AT&T today announced it has agreed to lease exclusive rights to its nearly 9,100 cell sites and sell outright an extra 600 towers to Crown Castle International Corp. for $4.85 billion in cash, filling the wireless company’s coffers for possible acquisitions, especially in Europe.

Under the terms of the deal, Crown Castle will lease and run the towers for an average of 28 years, after which the company will be able to buy them outright for around $4.2 billion.

Crown Castle will gain more than 9,000 AT&T cell towers available to lease to other providers.

AT&T is seeking to monetize its extensive portfolio of cell towers while protecting rights of access. The cell phone company is guaranteed subleases on the towers for at least a decade for $1,900 per month per site, with annual rent increases of 2 percent, with a right to renew for up to 50 years total. AT&T will also have access to reserve capacity on the towers for future use.

The money will help AT&T’s balance sheet as it undertakes a $14 billion network upgrade and plans a shareholder-friendly stock buyback that may top $11 billion.

After the deal closes, Crown Castle will become the nation’s largest provider of shared wireless infrastructure, operating more than 40,000 communications towers in the United States alone. This is not the first deal Crown Castle has done with major wireless carriers. In November 2012, Crown acquired exclusive rights to lease and operate 7,100 T-Mobile USA cell towers.

In the past, many wireless carriers refused to share tower space for competitive reasons. Today, the demand for wireless data is forcing carriers to increase the number of cell sites to improve coverage and capacity. Many carriers now share cell sites to cut costs and avoid the regulatory burdens and citizen objections that often come with a proposed new tower. Crown Castle’s acquisition is likely to boost leasing of the AT&T cell sites. At present, AT&T’s towers only have 1.7 tenants per site, including AT&T. Crown Castle towers average 2.8 tenants per site. Potential clients for the AT&T towers include the three other major carriers: Verizon Wireless, Sprint, and T-Mobile.