Two ways to read a stream
Imagine a friend reads out a long line of numbers. You write them down. How do you know when each number starts?
One way is for both of you to clap the same beat. Another way is for your friend to say "go" before each number. Cables face the same problem.
Key words
| Word | What it means |
|---|---|
| Synchronous | sending with a shared clock, so bits arrive on a steady beat |
| Asynchronous | sending with no shared clock: each byte starts and ends with a marker |
| Bandwidth | how much data a link can carry each second |
| Noise | unwanted signal that mixes with the real signal |
| Compression | storing data in fewer bits so it is faster to send |
| Lossless | compression that can give back the exact original |
| Lossy | compression that throws some detail away for good |
| Start bit and stop bit | markers that wrap a byte in asynchronous sending |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can compare synchronous and asynchronous sending, explain how a cable limits bandwidth and how noise harms a signal, and show how compression saves bits.
Starter: bits and bytes
Have a go
Starter: bits and bytes
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Synchronous and asynchronous
Synchronous sending uses a shared clock. The sender and receiver keep the same beat, so a long stream of bits flows with no gaps. It is fast and fits big transfers.
Asynchronous sending has no shared clock. Each byte gets a start bit and a stop bit. That wastes a little space, but it is simple and cheap.
Synchronous and asynchronous compared
| Feature | Synchronous | Asynchronous |
|---|---|---|
| Clock | Shared by both ends | None shared |
| Markers | Few, at the start of a block | A start bit and stop bit on every byte |
| Speed | Faster, little waste | Slower, 2 extra bits per byte |
| Typical use | Fast links that carry steady streams of frames | Simple serial links, such as a console cable |
The cells students fill in are left blank in this preview.
Bandwidth limits and noise
Every cable can carry only so much each second. That is its bandwidth limit. A thin cable or a long run limits it more. Better cable, such as Cat 6, carries more.
Noise is unwanted signal from motors, lights or other cables. Noise mixes with the real signal. If it is too strong, the receiver reads a 0 as a 1 and the data is damaged.
Compression: fewer bits to send
Compression makes data smaller so it takes less of the bandwidth. Lossless compression gives back the exact original, which a text file needs. Lossy compression drops some detail for good, which a photo or music can bear.
A simple lossless idea is run-length coding. Write "aaaa" as "4a".