Comcast and Charter announced an agreement to cooperate in their plans to sell mobile phone service, an agreement that also forbids each company from making wireless mergers and acquisitions without the other's consent for one year. "That agreement could stoke Wall Street speculation among investors and analysts that the two largest U.S. cable companies together could decide to make a play for a carrier like T-Mobile U.S. Inc. or Sprint Corp.," wrote The Wall Street Journal. Ars Technica reports: The deal could violate antitrust law, said Harold Feld, an attorney and senior VP of consumer advocacy group Public Knowledge. "One of the basic ideas of antitrust law is that when companies that compete with each other, or could compete with each other, make an explicit agreement to not compete with each other, that violates the antitrust laws," Feld told Ars today. "Agreeing to coordinate with each other to avoid competition is expressly a violation of the antitrust laws." But that doesn't mean Comcast and Charter won't be able to follow through with their plan. It's impossible to say with absolute certainty whether any specific agreement violates antitrust law, and "both Comcast and Charter have very good lawyers," Feld said. Comcast and Charter have a combined 47 million internet subscribers, dominating the US market for high-speed broadband, but they do not compete against each other in any city or town. The Comcast/Charter cooperation agreement fits in nicely with Comcast's mobile plans, because the company intends to sell smartphone data plans only to customers who also have Comcast home Internet service. Comcast's mobile service is scheduled to be available by the end of June, while Charter has said it intends to offer similar service in 2018.