Did You Know About The Alternative To 40G BiDi: 40G CSR? – Cisco Blogs

Many of you are familiar with the 40G BiDi pluggable optic, but did you know that the 40G CSR can serve similar use cases and is sometimes an attractive alternative?

As network operators increase their link bandwidth from 10G to 40G, they need to choose between two categories of 40G pluggable optics: Those that require ribbon fiber or those that use duplex fiber via advanced transceiver technology.  The advantage of a duplex MMF (Multi-Mode Fiber) 40G optic is that is uses the same fiber cable infrastructure as 10G SR optics. Therefore, upgrading from 10G to 40G does not require new fiber cabling.

A number of years ago Cisco introduced the “40G BiDi” (Bi-Directional) QSFP+ short-reach optic (QSFP-40G-SR-BD), which uses duplex MMF. More recently Cisco introduced the QSFP+ “40G CSR” optic (QSFP-40G-CSR-S), which also operates on duplex MMF at 40G and has longer reach.

Reach advantage of the QSFP+ 40G CSR

For reaches beyond 150m, the 40G CSR optic is an excellent alternative to 40G BiDi, because it provides connectivity up to 400m over duplex MMF. This type of fiber is very common in applications including:

• Enterprise: 40G optical connectivity between distribution switches and wiring closet switches.
• Data Center: 40G optical links between leaf and spines switches.
• Service Provider: 40G optical links connecting switches, routers, and other platforms

The 40G CSR provides extended reach optical connectivity of 400m over OM4/OM5 and 300m over OM3 using SWDM (Shortwave Wavelength Division Multiplexing) technology, while the 40G BiDi provides connectivity of 150M over OM4/OM5 and 100m over OM3 using 2x20Gbps PAM4 (Pulse Amplitude Modulation) bi-directional technology.

Comparison between 40G CSR and 40G BiDi

While the two optical transceivers use different technology they both provide 40G connectivity in the QSFP+ form factor and are available today, as shown in the following comparison:

On-going benefits of 40G BiDi

The 40G BiDi continues to be a great choice for applications requiring:

• Optical monitoring using the receive-only 40G BiDi (QSFP-40G-BD-RX) pluggable optic
• Interoperability with the dual-rate 40/100G BiDi (QSFP-40/100G-SRBD) pluggable optic

With Cisco Optics, you have multiple options for 40G MMF cable infrastructure.

Additional Resources: 

The Optics-to-Device compatibility matrix

Complete selection of Cisco pluggable transceivers from 1Gbps to 400Gbps

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