Sensex today opened nearly 112 points up on buying at existing lower levels amid a firm global trend. The 30-share index, which had lost 27.40 points in the previous session, bounced back to 18,040.17 level.Similarly, the wide-based National Stock Exchange's Nifty rose by 30 points to 5416.45. Brokers said revival of buying by funds, triggered by a firm global trend mainly boosted the trading sentiment. Hong Kong Hang Seng index went up 0.10 per cent in morning trade guided by better trend at the Wall Street last night.