The extract that would not open
Priya starts a download of a big sales extract before she leaves for the night. In the morning the file is there, but the analysis program says it is damaged.
Somewhere on the way, pieces went missing or changed. How does a computer notice that, and why do pieces go missing at all?
Key words
| Word | What it means |
|---|---|
| Packet loss | packets that never reach the other end |
| Congestion | too much traffic for a link or device to handle |
| Error handling | spotting that data is damaged and dealing with it |
| CRC | a check value worked out from the data, to spot damage |
| Cyclic redundancy check | the full name of a CRC |
| Bandwidth | how much data a link can carry each second |
| Check value | a short number worked out from data to spot damage |
| Interference | unwanted signals that spoil a link |
| Router | a device that joins networks and passes packets on |
| Packet | a small piece of a message, with its own labels |
| Network | devices joined so they can share things |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can give causes of packet loss and explain how a CRC spots a damaged packet.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Packets go missing for many reasons
Packet loss happens when a packet does not arrive. A little loss is normal. A lot makes a download slow, because the lost pieces must be sent again.
The usual cause is congestion: a router has too much traffic, its queue fills, and it drops new packets. Others are a weak Wi-Fi signal and interference. A faulty cable or port can drop packets. So can a wrong setting on a device. A damaged packet is also thrown away.
Causes of packet loss
| Cause | What happens | A fix |
|---|---|---|
| Congestion | A router queue is full and drops packets | More bandwidth or less traffic |
| Weak wireless signal | Packets are not heard at the other end | Move closer or add an access point |
| Interference | Signals are spoiled and packets are damaged | Better cable or another Wi-Fi channel |
| Faulty hardware | A bad cable, port or device drops packets | Replace the part |
| Wrong settings or bugs | A device sends packets nowhere | Fix the setting or update the software |
| Damage found by a check | A damaged packet is thrown away | The sender sends it again |
The cells students fill in are left blank in this preview.
Extra: Ping your own machine: nothing is lost
ping -c 3 -W 1 127.0.0.1A command for the lesson’s lab terminal. It is shown here and not run.
Extra: Ping an address nobody answers: all is lost
ping -c 3 -W 1 192.0.2.1A command for the lesson’s lab terminal. It is shown here and not run.
Check
Have a go
Check
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.