"Link" is preferred because "connection" has different connotations and established meanings. (First, "connection" may mean the process or the fact of being connected [permanently or for a period of time]. Second, see below.)

http://en.wikipedia.org/wiki/Telecommunications_link
"A telecommunications link is generally one of several types of information transmission paths accomplished by communication satellites to connect two points on earth." (+ I would add that also non-satellite links are often called the same: optical underground links, long- or short-distance wireless links.)

http://en.wikipedia.org/wiki/Connection
"Telecommunication circuit, the complete path between two terminals"
Explanation: a "connection" is the "link" PLUS the input/output component (įvadas).

"Connection" is also used loosely for links that are rather small, such as a household's or an office's connection to to the internet.

Regarding the adjective, you have to choose it in your context (so that it doesn't overlap with other terms and reflects <I><B>how big and important</B></I> is the link). For example, "backbone link" (or simply "backbone", noun) is usually one of the major links in a country or region. "Principal" would perhaps work quite well, since your network description seems to be quite general; it's a new network yet to be built or upgraded, isn't it?