Network models, topologies and the physical layer notes — Unit 1

A Computer Network is an interconnected collection of autonomous computers and devices that communicate with each other to share resources and information. The devices can be connected via wired (copper cable, fiber optics) or wireless (radio waves, infrared, satellite) media.

⇒1.1 Goals and Applications of Computer Networks

• Resource Sharing: Sharing of hardware (printers, scanners) and software (applications, databases).
• High Reliability: Replicating files on multiple machines ensures data availability even in case of hardware failure.
• Cost Reduction: Centralized resource sharing reduces the need for individual hardware components.
• Communication Medium: Facilitates email, video conferencing, and instant messaging.
• Scalability: Allows adding new components to the system effortlessly.

⇒1.2 Components of a Network

• Sender: The device that sends the data message.
• Receiver: The device that receives the message.
• Message: The information or data to be communicated (text, audio, video).
• Transmission Medium: The physical path by which a message travels from sender to receiver.
• Protocol: A set of rules that govern data communications.

Network topology refers to the physical or logical layout of a network. It defines how different nodes (computers, printers, routers) are connected to each other.

Hybrid Topology: A combination of two or more different topologies (e.g., Star-Bus or Star-Ring) to leverage their respective advantages.

Networks are primarily classified based on their geographical coverage. The main categories include PAN, LAN, MAN, and WAN.

⇒3.1 Local Area Network (LAN)

A LAN connects computers within a limited geographical area, such as a home, school, or office building. They usually employ Ethernet or Wi-Fi technology.

• Speed: Very high (up to 10 Gbps).
• Error Rate: Extremely low.
• Ownership: Typically privately owned.

⇒3.2 Wide Area Network (WAN)

A WAN covers a large geographical area, such as a country, continent, or the entire globe (e.g., the Internet). It interconnects multiple LANs.

• Speed: Generally slower than LANs due to distance and infrastructure.
• Error Rate: Higher compared to LANs.
• Ownership: Usually managed by multiple service providers (ISPs).

⇒3.3 Metropolitan Area Network (MAN) & Personal Area Network (PAN)

• MAN: Covers an entire city or a large campus. It serves as an intermediate between LAN and WAN (e.g., Cable TV networks).
• PAN: Connects devices within the immediate vicinity of a single person (typically within 10 meters). Bluetooth and USB are common PAN technologies.

Transmission mode defines the direction of signal flow between two linked devices. It is categorized into three types: Simplex, Half-Duplex, and Full-Duplex.

⇒4.1 Simplex Mode

In Simplex mode, communication is unidirectional. Data can only flow in one direction from sender to receiver.

• Characteristics: Only one device can transmit, the other can only receive.
• Examples: Keyboard to Monitor, Radio/Television broadcasting.

⇒4.2 Half-Duplex Mode

In Half-Duplex mode, data can flow in both directions, but not simultaneously. When one device is sending, the other must receive.

• Characteristics: The entire bandwidth is utilized by the transmitting device at any given time.
• Examples: Walkie-Talkies, Police radios.

⇒4.3 Full-Duplex Mode

In Full-Duplex mode, data flows in both directions simultaneously. Both devices can send and receive data at the same time.

• Characteristics: The link's capacity is shared between the two directions.
• Examples: Telephone networks, Modern Ethernet switches.

The Open Systems Interconnection (OSI) model is a conceptual framework created by ISO (International Organization for Standardization) in 1984. It standardizes the functions of a telecommunication or computing system into seven distinct abstract layers.

⇒5.1 Architecture of the OSI Model

The model is divided into 7 layers, categorized into Upper (Host) layers and Lower (Media) layers:

• Layer 7: Application Layer (Network process to application)
• Layer 6: Presentation Layer (Data representation and encryption)
• Layer 5: Session Layer (Interhost communication)
• Layer 4: Transport Layer (End-to-end connections and reliability)
• Layer 3: Network Layer (Path determination and logical addressing)
• Layer 2: Data Link Layer (MAC and LLC, physical addressing)
• Layer 1: Physical Layer (Media, signal, and binary transmission)

⇒5.2 Encapsulation and Decapsulation

As data moves down from the Application layer to the Physical layer at the sender, each layer adds its own header (and sometimes a trailer). This is Encapsulation. At the receiver side, as data moves up, headers are stripped off. This is Decapsulation.

