25G DAC FEC Class Description

1. 25G Cable Interface Types
      The IEEE's 25G standard defines different interface types, of which the ones for cable are 25GBASE-CR-S and 25GBASE-CR. 25GBASE-CR-S is known as the short-range interface, and is designed to support high-quality cables without the need for FEC to minimize latency. 25GBASE-CR is known as the full-range interface, and is designed to achieve the lowest possible cable cost and the longest possible distance, which requires the use of FEC functionality.

Table 1 25G Copper Interfaces in the IEEE

Physical Layer	Interface Name	Error Correction Method
Direct Connect Cable DAC	25GBASE-CR-S	BASE-R FEC or FEC disabled
Direct Connect Cable DAC	25GBASE-CR	BASE-R FEC or RS-FEC

2. BASE-R FEC and RS-FEC Description
      The options for FEC in the PHY include BASE-R FEC and RS-FEC.
BASE-R FEC, also known as Fire Code or FC-FEC, is suitable for correcting burst errors in the receive equalizer in the backplane channel, the algorithm introduces a network delay of about 80ns, refer to IEEE 802.3bj Clause 74 for details.
      RS-FEC, also known as Reed-Solomon Forward Error Correction Reed-Solomon Coded Forward Error Correction (RS-FEC) is a strong error correction algorithm widely used in data storage (e.g., CDs) and transmission applications, which introduces a network latency of about 250 ns, as described in detail in IEEE 802.3bj Clause For details, refer to IEEE 802.3bj Clause 91 and 802.3by Clause 108.
3. 25G DAC Classes in 802.3
      For different interface applications, and in conjunction with the different parameters of the directly connected cables, there are also three types of cables defined in 802.3by (110.10 Cable assembly characteristics), CA-25G-N, CA-25G-S and CA-25G-L. The CA-25G-N (CA-25G-S) is a cable assembly with the same parameters as the directly connected cables.
      CA-25G-N (Cable assembly no-FEC), which supports DACs used between two PHYs operating in no-FEC mode, has an achievable length of 3 m. The CA-25G-N (Cable assembly no-FEC), which supports DACs used between two PHYs operating in no-FEC mode, has an achievable length of 3 m.
      CA-25G-S (Cable assembly short), supports DAC between two PHYs operating in BASE-R FEC mode, with a realizable length of 3 m.
      CA-25G-L (Cable assembly long), which supports DAC between two PHYs operating in RS-FEC mode with error correction enabled on both receivers, with an achievable length of 5 m. The following table describes the CA-25G-S (Cable assembly short).
      The table below describes the insertion loss characteristics of the cables CA-25G-L, CA-25G-S and CA-25G-N. The maximum insertion loss of the CA-25G-L cable is greater than that of the CA-25G-S due to the improved error correction capability in RS- FEC mode. Similarly, the maximum insertion loss of the CA-25G-S cable is greater than that of the CA-25G-N due to the provision of error correction in BASE-R FEC mode.

Table 2 Definition of cable classes in 802.3by

4. Cable Ratings by Manufacturer
      Cisco's cables follow 802.3, and their datasheets classify cables in the following classes:

Table 3 Cisco Cable Definitions

      Mellanox's 25G DACs are divided into two grades, CA-25G-N and CA-25G-L, with 1m and 2m conforming to CA-25G-N, 4m and 5m conforming to CA-25G-L, and 3m products have different grades depending on the wire diameter used, with thick wire 26AWG processed with less insertion loss, conforming to CA-25G-N, and with thin wire The insertion loss of 30AWG is larger and conforms to CA-25G-L, as shown in the table below:

Table 4 Mellanox Cable Definitions

      Our products also follow the 802.3 standard, of which 1m and 2m comply with CA-25G-N, 3m using 28AWG or 30AWG can reach CA-25G-N rating, with better performance than Cisco and Mellonox, 4m reaches CA-25G-S, which is also better than Cisco and Mellonox, and 5m reaches CA-25G-L. same as these 2 products. The details are listed in the table below:

Table 5 Moduletek Optical Cable Definitions

5. Summary

      This article refers to the international standard IEEE802.3 briefly introduces the classification level of 25G DAC, and compares the performance of our products with those of Cisco and Mellanox. From the comparison, it can be seen that the performance of our products of various lengths meets or exceeds the nominal level of the products of these two companies.
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