EECS 3213 (Priority Deck)

What is circuit switching?

Establishing a dedicated end-to-end circuit before transmitting.

What is packet switching?

Splitting data into packets and transmitting them over shared network links without reserving a dedicated path.

What defines connectionless packet-switched service?

Packets can be transmitted without establishing a connection between the sender and receiver beforehand.

Circuit vs packet switching: key trade-off?

Circuits reserve capacity and can offer QoS; packets use capacity efficiently but may incur delay or loss.

Large vs small packets: main trade-off?

Large packets reduce header overhead (i.e. require fewer headers for the same amount of data transmitted) but make a retransmission costlier if lost or corrupted.

How does a bus topology operate?

Devices share a backbone; frames propagate along it. Simultaneous sends can collide.

How does a ring topology regulate access?

Each device connects to two neighbours; a circulating token grants permission to transmit.

Star topology: hub vs switch?

Hub broadcasts to all connected stations; switch forwards known frames only toward the destination.

What happens if a link fails in mesh, star, bus, or ring?

Mesh: alternate links remain. Star: one device affected. Bus backbone: whole LAN affected. Ring: may break ring.

What does IP best-effort delivery mean?

IP tries to forward packets but does not guarantee delivery, correct order, or freedom from corruption.

How is an IP address divided conceptually?

Net ID identifies the destination network; Host ID identifies a host within that network.

Why can routers use Net ID instead of a route for every host?

Hosts on the same network share a Net ID, so routers can use one routing entry for the entire network instead of one per host.

Why use network layering?

Abstract away underlying technologies so applications can use common services across different networks.

What is a networking protocol?

Rules and conventions governing communication between peer entities at the same layer.

What is a service in a layered architecture?

A function provided by one layer to the layer immediately above it.

What are peer processes?

Corresponding entities at the same network layer on different machines.

SDU vs PDU?

SDU: data passed down by the upper layer. PDU: that data plus the current layer’s protocol information.

What is encapsulation?

Each downward layer adds its own header or control information around the data.

Name the seven OSI layers, top to bottom.

Application, Presentation, Session, Transport, Network, Data Link, Physical.

Name the five practical TCP/IP layers, top to bottom.

Application, Transport, Network, Data Link, Physical.

What is the Physical Layer responsible for?

Transmitting bits as physical signals, including defining their signal levels, bit interval, and data rate.

What is the Data Link Layer responsible for?

Hop-to-hop frame delivery over a link, including framing, MAC addressing, error control, flow control, and access control.

What does Medium Access Control (MAC) do?

Decides which station may transmit when several share the same medium.

Data Link vs Network Layer: delivery scope?

Data Link moves frames across one link/hop; Network routes packets across multiple links/networks.

What functions belong to the Network Layer?

Logical addressing, routing, congestion control, and fragmentation/reassembly.

Network vs Transport: where is delivery targeted?

Network gets packets to the destination host; Transport gets data to the destination process.

Which addressing mechanism reaches the correct application on a host?

Transport-layer port addressing (the destination port number).

MAC vs IP vs port address: what does each identify?

MAC: local network interface (what allows you to connect to a network, such as wifi adapter or ethernet port); IP: host/network across the Internet; port: process/application on that host.

Why can't routers efficiently use MAC addresses for WAN routing?

MAC addresses identify devices on a LAN but don't indicate their network's location. Routes cannot be efficiently grouped by network, making routing tables too large.

How do Data Link and Transport Layer flow/error control differ?

Data Link control operates hop-by-hop between neighbouring devices, while Transport control operates end-to-end between the sending and receiving processes.

What are the data-unit names from Transport down to Physical?

Transport: segment; Network: packet; Data Link: frame; Physical: bit.

Which stack layers does a router normally implement?

Physical, Data Link, and Network (three layers in the 5-layer model).

Which addresses do TCP, IP, and Data Link headers carry?

TCP: ports; IP: source/destination IP addresses; Data Link: source/destination MAC addresses.

Do hosts on different LAN technologies need to understand each other’s Data Link protocols?

No. Each uses its local link technology; IP connects the dissimilar networks.

ipconfig vs ipconfig /all?

ipconfig displays basic network settings (IP address, subnet mask, gateway). ipconfig /all displays additional details, including MAC addresses, DNS servers, and DHCP information.

What do ping and tracert test?

ping: whether a destination is reachable and round-trip delay.
tracert: identifies routers (hops) along the path to a destination and measures round-trip times to those hops.

Analog vs digital DATA?

Analog data has continuously varying values; digital data has discrete values.

Analog vs digital SIGNAL?

Analog signal: continuous amplitudes. Digital signal: limited discrete signal levels.

What are the three sine-wave parameters?

Peak amplitude A, frequency f, and phase φ (offset relative to time zero).

What phase puts a sine wave at its positive maximum at t = 0?

+90° or +π/2 radians.

What does a periodic vs nonperiodic composite spectrum look like?

Periodic: discrete frequency spikes. Nonperiodic: a continuous range of frequencies.

What are the axes of time-domain vs frequency-domain plots?

