The QSFP28 100G SR4 and QSFP28 DAC (Direct Attach Copper) cables are both used for high-speed 100 Gbps data transmission in data centers and enterprise networks, but they differ significantly in their technology, use cases, and characteristics. Below is a detailed comparison:

Technology

• QSFP28 100G SR4:

• Type: Optical patch cord using a QSFP28 SR4 transceiver module.
• Medium: Multimode fiber (MMF), typically OM3 or OM4, with an MTP/MPO-12 connector.
• Operation: Uses four parallel optical lanes (4x25Gbps) operating at 850nm wavelength to transmit and receive data. Electrical signals are converted to optical signals by lasers and back to electrical signals by photo
• Components: Requires QSFP28 SR4 transceivers inserted into switch ports, connected via a separate MTP/MPO fiber patch cord.
  • QSFP28 DAC Cable:
• Type: Copper-based direct attach cable.
• Medium: Twin-ax copper cable with integrated QSFP28 connectors on both ends.
• Operation: Transmits electrical signals directly over copper without optical conversion. Available in passive (no signal amplification) or active (includes signal amplification for slightly longer reaches).
• Components: A complete cable assembly with fixed QSFP28 connectors, eliminating the need for separate transceivers or patch cords.

Transmission Distance

• QSFP28 100G SR4:

#.Supports up to 70m on OM3 multi-mode fiber or 100m on OM4 multi-mode fiber, making it suitable for short to medium-range connections within data centers.

#.Ideal for connections between racks or across larger data center environments.

 

• QSFP28 DAC:

#.Limited to shorter distances, typically 0.5m to 5m for passive DACs and up to 10m for active DACs due to signal degradation over copper.

#.Best for intra-rack or adjacent-rack connections, such as Top-of-Rack switch-to-server links.

Power Consumption

• QSFP28 100G SR4:

#.Higher power consumption, typically <3.5W per transceiver, due to the use of lasers and photo detectors for optical signal conversion.

#.Requires power for two transceivers (one at each end) plus the switch ports.

• QSFP28 DAC:

#.Lower power consumption, often <1W for passive DACs and slightly higher for active DACs, as no optical components are involved.

#.More energy-efficient, reducing cooling costs in data centers.

Cost

• QSFP28 100G SR4:

#.More expensive due to the cost of two QSFP28 SR4 transceivers and the MTP/MPO fiber patch cord.

#.Higher initial investment, but reusable transceivers allow flexibility in cabling.

• QSFP28 DAC:

#.Significantly cheaper, as it’s a single integrated cable assembly without optical components. DACs are a cost-effective alternative to optical solutions for short distances.

#.Ideal for budget-conscious deployments where short-range connectivity is sufficient.

 

 

Connector and Cabling

• QSFP28 100G SR4:

#.Uses MTP/MPO-12 connectors (8 fibers: 4 for Tx, 4 for Rx) to connect transceivers over multi-mode fiber.

#.Requires careful polarity management (Type B, key-up configuration) to ensure proper signal alignment.

#.Fiber cables are lighter, more flexible, and immune to electromagnetic interference.

• QSFP28 DAC:

#.Fixed QSFP28 connectors on both ends of a twinax copper cable, no separate transceivers or connectors needed.

#.Copper cables are thicker, stiffer, and have a larger bend radius, making cable management more challenging in tight spaces.

#.Susceptible to EMI, though shielding mitigates this.

Applications

• QSFP28 100G SR4:

#.Used for short to medium-range connections in data centers, such as switch-to-switch or switch-to-router links across racks.

#.Supports breakout configurations (e.g., 100G to 4x25G) using MTP-to-LC breakout cables to connect to 25G SFP28 ports.

#.Common in high-density 100G Ethernet networks requiring flexibility and longer reaches.

• QSFP28 DAC:

#.Ideal for short-range, high-density connections within a rack, such as ToR switch-to-server or switch stacking.

#.Available in direct (QSFP28 to QSFP28) or breakout (QSFP28 to 4x SFP28 or 2x QSFP28) configurations for hybrid connectivity.

#.Preferred in cost-sensitive, low-latency environments like high-performance computing (HPC) or network storage.

Latency

• QSFP28 100G SR4:

#.Slightly higher latency due to electrical-to-optical signal conversion in transceivers.

#.Suitable for applications where latency is not ultra-critical.

• QSFP28 DAC:

#.Lower latency since it transmits electrical signals directly over copper without conversion.

#.Preferred for latency-sensitive applications like HPC or financial trading systems.

Compatibility and Maintenance

• QSFP28 100G SR4:

#.Requires compatible QSFP28 SR4 transceivers and MTP/MPO cables, with attention to fiber type (OM3/OM4) and polarity.

#.Fiber connectors are prone to contamination, requiring careful cleaning to avoid signal loss.

#.Transceivers are hot-pluggable, allowing easy replacement or upgrades.

• QSFP28 DAC:

#.Plug-and-play with no separate components, reducing setup complexity.

#.Less maintenance since copper connectors are less sensitive to dust or contamination, but the entire cable must be replaced if damaged.

#.Compatibility with major vendors is ensured through rigorous testing.

Form Factor and Density

• QSFP28 100G SR4:

#.Uses standard QSFP28 ports, supporting high port density in switches.

#.Fiber cables are thinner, allowing better airflow and easier cable management in high-density racks.

• QSFP28 DAC:

#.Also uses QSFP28 ports, but the thicker, stiffer copper cables can complicate cable management and reduce airflow in dense deployments.

#.Still supports high port density but is less flexible in tight spaces.

Summary Table

Feature	QSFP28 100G SR4	QSFP28 DAC Cable
Technology	Optical (MMF, 850nm)	Copper (Twinax)
Distance	Up to 70m (OM3), 100m (OM4)	0.5m–5m (passive), up to 10m (active)
Power Consumption	<3.5W per transceiver	<1W (passive), slightly higher (active)
Cost	Higher (transceivers + fiber cable)	Lower (single cable assembly)
Connector	MTP/MPO-12 (8 fibers)	Fixed QSFP28 connectors
Applications	Switch-to-switch, cross-rack	ToR switch-to-server, intra-rack
Latency	Slightly higher (optical conversion)	Lower (direct electrical)
Maintenance	Requires cleaning, polarity management	Plug-and-play, minimal maintenance
Cable Management	Lighter, flexible fiber	Thicker, stiffer copper
EMI Susceptibility	Immune (fiber)	Susceptible (mitigated by shielding)

Choosing Between QSFP28 100G SR4 and QSFP28 DAC

• Choose QSFP28 100G SR4for:

• Longer distances (up to 100m) within or across racks.
• Environments requiring EMI immunity or flexible cabling.
• Breakout scenarios (100G to 4x25G) or future-proofing with reusable transceivers.
• Applications like 100G Ethernet in larger data centers, where fiber infrastructure is preferred.
  • Choose QSFP28 DACFor:
• Short-range (≤5m) connections within a rack or adjacent racks.
• Cost-sensitive deployments with low latency and power requirements.
• Simple, plug-and-play setups with minimal maintenance.
• High-performance computing or storage networks where latency is critical.