Engineering Guide
Communication Link Throughput, Latency, USB, and PoE Power Budgeting
A link succeeds only when useful data arrives before its deadline and powered endpoints receive adequate voltage and power. This guide joins data-plane and power-plane budgets without confusing protocol efficiency with electrical efficiency.
System-Level Link Workflow
| Step | Action | Engineering Check |
|---|---|---|
| Define useful work | State payload bytes, transaction frequency, deadline, direction, and power required at the endpoint. | |
| Declare units | Choose decimal or binary data units and distinguish bits/s from bytes/s. | |
| Account for overhead | Include framing, headers, gaps, acknowledgements, encoding, retries, and application records. | |
| Build latency | Add serialization, propagation, processing, queueing, scheduling, and turnaround delays. | |
| Check concurrency | Model arbitration, shared media, bursts, buffers, and full- versus half-duplex behavior. | |
| Budget delivered power | Start from negotiated source limits, then subtract cable, connector, conversion, and allocation losses. | |
| Test corners | Measure smallest and largest messages, congestion, loss, startup, cable length, temperature, and supply tolerance. | |
| Keep margin | Reserve throughput, latency, current, voltage, and thermal margin for growth and uncertainty. |
Payload Throughput and Transfer Time
Formula reference
Useful data rate
Payload efficiency = payload bits/total transmitted bitsEffective throughput = line rate x payload efficiency x utilizationTransfer time = payload bits/effective throughputUtilization = measured throughput/line rateVariable definitions
- Line rate is not application throughput
- Encoding, framing, acknowledgements, retries, and gaps consume capacity
- Use the Data Rate Converter only for consistent units
The Network Throughput Calculator supports first-pass efficiency and transfer estimates. The Data Rate Converter handles bits, bytes, SI, and IEC units but does not model protocol behavior.
Latency Budget
| Delay | First-Order Model | What Sets It |
|---|---|---|
| Serialization | Message bits / line bit rate | Frame size, encoding, protocol overhead |
| Propagation | Distance / propagation speed | Cable/fiber velocity factor and physical path |
| Processing | Measured or specified device time | PHY, switch, stack, firmware, application |
| Queueing | Traffic- and scheduler-dependent | Load, priority, buffering, arbitration |
| Turnaround | Protocol/device dependent | Half-duplex direction change and response preparation |
| Round trip | Forward + remote + return | Not always exactly twice one-way delay |
Formula reference
One-way latency
Serialization delay = transmitted bits/line bit ratePropagation delay = path length/propagation speedOne-way latency = serialization + propagation + processing + queueing + turnaroundSimplified symmetric RTT = 2 x one-way latencyVariable definitions
- Queueing is workload dependent
- Real RTT includes remote response and potentially asymmetric paths
- Report distributions and worst-case conditions, not only averages
Use the Communication Latency Calculator to organize deterministic terms. Protocol retries, OS scheduling, congestion, and application processing require measurement or higher-level models.
USB Power Budget
Formula reference
USB delivery model
Source power = Vsource x ICable drop = I x RloopCable loss = I squared x RloopLoad voltage = Vsource - cable dropRequired source power = load power/efficiencyVariable definitions
- Current and voltage depend on advertised or negotiated capability
- Rloop includes outgoing and return conductors
- The calculator is not a USB PD negotiation model
Use the USB Power Budget Calculator for source, cable, load, and multi-device planning. Verify source, sink, cable e-marker, hub, connector, startup, and thermal limits.
Ethernet PoE Power Budget
Formula reference
PoE delivery model
PSE power = VPSE x ICable loss = I squared x RloopPD input power = PSE power - cable lossUsable load power = PD input power x conversion efficiencySwitch budget = sum of allocated port power + system marginVariable definitions
- Applicable IEEE class and pair configuration set real limits
- Cable resistance varies with length, conductor, temperature, and pairing
- A simplified budget is not standards compliance proof
The Ethernet PoE Power Calculator supports source, cable, PD, and multi-port budget estimates. Check classification, negotiation, cable bundle temperature, switch total budget, and endpoint startup.
