The former is an external means of communication, essential in describing something probably outside the usual reader's knowledge, very good for textbooks.

"The syntax is less than perfect, but we should have no difficulty with essential communication."

Early monastic communities evolved simple hand signing for essential communications.

These systems provided essential communications links between air, ground, and naval forces.

In my (engineering based) profession, I find clear verbal and written communication essential.

He said the posts provided essential communications links in Iraq's sophisticated air defense system.

It will make both general essential communication much easier, and open you up to a lot of people who don't speak English.

Over the years the Army turned to off-the-shelf civilian radio equipment to provide essential communications.

"The equipment has to be reserved for essential military communications, and as officer on duty, that is my decision to make."

This is so you only receive essential communication from the agency.