Lucent Technologies Inc. is expected to announce today that it is acquiring Ascend Communications Inc., the data networking company, in a deal valued at about $20 billion, people close to the talks said.

And in the short run, at least, any data networking takeover would probably dilute the value of Lucent's stock.

Cisco Systems Inc., the maker of data networking equipment, will provide high-speed communications equipment that uses an advanced sort of technology known as asynchronous transfer mode.

In June, Northern Telecom Ltd. of Canada, one of the largest makers of telecommunications gear, said it would acquire Bay Networks Inc., a large data networking company.

In 1999, it acquired two Massachusetts-based private data networking companies - Castle Networks Inc and Argon Networks Inc.

Since then, Lucent has acquired Livingston Enterprises Inc. and the Prominet Corporation, two leading data networking equipment technology companies.

Modern Ethernet, the most common data networking standard, can use UTP cables.

The deal also gives added momentum to a wireless data networking standard popularized by Apple Computer over the last two years.

Meanwhile, Northern Telecom's data networking group, which is meant to work with Bay, will remain mostly in Ottawa and in Richardson, Tex.

Its products include high-powered direct-to-home broadcast satellites, commercial weather satellites, digital audio radio satellites and spot-beam satellites for data networking applications.