Library / Computer / CMP-04
Computer
Transfer and propagation delay
Time to send a file or message over a link of given rate and length.
- Ref
- CMP-04
- Rev
- v1
- Field
- Computer
- Result
- t [ms]
B message size in bits, Rb bit rate, d distance and vp the signal speed in the medium (about 200 000 km/s in fibre, 300 000 km/s in air or vacuum). Queueing, protocol and processing delays are not included.
Sending a message takes the time to push all its bits onto the link plus the time for the signal to travel along it. Big files are limited by the bit rate; short messages over long distances are limited by the distance.
Calculator
- Transmission time800 ms
- Propagation time25 ms
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- Small messages are dominated by propagation, big files by transmission. Geostationary satellite links add about 240 ms each way regardless of bit rate.
- Doubling the bit rate halves only the transmission part. Propagation is fixed by physics.
- Bytes and bits are different: multiply bytes by 8, and keep megabit and megabyte straight.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (ms) | Local step | Change in result |
|---|---|---|---|---|---|---|
| B Message size | 0.001 to 10,000 MB | 25.08 | 800,000 | 800,000 | +10% | +9.7 % |
| Rb Bit rate | 0.1 to 10,000 Mbit/s | 33 | 800,000 | 800,000 | +10% | −8.82 % |
| d Distance | 1 to 40,000 km | 800 | 1,000 | 200 | +10% | +0.303 % |
| vp Signal speed | 100 to 300 km/ms | 816.7 | 850 | 33.33 | +10% | −0.275 % |
Sweep of Bit rate
| Bit rate (Mbit/s) | Total delay (ms) |
|---|---|
| 0.1 | 800,000 |
| 0.4217 | 189,700 |
| 1.778 | 45,010 |
| 7.499 | 10,690 |
| 31.62 | 2,555 |
| 133.4 | 624.9 |
| 562.3 | 167.3 |
| 2,371 | 58.74 |
| 10,000 | 33 |
Sweep one variable
Variables and how each one contributes
Total delay
What the equation gives you. Everything else feeds it.
Message size
Size of the message (1 MB = 8 million bits). Scales the transmission part.
Bit rate
Link bit rate. Inversely proportional for the transmission part.
Distance
Length of the path. Adds distance / signal speed.
Signal speed
Propagation speed: 200 in fibre, 300 in free space. Inversely proportional for the propagation part.
Worked example with the values above
- Transmission: 10 MB × 8 = 80 Mbit, divided by 100 Mbit/s = 800 ms.
- Propagation: 5,000 km / 200 km/ms = 25 ms.
- Total delay = 825 ms.
Assumptions and limits
- Single hop; no queueing, protocol or processing delay.
- Constant bit rate and signal speed.
- Decimal megabytes (10⁶ bytes).
Common mistakes
- Confusing bytes with bits.
- Using speed of light in vacuum for a fibre link.
- Forgetting the overheads of headers and error correction.
Statistics and uncertainty
Real links add queueing and protocol delay that varies packet to packet (jitter); measure delay several times and report the mean and the 95th percentile. The calculation gives the minimum possible delay.
Where it is used
- Planning data-link and satellite communications.
- Estimating file transfer times in the field.
- Time-critical networks (fire control, sensor fusion).
Maths you need
- Units: bits, bytes and prefixes
- Speed = distance/time: propagation delay