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Classification of 25G DAC FEC Classes
Time: 2020-02-23
1. 25G Cable Interface Types
The IEEE 25G standard defines different interface types, among which the cable-related ones are 25GBASE-CR-S and 25GBASE-CR. 25GBASE-CR-S is referred to as the short-reach interface, designed to support high-quality cables without the need for FEC to minimize latency. 25GBASE-CR is called the full-reach interface, aiming to achieve the lowest possible cable cost and the longest possible transmission distance, which requires the use of FEC functionality.
Table 1: 25G Copper Cable Interfaces in IEEE Standards
|
Transmission Medium
|
Interface Name
|
Error Correction Method
|
|
Direct Attach Cable (DAC)
|
25GBASE-CR-S
|
BASE-R FEC or FEC Disabled
|
|
Direct Attach Cable (DAC)
|
25GBASE-CR
|
BASE-R FEC or RS-FEC
|
2. Description of BASE-R FEC and RS-FEC
The FEC options 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 of backplane channels. The network latency introduced by this algorithm is approximately 80 ns. For detailed information, refer to IEEE 802.3bj Clause 74.
RS-FEC, also known as Reed-Solomon Forward Error Correction, is an error correction algorithm with strong error correction capability. It is widely used in data storage (e.g., CDs) and transmission applications. The network latency introduced by this algorithm is approximately 250 ns. For detailed information, refer to IEEE 802.3bj Clause 91 and 802.3by Clause 108.
3. 25G DAC Classes in 802.3
For different interface applications, combined with the varying parameters of direct attach cables, three cable types are defined in 802.3by (110.10 Cable assembly characteristics), namely CA-25G-N, CA-25G-S, and CA-25G-L.
CA-25G-N (Cable assembly no-FEC): A DAC used between two PHYs operating in FEC-disabled mode, with a maximum achievable length of 3 m.
CA-25G-S (Cable assembly short): A DAC used between two PHYs operating in BASE-R FEC mode, with a maximum achievable length of 3 m.
CA-25G-L (Cable assembly long): A DAC used between two PHYs operating in RS-FEC mode with error correction functionality enabled on the receivers at both ends, with a maximum achievable length of 5 m.
The table below describes the insertion loss characteristics of CA-25G-L, CA-25G-S, and CA-25G-N cables. Due to the enhanced error correction capability in RS-FEC mode, the maximum insertion loss of CA-25G-L cables is higher than that of CA-25G-S cables. Similarly, because BASE-R FEC mode provides error correction, the maximum insertion loss of CA-25G-S cables is higher than that of CA-25G-N cables.
Table 2: Cable Class Definitions in 802.3by
|
Parameter Type
|
CA-25G-N
|
CA-25G-S
|
CA-25G-L
|
Unit
|
|
Maximum Insertion Loss @ 12.98 GHz
|
15.5
|
16.48
|
22.48
|
dB
|
|
FEC Mode
|
No FEC
|
BASE-R FEC
|
RS-FEC
|
-
|
|
Maximum Achievable Length
|
3
|
3
|
5
|
m
|
4. Cable Classes of Various Manufacturers
Cisco's cables comply with IEEE 802.3 specifications, and their cable classification in the data sheet is as follows:
Table 3: Cisco Cable Definitions
|
Cisco Product Model
|
Cable Length
|
Maximum Insertion Loss (dB)
|
IEEE Class
|
Application Notes
|
|
SFP-H25G-CU1m
|
1 m
|
15.5
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
SFP-H25G-CU2m
|
2 m
|
-
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
SFP-H25G-CU3m
|
3 m
|
16.48
|
CA-25G-S
|
Need to enable BASE-R FEC on the host
|
|
SFP-H25G-CU4m
|
4 m
|
22.48
|
CA-25G-L
|
Need to enable RS-FEC on the host
|
|
SFP-H25G-CU5m
|
5 m
|
-
|
CA-25G-L
|
Need to enable RS-FEC on the host
|
Mellanox's 25G DACs are divided into two classes: CA-25G-N and CA-25G-L. The 1 m and 2 m models comply with CA-25G-N, while the 4 m and 5 m models comply with CA-25G-L. The classification of the 3 m model varies depending on the wire gauge used: those manufactured with the thicker 26 AWG wire have lower insertion loss and comply with CA-25G-N, whereas those manufactured with the thinner 30 AWG wire have higher insertion loss and comply with CA-25G-L, as shown in the table below:
Table 4: Mellanox Cable Definitions
|
Mellanox Product Model
|
Cable Length
|
Cable Gauge
|
Maximum Insertion Loss (dB)
|
IEEE Class
|
Application Notes
|
|
MCP2M00-A001E30N
|
1 m
|
30 AWG
|
15.5
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
MCP2M00-A002E30N
|
2 m
|
30 AWG
|
-
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
MCP2M00-A003E26N
|
3 m
|
26 AWG
|
-
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
MCP2M00-A003E30L
|
3 m
|
30 AWG
|
22.48
|
CA-25G-L
|
Need to enable RS-FEC on the host
|
|
MCP2M00-A004E26L
|
4 m
|
26 AWG
|
-
|
CA-25G-L
|
Need to enable RS-FEC on the host
|
|
MCP2M00-A005E26L
|
5 m
|
26 AWG
|
-
|
CA-25G-L
|
Need to enable RS-FEC on the host
|
Our company's products also comply with the IEEE 802.3 standard. The 1 m and 2 m models meet the CA-25G-N class. The 3 m model, whether using 28 AWG or 30 AWG wire, can reach the CA-25G-N class, outperforming comparable products from Cisco and Mellanox. The 4 m model meets the CA-25G-S class, also superior to comparable products from Cisco and Mellanox. The 5 m model meets the CA-25G-L class, which is equivalent to the products of the two companies. Details are shown in the table below:
Table 5: Moduletek Cable Definitions
|
Moduletek Product Model
|
Cable Length
|
Cable Gauge
|
Maximum Insertion Loss (dB)
|
IEEE Class
|
Application Notes
|
|
DAC-SFP-25G-P-E-1M
|
1 m
|
30 AWG
|
15.5
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
DAC-SFP-25G-P-E-2M
|
2 m
|
30 AWG
|
-
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
DAC-SFP-25G-P-E-3M
|
3 m
|
28 AWG
|
-
|
CA-25G-N
|
No need to enable FEC on the host interface
|
|
DAC-SFP-25G-P-E-3.5M
|
3.5 m
|
28 AWG
|
16.48
|
CA-25G-S
|
Need to enable BASE-R FEC on the host
|
|
DAC-SFP-25G-P-E-4M
|
4 m
|
26 AWG
|
-
|
CA-25G-S
|
Need to enable BASE-R FEC on the host
|
|
DAC-SFP-25G-P-E-4.5M
|
4.5 m
|
26 AWG
|
22.48
|
CA-25G-L
|
Need to enable RS-FEC on the host
|
|
DAC-SFP-25G-P-E-5M
|
5 m
|
26 AWG
|
-
|
CA-25G-L
|
Need to enable RS-FEC on the host
|
5. Summary
Based on the international standard IEEE 802.3 series specifications, this document briefly introduces the classification of 25G DACs and compares the performance of our products with those of Cisco and Mellanox. It can be seen from the comparison that the performance of our products of all lengths meets or exceeds the nominal levels of the products of these two companies.
Moduletek is dedicated to serving you.
If you have any questions about the above content, please contact us via email: sales@moduletek.com
