter mingled from even the time of Indus Valley or a period immediately after that. Vedic period gives reminences of every part of this country. There are descriptions of the entire country including south india in Ramayana and Mahabharatha (including Chera, Chola, Pandya and Sinhala Kingdoms).
Now when we are talking about Sangan literature, we are talking about a source which is as older, if not older than, the vedic/puranic scriptures.