CS601: Data Communication — Complete Cheat Sheet

A Simple Communication Model

A simple communication model consists of a source generating data, a transmitter converting data to signals, a transmission system (medium) carrying signals, a receiver converting signals back to data, and a destination capturing the data.

Effectiveness of a Data Communication System

Depends on four fundamental characteristics:

1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.
2. Accuracy: The system must deliver data accurately. Data altered in transmission and left uncorrected are unusable.
3. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. For video/audio, this means delivering data as produced, in the same order, without significant delay (real-time transmission).
4. Jitter: Variation in the packet arrival time. Uneven delay in delivery of audio/video packets. E.g., if some packets arrive with 30 ms delay and others with 40 ms delay, uneven quality results.

Five Components of a Data Communication System

1. Message: The information (data) to be communicated. Forms: text, numbers, images, audio, video.
2. Sender: The device that sends the data message (computer, workstation, phone, camera, etc.).
3. Receiver: The device that receives the message (computer, workstation, phone, TV, etc.).
4. Transmission Medium: The physical path by which a message travels — twisted-pair wire, coaxial cable, fiber-optic cable, radio waves.
5. Protocol: A set of rules that governs data communications. An agreement between communicating devices. Without a protocol, two devices may be connected but not communicating.

Data Representation

Information comes in different forms:

• Text: Represented as a bit pattern — a sequence of bits (0s and 1s). Different sets of bit patterns are called coding systems (e.g., ASCII, Unicode).
• Numbers: Also represented by bit patterns. Not coded as text; directly converted to binary.
• Images: Composed of a matrix of pixels. Pixel color/intensity encoded into bit patterns (e.g., RGB).
• Audio: Continuous (analog) signal — recorded and converted to digital.
• Video: Sequence of images (frames) displayed in time — either continuous or a combination of images.

Data Flow (Transmission Modes)

Communication between two devices can occur in three modes:

Network Criteria

A network must meet certain criteria:

• Performance: Measured by Throughput (actual data transfer rate) and Delay (time for data to travel). Depends on number of users, type of medium, hardware/software capabilities.
• Reliability: Measured by frequency of failure, time to recover, and network robustness in a catastrophe.
• Security: Protection of data from unauthorized access and damage, implementation of policies for breach recovery.

Physical Structures

• Link: A communications pathway that transfers data from one device to another.
• Point-to-Point Connection: Dedicated link between two devices. Entire capacity reserved for those two devices.
• Multipoint (Shared) Connection: More than two devices share a single link. Capacity shared spatially or temporally.

Physical Topologies

Topology = the geometric representation of the relationship of all links and linking devices (Links + Nodes = Topology). Four basic topologies:

Network Classification

Networks are classified by: Size, Geographical Coverage, and Ownership.

Local Area Networks (LAN)

• Usually privately owned.
• Connects hosts in a single office, building, or campus.
• Can be as simple as two PCs and a printer in someone's home office, or extend throughout a company.
• Each device in a LAN has a host address.

Wide Area Networks (WAN)

• Wider geographical span than LAN — spans a town, state, country, or the world.
• Interconnects connecting devices such as switches, routers, or modems.
• Normally created and run by communication companies (ISPs, telcos).
• Point-to-Point WAN: Connects two devices through a transmission medium (e.g., leased line).
• Switched WAN: Connects multiple end systems through switching nodes (e.g., backbone of the Internet).
• Internetwork: When two or more networks are connected — the result is an internetwork (or internet with lowercase 'i').

Switching

The Internet

• An internet (lowercase 'i') = two or more networks that can communicate with each other.
• The Internet (uppercase 'I') = the worldwide communication system composed of thousands of interconnected networks.
• Users access the Internet through Internet Service Providers (ISPs).

Internet History

• Telegraph and telephone networks before 1960: constant-rate communication only.
• ARPANET — first packet-switched network (early 1970s).
• Birth of the Internet and TCP/IP (presented 1973, officially adopted 1983).
• MILNET — military network split from ARPANET.
• CSNET — Computer Science Network for universities without ARPANET access.
• NSFNET — National Science Foundation Network, backbone for academic and research traffic.
• Internet today — a massive global infrastructure.

