DDR4

Fourth generation memory - DDR4 RAM for your servers

In order to provide high system performance for demanding applications, server solutions and workstations require, among other things, a sufficient amount of high-performance main memory (RAM: Random Access Memory). DDR4, the fourth generation of the proven DDR standard (Double Data Rate), is widely used. The fifth generation has already been introduced as its successor in the consumer market, but it has hardly been relevant there due to the still limited availability, a correspondingly high price level and special requirements for processors and mainboards. Compatible server processors with support for DDR5 have already been announced, but the established DDR4 memory, with its excellent price-performance ratio and high compatibility with many existing server systems, remains the common standard in the professional sector.

How do DDR4 memory modules differ from the previous generation?

RAM modules for servers, workstations and desktop systems are also referred to as DIMMs (Dual Inline Memory Modules) and differ depending on the memory generation both in performance and electrical connection. DDR4 DIMMs and the corresponding slots on the mainboard have 288 contacts (pins) and are therefore not compatible with the previous generation DDR3, which only has 240 pins. In addition, the position of the notch in the module's connection area, which is used for correct alignment in the slot, differs for both standards.
For use in notebooks and systems with a particularly compact form factor, special main memory modules are used, so-called SO-DIMMs (Small Outline Dual Inline Memory Modules). These significantly smaller modules have a smaller amount of contact pins and the memory generations here are also not compatible with each other.
Although DDR4, like its predecessor, works with 8x prefetching, meaning that eight data bits per data pin are read out per read request and the output takes place in a data burst of length eight, the standards differ significantly in their performance. With DDR4 memory, the voltage has been lowered to 1.2 V and the clock frequencies have been significantly increased; in addition, chips with up to eight memory layers enable a higher maximum memory capacity.

In which specifications is DDR4 RAM available?

Depending on the clock frequency of the chips used, DDR4 RAM is available in various specifications defined by JEDEC (Joint Electronic Device Engineering Council). With DDR3 memory, the designation of the modules was derived from the data rate. This has changed with DDR4 memory and the designation of the modules here refers directly to the clock frequency of the memory chips used.

Memory chip | Module designation | Clock frequency | Data rate

• DDR4-1600 | PC4-1600 | 1600 MHz | 12,8 GB/s
• DDR4-1866 | PC4-1866 | 1866 MHz | 14,9 GB/s
• DDR4-2133 | PC4-2133 | 2133 MHz | 17,0 GB/s
• DDR4-2400 | PC4-2400 | 2400 MHz | 19,2 GB/s
• DDR4-2666 | PC4-2666 | 2666 MHz | 21,3 GB/s
• DDR4-2933 | PC4-2933 | 2933 MHz | 23,5 GB/s
• DDR4-3200 | PC4-3200 | 3200 MHz | 25,6 GB/s

The individual specifications are additionally available with different CAS latencies (column access strobe latency). This is calculated from the memory timings and defines the time between sending a read command and receiving the data. The lower this value, the higher the memory performance.

DDR4 memory for different areas of application

In addition to the defined specifications, RAM modules differ in other classifications, which are indicated with an additional indicator and are often decisive for the area of application of the RAM.

• Unbuffered modules: UDIMM

In the case of an unbuffered module, the system's memory controller has direct access to the memory chips. This type is suitable for standard PCs and is often offered without the additional indicator: e.g. PC4-2400 or PC4-2400U
Since unbuffered modules with integrated ECC error correction (Error Correcting Code) are also available, these are marked with the indicator E. They are often used in smaller servers and workstations: e.g. PC4-2400E

• Registered modules with ECC: RDIMM

When using registered modules, the system's memory controller does not communicate directly with the individual memory chips, but accesses a register chip integrated on the module. This reduces the load on the controller and makes it possible to manage large amounts of RAM. In addition, these modules almost always have the error correction technology ECC. This makes the memory suitable for high-performance servers and workstations that require a lot of memory and offer high reliability. Registered modules can be recognised by their additional indicators R, ECC or R ECC: e.g. PC4-2400R

• Load reduced modules: LRDIMM

Load reduced modules have a buffer memory instead of a register chip to reduce the load on the memory controller and enable a higher memory capacity. They are marked with the additional indicator L and the maximum memory configuration of modern servers can often only be achieved with LRDIMMs: e.g. PC4-2400L

DDR4 Memory for servers - The memory rank and its relevance

Memory modules for servers also have other indicators that refer to the respective memory rank of the module. This defines how many memory banks a RAM module has. A memory bank corresponds to a data area or data block that is created with the memory chips of the module. These data blocks have a width of 64 bits, with an additional 8 bits in the case of ECC memory, resulting in a width of 72 bits.
Depending on the construction of the module, one, two or four data blocks are possible, which leads to the designations Single Rank, Dual Rank and Quad Rank.
Since not only the module but also the individual chips have a different number of memory banks depending on their construction, the memory chips used are defined by the additional indicators x4 and x8.

In case of x4 chips, a total of 18 memory chips are required for a memory bank with a width of 72 bits: 18 x 4 = 72
In case of x8 chips, a total of 9 memory chips are required for a memory bank with a width of 72 bits: 9 x 8 = 72

Therefore, the number of memory chips and their construction results in the memory rank of a RAM module and its additional indicator, for example:

Single Rank with x4 chips: 1Rx4
18 chips per module (18 x 4 = 72)

Dual Rank with x4 chips: 2Rx4
36 chips per module (36 x 4 = 144 ; 144 / 72 = 2)

Dual Rank with x8 chips: 2Rx8
18 chips per module (18 x 8 = 144 ; 144 / 72 = 2)

Quad Rank with x8 chips: 4Rx8
36 chips per module (36 x 8 = 288 ; 288 / 72 = 4)

The memory rank can be a decisive factor in the optimal RAM configuration of a server. For example, a quad rank module with 32 GB capacity is comparable to four single rank modules of 8 GB memory capacity each, but takes up only one of the available slots. Provided that the system supports DIMMs with this capacity, the available RAM slots can be used efficiently when there is a high demand for RAM. It should be noted, however, that some server systems can only manage a limited amount of ranks. Therefore, when selecting suitable memory modules, attention should be paid to whether rank restrictions exist in order to optimally use the available slots of the system for the required RAM.

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