40G High-Speed DAC Cables vs. QSFP+ Optical Modules

High-speed 40G QSFP+ DAC cables and QSFP+ optical modules can be used as solutions for 40G networking at the data center level. 40G QSFP+ DAC high-speed cable applications mainly include short-distance transmission in data centers and convenient wiring. Whereas, Optical modules can provide communication over long and short distances at rates even beyond 40G.

The above exhibit shows that each of the two solutions has pros and cons. In the following passages, we will analyze the advantages and disadvantages of the two.

Introduction to 40G QSFP+ DAC Cables

40G High-Speed Cable (DAC) refers to a copper cable equipped with transceiver shape connectors at both ends. A 40G QSFP+ DAC cable can transfer data at 40Gbps and is a very economical solution for high-speed interconnection. These cables are offered in various lengths. However, we cannot use these cables to achieve long-distance connectivity. Passive DACs are recommended for

- 40G QSFP+ to QSFP+ DAC
- 40G QSFP+ to 4*SFP+ DAC
- 40G QSFP+ to 4XFP+ DAC

The first two types are mainly used in practice.

40G QSFP+ to QSFP+ DAC

The 40G QSFP+ to QSFP+ DAC consists of a factory assembled cable assembly with two 40G QSFP+ transceivers terminated at both ends. This high-speed cable can be used to connect existing 40G QSFP+ ports to 40G QSFP + ports. This kind of cable provides bi-directional communication through integrated, serial data links. The 40G QSFP+ to QSFP+ DAC is a cost-effective solution to achieve 40G connectivity between QSFP+ ports of routers and switches in a data center environment.

40G QSFP+ to 4 × SFP+ DAC

This cable consists of one 40G QSFP+ transceiver shape connector, copper wire, and four 10G SFP+ transceivers. The 40G QSFP+ to 4 × SFP+ DAC cables are mainly used to interconnect switches with 10G ports to the switches with 40G ports. With this cable, you will split one 40G port into four independent Ethernet ports to achieve greater port density. Hence, this type of DAC cable can be great for operators who are in the process of migrating from 10G to 40G Ethernet.

40G QSFP+ to 4XFP DAC

This cable comes with one 40G QSFP+ connector, copper core, and four 10G XFP connectors. Therefore, this high-speed cable can connect the existing 40G QSFP+ ports to 4 XFP ports. QSFP+ to 4 XFP DAC cables are not as common as the two types mentioned earlier of DAC cables.

Introduction to 40G QSFP+ Optical Modules

40G QSFP + optical module is a four-channel small 40Gbps plug-in optical module that can interconnect network equipment (such as switches, routers, media converters, and similar devices) and optical or copper cables. Following are some of the most common types of 40G QSFP+ optical modules;

- 40GBASE-SR4
- 40GBASE-LR4
- 40GBASE-ER4
- 40G LR4 PSM

40G QSFP+ SR4

The 40G QSFP+ SR4 optical module interface has 4 x independent full-duplex channels and is often used with multimode fiber. When using the OM3 fiber cable, its transmission distance is 100m, whereas up to 150m is achievable with OM4 cable.

40G QSFP+ LR4

The 40G QSFP+ LR4 optical module interface support LC duplex connectors. It has the advantages of high density, high speed, large capacity, low cost, and low power consumption. It is usually used with single-mode optical fiber, and the transmission distance can be up to 10 km.

40G LR4 PSM

The QSFP-40G-PSM-LR4 optical module interface uses Parallel Single Mode Technology (PSM). This transceiver offers 4 x independent transmit (Tx) and receive (Rx) channels – each capable of operating at 40Gb/s. The QSFP+ PSM LR4 module uses a 12-fiber MPO connector on the optical interface and a 38-pin z-pluggable connector on the electrical interface.

40G QSFP + ER4

40G QSFP + ER4 optical module is used with single-mode LC fiber as the 40G QSFP + LR4 optical module. The difference is that the 40G QSFP + ER4 optical module converts four channels of 10G data input to 4 channels of CWDM optical signals. Each channel's speed is up to 11.2Gbps. They are multiplexed into one channel for 40G optical transmission, mainly used in 40GB Ethernet links, Infiniband QDR and DDR interconnect, and 40G client telecommunication connections.

40G QSFP+ DAC Cable Advantages

• These cables offer higher data transmission rates than standard copper interfaces.
• DACs are hot-swappable and interchangeable with optical fiber modules.
• DAC cables are cheaper than active optical cables (AOCs) and optical transceivers for short-reach applications.
• Breakout type DACs are ideal and economical solution for splitting 40G ports into 10G ports.

Where to Use DACs?

• The active and passive DAC cables are suitable for adjacent racks or shorter ToR.
• Active DAC cables are a viable solution for applications where the distance remains • DAC cables are best for the end-of-row architectures provided that the distance does not exceed their design limits.

Conclusion

In the light of the facts as mentioned above, it can be concluded that;

• DACs and QSFP transceivers are two viable solutions available to achieve connectivity within the data centers
• DAC cables (active and passive) cost much lesser than the optical modules
• Optical modules should only be considered for long-distance connectivity
• DAC cables are better to establish 40G links between the QSFP port of routers and switches deployed within or across racks.
• Passive DAC cables are suitable for