Internet Standards & Administration

• Internet Draft: A working document submitted by individuals or groups. No official status yet.
• Request for Comments (RFC): A published document detailing Internet standards and information. Types:
  • Proposed Standard — a stable, well-understood specification.
  • Draft Standard — at least two independent implementations exist.
  • Internet Standard — fully mature specification.
  • Historic — obsolete or superseded.
  • Experimental — for testing purposes.
  • Informational — provides general information.
• Internet administration bodies include ISOC (Internet Society), IAB (Internet Architecture Board), IETF (Internet Engineering Task Force), IRTF (Internet Research Task Force).

Advantages & Disadvantages of Protocol Layering

• Advantages:
  • Modularity: Each layer can be independently designed and updated.
  • Separation of Service & Implementation: Each layer provides a service without exposing how it works.
  • Reduced Complexity & Cost.
• Disadvantages: None really! Protocol layering is universally beneficial.

Two Principles of Protocol Layering

1. Bidirectional Communication: Each layer must perform two opposite tasks, one in each direction.
2. Identical Objects: The two objects under each layer at both communicating sites should be identical.

Logical Connections

Each layer has an imaginary (logical) connection to the same layer at the other side. Data appears to flow horizontally between peer layers, though it physically flows vertically through the stack.

TCP/IP Protocol Suite (5 Layers)

• The protocol suite used in the Internet today.
• Each layer provides specific functionality; hierarchical protocol.
• Presented in 1973 and chosen as the official protocol of the Internet in 1983.

Identical Objects at Each Layer

• Layer 5 (Application): Identical objects are messages.
• Layer 4 (Transport): Identical objects are segments or user datagrams.
• Layer 3 (Network): Identical objects are datagrams.
• Layer 2 (Data Link): Identical objects are frames.
• Layer 1 (Physical): Identical objects are bits.

Encapsulation & Decapsulation

As a message travels down the sender's protocol stack, each layer adds its own header (and trailer at Data Link layer) — this is Encapsulation. At the receiver, as data moves up the stack, these headers are stripped — this is Decapsulation.

This is an important concept in Internet protocol layering. Each layer header contains control information needed by the peer layer at the other end.

Addressing in TCP/IP

Every communication needs at least two addresses: Source Address and Destination Address. The Physical Layer is an exception (no addressing).

The Open System Interconnection (OSI) Model

• Developed by the International Organization for Standardization (ISO), established in 1947.
• Close to three-fourths of countries represented.
• Introduced the OSI Model in the late 1970s.
• OSI is a 7-Layer Model.

OSI Model vs TCP/IP Protocol Suite

• Two layers of OSI are missing from TCP/IP:
  • Session Layer: Establishes, maintains, and synchronizes interactions (dialog control, synchronization points).
  • Presentation Layer: Handles data translation, encryption/decryption, compression, formatting.
• In TCP/IP, the Application layer = Application + Presentation + Session (of OSI).

Why the OSI Model Did Not Succeed

Three reasons OSI did not replace TCP/IP:

1. Bad Timing: OSI was completed when TCP/IP was already fully in place and widely adopted.
2. Bad Technology: Some layers in OSI were not fully defined; the model was complex.
3. Bad Performance: Performance of TCP/IP was better than that of OSI implementations.

Data Communication vs. Computer Networks

Data Communication covers the fundamentals of how data is transmitted: analog & digital transmission, transmission media, switching, error detection and correction, media access and data link control, wired and wireless LANs. Computer Networks focuses on the higher-level protocols and applications built on top.

Communication at the Physical Layer

The physical layer deals with the actual transmission of bits over a physical medium.

Analog vs. Digital Data

• Analog Data: Continuous values — has infinite possible values. Example: analog clock hands, human voice.
• Digital Data: Discrete values — has a limited number of defined values. Example: digital clock, data stored in a computer.