The lower layers (Layers 1, 2, and 3) are primarily concerned with the physical delivery of data across the network.

⇒Layer 1: Physical Layer

• Function: Responsible for transmitting raw bit streams over a physical medium.
• Key Duties: Defines hardware specifications (voltages, pins, cables), bit rate control, physical topologies, and transmission modes.
• Protocol Data Unit (PDU): Bits.

⇒Layer 2: Data Link Layer

• Function: Provides node-to-node data transfer and handles error correction from the physical layer.
• Key Duties: Framing, physical addressing (MAC addresses), flow control, error control (CRC), and access control (CSMA/CD).
• Protocol Data Unit (PDU): Frames.

⇒Layer 3: Network Layer

• Function: Responsible for routing packets from the source host to the destination host across multiple networks.
• Key Duties: Logical addressing (IP addresses) and routing (determining the best path).
• Protocol Data Unit (PDU): Packets.

The upper layers (Layers 4, 5, 6, and 7) focus on end-to-end communication, session management, and application-level features.

⇒Layer 4: Transport Layer

• Function: Provides reliable or unreliable end-to-end message delivery.
• Key Duties: Segmentation and reassembly, service-point (port) addressing, connection control, flow control, and error control.
• Protocol Data Unit (PDU): Segments / Datagrams.

⇒Layer 5: Session Layer

• Function: Establishes, manages, and terminates sessions between applications.
• Key Duties: Dialog control (half-duplex or full-duplex) and synchronization (adding checkpoints to data streams).

⇒Layer 6: Presentation Layer

• Function: Transforms data into the form that the application accepts.
• Key Duties: Translation (ASCII to EBCDIC), Encryption/Decryption for security, and Data Compression.

⇒Layer 7: Application Layer

Provides network services directly to end-user applications. (e.g., HTTP for web, SMTP for email, FTP for file transfer).

Unlike the theoretical OSI model, the TCP/IP Reference Model is a practical, implementation-driven framework that powers the modern Internet. It was developed by the DoD (Department of Defense).

⇒8.1 Layers of the TCP/IP Model

Both models are based on the concept of layered architecture, but they have distinct philosophical and practical differences.

• Development: OSI is a theoretical model created before the protocols were invented. TCP/IP is a practical model created after the protocols were developed.
• Layer Count: OSI has 7 layers. TCP/IP has 4 (or sometimes considered 5) layers.
• Strictness: OSI has strict boundaries between layers. TCP/IP allows for a more relaxed, flexible approach.
• Reliability: In OSI, the Transport layer guarantees delivery. In TCP/IP, the Transport layer can be reliable (TCP) or unreliable (UDP).

⇒Layer Mapping

The Physical Layer is the lowest layer of the OSI model. Its primary responsibility is the transmission of raw bit streams across a physical medium. It concerns itself with the physical characteristics of interfaces and media.

⇒10.1 Key Responsibilities

• Representation of Bits: Encoding digital 1s and 0s into electrical, optical, or radio signals.
• Data Rate: Defining the transmission rate (number of bits sent per second).
• Synchronization of Bits: The sender and receiver clocks must be synchronized.
• Line Configuration: Establishing point-to-point or multipoint connections.
• Physical Topology: Defining how devices are physically connected (Mesh, Star, Bus).
• Transmission Mode: Setting Simplex, Half-Duplex, or Full-Duplex communication.

The physical layer ensures that when a sender transmits a '1', the receiver reads it as a '1' and not a '0'.

Transmission media provide the physical pathway over which data travels. Guided media (wired media) use solid physical conductors to direct signals from one device to another.

⇒11.1 Twisted Pair Cable

Consists of two insulated copper wires twisted together to reduce electromagnetic interference (EMI) and crosstalk. Comes in two forms: Unshielded (UTP - common in LANs) and Shielded (STP).

