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What is 25GBASE-T ?

25GBASE-T, or 25G Ethernet over twisted-pair, has become a sought-after technology over the past few years driven by cloud computing. With 25Gbps on a single channel, 25GBASE-T can also be a viable solution to meet the ever-higher demands for bandwidths, making it especially suitable for data centers and server rooms. This article will give an all-round introduction of 25GBASE-T.

25g Ethernet adopts 25gb / s single channel physical layer technology, which can realize 100g transmission based on four 25gbps fiber channels. Since the sfp28 package is developed based on the SFP + package, and their sizes are similar, the 25g sfp28 port can be compatible with the 10g SFP + port, but the bandwidth of the 25g sfp28 optical module is 2.5 times that of the 10g SFP + optical module. Compared with 40g Ethernet, 25g Ethernet has significant performance advantages, which can provide higher port density and lower bandwidth cost. In recent years, 25g products (such as 25g switch, optical module, DAC, AOC, etc.) have been widely used in cloud data center and 5g network.

25g is mainly used for the access between tor switch and server (the server access of next generation data center), and 100g or higher rate network upgrade. 40g is mainly used for the connection between switches in the data center to help break the bottleneck of the connection between access switches and distribution switches. If 25g can be widely used in switch interconnection, it will promote the further development of 25g Ethernet. At the same time, with the arrival of 5g network, the demand of 5g forward transmission for 25g WDM optical module will make 25g Ethernet usher in a new peak of development. For 40g, because its efficiency is higher than 10g and cost is lower than 100g, it is still the best choice for high bandwidth.

 

25gbase-t standard and technology

25gbase-t standard was developed by IEEE in 2016. It can realize 25gbps data transmission through twisted pair.

Name Standard Transmission media Number of media Gigabaud per channel Maximum transmission distance RS-FEC   (802.3by CL 108)
25GBASE-T 802.3bq(CL 113) Cat8 Copper twisted pair 4 pairs 2 m30 Unsupported


25gbase-t technology is based on 10Base-T technology and can be compatible with 10gbase-t. after continuous development, it can be compatible with 40gbase-t. The parameter comparison of 10G / 25g / 40gbase-t technology is shown in the table below. By comparison, it can be found that the complexity of 25gbase-t technology is similar to that of 10GBASE-T technology. Meanwhile, 25gbase-t follows the 40gbase-t technology developed earlier, which will effectively reduce the complexity of 25gbase-t equipment and shorten the time to market. In addition, 25gbase-t uses SerDes technology, and Gigabit connection unit interface specification and sfp28 protocol are also reused by 25gbase-t.

Parameter 10GBAES-T 40GBASE-T 25GBASE-T
Passageway 100m,CL 55.7 30m,CL98 7d1.0 30m,CL98 7d1.0
Baud rate (MHz) 800 3200 2000
Receive significant bits 9.5-10 7.8 6.5-7.5
Channel IL (DB) 46.9 29.4 22.6
Channel round trip (baud) 880 1056 660
Echo cancellation (DB) 55 47(-6dB)~55 43(-12dB)~55
Next elimination (DB) 40 34(-6dB)~40 28(-12dB)~40
FEXT elimination (DB)) 25 22(-3dB)~25 19(-6dB)~25
Relative SNR margin (DB) 0 +2.7dB~0.2 dB +8.7 Db~+4 dB


What are the advantages of 25gbase-t?

Why is 25gbase-t so popular in a short time? The main reasons are as follows:

Excellent output / input ability and structure ability

25gbase-t can cost effectively expand the bandwidth capacity, and can meet the requirements of cloud data center and the next generation of data center servers and storage devices. 25gbase-t can achieve 25gbps transmission with a single channel. Compared with 10GBASE-T, it improves the network bandwidth performance by 2.5 times. At the same time, 25gbase-t can provide 4 times of switching port density for each switching chip. Compared with 40gbase-t, 25gbase-t has higher port density and better scalability.

Use automatic negotiation and high compatibility

According to the 25gbase-t standard, the port of the switch can automatically identify the transmission rate and create links at the same rate. Through the industry standard interoperability specification, the network upgrade is greatly simplified and the connection cost is effectively reduced. At present, the commonly used interoperability specifications are 10GBASE-T, 40gbase-t, 50gbase-t and 100gbase-t. 25gbase-t can not only be backward compatible with 10GBASE-T, but also provide forward compatible capability (such as 100g Ethernet) through automatic negotiation, which undoubtedly improves the connection efficiency and effectively saves the cost.

Flexible topology

It can be seen from the above that 25gbase-t can realize 30 meter data transmission with cat8 network jumper. Compared with 25G DAC high-speed cable, 25gbase-t has a long transmission distance, so 25gbase-t can support more topology cabling, such as the connection between cabinets or between EOR switch and server, MOR cabling, etc. For this reason, 25gbase-t is regarded by most large cloud data centers as an effective solution to manage switches in hundreds of cabinets.
Lower Costs
25GBASE-T can reduce capital expenditures (CAPEX) and operating expenses (OPEX). Utilizing a 1x25Gbps channel, 25GBASE-T enables a simple and seamless 10G-25G-100G migration path compared with the traditional 10G-40G-100G path. In addition, the 10G-25G-100G path also avoids rewiring and network redesign, significantly cutting down the required number of switches and cables, namely CAPEX. As to the OPEX, 25GBASE-T offers superior switch port density by requiring just one channel compared with the four-channel 40GBASE-T, which brings lower power requirement and lower cost per unit of bandwidth.

25GBASE-T Application
As mentioned earlier, the application advantage of 25GBASE-T lies in its 30 m reach zone over the same Cat8 cables as 40GBASE-T, filling the gap for longer distances to support MoR and EoR connections and making it popular in cloud data centers for switch-to-server applications. Read this article for more details: Cat8 Cable for 25G/40GBASE-T Network. It is shown that 25GBASE-T can work anywhere in data centers, including fat tree, leaf and spine architecture, interconnected fat tree fabrics, traditional three-tier architecture, for full-mesh, interconnected mesh, and centralized switching.

Conclusion
Nowadays 25GBASE-T has gained more and more attention, as it offers excellent network bandwidth performances and high scalability with good compatibility. It also enables a more cost-effective path for network migration to 100G and ever higher speeds without rewiring. And 25GBASE-T is suitable for MoR and EoR architectures while saving considerable costs for cabling and management. All in all, 25GBASE-T is an optimal solution for cloud data centers and server rooms.

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