Analog vs. Digital Signals

• Signals represent data — signals can be analog or digital.
• Analog Signal: Infinite levels of intensity over time — a smooth, continuous waveform.
• Digital Signal: Limited number of defined values — a step-function waveform with discrete levels.

Periodic & Non-Periodic Signals

• Both analog and digital signals can be periodic or non-periodic.
• Periodic Signal: Completes a pattern within a measurable time frame; repeats the pattern over identical periods (cycles).
• Non-Periodic Signal: No pattern; changes without repeating.
• In data communications, we use: Periodic ANALOG signals and Non-periodic DIGITAL signals.

Periodic Analog Signals

• Can be simple or composite.
• Simple Periodic Analog Signal: The sine wave — cannot be decomposed into simpler signals.
• Composite Periodic Analog Signal: Composed of multiple sine waves.

Sine Wave — Three Parameters

1. Peak Amplitude (\(A\))

• The absolute value of the signal's highest intensity.
• Proportional to the energy it carries.
• Measured in Volts (for electrical signals).

2. Frequency (\(f\)) and Period (\(T\))

• Period (T): Amount of time required to complete 1 cycle (in seconds).
• Frequency (f): Number of periods (cycles) in 1 second (in Hertz).
• Relationship: \(f = \frac{1}{T}\)   or   \(T = \frac{1}{f}\)

3. Phase (or Phase Shift, \(\phi\))

• Position of the waveform relative to time \(t = 0\).
• Describes the amount of shift; indicates the start of the first cycle.
• Measured in degrees or radians.
• \(0°\): starts at 0 amplitude going up. \(90° = \pi/2\) rad: starts at peak. \(180° = \pi\) rad: starts at 0 going down.

Wavelength (\(\lambda\))

Wavelength is the distance a simple signal travels in one period through a transmission medium. It binds the period/frequency to propagation speed:

$$\lambda = v \times T = \frac{v}{f}$$

Where \(v\) is the propagation speed (typically \(2 \times 10^8\) to \(3 \times 10^8\) m/s depending on medium). If propagation speed changes, wavelength changes but frequency remains the same.

Time Domain vs. Frequency Domain

• Time-Domain Plot: Shows changes in signal amplitude over time (amplitude vs. time). Phase is not explicitly shown.
• Frequency-Domain Plot: Shows peak amplitude vs. frequency. A single sine wave is a single spike. More compact when dealing with multiple sine waves.
• The frequency domain is more compact and useful when dealing with composite signals (e.g., three sine waves at different frequencies are three spikes).

Composite Signals

• A single sine wave can only carry limited information (one frequency component).
• A composite signal is made up of multiple simple sine waves.
• Can be periodic or non-periodic.
• Based on Fourier analysis, any composite signal can be decomposed into a set of simple sine waves with different frequencies, amplitudes, and phases.
• A periodic composite signal decomposes into discrete frequencies (e.g., \(f\), \(3f\), \(9f\), etc.).
• A non-periodic composite signal decomposes into a continuous range of frequencies.

Bandwidth

The bandwidth is normally the difference between the highest and lowest frequencies in a composite signal.

Digital Signals

• Information can also be represented by a digital signal.
• Example: 1 = positive voltage, 0 = zero voltage.
• A digital signal can have more than two levels — we can send more than one bit per level.

Signal Levels and Bits

If a signal has \(L\) levels, each level needs \(\log_2 L\) bits:

$$\text{Bits per level} = \log_2(L)$$

• 2 levels → \(\log_2 2 = 1\) bit per level.
• 4 levels → \(\log_2 4 = 2\) bits per level.

Bit Rate

• Bit Rate: Number of bits sent in 1 second, expressed in bits per second (bps).
• Most digital signals are non-periodic, so period and frequency are not appropriate. We use bit rate instead.

