Technology companies have been beefing up their portfolios by buying up service providers, which are proven to be attractive targets with their relatively high-margin businesses and stability during tough economic times.

On Monday, Xerox Corp said it plans to buy Affiliated Computer Services Inc in a deal valued at $5.4 billion.

Xerox Chief Executive Ursula Burns said the company's revenue from services will triple to an estimated $10 billion next year from $3.5 billion in 2008 after the deal.

Other recent events:

-- On Sept. 21, Dell Inc offers to buy Perot Systems Corp for about $3.9 billion, paying a steep 67.5 percent premium.

-- In August 2008, HP acquired EDS for $13 billion. WORLD'S BIGGEST IT SERVICES COMPANIES 1) IBM Market capitalization: : $159 billion Percentage of services revenue: 57.9 percent Employees : 398,455 Specialization : Servers, IT services, software 2) Hewlett-Packard Market capitalization: : $111 billion Percentage of services revenue: 35.3 percent Employees : 321,000 Specialization : PCs, servers, printers, IT services 3) Dell Market capitalization: : $30 billion Percentage of services revenue: 16.4 percent Employees : 76,500 (excl Perot) Specialization : PCs, servers, IT services 4) Infosys

Market capitalization: : $27 billion Percentage of services revenue: 96 percent Employees : 103,905 Area of specialization : IT outsourcing 5) Accenture Market capitalization: : $26 billion Percentage of services revenue: Consulting - 57/Outsourcing - 43 Employees : 186,000 Areas of specialization : Consulting, IT services, outsourcing 6) Tata Consultancy Services

Market capitalization: : $24 billion Employees : 143,000 Area of specialization : IT outsourcing 7) Wipro

Market capitalization: : $17 billion Percentage of services revenue: 79 percent Employees : More than 95,000 Area of specialization : IT outsourcing 8) Cognizant

Market capitalization: : $11 billion Employees : 64,100 Specialization : IT services, outsourcing, consulting 9) Computer Sciences

Market capitalization: : $7.7 billion Employees : 92,000 Specialization : IT services, outsourcing (Sources: Company websites and filings, Reuters data) (Reporting by S. John Tilak, Manasi Phadke in Bangalore)