Start with the link scenario
A link within the same rack has different requirements than a connection between halls or buildings. Start with the distance, cable environment and the connectors that already exist. Then choose the optical variant. The cheapest module is not always the most cost-effective solution if it requires new cabling or additional conversions.
Also check the port's support for 100G, breakout, FEC and the current transceiver's encoding. QSFP28 describes the casing; it does not guarantee that all QSFP28 modules are supported in all QSFP28 ports.
DAC and AOC for short connections
Passive DAC is a copper cable with fixed QSFP28 ends. It is often energy-efficient and cost-effective for very short links within or between nearby racks. Limitations include cable thickness, bend radius and the platform's supported lengths.
AOC is an active optical cable where transceivers and fiber cable form a complete unit. It is lighter and can handle longer distances than passive DAC, but the fixed modules cannot be swapped separately. AOC is used for direct point-to-point connections, for example between switches and servers, where a complete optical link is preferred over separate transceivers and fiber patch cables.
SR4 for parallel multimode
100GBASE-SR4 uses parallel transmit and receive lanes over multimode fibre and typically MPO/MTP connectors. It is an established option for short data centre links. Check fibre class, polarity, number of active fibres, cassette structure and whether the link should be able to break out.
MPO links require discipline around polarity and contact integrity. A single contaminated multi-fibre end can affect multiple channels simultaneously. Inspect and clean both sides before connection.
DR and FR for serial singlemode
Single-lambda variants such as DR and FR typically use LC duplex and singlemode fibre. They can simplify the cable infrastructure compared to parallel optics. Exact range and FEC requirements must be obtained from product and platform documentation.
DR is typically used for shorter singlemode links, while FR covers longer data centre and campus distances. Designations close to these families may be standardised or vendor-specific; verify complete item data.
LR4, ER4 and longer ranges
LR4 transmits four optical wavelengths over a LC duplex pair and is typically used for longer singlemode links. ER4 and ZR4-like solutions extend the range further but impose higher requirements on power budget, dispersion, receiver levels, and sometimes attenuation on short links.
For 20–80 km, the design should include actual fibre loss, connectors, splices, temperature, ageing, and any passive components. Also check if the product is standard-based, multi-source agreement-based, or vendor-specific.
A simple selection matrix
- 0–5 metres within rack: start by assessing passive DAC if the platform supports the length.
- Short link where low weight and easy installation are prioritised: assess AOC.
- Short multimode link with MPO infrastructure: assess SR4.
- Short to medium-length singlemode link with LC: assess DR or FR.
- Longer singlemode link: assess LR4, ER4 or another range class after budget calculation.
Purchase check points
- Exact switch, router or NIC model and software version.
- 100G port mode, FEC and any breakout configuration.
- Fibre class, connector, polarity and total channel length.
- Optical power budget and maximum receiver power.
- Commercial or industrial temperature class.
- Coding, DDM/DOM, warranty and spare parts strategy.
Sometimes, but check maximum receiver power and need for attenuators.
No. They are different range and application classes; also check exact standard and FEC requirements.
The standard implementation uses parallel multimode via MPO/MTP. LC-based 100G solutions use other optical architectures.
Next step
Verify switch model, link type and desired range. We help you match QSFP28 module, fibre and connector before you order.