Bit Length

Analogous to wavelength for analog signals. Bit length = distance one bit occupies on the medium:

$$\text{Bit Length} = \text{Propagation Speed} \times \text{Bit Duration}$$

Digital Signal as a Composite Analog Signal

• Based on Fourier analysis, a digital signal is a composite analog signal.
• Vertical line in time domain → frequency of infinity.
• Horizontal line in time domain → frequency of zero.
• Therefore, a digital signal has an infinite bandwidth (frequencies from 0 to ∞).

A digital signal (periodic or non-periodic) is a composite analog signal with frequencies between zero and infinity — infinite bandwidth. Two approaches for transmission:

Baseband Transmission

• Sending a digital signal without changing it to an analog signal.
• Requires a low-pass channel — a channel with bandwidth starting from 0 Hz.
• Dedicated medium required (e.g., cable between two devices).
• With a wide low-pass channel, we can approximate the digital signal well.
• With a narrow low-pass channel, only the lower harmonics pass — the received signal is a rough approximation.

Broadband Transmission (Modulation)

• Changing the digital signal to an analog signal for transmission.
• Modulation allows us to use a bandpass channel — a channel with bandwidth that does not start from zero.
• Bandpass channels are more available than low-pass channels.
• The digital signal modulates a carrier signal (analog) for transmission.

Transmission media are not perfect. The signal sent is not the same as the signal received. Three causes of impairment:

1. Attenuation

• Loss of energy as a signal travels through a medium.
• The medium's resistance converts some signal energy to heat.
• To compensate, amplifiers are used to boost the signal.

Decibel (dB)

• Unit measuring relative strength of two signals or one signal at two different points.
• Negative if signal is attenuated; positive if signal is amplified.

$$dB = 10 \log_{10}\left(\frac{P_2}{P_1}\right)$$

dBm (Decibel-milliwatt)

Measures signal power relative to 1 milliwatt:

$$dBm = 10 \log_{10}\left(\frac{P_m}{1\text{ mW}}\right)$$

2. Distortion

• Signal changes its form or shape.
• Occurs in composite signals made of different frequencies.
• Each frequency component has its own propagation speed and therefore its own delay.
• Differences in delay create differences in phase if the delay is not exactly the same as the period duration.
• Result: signal components arrive out of phase, distorting the composite signal.

3. Noise

Random, unwanted signals that corrupt the transmitted signal. Four types:

• Thermal Noise: Random motion of electrons in a wire creates extra signal. Present in all electronic media. Cannot be eliminated.
• Induced Noise: From external sources such as motors, appliances, electromagnetic interference.
• Crosstalk: Effect of one wire on another — unwanted coupling between parallel wire pairs.
• Impulse Noise: High-amplitude spikes of very short duration. Caused by power surges, lightning, etc. Major source of errors in digital data.

Signal-to-Noise Ratio (SNR)

Measures the ratio of average signal power to average noise power:

$$\text{SNR} = \frac{\text{Average Signal Power}}{\text{Average Noise Power}}$$

$$\text{SNR}_{dB} = 10\log_{10}(\text{SNR})$$

• High SNR = clean signal.
• Low SNR = noisy, corrupted signal.

How fast can we send data (in bps) over a channel? Data rate depends on three factors:

1. The bandwidth available.
2. The level of the signals we use.
3. The quality of the channel (level of noise).

Two theoretical formulas were developed to calculate data rate limits:

Noiseless Channel: Nyquist Bit Rate

For a noiseless channel, the Nyquist formula defines the theoretical maximum bit rate:

Finding a balance between bit rate and system reliability — increasing levels increases bit rate but makes the system more susceptible to noise.

Noisy Channel: Shannon Capacity

In reality, channels are always noisy. Claude Shannon (1944) introduced the formula for theoretical highest data rate:

Shannon formula defines an upper limit; gives capacity regardless of signal levels. It is independent of the number of signal levels.

Using Both Limits in Practice

• Shannon's formula gives the upper limit on data rate.
• Nyquist formula gives the required number of signal levels.
• In practice: Use Shannon to find max capacity → choose a target rate below it → use Nyquist to find the needed signal levels.

