High-Speed Cable Basics: How a Cable's Name Is Defined
After about seven years in optical communications, working on high-speed cable products such as QSFP, USB and HDMI, I want to briefly explain how a cable's name is actually defined.
The question customers asked me most often was:
"Do you have a QSFP28 100G product?"
"Do you make USB Type-C?"
My answer was always another question: what length do you need, and what signal format will it carry?
Whether it is a copper cable or an Active Optical Cable (AOC), a cable's name generally contains the following elements:
1. Connector:
A connector integrates optical fiber or copper into a single plug so users can easily attach the cable to equipment. Most industrial connectors are defined in the SNIA specifications (https://www.snia.org/technology-communities/sff/specifications). For example, the QSFP28 connector, the most common one in data centers, is defined in SFF-8665. That spec defines the connector's packaging, dimensions, and hardware/software functions.
The figure below shows connector types (optical and electrical) commonly used in Ethernet applications
Source: https://ethernetalliance.org/
Beyond industrial connectors, the familiar USB comes in different connector types such as Type-A, Type-B and Type-C, as shown below
HDMI works the same way. Each of these two connectors is specified by its own association:
USB: https://www.usb.org/
HDMI: https://www.hdmi.org/
2. Signal format:
Next comes the signal format. Start with specs we see in everyday life: HDMI 2.0 supports 4K video, while HDMI 2.1 carries even higher-resolution 8K video.
Different associations define signal formats for different applications, and they update their signal-processing schemes year after year to keep up with today's ever-growing data traffic.
Common signal formats include:
HDMI: https://www.hdmi.org/
USB: https://www.usb.org/
Ethernet: https://ethernetalliance.org/
InfiniBand: https://www.infinibandta.org/
PCIe: https://pcisig.com/
Each of these specifications runs to several hundred pages, detailing signal transmission, decoding, test methods and more.
3. Length:
Finally, the most intuitive element: length, which varies by requirement and use case. A 0.5–1.0 m cable is more than enough for a phone, while cables inside a data center can run from hundreds of meters to several kilometers. As mentioned above, as data volumes rise, the bandwidth required of each cable keeps climbing, and optical fiber communication is the only solution for high-bandwidth, long-distance transmission.
Putting it all together, I usually break a product down into three parts: connector + signal format + length. For example, "QSFP28 100GbE Cable, 1m" means
Connector: QSFP28
Signal format: 100G Ethernet
Length: 1 m
Likewise, "USB Type-C HDMI2.0, 5m" means
Connector: USB Type-C
Signal format: HDMI 2.0
Length: 5 m
And the product below has never appeared on the market, but it is technically feasible. Pairing this signal format with this connector simply wastes the connector's pins and would likely add unnecessary cost
QSFP+ HDMI2.0, 5m
Connector: QSFP+
Signal format: HDMI 2.0
Length: 5 m
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