The top executive at the National Cable & Telecommunications Association announced Monday that he would step down, possibly as early as this year. Robert Sachs has been president and chief executive officer of the Washington-based cable trade group since 1999. His contract runs out this year but told the NCTA's board that he would be willing to stay into next year to help the board find his successor. Sachs said his decision was personal, since he commutes between his home in Boston and the NCTA's headquarters. Sachs, a former communications attorney, is a director of CableLabs, Cable in the Classroom and The Ad Council.