The growing popularity of sequence charts, first of all Message Sequence Charts and UML Sequence Diagrams, for the description of communication behavior has evoked criticism regarding the semantics of the charts which led to extensions of these standardized visual formalisms. One such extension are Live Sequence Charts which allow to distinguish mandatory and possible behavior in protocol specifications. In the original language definition for LSCs the semantics are only described informally, although a sketch for a possible formalization has been provided as well. In this paper we intend to fill in the semantic blanks of the original LSC definition. Following the sketched path we define the semantics of an LSC by deriving a Timed Böchi Automata from it. We also consider qualitative and quantative timing aspects and sub-charts.