) --

Nokia Siemens Networks

and private-equity firm One Equity Partners have jointly bid for Nortel's optical networking and carrier ethernet business, challenging


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bid of $526 million, a source familiar with the matter told



Nortel has been selling off assets following its bankruptcy filing in January.

Ciena's offer

included $390 million in cash and 10 million in Ciena shares, which is equivalent to $136 million based on Wednesday's closing stock price. But its bid last month received flak from analysts and investors who were concerned about the potential integration issues of the acquired assets with Ciena's portfolio.

Nortel said the final auction will be held Friday.


says it's unclear if another bidder also would put in an offer to challenge the offer from Nokia Siemens and One Equity.

Nokia Siemens is a joint venture of Finland's


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and Germany's