What is one difference between a QSFP28 LC BiDi transceiver and a QSFP28 MPO SR4 transceiver?

Question

Options

• AThe BiDi transceiver uses fewer fiber strands than the SR4 transceiver
• BThe BiDi transceiver supports splitting the bandwidth into multiple lower bandwidth links
• CThe BiDi transceiver requires single-mode fiber, while the SR4 transceiver supports multi-mode
• DThe BiDi transceiver supports lower bandwidth than the SR4 transceiver

• A
  94% (30)
• B
  3% (1)
• C
  3% (1)

Explanation

Option A is correct because a QSFP28 LC BiDi (Bidirectional) transceiver uses only 2 fiber strands - it sends and receives on the same fiber pair using different wavelengths (WDM). A QSFP28 MPO SR4 transceiver uses 8 fiber strands (4 transmit + 4 receive lanes) via an MPO connector, one dedicated strand per lane.

Why the distractors are wrong:

• B is incorrect - splitting into multiple lower-bandwidth links (breakout) is a feature of the SR4/MPO architecture, not BiDi. An MPO cable can break out into 4x25G lanes; BiDi cannot.
• C has the relationship backwards - SR4 uses multimode fiber (OM3/OM4), and while some BiDi variants exist in both single-mode and multimode flavors, this choice incorrectly frames SR4 as the multimode-capable one when it requires multimode.
• D is incorrect - both are QSFP28 form factor, meaning both deliver 100Gbps. They differ in how they achieve that bandwidth, not the total bandwidth itself.

Memory tip: Think "BiDi = 2 directions, 2 fibers." SR4 = "4 lanes out, 4 lanes in" = 8 fibers. The more lanes, the more fiber strands needed.

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