Time domain: amplitude vs time. Frequency domain: amplitude vs frequency.

In a frequency-domain spike plot, what do location and height mean?

Horizontal position = component frequency; spike height = peak amplitude.

How is signal bandwidth defined?

Highest frequency minus lowest frequency present: B = f_high − f_low.

What is the frequency bandwidth of a pure sine wave?

0 Hz: it contains just one frequency, regardless of the frequency value.

What is the theoretical bandwidth of an ideal digital signal?

Infinite: abrupt changes (in a signal’s amplitude over time) require an unlimited set of frequency components.

Signal bandwidth vs channel bandwidth?

Signal: frequencies the signal contains. Channel: frequencies the medium passes satisfactorily.

Low-pass channel vs band-pass channel?

Low-pass passes from 0 Hz to a cutoff; band-pass passes a range starting above 0 Hz.

Three main signal impairments?

Attenuation (loss of strength), delay distortion (shape changes), and noise (unwanted signals).

What does the sign of a power change in dB mean?

Negative dB = power loss; positive dB = gain; 0 dB = no change.

dB vs dBm vs dBW?

dB: how much signal power has increased or decreased compared to another power level. dBm: absolute power referenced to 1 mW. dBW: absolute power referenced to 1 W.

What is delay distortion?

Different frequencies arrive with different delays, changing signal shape and causing intersymbol interference (when one transmitted symbol can overlap with the next, making it harder for the receiver to distinguish them).

side note: symbol = one unit of information transmitted as a signal. Usually represented by a pulse or particular signal state.

What does signal-to-noise ratio (SNR) measure?

Useful signal power divided by noise power; higher SNR generally means clearer transmission.

What transmission uses low-pass vs band-pass channels?

Baseband digital transmission: low-pass. Broadband/carrier-based transmission: band-pass.

side notes:
Baseband: sends digital signals directly through medium, without shifting them onto a carrier frequency.

Broadband: uses a high frequency carrier wave to transmit data through modulation.

How are digital data and analog data sent as digital signals?

Digital data → line coding; analog data → PCM (digitization).

What methods support digital transmission of digital data?

Line coding maps bits to digital signals; block coding adds redundant bits before line coding.

What is line coding?

Mapping a stream of binary data bits onto discrete digital signal levels.

What are the two big design concerns for line coding?

Avoid unwanted DC components and maintain clock synchronization.

Side notes:

• DC component: Nonzero average voltage. Undesirable because some equipment blocks DC, and it wastes energy.

• Clock synchronization: Long sequences of identical bits may lack signal transitions, making bit boundaries unclear (e.g., 8 vs. 9 bits).

What is unipolar line coding, and its weakness?

Represents bits using 0 V and one nonzero voltage (e.g., +5 V). Causes unwanted DC components and poor synchronization during long sequences of identical bits.

NRZ-L vs NRZ-I: what encodes bits?

NRZ-L: The voltage level represents the bit (e.g. +5V = 0, -5V = 1).

NRZ-I: A voltage transition represents 1; no transition represents 0.

Side notes:

• Both are polar line-coding schemes (digital data → digital signal).

• NRZ-L: Long sequences of identical bits cause synchronization problems.

• NRZ-I: Transitions on 1s help synchronization, but long runs of 0s remain problematic.

• NRZ = “Non-return to zero”

Which bit runs cause synchronization problems in NRZ-L and NRZ-I?

NRZ-L: long runs of either bit. NRZ-I: long runs of 0s (since voltage only changes when the bit = 1)

What is RZ line coding, and what is its trade-off?

RZ (Return-to-Zero) is a polar line-coding scheme that uses positive/negative voltage levels to represent bits, returning to 0 V halfway through each bit. It provides good synchronization but requires more bandwidth.

What is Manchester encoding, and what is its trade-off?

Manchester is a polar line-coding scheme where a mid-bit voltage transition represents each bit:

• 0: Positive → Negative

• 1: Negative → Positive

Provides reliable clock synchronization but requires greater bandwidth.

What is Differential Manchester, and how does it encode 0 vs. 1?

Differential Manchester is a polar line-coding scheme that always has a mid-bit voltage transition for synchronization.

• 0: Additional voltage transition at the beginning of the bit.

• 1: No additional transition at the beginning.

How does AMI coding encode bits?

AMI (Alternate Mark Inversion) is a bipolar line-coding scheme that uses three voltage levels. How it works:

• 0: Zero voltage.

• 1: Alternating positive and negative voltages (e.g. +-5V)

Requires less bandwidth than Manchester, but long runs of 0s can cause synchronization problems.

What is AMI’s main synchronization weakness?

A long run of 0s has no transitions, so the receiver may lose bit timing.

What does B8ZS scrambling do?

Replaces eight consecutive AMI zeros with 000VB0VB to introduce timing transitions.

What is 2B1Q line coding and its trade-off?

2B1Q (2 Binary, 1 Quaternary) is a multilevel line-coding scheme that represents pairs of bits using one of four voltage levels.

Advantages: Reduced bandwidth requirements.

