25G Ethernet vs 40G Ethernet: Which to Choose?

In today’s ever evolving data center, the growth in demand for bandwidth can be achieved by a shift to higher transmission speed-100G. To upgrade to 100G, there’re two approaches, 10G to 40G to 100G and 10G to 25G to 100G. So which one should you choose? Here focuses on 25G Ethernet and 40G Ethernet, and makes a comparison in the application, network switch selection and cabling to help users make a wise decision.

Figure 1: 25G vs 40G, which one to choose?

25G Ethernet Overview

25G Ethernet standard was put forward by the 25 Gigabit Ethernet Consortium, which consists of leading companies including Arista, Broadcom, Google, Mellanox Technologies and Microsoft in 2014. The goal is to promote the standardization and use of 25G Ethernet. Since the business needs to upgrade constantly, 10G network will be replaced in the future. Up to till now, 25G products like 25G switch or SFP28, 25G DAC and AOC cables have received more and more recognition. Many providers like Brocade, Dell and FS have published 25GbE switches to achieve the optimized network performance. These 25G network switches are often used as the ToR or leaf switches to connect the servers and terminal equipment. 25G Ethernet is an incremental update from 10G Ethernet. So for 25G and 10G Ethernet switches with the same port density, 25G switches and optics provide 2.5 times more performance and bandwidth than 10G Ethernet solution, which helps reduce the power and cost.

40G Ethernet Overview

Official development of 40G Gigabit Ethernet began in 2008, and is approved in 2010. 40G Gigabit Ethernet (40GbE) is a standard developed by the IEEE 802.3ba Task Force. It enables the transfer of Ethernet frames at 40 Gigabit per second. 40GbE runs on Quad Small Form Factor Pluggable (QSFP+) cabling, which can support high-bandwidth applications like video on demand and high-performance computing. Compared with 25G Ethernet, 40G standard is more mature. There’re many types of products for 40G applications, including MTP/MPO trunk cable assemblies, 40G QSFP+ optical transceivers and 40Gb switch, which are widely used for networking. In data centers, 40G switches are used as the aggregation/leaf switches, while they can act as core/spine switches for small and medium enterprises. When upgrading 10G to 40G Ethernet, users can use 40G QSFP+ to 4x10G SFP+ DAC/AOC cables to achieve this upgrade, or choose MTP/MPO to LC breakout cables and 40G transceivers solution according to their actual budget and transmission distance. For connecting two 40GbE switches, they need to choose MTP/MPO trunk cables or 40G QSFP+ to QSFP+ DAC/AOC cables.

25G vs 40G Ethernet: Which to Choose?

There’re some factors that you should consider before making the final decision.

Application

Generally, IT staff uses 25G Ethernet for switch to server applications, while use 40G for switch to switch applications. Thus, users should know their specific practices before designing the network.

Network Switch Selection

25G and 40Gnetwork switches are designed to meet customers’ different uses. But note that, 25G can be more easily upgraded to 50G (2x25G) and 100G (4x25G). Due to the high performance 25G chips with single-lane 25G serializer-deserializer technology, the port density and total broadband of 25G switch is higher than 40G switch when both 25G and 40G switches use the chip with 128 channels (see the following table). And it also can save captial expenditure and operating expenses.

Speed of port	Channel rate (Gb/s)	Number of channels per port	Available port	Total broadband (Gb/s)
10G	10	1	128	1280
25G	25	1	128	3200
40G	10	4	32	1280
100G	25	4	32	3200

Cabling

25G Ethernet uses the SFP28 form factors. Note that, 25GBASE-SR SFP28 transceiver module is a working wavelength of 850nm 25G fiber transceiver. It’s a vertical cavity surface emitting laser. Thus, this optical module needs to be connected with OM3 and OM4 fibers. For interconnections in data center switches, using SFP28 DAC/AOC cables is a more suitable and cost-effective solution than using 25G transceivers. But note that, the DAC/AOC cables’ lengths are limited. Because performance of these cables may be affected if they are too long ( FS recommends the max length to use DAC is 10m, while AOC is 30m). When upgrading 10G to 25G, users just need to upgrade 10G SFP+ to SFP28 transceivers. Because they can reuse the existing cabling while still achieve higher bandwidths, avoiding costly and complex changes.

On the other hand, 40G Ethernet commonly uses QSFP+ transceiver form factor. This kind of transceiver has four optical signal transmission channels and receiving channels, which can support single mode and multimode MTP/MPO cables and copper cabling. Specific cabling of 40G Ethernet is shown in figure 2. But note that, the option that uses 40G transceivers with single mode cabling is typically not applied in data centers. Because it’s built for long distances. Thus the use of OM3 and OM4 is a more favored cabling solution.

Figure 2: 40G Ethernet cabling.

Conclusion

Both 25G Ethernet and 40G Ethernet are the solutions to implement more robust and cost-effective networks. They can be applied in data centers with different uses and cabling solutions. So users should choose one depends on the actual demand and usage environment. At FS, we offer our customers a variety of 25G and 40G networking products with good quality, to help you build a good network.