Networks Chapter 9 - Physical Layer

What does the physical layer do?

Converts digital data to signals, establishes/ends connections, and handles flow control and collision recovery.

What is analog bandwidth?

Range between upper and lower frequencies, measured in Hz.

What is digital bandwidth?

Bit rate measured in bit/s (kbit/s, Mbit/s, Gbit/s).

What is modulation?

Varying a carrier's properties to transmit data.

What is AM?

Amplitude Modulation - varies amplitude.

What is FM?

Frequency Modulation - varies frequency.

What is PM?

Phase Modulation - varies phase.

What is ASK?

Amplitude-Shift Keying - discrete amplitude levels.

What is FSK?

Frequency-Shift Keying - discrete frequencies.

What is PSK?

Phase-Shift Keying - discrete phase shifts.

What is QAM?

Combines ASK + PSK to send multiple bits per symbol.

What is PCM?

Pulse-Code Modulation - digitally samples an analog signal.

What is bus topology?

All nodes share one cable; signal sent both directions.

What is star topology?

All devices connect to a central host/switch.

What is ring topology?

Data travels around a loop in one direction.

What is mesh topology?

Every node connects directly to all others.

What is twisted pair?

Two copper wires twisted to cancel EMI; inexpensive; 100 m max.

What is coaxial cable?

Inner conductor + shield; longer runs; used in early Ethernet and cable.

What is fiber optic?

Sends light pulses; very high bandwidth and distance; immune to EMI; costly.

What is Ethernet (802.3)?

10 Mb/s - bus/star, Manchester encoding, CSMA/CD.

What is Fast Ethernet?

100 Mb/s - star, uses 4B/5B and NRZI or PAM-3/5.

What is Gigabit Ethernet?

1 Gb/s - uses PAM-5, FEC, Trellis coding.

What is 10/40/100 Gigabit Ethernet?

Higher speeds over fiber/copper with pluggable PHY modules.

What is Manchester coding?

Each bit has a mid-bit transition; self-clocking.

What is NRZI?

Transition = 1, no transition = 0.

What is RZ?

Signal returns to 0 between bits.

What is NRZ?

No return to 0; fewer transitions.

What is 4B/5B?

Maps 4 bits to 5 to ensure transitions.

What is PAM-5?

Uses five voltage levels (-2 to +2) for 2 bits + FEC.

What is a modem?

Modulator-demodulator that converts digital data to analog signals and back.

What is FDD?

Frequency-Division Duplexing - send/receive on separate frequencies.

What is TDM?

Time-Division Multiplexing - users share a channel in time slots.

What is DSSS?

Direct-Sequence Spread Spectrum - spreads signal using pseudo-random noise code.

What is ATM?

Asynchronous Transfer Mode; connection-oriented WAN using fixed 53-byte cells and time-division multiplexing.

What is 802.11b?

2.4 GHz, 11 Mb/s DSSS.

What is 802.11g?

2.4 GHz, 54 Mb/s OFDM.

What is 802.11a?

5 GHz, 54 Mb/s OFDM.

What is 802.11n?

2.4/5 GHz, MIMO, up to 600 Mb/s.

What is the Hidden Station problem?

Two stations can't hear each other → collisions; fixed by RTS/CTS handshake.

What is the Exposed Station problem?

A station refrains from sending even though it wouldn't interfere; also handled by RTS/CTS.

What does a hub do?

Layer 1; repeats signal to all ports.

What does a bridge/switch do?

Layer 2; forwards based on MAC addresses.

What does a router do?

Layer 3; forwards packets based on IP addresses.