"Consistent with our focus on the top 100 US markets, nearly half of the AT&T towers are located in the top 50 markets, where we expect the majority of network densification and upgrade activity to occur.
The deal is expected to close by the end of the year.
AT&T, the No. 2 wireless carrier in the United States, had said last month that it was exploring the sale of its towers but that its ability to reach a deal would depend on the terms it is able to reach with the buyer for its ongoing use of the towers.
When wireless service providers sell broadcast towers they typically lease back space from tower operators so they can continue to offer their services without interruption
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"This deal will let us monetize our towers while giving us the ability to add capacity as we need it," said Bill Hogg, Senior Vice President - Network Planning and Engineering, AT&T Services.
"And we'll get additional financial flexibility to continue to invest in our business, maintain a strong balance sheet and return value to our shareholders," he added.
Under the deal AT&T will keep its communications facilities on the towers for a minimum of 10 years with monthly rent of $1,900 per site and fixed annual rent escalators of 2 percent.
AT&T will also have access to additional space on the towers for its future use, the company said.
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Crown Castle will have the right to sublease other available capacity on the towers and expects to accommodate at least one additional tenant per tower.
In addition, it said nearly half of the towers were located in the top 50 markets, where it aims to benefit from increasingly dense networks and future network upgrades.