With　the　increasing　complexity　and　shortening　design　cycles　of　embedded　systems,　system-level　design　methods　are　becoming　more　and　more　important.　System-level　design　methods　can　determine　whether　the　initial　design　alternative　is　feasible　or　not　by　the　development　of　executable　model　of　the　system,　which　allows　qualitative　and　quantitative　properties　to　be　analyzed　before　the　system　is　actually　realized　in　terms　of　hardware　and　software　components.　In　this　way,　design　errors　can　be　detected　in　the　earliest　design　phases.　Furthermore,　expensive　and　time-consuming　design　iterations　are　prevented.　Network　protocols　have　many　complex　concurrent　and　distributed　characteristics,　so　it　is　very　difficult　to　analyze　them　using　mathematical　methods.　One　of　the　most　promising　solutions　to　this　problem　is　system-level　modeling　and　simulation.　This　paper　describes　system-level　design　methods　in　detail.　The　modeling　language　Parallel　Object-Oriented　Specification　Language　(POOSL)　and　its　extended　component　library　are　also　introduced.　Based　on　that,　a　system-level　model　for　the　Selective-Repeat　Sliding　Window　Protocol　was　built.　Several　results　of　performance　evaluation　are　illustrated.