Developed by IEEE 802.3 task force P802.3by, 25G Ethernet is a new standard for Ethernet connectivity in a data center environment. This standard was derived from 100G Ethernet standard, however, its operation works as a single lane connection with 25Gbps that can be run on fibers or coppers. To support 25G Ethernet, there are three most common cable solutions: SFP28 optical transceiver, SFP28 twinax cable and cat8 cable, which one is better for your 25G network? Please continue read the post, and you will find the answer.
Fiber optic module and fiber patch cable are always the first choice in network connectivity. In this connection method, two things will be required: 25G SFP28 modules and duplex LC optical jumper. You need firstly plug the patch cable into SFP28 modules, then plug the whole assembly into the both devices. There are main two types of SFP28 transceivers used to transmit data across fiber optic cables with different maximum distances as shown in the following table.
Pros: SFP28 module and fiber cable is the best choice when the distance between two data center servers is very long.
Cons: Fiber and module is often more expensive than other two cabling solutions.
Intended for short runs (up to 5 meters), SFP28 twinax cable or SFP28 direct attach twinax cable (DAC) is a cable assembly which is terminated with two SFP28 connectors at both ends. As the other DAC cables (eg: SFP+ DAC or QSFP+ DAC) that we are more familiar with, the SFP28 connector is also not real transceiver module, which doesn’t have expensive optical lasers, thus making this solutions more cost-effective than SFP28 transceiver solution. SFP28 twinax cable is ideal for high density, high speed I/O data center applications in the networking, telecom and data storage markets where maximum overall network efficiency and lower overall cost are desired.
Pros: It provides an extremely efficient increase in speed to top-of-server (ToR), and it is very suitable for very short links and offer a highly cost-effective way to establish a 25-Gigabit link between SFP28 ports of switches within racks and across adjacent racks.
Cons: Transmission distance is usually less than 10 meters.
While 25G over twinax DAC assemblies will fulfill the ToR server environment where a distance of 3 to 5 meters is more than adequate, there is also the need for longer distances to support the middle of the row (MoR) topologies to about 15m and end of row (EoR) to 30m. That’s where a 25GBase-T application over balanced twisted-pair copper cabling has potential to fill the gap. The Cat8 cabling is designed to support emerging 25GBase-T and 40GBase-T applications, which are specified over an extended bandwidth of 2GHz for a distance over 30 meters. It is fully backward compatible with Cat6a cabling, including RJ45 connectivity, and supports all Cat6a applications such as 10GBase-T for a distance of 100 meters.
Pros: Longer transmission distance than SFP28 twinax cable, up to 30 meters. Fully backward compatible with Cat6a cabling, which will provide easier migration from 1G to 10G to 25G.
Cons: The technology of 25GBase-T hasn’t been very mature yet, so there is not many devices that can support 25GBase-T standard.
Taking the advantages of high density and low cost consumption with no changes in architecture required, there is no doubt that 25G Ethernet will have a broad market potential in the server interconnect world. In the previous text, we have introduced three cabling solutions: 25G Ethernet over fiber, twinax and fiber. You can choose the right solution according to your demands.