What Wall Street giveth, Wall Street taketh away.

Ciena Corp. (NYSE: CIEN) shares are starting to fall -- down $0.05 (1.08%) to $4.59 in late afternoon trading on Friday -- as the stock was downgraded to a Sell by Banc of America Securities LLC .

This morning's setback shows Ciena might be an exception to the otherwise upward swing that several optical-networking-related stocks have seen lately. JDSU (Nasdaq: JDSU; Toronto: JDU), for instance, is up $0.26 (7.45%) to $3.75 this morning after CNBC stock picker Jim Cramer raised his target for the stock to "four smackers."

The researchers at B of A had concerns over Ciena's valuation and doubts as to whether the company could continue to increase revenues and gross margins. They set a $2.50 price target for Ciena's shares -- a number that's 46 percent off yesterday's close.

"While we see CIENA’s financial models improving and excitement around broadband, we believe the stock more than reflects these trends. We chose to focus on companies with better valuation, strong margins and positive cash flow," writes Banc of America analyst Tim Long, in his note.

Ciena shares had been gaining steadily during the week, peaking after the company reported a lower than expected quarterly loss on Thursday. (See Ciena Narrows Q1 Loss.)

Even analysts that were a little more bullish on Ciena note that its operating margins are a concern, given the stock's valuation.

"While Ciena is growing faster than the comparable companies, we're also less certain that the company's normalized operating margins will get back into the double digit + range – which we think should be necessary to support the company's current valuation," writes George Notter of Jefferies & Co. Inc. in a note to clients this morning.

"Ciena guided both gross margin and opex roughly flat q/q… We believe guidance suggests that gross margins may be peaking now that they are inline with peers Nortel and Lucent," writes JP.MorganChase analyst Ehud Gelblum, in his review of Ciena's earnings.

— Phil Harvey, News Editor, Light Reading