CAP7001 · Unit 1: Network basics
Data communication: characteristics, components and data representation
10 practice questions · in the MTE and ETE syllabus
Key points
- Five components: message, sender, receiver, medium, protocol
- Four characteristics: delivery, accuracy, timeliness, jitter
- Jitter = variation in arrival delay
- Image bytes = pixels x bits per pixel / 8
Data communication is the exchange of data between two devices over a transmission medium. A system has five components: message, sender, receiver, medium and protocol (the agreed rules).
Four characteristics define quality:
| Characteristic | Meaning |
|---|---|
| Delivery | Data reaches the correct destination only |
| Accuracy | Data arrives unaltered |
| Timeliness | Data arrives in time to be useful |
| Jitter | Variation in packet arrival delay |
Data representation: text (ASCII 7-bit, extended ASCII 8-bit, Unicode), numbers (binary), images (pixels, e.g. 24 bits per RGB pixel), audio and video.
Worked example: a 100 x 100 image at 24 bits per pixel needs 100 x 100 x 24 / 8 = 30,000 bytes.
Exam pitfall: jitter is uneven arrival spacing, not slow arrival. Packets that are all correct but arrive at irregular intervals violate jitter, not accuracy.
Try two questions
Which statement about representing text in a data communication system is correct?
- ASCII uses 16 bits per symbol, so it covers every script, whereas Unicode is meant for English alone
- ASCII uses 7 bits per symbol, whereas Unicode uses longer codes to cover the scripts of many languages
- Unicode uses 7 bits per symbol, whereas ASCII needs longer codes to cover many language scripts
- ASCII and Unicode use the same 8-bit codes, which is why both can represent world scripts equally
Show the answer
B. Standard ASCII is a 7-bit code with 128 symbols, while Unicode uses up to 32 bits per symbol to cover world scripts.
Why not A: ASCII is a 7-bit code and is not able to represent most world scripts.
Why not C: The sizes are swapped here; Unicode is the longer code.
Why not D: The two codes differ in size, and 8-bit codes cannot cover all world scripts.
A 2500-character English text is stored twice, each copy with a 64-byte header: once in 8-bit ASCII (1 byte per character) and once in 32-bit Unicode (4 bytes per character). How many more bytes does the Unicode copy need?
- 7500 bytes
- 7564 bytes
- 10000 bytes
- 2500 bytes
Show the answer
A. The identical headers cancel, leaving 2500 x 4 - 2500 x 1 = 7500 bytes. Adding the header to the difference or quoting the whole Unicode size are the usual slips.
Why not B: That adds one header to the difference although both copies carry the same header.
Why not C: That is the total size of the Unicode body, not the difference between the two copies.
Why not D: That is the size of the ASCII body, not the extra space used by Unicode.