Memory manufacturers stated that it was impractical to mass-produce DDR1 memory with effective transfer rates in excess of 400 MHz (i.e. 400 MT/s and 200 MHz external clock) due to internal speed limitations. DDR2 picks up where DDR1 leaves off, utilizing internal clock rates similar to DDR1, but is available at effective transfer rates of 400 MHz and higher. DDR3 advances extended the ability to preserve internal clock rates while providing higher effective transfer rates by again doubling the prefetch depth.