Engineering Reference
USB, Ethernet and Data-Rate Units Reference
Lookup for USB and Ethernet power terminology, PoE PSE and PD limits, bit and byte units, payload throughput, transfer time, propagation delay, latency, and overhead.
- Reading Time
- 12 min
- Format
- Units and interface lookup
- Updated
- September 29, 2026
Versioned Standards Boundary
USB, USB PD, Ethernet and PoE requirements evolve. The project tables are calculation references, not certification limits. Confirm current normative specifications, source/sink declarations, cable ratings and negotiated operating modes.
Data-Rate and Latency Terms
| Term | Unit | Meaning | Boundary |
|---|---|---|---|
| Bit rate / line rate | bit/s | Physical or logical bits transferred per second | Not necessarily useful payload throughput |
| Byte rate | B/s | Bytes transferred per second | 1 B = 8 bits; capitalization matters |
| Payload throughput | bit/s or B/s | Useful application data delivered per second | Excludes some or all framing, headers, gaps, coding and retransmissions |
| Protocol efficiency | % | Payload throughput divided by selected line-rate basis | The basis and included overhead must be stated |
| Bandwidth utilization | % | Traffic rate divided by available link capacity | High average utilization leaves less burst and retransmission margin |
| Transmission delay | s | Data size in bits divided by bit rate | Does not include propagation, queue or processing delay |
| Propagation delay | s | Distance divided by propagation velocity | Independent of packet size |
| Latency | s | End-to-end time under a defined path and load | Not interchangeable with throughput |
| Round-trip time | s | Elapsed time for travel to a destination and response path | Not always exactly twice one-way latency |
| Power budget | W | Source power minus cable, conversion and allocation losses | Negotiation and classification still govern availability |
Decimal and Binary Units
| Symbol | Definition | Exact value | Use |
|---|---|---|---|
| kb/s | 10³ bit/s | 1000 bit/s | Decimal data-rate prefix |
| Mb/s | 10⁶ bit/s | 1,000,000 bit/s | Decimal data-rate prefix |
| Gb/s | 10⁹ bit/s | 1,000,000,000 bit/s | Decimal data-rate prefix |
| kB | 10³ bytes | 1000 B | Decimal storage/data-size prefix |
| KiB | 2¹⁰ bytes | 1024 B | Binary data-size prefix |
| MB | 10⁶ bytes | 1,000,000 B | Decimal data-size prefix |
| MiB | 2²⁰ bytes | 1,048,576 B | Binary data-size prefix |
| GB | 10⁹ bytes | 1,000,000,000 B | Decimal data-size prefix |
| GiB | 2³⁰ bytes | 1,073,741,824 B | Binary data-size prefix |
USB Power Reference Points
| Reference | Voltage | Current | Power | Boundary |
|---|---|---|---|---|
| USB 2.0 | 5 V | 0.5 A | 2.5 W | Legacy USB default power reference. |
| USB 3.0 | 5 V | 0.9 A | 4.5 W | Common SuperSpeed USB current reference. |
| USB 3.1 | 5 V | 0.9 A | 4.5 W | Uses the same basic 5 V, 900 mA reference for simple power budgeting. |
| USB 3.2 | 5 V | 0.9 A | 4.5 W | This calculator treats USB 3.x as a simple power reference, not a protocol-speed model. |
| USB Type-C Default 1.5 A | 5 V | 1.5 A | 7.5 W | USB-C default current reference when the source advertises 1.5 A. |
| USB Type-C 3 A | 5 V | 3 A | 15 W | USB-C current reference when the source and cable support 3 A. |
| USB PD Reference | 20 V | 5 A | 100 W | Power-only USB PD reference. This calculator does not model PDO, RDO, CC logic or negotiation. |
These rows mirror the calculator's current simplified reference set. They do not enumerate every USB PD revision, PDO, EPR mode, cable requirement or negotiation rule.
Ethernet PoE Reference Points
| Standard | Type | PSE reference | PD reference | Boundary |
|---|---|---|---|---|
| IEEE 802.3af | PoE Type 1 | 15.4 W | 12.95 W | Common legacy PoE reference for lower-power devices. |
| IEEE 802.3at | PoE+ Type 2 | 30 W | 25.5 W | Common PoE+ reference for cameras, radios and higher-power endpoints. |
| IEEE 802.3bt Type 3 | 4PPoE Type 3 | 60 W | 51 W | Four-pair PoE reference for higher-power powered devices. |
| IEEE 802.3bt Type 4 | 4PPoE Type 4 | 90 W | 71.3 W | High-power four-pair PoE reference. PD available power is commonly referenced around 71 W to 73 W. |
Canonical Calculation Anchors
| Case | Result | Interpretation |
|---|---|---|
| 8 Mb/s to MB/s | 1 MB/s | Exact bit/byte conversion before protocol overhead |
| 100 Mb/s at 92% efficiency | 92.0 Mb/s payload | Efficiency basis must be stated |
| 100 MB at 100 Mb/s | 8.0 s ideal | Uses decimal 100 MB and excludes overhead |
| 1500 B at 100 Mb/s | 120.0 µs transmission delay | Packet-size-dependent serialization |
| 100 m at VF 0.67 | 0.498 µs one-way propagation | Independent of packet size |
Common Errors
- Confusing b/s with B/s.
- Treating MB and MiB as identical.
- Calling line rate payload throughput.
- Ignoring framing, coding and retransmission overhead.
- Treating bandwidth and latency as interchangeable.
- Assuming higher bit rate changes propagation velocity.
- Assuming USB Type-C guarantees USB PD.
- Ignoring USB cable current and e-marker requirements.
- Using PSE PoE power as PD load power.
- Ignoring cable loss and DC conversion efficiency.
- Mixing one-way delay with round-trip time.
- Using versioned standard values without checking current specifications.
Support reference
FAQ
What is the difference between Mb/s and MB/s?
Mb/s is megabits per second; MB/s is megabytes per second. Ignoring overhead, divide bits per second by eight to obtain bytes per second.
Are MB and MiB the same?
No. MB is 1,000,000 bytes under the decimal SI convention; MiB is 1,048,576 bytes under the binary IEC convention.
Why is payload throughput lower than Ethernet link speed?
Frames, preambles, interpacket gaps, headers, coding, acknowledgments, retransmissions and higher-layer overhead consume part of the selected line-rate basis.
Is USB Type-C the same as USB Power Delivery?
No. Type-C describes a connector and related interface behavior. USB PD is a negotiated power protocol; connector shape alone does not guarantee a power level.
Does a USB cable always support the source's maximum power?
No. Source, sink, cable current capability, e-marker requirements, negotiated contract and voltage drop all constrain usable power.
What is the difference between PoE PSE and PD power?
PSE power is supplied at the sourcing equipment. PD power is the amount guaranteed or available at the powered-device interface after allowed channel loss under the applicable class.
Can I use the PoE class wattage as DC load power?
Not directly. Account for cable loss, conversion efficiency, startup, temperature, class negotiation and device margin.
What is transmission delay?
It is packet or data size in bits divided by link bit rate. It is separate from propagation, processing and queueing delay.
Does higher bandwidth always reduce propagation delay?
No. Propagation delay is distance divided by signal velocity. Higher bit rate reduces serialization time but does not materially increase propagation speed.
Are the displayed USB and PoE values a compliance specification?
No. They are calculator reference points. Verify current USB-IF or IEEE requirements, device declarations, cable ratings and regional safety obligations.
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