Trunking can be describe In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of this include telephone systems and the VHF radios commonly used by police agencies. More recently port trunking has been applied in computer networking as well.
A trunk line is a circuit connecting telephone switchboards (or other switching equipment), as distinguished from local loop circuit which extends from telephone exchange switching equipment to individual telephones or information origination/termination equipment.
When dealing with a private branch exchange (PBX), trunk lines are the phone lines coming into the PBX from the telephone provider . This differentiates these incoming lines from extension lines that connect the PBX to (usually) individual phone sets. Trunking saves cost, because there are usually fewer trunk lines than extension lines, since it is unusual in most offices to have all extension lines in use for external calls at once. Trunk lines transmit voice and data in formats such as analog, T1, E1, ISDN or PRI. The dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks.
TRUNKED RADIO SYSTEM
A trunked radio system is a complex type of computer-controlled radio system. Trunked systems use a few channels (the actual frequencies), and can have virtually unlimited talkgroups. The control channel computer sends packets of data to enable one talkgroup to talk together, regardless of frequency. The primary purpose of this type of system is efficiency; many people can carry many conversations over only a few distinct frequencies.  Trunking is used by many government entities to provide two-way communication for fire departments, police and other municipal services, who all share spectrum allocated to a city, county, or other entity.
When there is single transmission channel between two points, each point being either the switching center or the node is called TRUNK.quote's from From Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Trunking#Trunk_line )