Due　to　the　increasing　complexity　of　the　embedded　systems,　the　gap　between　a　product　idea　and　a　working　system　expands　rapidly.　To　manage　the　complexity　and　shorten　the　design　cycles,　this　article　puts　forward　a　method　of　system-level　design,　which　focuses　on　the　development　of　executable　models　which　allow　qualitative　and　quantitative　properties　to　be　analyzed　before　the　system　is　actually　realized　in　terms　of　hardware　and　software　components.　System-level　design　methods　should　be　based　on　well-defined　modeling　languages　to　support　creating　executable　models　that　properly　represent　the　system　to　be　designed.　The　parallel　object-oriented　specification　language　(POOSL)　is　a　system-level　modeling　language　for　complex　embedded　systems.　It　has　been　demonstrated　to　be　very　effective　for　modeling　systems　such　as　internet　router　and　network　processor.　A　system-level　model　for　SWP　is　built　by　using　POOSL.　Through　the　simulation　of　the　model,　results　of　the　quantitative　performance　analysis　on　the　influence　of　the　parameters　such　as　the　window　size,　the　packet　size　and　the　timeout　period　are　given.　Furthermore,　some　key　parameters　can　be　determined　during　the　earliest　phases　of　the　design　process.　The　system-level　method　can　assess　the　schemes　in　the　earliest　phases　of　system　design.