In one of its simpler forms, a repeater based radio communications system consists of two or more remote stations with a repeater in the middle.  One persons talks in the assigned frequency and the others listen.  When this person is done talking, the others can take their turns.  This form of communications has been in use since its inception, but what happens when the number of remote stations is so large that the possibility of people wanting to have conversations simultaneously is considerable?  What about privacy in conversations?  You could assign different frequency to different conversations, but then you have the problem of coordination (what frequency are you going to be listening on?), and nothing keeps a third party from simply tuning into that frequency and listening. Assigning a fixed frequency to a single station would be a waste of valuable bandwidth, unless said station were to be transmitting constantly.

This is where trunking comes in, as a more efficient, somewhat more private means of frequency reutilization.  In this case, the FCC of whatever entity regulates the use of the airwaves, assign a number of frequency to a communications company.  This company then sets up this frequencies (trunks) in its system and programs all the remote stations to it.  What happens then, is that whenever a person wants to initiate a conversation, the central station will assign a trunk from its pool of available frequencies to this conversation.  When this person is done talking, the station will mark the trunk as available again.  When the other party in the conversation wants to talk, he or she might be doing it in a different frequency.  The central station takes care of keeping both parties always in the same frequency.

How is this more private?  It's harder to follow a conversation that might be hopping from one frequency to another, unless you have a scanner sophisticated enough to be able to follow trunked conversations (and as times goes by, these scanners are getting cheaper, but at the same time, trunking systems are improving its means of countering eavesdroppers).

Just like ISPs and other communications carriers, this method works on the assumption that individuals use the system onle a small fraction of the time, allowing for oversubscription, hence increased efficiency.

Sources: Trunking Radio Systems.
Trunking Communications Overview.

Trunking, with relation to Ethernet, is the process of aggregating and tagging of information from multiple VLANs within a single switch and transmitting this information across a single physical link.

A single switch can contain multiple VLANs. These are logically segmented networks. If there is a need to expand these VLANs to another switch, one could assign one uplink port per VLAN. This means that if one has 5 VLANs, one would have to use 5 ports to pass information from that VLAN to another switch.

If both switches support a trunking protocol, such as Cisco's ISL or IEEE's 802.1q, information from all the VLANs can be tagged and aggregated together to be transmitted across a single trunk link. The receiving switch will then read the tags, remove them, and place the frames in the corresponding VLAN. This means that instead of one link per VLAN, you now have one link carrying the traffic from all the VLANs.

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