Connected but not right
A video call freezes and the voice breaks up. The link light is green. The cable is plugged in. Nothing is broken, yet nothing works well.
These are performance faults. The link is alive but the signal or the timing is wrong. Tools and counters show them, if you know what to read.
Key words
| Word | What it means |
|---|---|
| Attenuation | the loss of signal strength as it travels |
| Latency | the delay between sending and receiving |
| Jitter | delay that varies from one packet to the next |
| Crosstalk | signal leaking from one pair of wires into another |
| EMI | electromagnetic interference from nearby electrical kit |
| Bottleneck | one slow part that holds back the whole link |
| Polarity | which fibre sends and which receives at each end |
| CRC | a check number on each frame that shows damage |
| Benchmark | a measured figure that shows how well a link should perform |
| Packet | a small labelled piece of a message |
| Datagram | a packet sent on its own, with no reply expected |
| Sequence number | a count that puts packets in order |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can name the technical faults, say how each is detected, and compare a result with a benchmark.
Starter: physical faults
Have a go
Starter: physical faults
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Technical faults and their causes
| Fault | What it is | Common cause |
|---|---|---|
| Attenuation | The signal is too weak when it arrives | A run too long, bends, dirty joins |
| Latency | A long delay | Too many devices, a slow link, a busy device |
| Jitter | Delay that keeps changing | A busy link, queues, poor cable |
| Crosstalk | A pair leaks into another | Untwisted wires at a plug, split pairs |
| EMI | Noise from nearby kit | Cable beside power cable or a motor |
| Transceiver mismatch | Two modules that do not suit each other | Wrong wavelength or speed |
| TX and RX reversed | Send and receive swapped, called a polarity or fibre mismatch | A duplex fibre pair crossed the wrong way |
| Bottleneck | One part holds back the link | A slow port or an old patch cord |
| Hardware errors | A device itself is faulty | A failing port or power fault |
The cells students fill in are left blank in this preview.
Status lights
The first clue is the LED lights on a port. No light often means no link. A steady light means a link. A flickering light shows traffic. A light of a different colour may show a slower speed or an error.
Read the maker's sheet for what each colour means. Then go to the counters.
How performance failures are detected
A device counts what goes wrong. A rising CRC error count shows damaged frames. Frame loss and dropped packets show data that never arrived. A count of dropped datagrams shows the same for datagrams.
Address conflicts show two devices with one address. Missing sequence numbers show packets that were lost on the way.