Data transmission over a network: how good is our network? Three characteristics of network performance:

• Bandwidth
• Throughput
• Latency (Delay)

Bandwidth

Used in two contexts: Bandwidth in Hertz (range of frequencies in a composite signal) and Bandwidth in bps (number of bits a channel/link/network can transmit).

Throughput

• Measure of how fast we can actually send data through a network.
• Bandwidth ≠ Throughput. A link may have bandwidth \(B\) bps, but we can only send \(T\) bps with \(T \leq B\).
• Throughput is always less than or equal to bandwidth.

Latency (Delay)

Defines how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent.

Propagation Time

$$\text{Propagation Time} = \frac{\text{Distance}}{\text{Propagation Speed}}$$

Propagation speed in cable ≈ \(2.4 \times 10^8\) m/s.

Transmission Time

$$\text{Transmission Time} = \frac{\text{Message Size (bits)}}{\text{Bandwidth (bps)}}$$

Queuing Time & Processing Delay

• Queuing Time: Time spent waiting in buffers at intermediate routers/switches. Varies dynamically based on traffic load.
• Processing Delay: Time required by routers to read packet headers, check errors, determine routing paths.

Bandwidth-Delay Product (BDP)

Defines the number of bits that can fill a link. Think of the link as a pipe — cross-section = bandwidth, length = delay.

$$\text{BDP (bits)} = \text{Bandwidth (bps)} \times \text{Delay (s)}$$

• Case 1 (Narrow pipe): Bandwidth = 1 bps, Delay = 5 s → BDP = 5 bits.
• Case 2 (Wide pipe): Bandwidth = 4 bps, Delay = 5 s → BDP = 20 bits.

Jitter

• Jitter is a problem if different packets encounter different delays and the application is time-sensitive (audio/video).
• Example: First packet delay = 20 ms, second = 45 ms, third = 40 ms → the application endures jitter.
• Causes uneven quality in real-time playback.

Digital Transmission Overview

Digital Transmission includes: Digital-to-Digital Conversion (Line Coding, Block Coding, Scrambling) and Analog-to-Digital Conversion (PCM, Delta Modulation).

Signal Element vs. Data Element

• Data Element: Smallest entity representing a piece of information — the bit. Data elements are carried.
• Signal Element: Shortest unit (pulse/level) of a digital signal. Signal elements are carriers.
• Ratio \(r = \frac{\text{Data Elements}}{\text{Signal Elements}}\).

Data Rate vs. Signal Rate

• Data Rate (Bit Rate): Number of data elements (bits) sent in 1 sec → bps.
• Signal Rate (Baud Rate, Pulse Rate, Modulation Rate): Number of signal elements sent in 1 sec → baud.
• Relationship: \(S = c \times N \times \frac{1}{r}\) baud, where \(c\) = case factor (typically 1/2 for worst or 1 for best case), \(N\) = data rate, \(r\) = data elements per signal element.

Line Coding

Converts a sequence of bits to a digital signal. Data stored as sequences of bits in computer memory; line coding converts these to signal elements for transmission over a low-pass channel.

Five broad categories of line coding schemes:

Block Coding (mB/nB)

• Changes a block of m bits into a block of n bits (where \(n > m\)).
• Uses mB/nB encoding technique.
• Adds redundancy to ensure synchronization.
• Block coding gives us redundancy and improves line coding performance.

4B/5B Encoding

• Converts 4-bit groups into 5-bit groups.
• Mapping designed so that no more than 3 consecutive 0s appear.
• Used with NRZ-I line coding (NRZ-I syncs on 1s; limiting zeros ensures sync).
• 4B/5B with NRZ-I solves the synchronization issue of NRZ-I.
• The 4B/5B mapping has specific codes: e.g., 0000 → 11110, 0001 → 01001, etc.

8B/10B Block Encoding

Maps 8-bit groups to 10-bit groups. Provides even greater redundancy and better error detection capabilities. Used in high-speed networks.

