Goldman Sachs analyst Doug Clark downgraded Infinera (INFN) to Sell saying the company's transition from a single product portfolio to a full solution set will bring margin headwinds and potential downside to consensus estimates. Shares, after rallying over 100% in the past 12 months, trade at a premium valuation, Clark tells investors in a research note. He cut his price target for shares to $19 from $24 and prefers investors switch into Buy-rated Ciena (CIEN). Infinera, which engages in optical transport networking systems, closed Friday down 51c to $21.41.