Adtran (ADTN) is sinking sharply after the company warned in a regulatory filing that its fourth quarter revenue could drop by teen percentage levels, compared with the previous quarter. Adtran, which provides services and support for communications networks, cited "typical seasonality" and "the current telecom equipment spending environment" as reasons for the decline. Adtran's statement today comes shortly after the company reduced its third quarter guidance on September 28, when it estimated that its Q3 revenue would be $162M. Analysts' consensus estimate for the company's Q4 revenue is currently $173.35M. In mid-afternoon trading, the company's stock dropped $1.04, or 6.08%, to $16.07.