Brian Roberts, Chairman and CEO of ComcastDavid A. Grogan | CNBCShares of Comcast closed down 5.5% Monday after AT&T announced a deal to combine its content unit WarnerMedia with Discovery to form a new media giant.The new media company, which could be worth well over $100 billion, will compete against other players that have invested heavily in streaming, including Comcast’s NBCUniversal, Netflix and Disney. Netflix stock closed down nearly 1%, while Disney shed more than 2%.Executives said the two companies already spend a combined $20 billion per year on content, including programming for their linear networks, which is comparable to Netflix’s projected estimate of $17 billion spent this year.The deal also could put pressure on Comcast’s internet business. By shedding its media assets, AT&T can focus on its core connectivity business, touting the power of 5G. AT&T Chief Executive John Stankey said in a call with reporters Monday that he plans to focus his company’s capital on fiber infrastructure to improve AT&T’s 5G network, which could compete against Comcast as a home broadband alternative.”Connectivity demand is higher than ever,” an AT&T spokesperson told CNBC earlier Monday. “Especially connectivity with symmetrical upload and download speeds. This transaction allows us to step up our investment in spectrum and fiber.”As part of the deal, AT&T said it would receive an aggregate amount of $43 billion in a combination of cash, debt and WarnerMedia’s retention of certain debt.Disclosure: Comcast is the owner of NBCUniversal, the parent company of CNBC.CNBC’s Alex Sherman contributed to this report.