Server network card access usually uses fiber modules with duplex LC interfaces (such as SR4/FR4), or uses direct attach fiber cable for short distances.

This depends on the specific transmission distance, and the core principle is: <5 meters use DAC, >5 meters use optical fiber. 

Typical application scenarios of three types of connectors

After long-term experience accumulation, YINGDA has summarized that the three types of connectors have different access and applicable scenarios, and users need to make decisions based on their actual needs and budget. The details are as follows:

Solution	QSFP56 200G DAC	QSFP56 200G SR4 MM850 fiber module + MPO patch cable	QSFP56 200G FR4 fiber module + LC duplex patch cord
Transmission medium	Copper cable (Twinax)	Multimode fiber (OM3/OM4)	Single mode fiber OS2
Interface type	QSFP56 (integrated cable, non separable)	MPO-12 connector ( fiber module+ fiber jumper can be separated)	LC duplex connector (fiber module+fiber jumper can be separated)
Transmission distance	3-5 meters (passive DAC ≤ 3m, active DAC ≤ 5-7m)	OM3 70m, OM4 100m	2km
Power consumption	Extremely low (passive DAC<0.1W, active DAC~1.5W)	<5W (typical 4.5-5W)	<7W (higher)
Delay	<100 ns (round-trip)	~200-600 ns (round-trip)	~130-2630 ns (round-trip)
Electromagnetic Interference Resistance	Poor (Copper cables are susceptible to EMI)	Excellent (fiber optic fully EMI resistant)	Excellent (fiber optic fully EMI resistant)
Cable flexibility	Coarse and harder, with a large bending radius	Thin, light, flexible, easy to wire	Thin, light, flexible, easy to wire
Cost	Minimum (without optical components)	Medium (requiring optical components)	Higher (requiring optical components), 2-3 times that of SR4
Typical application scenarios	Inside the server cabinet ↔TOR Switch	Cross rack (≤100 m, interconnected within the data center)	Cross machine room/cross building access, up to 2km

Option 1: Short distance access (<5 meters) – Direct attach fiber cable

QSFP56 200G DAC is a copper cable component pre installed with QSFP56 interface at both ends, with copper twinax cable inside. The signal is transmitted directly in the form of electrical signal in the copper cable without photoelectric conversion. This is the most mainstream solution for server access, suitable for connecting servers to top switches in cabinets. According to the type of twinax fiber cable, it can be divided into active DAC and passive DAC, and the general differences between the two are as follows:

DAC Types	Transmission distance	Power consumption	Features
Passive DAC	≤3 m	<0.1W	No electronic components, almost zero power consumption, lowest cost
Active DAC (AEC/ACC)	5-7 m	~1.5W	Built in Redriver/Timer chip to compensate for signal attenuation

Core strengths

• Extremely low power consumption: Passive DAC power consumption is almost negligible (<0.1W), which can significantly reduce data center PUE during large-scale deployment.
• Ultra low latency: The signal is transmitted directly in passive copper cable without the need for photoelectric conversion, with a round-trip delay of less than 100 nanoseconds.
• Lowest cost: No need for fiber modules or optical fibers, making it the most economical solution for short-range interconnection within the cabinet.
• Plug and play: Passive DAC is integrated design, no need to purchase additional optical fiber modules and fiber patch cables.

Applicable scenarios

• Standard server access: The server network card and the top of cabinet (TOR) switch are located in the same cabinet, with a distance of ≤ 3 meters → Passive DAC.
• A slightly longer connection inside the cabinet: The server and switch are not in the same cabinet, but the distance is 3-7 meters → Active DAC (AEC/ACC).

Usage restrictions

• Distance bottleneck: The maximum distance for 200G passive DAC is 3 meters, while active DAC can only reach 5-7 meters. I am powerless for cross cabinet connections.
• Coarse cables: Copper cables are thicker and harder than fiber optic cables, and there is greater pressure on the wiring at the back of the cabinet during high-density cabling.
• Electromagnetic interference sensitivity: In strong EMI environments (such as near power equipment), signal quality may decrease.

Option 2: Medium to Long Distance Access (>5 meters) – MPO Interface

When the server and switch are not in the same cabinet, or when cross cabinet cabling is required, the standard practice is to use 200G SR4 fiber modules + MPO patch cable (cross cabinet standard).

The 200G QSFP56 SR4 is a 4-channel parallel multi-mode fimodule, with core technologies including:

• PAM4 modulation: 4 channels x 50G PAM4=200G bandwidth, with spectral efficiency doubled compared to NRZ.
• 850nm VCSEL array: Low power, high reliability laser designed specifically for short-range multimode fibers.
• Parallel fiber interface: MPO-12 APC connector is used to achieve full duplex 200G transmission through 8-core fiber (4 receivers and 4 transmitters).

Core strengths

• Long transmission distance: OM4 multimode fiber can reach up to 100 meters, far exceeding DAC.
• Completely resistant to electromagnetic interference: Fiber optic transmission of optical signals, unaffected by EMI, suitable for high-density, high-power environments.
• Flexible wiring: MPO patch cables are thin, light, and flexible, with a small bending radius, making it easy to manage cables at the back of the cabinet.
• Support breakout applications: 1 × 200G can be split into 2 × 100G or 8 × 25G through MPO beakout cable to achieve flexible networking.

