Telephony Links and User Equipment

While data and telephony networks share much in common, one of the major ways in which they differ is with respect to the terminology used to describe network elements such as the links used to interconnect different types of equipment. For example, you’ve already learned that the link between a telecommunication carrier’s central office (CO) and a user’s home is known as the local loop. In most locations worldwide, the local loop is still implemented using a pair of copper wires sometimes referred to as the “tip and ring”. When two telephone systems are interconnected, such as two PSTN switches or a PSTN switch and a PBX, the link is described as a trunk link. The points below outline the different terms used to describe the various links on the telephone network.

Local loop. A link that interconnects a CO switch to a user home, typically implemented using a pair of copper wires.

Tie trunk. A link that interconnects PBXs.

CO trunks. A link that interconnects a CO switch and a PBX.

PSTN switch trunks. A link that interconnects switches on a carrier network.

The local loop connection between the CO switch and a user’s home is often referred to more generically as a phone line, and this is also correct. However, in an office environment where user telephones connect to a PBX, the correct term used to describe the connection is “station line”.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.