What happens when a tracker loses coverage?
A tracker that cannot reach its server keeps recording. It writes each position and event to its own memory, a buffer, and sends the stored records when the network comes back. This is called store and forward. The question is how long the buffer lasts before it is full.
How many records does a device write in an hour?
Most trackers write a record when the first of several rules fires: a time interval, a distance, or a change of heading. Events add records of their own. While parked, many devices write much less.
Worked example
- Moving: 3600 / 30 = 120 records an hour by time, 60 x 1000 / 500 = 120 by distance. The faster rule gives 120, plus 20 events = 140 an hour.
- Parked: 3600 / 3600 = 1 an hour. With 40% of the gap moving: 140 x 0.4 + 1 x 0.6 = 56.6 an hour.
- Capacity: 2 MB x 1,048,576 x 0.9 / 64 bytes = 29,491 records.
- Buffer: 29,491 / 56.6 = 521 hours, about 21.7 days. A 48-hour gap is covered with a wide margin.
- Switch to one record a second while moving and the rate becomes 3,620 an hour moving and 1,448.6 during the gap. The buffer drops to about 20 hours and a 48-hour gap overflows.
What happens when the memory is full?
| Behaviour | What is lost | What to check |
|---|---|---|
| Overwrite the oldest | The start of the gap. The latest records, near the end of the trip, are kept. | Whether alarms are kept apart from positions. |
| Stop writing | The end of the gap, often the part you need for an incident. | Whether the device warns that the memory is full. |
How long does the catch-up take?
Stored records leave in packets. Each packet adds headers, so the data on the air is a little more than records times record size. At the edge of coverage the signal is weak and the speed is low, so a long backlog can take minutes. Short stops in coverage may end before the upload does.
Questions for the device maker
- How many records, or how much memory, does the model keep for positions and events?
- How large is one record with the inputs and sensors we use?
- When the memory is full, does it overwrite the oldest records or stop?
- Are alarms kept in a separate area so they are not overwritten?
- Can the reporting rules change by themselves when there is no coverage?
- In what order are stored records sent when coverage returns, and how many per packet?
Assumptions and limits
- Memory, record size, packet size and upload speed are typical values, not Tracom specifications. Replace them with your datasheet and your own measurements.
- 1 MB is taken as 1,048,576 bytes. A datasheet that means 1,000,000 bytes gives about 5% fewer records.
- The rate is an average. A trip with many events or stops writes more records than the average suggests.
- A buffer does not replace coverage for live alarms. A panic alarm in a dead zone reaches the control room only when the signal returns, unless another link carries it.
- Everything is calculated in your browser. If you ask for the emailed report, your inputs are sent to our server with the request and kept for up to 45 days.