Disadvantages: Harder to distinguish voltage levels (more noise-sensitive), and long runs of identical bit pairs cause synchronization problems.

Line coding vs block coding?

• Line coding: Converts a stream of bits into a digital signal.

• Block coding: Groups bits and maps them to codewords containing extra (redundant) bits for synchronization and error detection, before line coding.

How does 4B/5B block coding help NRZ-I?

Replaces each group of 4 bits with a 5-bit codeword, ensuring no more than 3 consecutive zeros occur. This creates enough voltage transitions for NRZ-I to maintain synchronization.

Data rate vs signal (baud) rate?

Data rate = bits per second; baud rate = signal elements per second. One element may encode several bits.

Which two methods digitize analog data for digital transmission?

Pulse Code Modulation (PCM) and Delta Modulation (DM).

What is PCM used for?

Converting analog data into digital bits through sampling, quantization, and binary encoding.

What are the three stages of Pulse Code Modulation (PCM)?

Sampling in time → quantization of amplitude → binary encoding.

Side note (how it works):

1. Sampling: Measure the analog signal's amplitude at regular time intervals.

2. Quantization: Round each measurement to the nearest permitted amplitude level.

3. Binary encoding: Convert each quantized value into a binary code (0s and 1s).

What is the Nyquist sampling rule?

To reproduce a band-limited analog signal, sample at least twice its highest frequency: f_s ≥ 2f_max.

Why is bandwidth alone insufficient to sample a band-pass signal?

Sampling depends on f_max, not just bandwidth; you must know where the band ends.

What happens when an analog signal is undersampled?

Aliasing. When an analog signal is sampled below the Nyquist rate, its reconstructed waveform can appear to have the wrong frequency.

What is quantization in PCM?

Rounding each sampled amplitude to its nearest allowed discrete level.

How many quantization levels are possible with m bits per sample?

M = 2^m levels; equivalently m = log₂(M) bits per sample.

What is quantization error/noise?

Difference between the original sample and the rounded quantized value; prevents exact recovery.

How is PCM bit rate determined?

Samples per second × bits per sample. (fs x m).

PCM vs Delta Modulation (DM)?

PCM encodes absolute sample amplitudes; DM encodes whether a stepwise approximation moves up or down.

How does Delta Modulation represent each sample?

Using 1 bit to move a staircase approximation of the analog signal up (1) or down (0) by a fixed step.

What two errors can affect Delta Modulation?

Slope-overload distortion (if steps cannot follow fast changes); quantization noise (from fixed-step approximation).

Bit interval vs bit rate?

Bit interval is the time required to send a single bit (s). Bit rate is the number of bits transmitted per second (bps).

Data Rate

Number of data elements (bits) sent in 1 second. unit: bps

Signal Rate

Number of signal elements (pulses) sent in 1 second. unit: baud

Describe Modulation

The process of converting digital data or a low-pass analog signal to band-pass (higher frequency) analog signal.

Types of Digital-to-Analog Modulation

Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK), and Quadrature Amplitude Modulation (QAM).

What is Quadrature Amplitude Modulation

Varying the amplitude and phase of a carrier wave based on the digital information to be transmitted.

What is Amplitude Key Shifting (ASK)

Digital modulation that represents bits by varying carrier amplitude (commonly 1 = wave present, 0 = absent), while frequency and phase remain constant.

Simple to implement/demodulate.
Highly susceptible to noise (since noise affects amplitude).

What is Frequency Shift Keying (FSK)

Digital modulation that represents bits using different carrier frequencies (0 = f1, 1 = f2), while phase and amplitude remain constant.

More resistant to noise than ASK, but requires roughly twice the bandwidth.

What is Phase Shift Keying (PSK/BPSK), and what are its implications

Digital modulation that represents bits by changing the carrier’s phase (e.g. 1 = 0 degrees, 0 = 180 degrees), while amplitude and frequency remain constant.

Better noise resistance than ASK while requiring similar bandwidth.

Demodulation is more complex (since phase differences are compared to a reference wave).

What is Quadrature Phase Shift Keying (QPSK)

Digital modulation that uses four carrier phases separated by 90 degrees to represent 2 bits per signal element.

Twice the bit rate of BPSK at the same baud rate, with approximately the same bandwidth.

More phases to distinguish → more susceptible to noise.

Describe a Constellation Diagram

Visually represents the possible signals a modulation scheme can represent.

What Three Factors Determine the Maximum Data Rate of a Channel?

The bandwidth of the channel, the quality of the channel (noise levels), the number of distinct signal levels used.

State Nyquist’s Law

In a noiseless channel, greater bandwidth and more signal levels allow higher bit rates. More levels are harder to distinguish at the receiver (more susceptible to noise).

State Shannon’s Theorem

Establishes the maximum achievable bit rate of a noisy channel based on bandwidth and SNR, regardless of the number of channels used.

Data Link Layer (Main Goal)

Provide reliable communication over the physical layer interface.

Data Link Layer (Responsibilities)

Framing, physical addressing, error control, medium access control, flow control.