Maryland-based Ciena Corp. is offering cash and stock worth $521 million to acquire some of Nortel's most-prized remaining business units. The fallen telecommunications company continues to liquidate.

The deal covers the Nortel Optical Networking and Carrier Ethernet businesses, which have been considered among the best assets yet to be sold by Nortel.

Nortel said its agreement with Ciena calls for the company to offer jobs to at least 2,000 employees, or more than 85 percent of total at Nortel's Optical Networking and Carrier Ethernet business units.

However, there's no guarantee the deal will close as announced since Ciena's offer will be used as an opening bid for an auction by Nortel.

Nortel once accounted for one-third of the market value on the entire Toronto Stock Exchange. It filed for bankruptcy protection in Canada and the U.S. in January.