Circuit switching characterizes a network in which connections must be reserved prior to data transmission but afford network end systems the ability to send/receive during the entirety of a session.
As opposed to packet-switching, circuit switching only allows a single network system to communicate over an allocated link during a given period of time. However, given that circuit switching connections are reserved for the duration of a session, that network system is free to transmit without queuing delays or interruptions.
Circuit switching can be configured with either Time-Division Multiplexing (TDM) or Frequency-Division Multiplexing (FDM). Both approaches divide network resources equally among connections but take considerably different approaches.
FDM allocates resources in a continuous manner, breaking a communications spectrum into smaller bands with proportionally-smaller transmission capacity.
TDM breaks a full-bandwidth transmission capacity into time frames during which systems are allowed to transmit data.