SBA Communications owns a portfolio of about 45,000 wireless towers throughout North America, South America, and Africa. It leases space on its towers to wireless service providers, who install equipment to support their wireless networks. The company has a very concentrated customer base, with most revenue in each market being generated by the top few mobile carriers. It owns more than 17,000 towers in the US, which account for about 75% of leasing revenue. Internationally, SBA's largest presence is in Brazil, where it owns roughly 10,000 towers. SBA operates as a real estate investment trust.