Dozens of companies signed up last week to sell short-term IOUs called commercial paper to the Fed, which will purchase three-month debt that has high credit ratings.

Federal Reserve financing programme was recently extended to commercial mortgages but many are or will become ineligible under its present rules because of ratings downgrades.

The paper studies the main content of Internal Ratings Based Approach and international commercial banks' IRB system, and analysis the key factors that implement IRB.