Scrambling

Biphase schemes are suitable for LAN but not for long distance. Block coding + NRZ-I solves sync but has DC component. Bipolar AMI has narrow bandwidth (no DC) but synch issue (long series of 0s). Scrambling is the solution.

• Scrambling replaces consecutive zero pulses with violations without increasing the data rate.
• The system inserts required pulses based on defined scrambling rules.
• AMI is used with scrambling to achieve both narrow bandwidth and synchronization.

B8ZS (Bipolar with 8-Zero Substitution)

• Replaces 8 consecutive zeros.
• If last non-zero pulse was positive (+): replace with 000+-0-+.
• If last non-zero pulse was negative (−): replace with 000-+0+-.
• Violations occur at positions 4 and 7.

HDB3 (High-Density Bipolar 3-Zero)

• Replaces 4 consecutive zeros.
• Decision based on number of non-zero pulses since last substitution being odd or even.

Process of converting analog data (e.g., voice, music) to digital data — Digitization. Two techniques:

1. Pulse Code Modulation (PCM)

Most common technique. Employs a PCM Encoder with three processes:

Step 1: Sampling (PAM)

• The analog signal is sampled at periodic intervals.
• Three different sampling methods: ideal sampling, natural sampling, flat-top sampling.
• Nyquist Sampling Theorem: The sampling rate must be at least twice the highest frequency:

  $$f_s \geq 2 \times f_{max}$$

• Example: Sampling a sine wave at three rates:
  • \(f_s = 4f\) (2× Nyquist rate) → excellent recovery.
  • \(f_s = 2f\) (exactly Nyquist rate) → acceptable recovery.
  • \(f_s = f\) (half Nyquist rate) → aliasing, signal cannot be recovered.

Step 2: Quantization

• Sampling produces a series of pulses with amplitude values between min and max signal amplitude.
• These are an infinite set with non-integral values — not suitable for encoding.
• We quantize the sampled output into certain levels based on the range of amplitudes and accuracy needed.
• The quantized values are rounded to the nearest predefined level.
• Quantization introduces quantization error (difference between actual and quantized value).

Step 3: Encoding

• After quantization, each quantized level is assigned a binary code (codeword).
• The number of bits per sample = \(\log_2(\text{number of quantization levels})\).

PCM Decoder (Signal Recovery)

The decoder reverses the process: binary codes → quantized levels → low-pass filter → reconstructed analog signal.

2. Delta Modulation (DM)

• A simpler technique than PCM.
• PCM finds the value of the signal amplitude for each sample; DM finds the change from the previous sample.
• No code words needed — only sends 1 bit per sample:
  • If current sample > previous → send 1 (step up).
  • If current sample < previous → send 0 (step down).
• Delta Modulation Components: Modulator (with staircase function generator and comparator) and Demodulator (with staircase function generator and low-pass filter).

How do we transmit data? Wiring and data stream considerations. Do we send 1 bit at a time, or group bits?

Parallel Transmission

• Binary data (1s and 0s) organized in groups of n bits.
• We send n bits at a time instead of just one.
• n wires required to send n bits simultaneously.
• Advantage: Fast. Disadvantage: Expensive (many wires), only practical for short distances.

Serial Transmission

• One bit follows another — only one communication channel needed.
• Cheaper than parallel for long distances.
• Three types of serial transmission:

Asynchronous Transmission

• Timing of signal is not important — the receiver re-synchronizes at the start of each character.
• Each character preceded by a start bit (0) and followed by a stop bit(s) (1).
• Gap between characters can vary.
• Simple and cheap but has overhead due to start/stop bits.

Synchronous Transmission

• Bits are sent as a continuous stream of frames without start/stop bits.
• Frames include synchronization flags (e.g., 01111110).
• Sender and receiver clocks are synchronized.
• More efficient than asynchronous for large data blocks.

Isochronous Transmission

• For real-time audio and video.
• Synchronization between characters is not enough — the entire stream must be synchronized.
• Guarantees fixed-rate data delivery.
• Ensures data arrives at a fixed rate to prevent jitter in real-time applications.

