DDR3, DDR4

DDR4 Transfer Rate

The initial 1.6GHz DDR4 speed grade is identical to the highest DDR3 speed range.

These are the planed data rates; 1.6GHz (2011), 2.0GHz (2011), 2.4GHz (2012), 3.2GHz (2013)
The estimate for release date (provided after the speed) is of production ramp-up. [I believe these are JEDEC estimates for dates and may be different for each company that produces the parts]
As the name implies, Double Data Rate interfaces provides two data transfers per clock. The clock is differential. The data is registered when the CK goes high [the + side], and /CK goes low [the - side]. DDR3 memory represents the third generation in DDR memory. DDR3 begins with a speed grade of 800Mbps as the lowest speed grade. The 800Mbps speed grade is also used as the highest speed grade for DDR II. Here is a Comparison of speed grades.

DDR2 Standard

JESD79-2B: DDR2 SDRAM Specification, January 2005
This may not be the latest revision of the DDR-2 standard.

DDR2 Electrical Interface

Double Data Rate III IC's use 1.5 volt SSTL_15 compatible I/O [class II], how ever the supply voltage may be higher.

DDR2 Terminations

DDR3 modules contain the require resistor termination located on the memory chips using a technique called On-Die Termination [ODT]. While DDR1 modules have the necessary resistive termination located on the motherboard. Using ODT, DDR3 are able to reduce the parts count required for mother board while at the same time locate the terminations closer the the signal destination. The ODT termination can be turned on or off by the DRAM controller. Normally the terminations are turned on for Writes and disable for Reads. The value of the ODT termination is selectable based on the number of modules in the system. With one DIMM module the ODT value is set at 150 ohms [300W pull-up and 300W pull-down]. When two modules are loaded into the system the ODT value is changed to 75 ohms [150W pull-up and 150W pull-down] for the DIMM not being written to while the DIMM being accessed has its ODT turned off. Writing to the Extended Mode Register [EMR] controls the ODT presence and value. Three combinations are allowed; termination disabled, 75 ohms, and 150 ohms [ also 50 ohms]. The newest revision adds 50 ohm termination values. ODT improves the eye-diagram over SSTL for either Single-Rank or Dual-Rank modules.

DDR4 Power Dissipation

Power Dissipation for reference only.
A DDR DIMM needs 5.4 watts, a DDR2 DIMM needs 4.4 watts and a DDR2 FB-DIMM needs 10.4 watts. DDR3 provide a 30% reduction in power consumption.