With data being transferred 8 bytes at a time DDR RAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 8 (number of bytes transferred). Thus with a bus frequency of 100 MHz, DDR-SDRAM gives a max transfer rate of 1600 MB/s.
DDR-200: DDR-SDRAM memory chips specified to run at 100 MHz
DDR-266: DDR-SDRAM memory chips specified to run at 133 MHz
DDR-333: DDR-SDRAM memory chips specified to run at 166 MHz
DDR-400: DDR-SDRAM memory chips specified to run at 200 MHz
PC-1600: DDR-SDRAM memory module specified to run at 100 MHz using DDR-200 chips, 1.600 GByte/s bandwidth per channel.
PC-2100: DDR-SDRAM memory module specified to run at 133 MHz using DDR-266 chips, 2.133 GByte/s bandwidth per channel
PC-2700: DDR-SDRAM memory module specified to run at 166 MHz using DDR-333 chips, 2.667 GByte/s bandwidth per channel
PC-3200: DDR-SDRAM memory module specified to run at 200 MHz using DDR-400 chips, 3.200 GByte/s bandwidth per channel
Note: All RAM speeds in-between or above these listed specifications are not standardized by JEDEC — most often they are simply manufacturer optimizations using higher-tolerance chips.
The package sizes in which DDR SDRAM is manufactured are also standardised by JEDEC.
There is no architectural difference between DDR SDRAM designed for different clock frequencies, e.g. PC-1600 (designed to run at 100 MHz) and PC-2100 (designed to run at 133 MHz). The number simply designates the speed that the chip is guaranteed to run at. Hence you can run DDR SDRAM at lower clock speeds than it was made for (underclocking) or higher clock speeds than it was made for (overclocking). Note that overclocking should only be done with high-quality memory and by those that know what they are doing (see overclocking for details on why).
DDR SDRAM DIMMs have 184 pins (as opposed to 168 on SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two). DDR operates at a voltage of 2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce power usage.
Some new chipsets use these memory types in dual or even quad channel configurations, which doubles or quadruples the effective bandwidth. In the dual-channel configuration it is recommended to use a matched pair of memory modules to optimize performance. The modules in a pair have the same size, speed, and latency timings, enabling the chipset to interleave accesses with maximum efficiency.
DDR is slowly being replaced by DDR-2, which has some modifications to allow higher clock frequency, but operates on the same principle as DDR. Competing with DDR-2 will be Rambus XDR-DRAM. It is expected that DDR-2 will become the standard, since QDR (Quad Data Rate) is too complex to implement, while XDR is lacking support.
Memory manufacturers have stated that it is impractical to mass-produce DDR-1 memory with effective clock rates in excess of 400 MHz. DDR-2 picks up where DDR-1 leaves off, and is available at clock rates of 400 MHz and higher.