LEARNING OBJECTIVES
What you will be able to do
- describe packets and packet switching
- compare serial, parallel, simplex and duplex transmission
- explain USB and error-detection methods
- distinguish symmetric and asymmetric encryption
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Explain how data travels reliably and securely between devices.
Networks divide data into manageable packets that may travel along different routes before being reassembled. Transmission choices balance speed, distance, cost and direction.
Noise and interception create risk, so systems detect damaged data and encrypt sensitive content.
SECTION 01
Packets and packet switching
The header can contain source and destination addresses and packet number; the payload carries data; the trailer can contain error-checking information. Routers select paths and packets may arrive out of order.
At the destination, packet numbers support reassembly and missing or damaged packets can be requested again. Packet switching shares links efficiently and reroutes around failure, but delay varies and headers add overhead.
SECTION 02
Transmission modes and USB
Serial transmission sends one bit at a time and is reliable over longer distances with fewer wires. Parallel sends several bits together and can be faster over short distances but suffers skew and needs more conductors.
Simplex communicates one way, half-duplex both ways but not simultaneously, and full-duplex both ways simultaneously. USB is a standard serial interface with automatic device recognition, power supply and broad compatibility, but connector types and cable length or bandwidth can limit it.
| Mode | Direction | Example |
|---|---|---|
| Simplex | one direction only | sensor broadcast |
| Half-duplex | both directions, one at a time | walkie-talkie |
| Full-duplex | both directions simultaneously | telephone call |
SECTION 03
Error detection and checking
Parity adds a bit so the count of 1s is even or odd; two-dimensional parity can help locate a single error. A checksum is calculated from a block and compared after transmission. An echo check returns data to the sender for comparison.
These methods detect many errors but do not prove data is correct in every case. Check digits validate manually entered identifiers using a defined calculation; verification methods such as double entry compare input with the source but do not test reasonableness.
Using odd parity
- The seven data bits are 1011001, containing four 1s.
- Odd parity requires an odd total number of 1s.
- Add parity bit 1, producing five 1s in the transmitted byte.
Answer: One valid transmitted pattern is 11011001 when the parity bit is placed first.
SECTION 04
Encryption
Encryption transforms plaintext into ciphertext using a key. Symmetric encryption uses the same secret key for encryption and decryption, so secure key exchange is difficult. Asymmetric encryption uses a public/private key pair.
A sender can encrypt for confidentiality using the receiver's public key, and only the receiver's private key decrypts it. Encryption reduces the value of intercepted data but does not prevent interception, deletion or malware.
| Method | Advantage | Challenge |
|---|---|---|
| Symmetric | fast for large data | secret key distribution |
| Asymmetric | public key can be shared | slower and more processing |
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Packets include routing, sequencing and checking information.
- Transmission method must suit distance and direction.
- Error detection supports retransmission but cannot detect every possible fault.
- Encryption protects confidentiality, not system availability.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- describe packets and packet switching
- compare serial, parallel, simplex and duplex transmission
- explain USB and error-detection methods
- distinguish symmetric and asymmetric encryption