Process of changing one of the characteristics of an analog signal (carrier) based on information in digital data. A sine wave has 3 characteristics: Amplitude, Frequency, Phase. By changing one, we represent digital data.

Aspects of Digital-to-Analog Conversion

Bit Rate vs. Baud Rate

• In analog transmission of digital data, baud rate ≤ bit rate.
• Data element vs. signal element: A signal element can carry multiple data elements.
• Bandwidth required is proportional to signal rate (except FSK).
• Carrier Signal: A high-frequency signal that serves as a base for the information signal. Modified by modulation.

Types of Digital-to-Analog Conversion

1. Amplitude Shift Keying (ASK)

• Amplitude of the carrier signal is varied to create signal elements.
• Frequency and phase remain constant while amplitude changes.
• Binary ASK (BASK) / On-Off Keying (OOK): 1 = carrier amplitude; 0 = zero amplitude.
• Bandwidth: \(B = (1+d) \times S\) where \(d\) is a factor between 0 and 1, \(S\) = baud rate.
• Implementation: Uses a multiplier — carrier signal × digital signal.

2. Frequency Shift Keying (FSK)

• Frequency of the carrier signal is varied to represent data.
• Peak amplitude and phase remain constant.
• Binary FSK (BFSK): Two different frequencies represent 0 and 1.
• Frequency is constant for the duration of one signal element but changes if the data element changes.
• Implementation: Uses voltage-controlled oscillator (VCO).

3. Phase Shift Keying (PSK)

• Phase of the carrier is varied to represent two or more different signal elements.
• Peak amplitude and frequency remain constant.
• PSK is relatively more common than ASK or FSK.
• Binary PSK (BPSK): Two phases (0° and 180°) represent 0 and 1.
• QPSK (Quadrature PSK): Four phases (45°, 135°, 225°, 315°) — each signal element carries 2 bits.
• Implementation of BPSK: Uses a multiplier with in-phase carrier.

Constellation Diagram

• Helps define the phase and amplitude of a signal element using two carriers (one in-phase, one in quadrature).
• Signal element represented as a dot in the diagram.
• X-axis = in-phase carrier amplitude; Y-axis = quadrature carrier amplitude.
• Distance from origin = amplitude; angle = phase.

4. Quadrature Amplitude Modulation (QAM)

• PSK is limited by equipment's ability to distinguish small phase differences.
• Why not combine ASK and PSK? → QAM.
• QAM changes both phase and amplitude.
• Allows more signal elements (higher bit rate) in the same bandwidth.
• Common QAM variants: 4-QAM, 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256-QAM.
• Constellation diagrams show different QAM configurations.

Representation of analog information by an analog signal. Required when the medium can only pass a bandpass (not baseband) channel. Three types:

1. Amplitude Modulation (AM)

• The amplitude of the carrier is varied to reflect changes in the modulating signal.
• Bandwidth of AM: \(B_{AM} = 2B\) where \(B\) is the bandwidth of the modulating signal.
• Carrier frequency should be at least \(2f_{max}\) of the modulating signal.

2. Frequency Modulation (FM)

• The frequency of the carrier is varied to reflect changes in the modulating signal.
• Bandwidth of FM: \(B_{FM} = 2(1 + \beta)B\) where \(\beta\) is the modulation index.
• FM requires more bandwidth than AM but provides better noise immunity.

3. Phase Modulation (PM)

• The phase of the carrier is varied to reflect changes in the modulating signal.
• Bandwidth of PM: \(B_{PM} = 2(1 + \beta)B\) — same formula structure as FM.
• Implementation uses a Voltage Controlled Oscillator (VCO) and a differentiator (\(d/dt\)).

A multiplexer (MUX) combines signals; a demultiplexer (DEMUX) separates them. The link is divided into channels.

Categories of Multiplexing

• Frequency-Division Multiplexing (FDM) — analog technique.
• Wavelength-Division Multiplexing (WDM) — analog, for fiber optics.
• Time-Division Multiplexing (TDM) — digital technique.

