How to Choose DAC and AOC Cables

DAC and AOC: Cost-Effective Alternatives to SFP Transceivers and Fiber

In a modern data center, each server has 2× 25G NICs connected to 2 different ToR leaf switches. Multiply this by 200-500 servers, and you have more than 1,000 links to cable. If you install a 25G SFP28 (€300) and an LC-LC patch cable (€30) at each end, the cost is €660 per link × 1,000 = €660,000. By switching to DAC (Direct Attach Copper) or AOC (Active Optical Cable), the cost drops to €50-120 per link—saving more than €500,000 on a data center project.

DAC and AOC cables are not always interchangeable. DAC is less expensive but limited to a maximum of 5m. AOC supports distances of up to 100m but costs more. This guide will help you choose the right cable for each use case.

DAC (Direct Attach Copper): Integrated Copper Cabling

A DAC is a copper cable with transceivers permanently attached at both ends. It plugs directly into SFP+/SFP28/QSFP28 ports like a transceiver module. There is no electrical-to-optical-to-electrical conversion: the signal remains electrical over copper throughout the cable. Benefits include minimal latency (~1ns vs ~3ns for SFP transceivers and fiber), a price 60-80% lower than SFP transceivers and fiber, no configuration, and 0.5W power consumption (vs 1W for an SFP+ module).

• SFP-H10GB-CU1M (Cisco): 10G SFP+ DAC, 1m. OPTINOC price: ~€45 excl. VAT.
• SFP-H10GB-CU3M (Cisco): 10G SFP+ DAC, 3m. Price: ~€60 excl. VAT.
• SFP-H10GB-CU5M (Cisco): 10G SFP+ DAC, 5m. Price: ~€80 excl. VAT.
• SFP-25G-CU3M (Cisco): 25G SFP28 DAC, 3m. Price: ~€80 excl. VAT.
• QSFP-100G-CU3M (Cisco): 100G QSFP28 DAC, 3m. Price: ~€150 excl. VAT.
• QDD-400-CU2M (Cisco): 400G QSFP-DD DAC, 2m. Price: ~€300 excl. VAT.
• Disadvantage: limited reach (maximum of 1m, 3m, or 5m depending on the data rate). 7m versions are available for 10G but are uncommon.

AOC (Active Optical Cable): Integrated Fiber-Optic Cabling

An AOC combines the flexibility and reach of fiber optics with the simplicity of a DAC. The non-detachable integrated transceivers are connected by internal optical fiber. Reach: 3m to 100m. AOCs cost more than DACs but less than separate transceiver modules and fiber patch cables.

• SFP-10G-AOC3M (Cisco): 10G SFP+ AOC, 3m. OPTINOC price: ~€70 excl. VAT.
• SFP-10G-AOC10M: 10G SFP+ AOC, 10m. Price: ~€120 excl. VAT.
• SFP-10G-AOC30M: 10G SFP+ AOC, 30m. Price: ~€200 excl. VAT.
• SFP-25G-AOC10M: 25G SFP28 AOC, 10m. Price: ~€200 excl. VAT.
• QSFP-100G-AOC10M: 100G QSFP28 AOC, 10m. Price: ~€300 excl. VAT.
• Benefits over separate fiber cabling: no patch cables to handle, no LC connectors to contaminate, and integrated fiber protected by a reinforced jacket.

Breakout: QSFP to 4× SFP DAC/AOC

A breakout cable connects 4× 25G SFP28 servers to 1× 100G QSFP28 switch port. This is standard practice in modern spine-leaf data centers.

• QSFP-4SFP25G-CU3M (Cisco): QSFP28 to 4× SFP28 DAC breakout cable, 3m. Price: ~€200 excl. VAT.
• QSFP-4SFP10G-CU3M: 40G QSFP+ to 4× 10G SFP+ DAC breakout cable. Price: ~€150 excl. VAT.
• QSFP-100G-AOC-4×25G-10M: 10m AOC breakout cable. Price: ~€400 excl. VAT.

DAC vs AOC vs Transceiver Modules and Fiber: Making the Right Choice

• Within a rack (<3m): always use DAC. Price: €45-80. Minimal latency.
• Between racks in the same room (3-10m): use a 5m DAC where possible; otherwise, use AOC. Savings compared with separate fiber cabling: 30-50%.
• Data center aisle (10-30m): use a 10-30m AOC. It costs more than DAC but less than separate transceiver modules and fiber.
• Between data center rooms or across a campus (30-100m): use a 100m AOC where available; otherwise, use SFP+ SR transceiver modules and multimode fiber.
• >100m: use SFP+ SR or LR transceiver modules with separate fiber. AOC is not suitable beyond this distance.

Multi-Vendor Compatibility

Like SFP transceiver modules, DAC and AOC cables have EEPROM coding. A Cisco DAC works in a Cisco switch. A Juniper switch or FortiGate appliance may reject it or operate with a warning. Recommendation: purchase DAC/AOC cables from the same vendor as the switches at both ends to avoid compatibility issues. If the two endpoints are from different vendors, purchase a third-party "compatible" DAC (Flexoptix, ProLabs) programmed with the appropriate coding for both vendors.

OPTINOC Recommendations

• Spine-leaf data center project with 100 servers: ~200× 3m SFP28 DAC cables + ~50× 3m QSFP28 DAC cables. Budget: ~€25K.
• Data center expansion using breakout cables: use QSFP-4SFP25G-CU3M to connect 4 servers to 1 spine port. Savings: 3 switch ports × $500 = $1,500 per breakout cable.
• Short campus uplinks: 10m 40G QSFP+ AOC between the distribution and core layers.

OPTINOC supplies Cisco, Juniper, HPE, Fortinet, and Mellanox/NVIDIA DAC/AOC cables with manufacturer warranties. Products are in stock, with delivery in 24-72h. Volume discounts are available for orders of 20 cables or more.