FB-DIMM (Fully Buffered DIMM) is a memory module technology targeted to servers developed recently created in order to increase the memory speed and the maximum memory capacity of a server. In this tutorial we will explain what are the main features of FB-DIMM modules and the difference between DIMM and FB-DIMM modules.

The main difference between FB-DIMM modules and regular DIMM modules is that on FB-DIMM the communication between the memory controller (chipset) and the module is serial, in the same way that occurs with PCI Express, while on standard DIMM modules this communication is parallel.

Using serial communication the number of wires needed to connect the chipset to the memory module is lower and also allows the creating of more memory channels, what increases memory performance. With FB-DIMM technology it is possible to have up to eight modules per channel and up to six memory channels. So this technology increases both memory capacity and speed.

Each extra memory channel that is added to the system increases the memory subsystem transfer rate. For example, if you use a single DDR2-533 channel, the memory transfer rate will be of 4,264 MB/s. If you use two DDR2-533 channels, as it happens on dual channel configuration available today, the memory transfer rate will be of 8,528 MB/s. With four channels, the memory transfer rate jumps to 17,056 MB/s, and so on.

FB-DIMM serial communication uses 10 pairs of wires between the chipset and the memory sockets and 12 or 14 pairs of wires between the memory sockets and the chipset. Each pair of wires use differential transmission, i.e., the signal is transmitted on a wire and the same signal but inverted is transmitted on the other wire from the pair, using the same idea used on twisted pair networking cables. This is done to ensure that the receiver is getting the data correctly. Read this tutorial to understand how this kind of transmission works.

Another important aspect of FB-DIMM is that it uses different paths for data transmission and data reception. Standard DIMM modules use the same path to both transmit and receive data. The system used on FB-DIMM modules helps to increase the performance of the memory subsystem.

click to enlarge
Figure 1: How FB-DIMM modules work.

Initially FB-DIMM modules will use DDR2 memory, migrating to DDR3 when this kind of memory becomes available. It is very important to notice that FB-DIMM modules will have the same physical size as DDR2-DIMM modules and will keep this same size when DDR3 memories are released.