Frequency-Division Multiplexing (FDM)

• An analog technique applied when the bandwidth of a link (in Hz) is greater than the combined bandwidths of the signals to be transmitted.
• Signals generated by each sending device modulate different carrier frequencies.
• These modulated signals are combined into a single composite signal for transport.
• Guard bands: Small bandwidth gaps between channels to prevent interference.
• FDM Multiplexing: Modulate → combine → send over single link.
• FDM De-Multiplexing: Filter → demodulate → separate signals.

The Analog Carrier System (Analog Hierarchy)

• Telephone companies multiplex signals from lower-bandwidth lines onto higher-bandwidth lines using FDM.
• Hierarchical structure: Groups → Supergroups → Mastergroups → Jumbogroups.

Wavelength-Division Multiplexing (WDM)

• Designed to use the high data-rate capability of fiber-optic cable.
• Fiber data rate is higher than metallic cable — using fiber for a single line wastes bandwidth.
• WDM combines several light signals of different wavelengths onto a single fiber.
• Conceptually the same as FDM but operates on light frequencies.
• Uses prisms (or diffraction gratings) to multiplex and demultiplex light signals.
• Multiplexer combines different wavelengths; demultiplexer separates them.

Time-Division Multiplexing (TDM)

• A digital process that allows several connections to share the high bandwidth of a link.
• Time is shared — each connection occupies a portion of time in the link.
• Two types: Synchronous TDM and Statistical (Asynchronous) TDM.

Synchronous TDM

• Each input is assigned a fixed time slot in each frame, regardless of whether the source has data to send.
• Time slots are organized into frames; a frame contains one slot per input.
• Interleaving: Taking turns — data from each source placed into successive slots.
• If a source has no data, its slot is empty (wasted).

TDM Techniques

• Multilevel Multiplexing: Multiple lower-rate inputs are combined to match a higher-rate output.
• Multiple-Slot Multiplexing: A source with higher data rate gets multiple slots per frame.
• Pulse Stuffing: When input rates don't perfectly divide the output rate, dummy bits are stuffed to equalize rates.

Digital Hierarchy (DS and T/E Lines)

T-1 Line: Carries 24 voice channels (24 × 64 kbps) plus 1 framing bit per frame = 24 × 8 + 1 = 193 bits per frame. At 8000 frames/sec: \(193 \times 8000 = 1.544\) Mbps.

Statistical TDM (Asynchronous TDM)

• In synchronous TDM, empty slots waste bandwidth.
• Statistical TDM allocates slots dynamically — only active sources get slots.
• Each slot carries an address to identify the source.
• More efficient utilization of bandwidth, but requires addressing overhead.

In wireless applications, stations must share the medium without interception by eavesdroppers and without being subject to jamming from malicious intruders. Spread spectrum adds redundancy and spreads the original spectrum needed for each station.

Principles of Spread Spectrum

• Bandwidth allocated to each station is larger than what is needed — to allow redundancy.
• The spreading process should be independent of the original signal.
• The receiver uses the same spreading code to recover the original signal.

Two Spread Spectrum Techniques

1. Frequency Hopping Spread Spectrum (FHSS)

• M different carrier frequencies are modulated by the source signal.
• At one moment, the signal modulates one carrier frequency; at the next moment, it modulates another.
• The pattern of frequency changes is determined by a pseudorandom code known to both sender and receiver.
• Frequency selection: A pseudorandom number generator creates a pattern that controls the frequency synthesizer.
• FHSS Cycles: The system hops through a predetermined sequence of frequencies.
• Bandwidth sharing: Multiple stations can use the same band by using different hopping patterns.

2. Direct Sequence Spread Spectrum (DSSS)

• DSSS also expands the bandwidth of the original signal, but the process is different.
• We replace each data bit with n bits using a spreading code.
• Each bit is assigned a code of n bits called chips.
• The chip rate is n times that of the data bit rate.
• The bandwidth of the transmitted signal is n times the bandwidth of the original signal.
• The receiver uses the same spreading code (chips) to recover the original data.