40GBASE-SR BiDi QSFP Transceiver Tutorial

One of the biggest challenges during the migration to 40G network is cabling infrastructure, especially 40G transmission in short distances. For most 40G transmission in long distances, duplex LC interface is commonly used, like 40GBASE-LR4 QSFP+ optics and 40GBASE-PLR4 QSFP+ optics. However, for 40G transmission in short distance, 40GBASE-SR4 and 40GBASE-CSR4 are used. These 40G transceivers require 12-fiber MTP/MPO connectors and 12 fibers to accomplish the transmission, which is very much different from the traditional 10G connections. Thus, during the upgrading of network from 10G to 40G, the cabling infrastructure should be changed and additional products like MTP/MPO cassettes are usually needed for better cabling. Money and time increases. If we can transmit 40G over duplex multimode fiber optic cable, there is no need to change the cabling infrastructure for 40G migration. And the cost and required time could be reduced dramatically.

40GBASE-SR BiDi QSFP Transceiver Transmitting 40G Over Duplex MMF

Is it possible to transmission 40G over duplex multimode fiber optic cables? With 40GBASE-SR BiDi QSFP transceivers, the answer is yes. The following will introduce the details of this optics. Compare with the traditional 40G QSFP+ transceivers like 40GBASE-SR4 and 40GBASE-CSR4, 40GBASE-SR BiDi QSFP optics use a much more straightforward transmission mode.

The traditional 40G QSFP+ transceivers like 40GBASE-SR4 and 40GBASE-CSR4 use 4*10G transmission mode occupying 4 fibers for sending and 4 fibers for receiving as shown in the following picture. Usually a 12-fiber MPO connector is used with 4 fibers unused.

MPO interface

For 40GBASE-SR BiDi QSFP optics only a duplex LC patch cord can finish 40G transmission over MMF and can perfectly achieve the migration from 10G to 40G. 40GBASE-SR BiDi QSFP take advantages of fiber optic multiplexing.

How Does 40GBASE-SR BiDi QSFP Working?

There are mainly three steps for 4*10G signals transmitting via 40GBASE-SR BiDi QSFP module. The first step is electrical. The four 10G signals are electrically combined together into 20G and 20G. The second step is an optical combining using two different wavelengths going on the same fiber strand. In the third step, the two 20G signals are sent to the other 40GBASE-SR BiDi QSFP module on the target device via the single optical fiber. The 40G signals are receiveds from the other 40GBASE-SR BiDi QSFP module on the other end of the optical fiber. The process for receiving is reverse. This is how a pair of BiDi QSFP transmit 40G signals over a duplex fiber optic patch cable. The following picture shows the basic working principle of this 40G BiDi QSFP module.

40G bidi QSFP working principle

The most commonly used two wavelengths for 40G BiDi QSFP module are 850 nm and 900 nm. The two wavelengths support reliable transmission over multimode fiber optic cables up to 150 meters over OM4 and 100 meters over OM3, which can perfectly fit the 40G short distance transmission in data center. To help you better understand this module. The following picture shows the internal structure of the 40G BiDi QSFP module and how a pair of BiDi QSFP modules working with each other.

40G bidi QSFP working principle

High Density Cabling Solution for 40GBASE-SR BiDi QSFP Modules

With 40GBASE-SR BiDi QSFP module, cabling in data center or server room would be much easier. As above mentioned, you do not have to change your cabling infrastructure for 40G BiDi QSFP. A pair of duplex LC-LC multimode fiber optic patch cable is enough. OM3 and OM4 are suggested. For high density cabling, a new version of LC patch cord—LC-HD duplex multimode fiber optic cable is suggested. This type of patch cable has a tab attached on the connector, which offers easier finger reach for telecom engineers. With 40GBASE-SR BiDi QSFP modules, you can scale your 40G network easily and cost-effectively.



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