With Intel processors clock frequencies are determined by two things. The level of the base clock frequency (bClk) and the memory multiplier. If you set the memory in the BIOS to DDR3-1600, then the multiplier goes to 8x. That's because DDR3-1600 modules run at 800 MHz, and 8 x 100 = 800. If you want the memory to run faster, you can set a higher multiplier.
With Sandy Bridge you can choose between DDR3-1333, -1600, -1866, -2133 and -2400. Intel expanded upon this quite a bit with Ivy Bridge. Now also available are: DDR3-1800, -2000, -2200, -2666 and -2800.
The bClk of Ivy Bridge processors can be overclocked a maximum of 7 percent, to 107 MHz. Extreme overclockers can take it a little further, but not much. However, with a combination of the bClk settings and the multiplier it's possible to achieve almost any memory speeds. For example, a 103 MHz bClk with DDR3-1866 multiplier creates RAM running at DDR3-1921 (1.03 x 1866 = 1921).
Configuring the right clock frequency and timings for the RAM you can do in the BIOS, but nowadays there's an even easier way. Modules have a so-called SPD chip, which contains information about supported clock frequencies, timings and the required voltage. There you will only find the speeds that adhere to official standards, even with faster modules the maximum setting according the SPD is often DDR3-1600. Intel therefore expanded on the SPD data in the form of XMP, short for eXtreme Memory Profiles. The XMP information of a memory module contains the highest validated clock frequencies, including the required timings and voltages. When you set your motherboard's BIOS to XMP, these settings are adopted and a DDR3-2133 kit will run at 2133 MHz with the settings validated by the manufacturer.
Not all memory modules have XMP, but the standard is gaining in popularity. Considering we requested RAM specifically optimised for Ivy Bridge, it's no surprise that all modules we received support XMP.
With RAM modules manufacturers indicate the clock frequency and the access time, for example CL10. This so-called CAS latency is the delay time between the moment a memory controller tells the memory module to access a particular memory column on a RAM memory module, and the moment the data from given array location is available on the module's output pins. In general, the lower the CAS latency, the better. CL is not the only timing. If you look at RAM specs, you will see modules described as DDR3-2133 9-11-10-31-2T.