Technical Parameters

parameter	Typical value
transmission rate	212.5Gbps (4 x 53.125Gbps PAM4), effective data rate 200Gbps
transmission distance	OM3 multimode fiber: 70 m, OM4 multimode fiber: 100 m
power consumption	<5W (typical 4.5-5W)
working wavelength	850nm VCSEL
interface	MPO-12 APC
protocol standard	IEEE 802.3cd 200GBASE-SR4, Compatible with InfiniBand HDR

Applicable scenarios

• Cross cabinet connection: The server and switch are not in the same cabinet, with a distance of 5-100 m → QSFP56 200G SR4 + MPO patch cable.
• AI/GPU Cluster: The standard short-range solution for NVIDIA InfiniBand HDR networks, supporting GPUDirect RDMA with latency<600ns.
• Data center spine architecture: 200G short-range interconnection between Leaf Spine switches.
• High EMI environment: scenes near strong interference sources such as power equipment and air conditioning.

Option 3: Long distance access (>100m) – LC optical interface

When the server and switch are not in the same data center and must be wired across data centers, 200G FR4 fiber modules and single-mode fiber LC Duplex patch cords must be used. The 200G FR4 (Single Mode Single Lambda) solution paired with LC duplex jumpers is a high-end, long-distance dedicated solution for server network card access.

Core strengths

• Ultra long transmission distance: 2km transmission distance. This breaks through the physical limitations of DAC 3m and SR4 100m, allowing servers and switches to be distributed on different floors or even in different buildings.
• Simple wiring architecture: LC duplex is only 2 cores SMF, greatly saves cable tray space,more easy to insert and maintenance.
• Strong infrastructure compatibility: The backbone cabling of data centers is usually covered with single-mode optical fibers (OS2). By using FR4, existing single-mode fibers can be directly reused.
• High signal quality (low dispersion): Single mode fiber transmission does not have the common “mode to mode dispersion” problem of multimode fiber. At frequencies of 10 gigahertz and higher, the signal consistency of FR4 is better and the error rate is easier to control.

Application scenarios

• Cross data center/cross floor computing cluster: When a high-performance computing (HPC) cluster must be split into different data centers due to power, cooling, or space limitations, FR4 is the only reliable solution to maintain 200G high-speed interconnection.
• Remote disaster recovery and high-speed synchronous backup: used to connect high-speed storage servers in two locations (within 2km), ensuring real-time mirror synchronization of massive data within millisecond level latency.
• Park level edge computing access: In large enterprise parks, the core switches are located in Building A, while high-performance AI servers are deployed in Building B. The FR4 scheme can achieve direct connection without the need for a relay switch.
• Physical isolation/high security requirements: The server needs to be placed in a protected room with special shielding or physical isolation, while the switch is located in a public data center. Realize remote access through the long-distance characteristics of FR4.

Technical Features

• CWDM4 Technology: Within FR4, 50G signals of four different wavelengths are multiplexed onto a single optical fiber. For a network card, it sees a logical whole (a single 200G channel) rather than four independent 50G channels.
• No Breakout: This is the biggest feature of this solution. FR4 cannot be divided into four parts. The network card must have a native 200G interface and cannot be split into 4 channels of 50G for four small traffic devices like SR4.
• Forced FEC (Forward Error Correction): Based on PAM4 modulation signals, the FR4 scheme must enable FEC functionality at both the network card and switch ends to correct signal fluctuations during long-distance transmission, which can result in a fixed delay of approximately 100-200ns.
• Cost and power consumption: Due to the internal integration of four lasers and a multiplexer/demultiplexer, the cost of the FR4 module is relatively high (about twice that of the SR4), and the power consumption is usually around 4.5W-5W, which has certain requirements for the heat dissipation of the network card.

Option 4: Breakout Scenario

In addition to the direct connection mentioned above, DAC and MPO connectors also support breakout. Here the breakout is not simply a physical connector conversion, it involves port splitting mode and requires support from both network cards and switches.

200G Fiber modules + MPO breakout cable

Breakout type	MPO cable type	Network card side (fiber module)	The other end
1×200G → 2×100G	MPO-12 to 2×MPO-12	200G QSFP56 SR4	Connect two 100G switches or servers
1×200G → 8×25G	MPO-16 to 8×LC	200G QSFP-DD SR8	Connect eight 25G servers (requires module support)

200G DAC breakout Cable

Direct attach fiber cable also support breakout cables; however, due to the distance limitations inherent to copper cable, passive DACs are limited to ≤3m, while active DACs range from 5 to 7 m.

Breakout type	DAC breakout cable type	Network card end	The other end
1×200G → 2×100G	QSFP56 to 2×QSFP28 DAC	200G QSFP56 port	Connect 2 x 100G devices within the same cabinet
1×200G → 4×50G	QSFP56 to 4×SFP56 DAC	200G QSFP56 port	Connect 4 x 50G devices within the same cabinet
1×200G → 8×25G	QSFP56 to 8×SFP28 DAC	200G QSFP-DD port	Connect 8 x 25G servers within the same cabinet

Conclusion

Simply make your selection based on the actual distance and cabling environment.

• For connections within a cabinet (<3 meters), choose DAC. DAC reigns supreme for short-distance cost-effectiveness, featuring near-zero power consumption and latency under 100 ns;
• For connections between cabinets (≤100 meters), choose 200G SR4 paired with MPO patch cords. SR4 serves as the standard solution for inter-cabinet links, offering 100-meter coverage, complete immunity to interference, and power consumption under 5 W.

Notes on Other Special Circumstances:

• High-density deployment or limited cabling space: Fiber optic solutions (SR4/AOC) feature thinner, more flexible cables, making them superior to DACs.
• Environments with strong electromagnetic interference (e.g., near power supply equipment): Fiber optic solutions (SR4/AOC) are mandatory.
• Integration with existing 100G networks: Select SR4 modules or DAC cables that support breakout configurations.
• Compatibility with InfiniBand HDR networks (NVIDIA/Mellanox equipment): Certified SR4 fiber modules or direct attach fiber cable must be used.