I'm a new in financial engeneering and trying to understand basic principles of volatility modelling. I wrote many papers and articles about different models (garch, local vol, stoch vol and ect.) and concluded that one of the main parts of that is calibration model parameters to market prices. It's understood and OK.
But I'm concerned about basic thing. Let's suggest that we haven't options market at all or have very illiquid market (1-2 OTM strikes). How can we quote let's say ATM strikes for some maturities? What are the ways of measure implied volatility in that case?