Synchronous Dynamic RAM was an improved version of DRAM that synchronised all its functions to a single frequency, usually the system's FSB frequency. The frequency refers to the rate at which the RAM would perform an action, namely refreshing, reading, or storing. SD RAM is available in various frequencies, with the fastest modules capable of running at 266 MHz (high performance modules, targeted at enthusiasts, which perform at higher frequencies, are also available). SD RAM modules have 168 contacts. After the release of DDR SDRAM, the original SDRAM began to be referred to as SDR SDRAM (Single Data Rate SDRAM).