Protocol Scope Boundary
The Serial Interface Timing, Pull-Ups, Termination, and Cabling Guide owns UART, I2C, SPI, CAN, RS-232, and RS-485 physical and timing decisions. This page owns end-to-end useful-data, delay, USB-power, and PoE-power budgeting.
Worked Examples
Payload throughput
A 100 Mbps link at 80% payload efficiency carries 80 Mbps useful data in the simplified model.
Transfer time
A 100 MB decimal payload at 80 Mbps takes 100 x 8 / 80 = 10 seconds before startup and application overhead.
Serialization delay
A 1500-byte frame at 100 Mbps takes 1500 x 8 / 100e6 = 120 us to serialize, excluding extra link overhead.
Propagation delay
A 100 m cable at 0.66c adds about 505 ns one way; processing may dominate on short links.
Latency budget
120 us serialization + 0.5 us propagation + 40 us processing + 200 us queueing gives 360.5 us one way.
Small-packet overhead
If a 64-byte payload needs 30 bytes of total overhead, payload efficiency is 64/(64+30) = 68.1%.
USB cable drop
A 5 V, 2 A load through 0.2 ohm round-trip cable resistance loses 0.4 V and 0.8 W; the load receives 4.6 V.
USB source budget
Four 2 W devices require 8 W load power. At 85% delivery efficiency, source demand is 9.41 W before startup margin.
PoE delivery
At 50 V and 0.6 A, source power is 30 W. A 4-ohm loop loses 1.44 W, leaving 28.56 W before conversion loss.
Multi-port PoE
Eight endpoints allocated 15 W each require 120 W delivered budget plus switch conversion and system margin.
Common Mistakes
Related Calculators
Related Resources
Support reference
FAQ
What is the difference between bandwidth and throughput?
Bandwidth is link capacity; throughput is the useful rate achieved after overhead, utilization, retries, and implementation limits.
How do I estimate effective throughput?
Multiply line rate by a justified payload efficiency for first-pass planning, then validate with realistic packet sizes, traffic, endpoints, and measurement tools.
How do I calculate transfer time?
Convert payload to bits and divide by effective payload throughput. Add setup, request, response, storage, or application time when those are part of the transaction.
What contributes to communication latency?
Serialization, propagation, processing, queueing, scheduling, arbitration, protocol turnaround, retries, and application work can all contribute.
Why can a high-bandwidth link still have high latency?
Higher bandwidth reduces serialization time, but distance, processing, buffering, scheduling, congestion, and protocol exchanges can still dominate.
Is round-trip time always twice one-way latency?
No. Forward and return paths, processing, queueing, and response generation may differ. Twice one-way is only a simplified symmetric estimate.
How is USB cable voltage drop calculated?
Use the total round-trip power-path resistance: voltage drop equals current times resistance, and cable power loss equals current squared times resistance.
Does USB Type-C automatically provide the maximum current?
No. Available current and voltage depend on source, sink, cable, CC advertisement, and for USB PD, successful profile negotiation.
How is PoE cable loss calculated?
For a simplified DC model, use current squared times effective loop resistance. Pair configuration, class, standard limits, temperature, and cable construction also matter.
What is the difference between PSE and PD power?
PSE power is supplied at the source end; PD power is available at the powered device after cable and connector losses, subject to the applicable PoE classification.
Can one efficiency percentage represent a whole communication system?
Only if its boundary is declared. Protocol payload efficiency and electrical power efficiency are different metrics and should not be combined casually.
How should I validate a communication budget?
Measure payload throughput, latency distribution, loss/retries, endpoint utilization, delivered voltage, current, temperature, and startup behavior at realistic corners.
Summary
Separate line rate from useful throughput, decompose latency into physical and implementation terms, and treat USB or PoE power as a negotiated source-to-load budget. First-order equations expose missing margin; realistic traffic and powered-hardware tests establish actual performance.