40GBASE-SR4 vs. 40GBASE-SR BiDi: Which One Provides Higher Return of Investment (ROI)?

With data center consolidation, and server virtualization, the data center network is shifting to 10 Gbps at the access layer and 40 Gbps at the aggregation layer. To support 40G connectivity, data center architects are challenged by the need for a major upgrade of the cabling infrastructure. This article will introduce the 40GBASE-SR4 cabling solution and 40GBASE-SR BiDi cabling solution to see which one provides more cost-effective solution for data center short distance transmission.

40GBASE-SR4 QSFP+ Overview

40GBASE-SR4 QSFP+, Short-reach (SR) transceivers for 40G connectivity in a QSFP form factor, uses independent transmitter and receiver sections, each with 4 parallel fiber strands. It can transmit 150 meters over OM4 MMF. For a 40G connection, 8 fiber strands are required, and 12-fiber MPO connectors are used. Consequently, 4 fiber strands in each connection are wasted as shown in the following picture.


For most data center networks, the aggregation-layer fiber infrastructure is built for 10G connectivity. That either supports direct connections between devices over LC-to-LC MMF, or uses LC-to-LC fibers to attach devices to patch panels. Regular duplex LC-to-LC fibers cannot be directly reused for 40G connectivity using traditional 40G transceivers.

40GBASE-SR BiDi QSFP+ Overview

The QSFP BiDi transceiver, firstly launched by Cisco (QSFP-40G-SR-BD), addresses the challenges of fiber infrastructure by providing the capability to transmit full-duplex 40G traffic over one duplex MMF cable with LC connectors. In other words, the QSFP BiDi transceiver supports 40G connection over one pair of MMF cables while allowing 40G to be deployed using the same infrastructure as 10G (10GBASE-SR) without the need to add any fibers.


The QSFP BiDi transceiver is a short-reach optical transceiver that delivers 40 Gbps over a duplex OM3 or OM4 MMF connection, which can reach 100 meters on OM3 or up to 150 meters over OM4. It has two 20G channels, each transmitted and received simultaneously on two wavelengths over a single MMF strand. The result is an aggregated duplex 40G link over a duplex fiber of two MMF strands shown in the above picture.

Cabling Cost With 40GBASE-SR4 vs. Cabling Cost With 40GBASE-SR BiDi

A structured cabling system is commonly deployed in data center networks to provide a flexible and scalable cabling infrastructure. Structured cabling uses short patch cords to attach devices to a patch panel and then uses fiber trunk cables either to consolidate the cables in a central location for additional connectivity or to direct the cables to another patch panel, to which the remote devices are attached. The following presents two 40G cabling solutions.

Soultion 1: 40G structured cabling with QSFP SR4 transceivers


Cabling costs with 40GBASE-SR4 transceivers are listed in the following table.

Product Description Price
12 Fibers 10G OM4 Multimode 12 Strands MTP Female 0.35dB Trunk Cable $ 53.00
12 Fibers 10G OM4 Multimode 12 Strands MTP Male 0.35dB Trunk Cable $ 53.00
48 Ports 1U Rackmount Fiber Enclosure, loaded with 4 FAPs 12x MTP Key-up/Key-down $ 510.00
Total Cost $ 1179.00


Solution 2: 40G structured cabling with QSFP BiDi transceivers


Cabling costs with 40GBASE-SR BiDi transceivers are listed in the following table.

Product Description Price
5M LC UPC to LC UPC Duplex 2.0mm PVC(OFNR) 10G OM4 Fiber Optic Patch Cable $ 5.20
12 Fibers 10G OM4 LC Duplex 12 Strands MTP Male 0.35dB Standard LGX Cassette $ 82.00
24 Fibers 10G OM4 12 Strands MTP Trunk Cable 3.0mm LSZH/Rise $ 110.00
Total Cost $ 284.4



From what illustrated above, we can see that the 40GBASE-SR BiDi transceiver removes 40G cabling cost barriers in data center networks. It provides immense savings and simplicity compared to other 40G QSFP (MMF) transceiver (40GBASE-SR4). Moreover, it allows organizations to migrate the existing 10G cabling infrastructure to 40G at no cost and to expand the infrastructure with low capital investment. FS.COM provides all the above products used in the 40G connection, and the prices are much lower than other vendors. If you want to save more costs, FS.COM is your primary option.

Related Article: 40G QSFP BiDi Transceiver – Cost-effective Solution for Migration From 10G to 40G