400G Transceiver

400G transceivers are high-speed optical modules based on optical fiber transmission technology that can achieve data transmission speeds of up to 400Gbps. The high bandwidth, extended transmission range, and low latency of 400G transceivers position them as primary high-speed connectivity solutions within data centers and enterprise networks.

According to different 400G transceiver form factors, there are CFP8, QSFP-DD, OSFP, COBO, etc., of which the most common type is 400G QSFP-DD. These transceivers are different from each other in terms of transmission distance, connector, media, and so on.

Usually, the 400G SR8 modules use an MPO-16 connector to connect to 8 fiber pairs, realizing 400G transmission. The 400G DR4/XDR4/PLR4 modules use an MPO-12 connector to connect to 4 fiber pairs. Unlike the 400G SR8 and 400G DR4, the 400G FR4 optical modules use a duplex LC optical connector.

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400G DAC Cable

400G Direct Attached Cable (DAC) mainly refers to passive direct attach cables, which use conductive copper wires for a direct connection between two ends. The internal structure of DAC typically involves a shielded Twinax design and does not include active components. DAC copper cables are primarily utilized for intra-system rack connections, linking compute servers to storage subsystems, with a maximum length of 3 meters.

400G AOC Cable

400G Active Optical Cable (AOC) utilizes optical fiber as the transmission medium, converting electrical signals into optical signals. Since the fiber core of AOC is made of insulating material, it is less susceptible to interference from electromagnetic, lightning, or radio signals during data transmission. The maximum transmission distance for 400G AOC can reach up to 100 meters.

Which is Best Suited for You?

There are three main options concerning 400G: transceivers, DACs, and AOCs. But which one is best suited for you?

First, we need to understand the kind of data you need to transmit, how far it travels, and the environmental conditions. After that is finalized, you can narrow down your options. The table below shows a general guide for your selection:

Differences Between 400G DAC and 400G AOC

According to the table, we know that the main differences between 400G DAC and 400G AOC are transmission distance and the available types on the market. At present, 400G DAC can provide more breakout cables and better satisfy your different connection requirements. Apart from that, 400G DAC and AOC differ from each other in the following aspects.

Transmission medium - 400G DAC uses copper wires as the transmission medium, with signals transmitted through electrical currents in the copper wires, while 400G AOC utilizes optical fibers as the transmission medium, with signals transmitted through optical pulses in the fibers.

Transmission distance - 400G DAC cables are suitable for short-distance connections, such as within a cabinet or between racks. Different from 400G DAC cables, 400G AOC cables support longer-distance transmission, up to 100m, and are usually used for connections between different cabinets in data centers.

Weight and volume - With fiber optic cable as transmission media, 400G AOC has about half the volume and only a quarter the weight of 400G copper DAC. Also, its cable bending radius is smaller than 400G DAC.

Interference-resistance - Since 400G AOC with fiber optic cable doesn't conduct electrical currents, it is resistant to interference from electromagnetic, lightning, or radio signals during data transmission. While 400G DAC with copper cable is vulnerable to power lines, lightning, and signal-scrambling.

Price - On today's 400GbE cable market, the price of the 400G AOC is often higher than that of 400G DAC, of course, with the same level. If both of them can meet your needs, you can choose a 400G DAC to save costs.

FAQ about 400G DAC/AOC

Q: Why does 400G DAC/AOC adopt PAM4 modulation?

A: PAM4 is a more efficient modulation technology that can effectively improve the bandwidth utilization efficiency. With same Baud rate, PAM4 signal can transmit twice faster than the traditional NRZ signal. Also, the transmission costs are greatly reduced.

Q: What's the key technology of 400G DAC/AOC?

A: The core technologies of 400G DAC/AOC are PAM4 and DSP. Since PAM4 is more sensitive to noise than NRZ especially in 400G AOC, DSP is introduced to make up for the disadvantage of PAM4. As a high-speed digital processing chip, DSP not only owns the function of recovering signal provided by the traditional CDR but also can make dispersion compensation and remove noise, nonlinear disturbance as well as other interferences.

Q: Can I plug 400G QSFP-DD DAC into an OSFP port?

A: No, since their form factors differ. The 400G OSFP DAC should be plugged into the OSFP port and the 400G QSFP-DD DAC into the 400G QSFP-DD port.