Resistive plate chamber commissioning and performance in CMS

The CMS muonsystem is conceived for trigger and muontrack reconstruction. The redundancy and robustness of the system are guaranteed by three complementary subsystems: drift tube in the barrel, cathode strip chamber in the end-cap and resistive plate chamber in barreland end-cap. The installation of muonstations and read-out trigger electronic has been completed in middle 2007. Since than, a remarkable effort has been addressed to the detector commissioning in order to ensure the readiness of the hardware/software chain for the LHC start up operation. At the end of 2007, a test of an entire CMS slice has been performed, involving about 5% of muonstations. Several thousand cosmic muonsevents have been collected. Performance of the barrelchambers are reported.