The field was created by the work of A. K. Erlang in whose honour the unit of telecommunication traffic intensity, the Erlang is named. His Erlang distributions are still in common use in telephone traffic engineering.
The crucial observation in traffic engineering is that in large systems the law of large numbers can be used to make the aggregate properties of a system over a long period of time much more predictable than the behaviour of individual parts of the system.
The most notable difference between these sub-fields is that packet-switched data traffic appears to be self-similar. This weakens or invalidates some of the assumptions used for telephone traffic engineering.
In the two sub-fields, the traffic engineering term "Quality of Service" means two related but distinct things. In circuit-switched networks it refers to the probability of being able to initiate a call to another party. In packet-switched networks it refers to the probability of the network meeting a given traffic contract, or in many cases is used informally to refer the probability of a packet passing between two points in the network.