Chapter 4 Flashcards
(35 cards)
What does the data link layer do?
It controls message transmission over physical media, handling media access, message delineation, and error control.
What is media access control?
Media access control determines when a message is sent, especially important in shared circuits to prevent collisions.
When is media access control unnecessary?
In point-to-point full duplex circuits, since both computers can transmit freely without interference.
What is roll-call polling?
The front-end processor polls each terminal in sequence to see if it has data to send.
What is hub polling (token passing)?
A poll is passed from one device to the next on a shared circuit; used in LANs.
What is contention?
Devices transmit when the circuit is free; used in Ethernet networks.
Which is better: controlled access or contention?
Contention is better for low-volume networks; controlled access is better for high-volume networks.
What are the two fundamental types of errors?
Human errors and network errors (corrupted or lost data).
When do errors usually occur?
In bursts — multiple bits are affected at once.
Do lower-speed lines have higher error rates?
Yes, due to less stable transmission conditions and unconditioned dial-up lines.
What is noise?
Undesired electrical or light signals that interfere with data transmission.
What is white noise (Gaussian)?
Thermal noise present in all circuits; usually not harmful unless very strong.
What is impulse noise?
Spikes caused by voltage changes, lightning, or poor connections; major cause of data errors.
What is cross-talk?
Signal interference from adjacent wires or circuits.
What are echoes?
Reflected signals from bad connections; can cause errors.
What is attenuation?
Loss of signal strength over distance; can distort data transmission.
What is intermodulation noise?
Combination of signals forming interference in another frequency band.
What is jitter?
Minute variations in signal phase or amplitude that can distort transmission.
What is harmonic distortion?
Output of a circuit doesn’t match its input, usually due to amplifier issues.
What are phase hits?
Short-term shifts in signal phase, which may cause errors.
How do amplifiers differ from repeaters?
Amplifiers boost analog signals (and noise); repeaters regenerate clean digital signals.
Name three ways to reduce noise.
Shielding wires, separating cables from power sources, tuning/maintaining equipment.
What is parity checking?
Adding a bit to each byte to make the number of 1s either even or odd; detects simple errors.
How does checksum work?
Adds byte values in a message, divides by 255, and compares the remainder at both ends.
