Antec Corp., Rolling Meadows, IL, and Toronto-based Northern Telecom have signed a letter of intent for three proposed interrelated business relationships. A systems integration joint venture company may provide integration services for hybrid fiber/coaxial-cable networks as they evolve to include voice, data, and interactive video applications. Also being considered is a joint-venture company that focuses on the product development of Northern Telecom`s integrated digital-access technology and Antec`s digital-video technology. Another business relationship involves marketing and sales arrangements for the Cornerstone voice and data product line, as well as products to be developed by the joint venture. It is expected that Antec and Northern Telecom will be the primary sales channels to the cable-TV and telephone industries, respectively.