Mobile communications Internet  International fixed communications  Others  Income from operations and Operating Margin  79 Domestic Fixed  Communications Mobile  Communications Internet International  Fixed  Communications Others Adjustment Total (in billions of NT$) For the year ended  December 31, 2011 Revenues from  external customers  79.4 93.0 24.8 15.2 5.1 — 217.5 Intersegment service  revenues  15.4 7.0 1.9 2.6 0.7 (27.6) — Interest income  — — — — 0.7 — 0.7 Other income  0.4 0.1 0.1 0.6 1.2 95.2 100.1 26.7 17.9 7.1 (27.6) 219.4 Segment income  before tax  18.5 27.8  9.6 2.0 (1.2) — 56.7 For the year ended  December 31, 2010 Revenues from  external customers  70.7 89.0 24.5 15.5 2.7 — 202.4 Intersegment service  revenues  14.7 2.1 1.1 1.7 0.7  (20.3) — Interest income  — — — — 0.5 — 0.5 Other income  0.3 0.1 0.3 (0.2) 85.4 91.4 25.6 17.3 4.2 (20.5) 203.4 Segment income  before tax  18.1 29.3 9.8 2.7 (2.2) — 57.7 The costs and expenses from our other business increased by 32.0 % from NT$6.1 billion in 2010 to NT$8.0 billion (US$0.3  billion) in 2011. The increase was primarily due to an increase of NT$1.4 billion (US$46.5 million) in construction costs and an  increase of NT$0.5 billion (US$16.5 million) in research and development expense.  As a result of the foregoing, our income from operations decreased by 4.0% from NT$57.4 billion in 2010 to NT$55.1 billion  (US$1.8 billion) in 2011. Our operating margin decreased from 28.3% in 2010 to 25.3% in 2011. The following table sets forth certain information regarding our operating income by business segment for the periods  indicated.  Our international fixed communications costs and expenses increased by 9.2% from NT$14.5 billion in 2010 to NT$15.8  billion (US$0.5 billion) in 2011. The increase was primarily due to an increase of NT$0.9 billion (US$28.4 million) in international  interconnection expenses.  Our internet operating costs and expenses increased by 9.1% from NT$15.8 billion in 2010 to NT$17.2 billion (US$0.6  billion) in 2011. This increase was primarily due to an increase of NT$0.8 billion (US$27.4 million) in leased line expenses for  broadband services and an increase of NT$0.4 billion (US$12.9 million) in corporate solution services and ICT costs.  Our mobile communications operating costs and expenses increased by 16.4% from NT$62.1 billion in 2010 to NT$72.3  billion (US$2.4 billion) in 2011. This increase was primarily due to an increase of NT$7.9 billion (US$262.6 million) in cost of  handsets sold and an increase of NT$1.6 billion (US$53.5 million) in leased line and internet access expenses.  P AR T I