⇒11.2 Coaxial Cable

Features a central copper core surrounded by an insulating layer, a braided metal shield, and an outer plastic jacket. It carries signals of higher frequency ranges than twisted pair (e.g., Cable TV).

⇒11.3 Fiber Optic Cable

• Uses glass or plastic core to transmit data in the form of light pulses.
• Operates on the principle of Total Internal Reflection.
• Advantages: Extremely high bandwidth, immune to EMI, highly secure, and supports very long distances.
• Disadvantages: Expensive, fragile, and requires specialized installation.

Unguided media transport electromagnetic waves without using a physical conductor. Signals are broadcast through the air or vacuum.

⇒12.1 Radio Waves

• Frequency: 3 kHz to 1 GHz.
• Omnidirectional (propagate in all directions).
• Can penetrate solid objects like walls.
• Used for FM radio, television, and standard Wi-Fi communication.

⇒12.2 Microwaves

• Frequency: 1 GHz to 300 GHz.
• Unidirectional (travel in a straight line). Requires line-of-sight.
• Cannot easily penetrate solid objects (affected by rain and obstacles).
• Used for mobile phone networks, satellite communication, and radar.

⇒12.3 Infrared Waves

• Frequency: 300 GHz to 400 THz.
• Used for short-range communication in closed areas.
• Cannot penetrate walls, preventing interference between adjacent rooms.
• Examples: TV remote controls, wireless keyboards.

Switching is the mechanism used to route data between nodes in a large network.

⇒13.1 Circuit Switching

A dedicated physical path is established between sender and receiver before data transmission begins. The path remains active for the entire session. (Example: Traditional Telephone Networks).

⇒13.2 Message Switching

No dedicated path is established. The entire message is treated as a single data unit, sent from node to node using a 'store-and-forward' mechanism. It can cause long delays and requires large buffers at intermediate nodes.

⇒13.3 Packet Switching

The data is divided into smaller, fixed-size chunks called packets. Each packet is routed independently through the network based on the destination address. This is highly efficient and forms the foundation of the Internet.

• Datagram approach: Packets are routed independently. No setup required.
• Virtual Circuit approach: A logical path is set up, and all packets follow that same path.

Multiplexing is the process of combining multiple signals into one signal over a single shared medium. The reverse process at the receiving end is called Demultiplexing.

⇒14.1 Frequency Division Multiplexing (FDM)

An analog technique where the total bandwidth of a link is divided into multiple frequency bands. Each sender is assigned a specific frequency band (e.g., Cable TV, FM Radio).

⇒14.2 Time Division Multiplexing (TDM)

A digital technique where the transmission time is divided into distinct time slots. Each sender transmits data only during its allocated time slot.

• Synchronous TDM: Time slots are pre-assigned regardless of whether a sender has data (can waste bandwidth).
• Statistical TDM: Time slots are allocated dynamically on demand.

⇒14.3 Wavelength Division Multiplexing (WDM)

An analog technique specific to fiber optic cables. Different signals are transmitted using different wavelengths (colors) of light simultaneously over the same fiber.

Signals traveling through a physical medium are never perfect; they degrade. This degradation is known as transmission impairment.

⇒15.1 Attenuation

The loss of signal strength as it travels over a distance. Overcoming attenuation requires amplifiers (for analog signals) or repeaters (for digital signals). Attenuation is typically measured in decibels (dB).

⇒15.2 Distortion

Occurs primarily in composite signals (signals made of multiple frequencies). Different frequency components travel at different speeds through the medium, causing the signal to arrive out of phase and changing the signal's shape.

⇒15.3 Noise

Unwanted electrical or electromagnetic energy that degrades the quality of signals and data. Types of noise include:

• Thermal Noise: Random motion of electrons in a wire.
• Induced Noise: Interference from external sources like motors or appliances.
• Crosstalk: Interference between adjacent wires (e.g., hearing another conversation on the phone).
• Impulse Noise: Sudden, high-energy spikes (e.g., lightning strikes), the primary cause of burst errors.

The maximum rate at which data can be reliably transmitted over a channel is its data rate or channel capacity.

⇒16.1 Nyquist Bit Rate (Noiseless Channel)

For a theoretical channel with absolutely no noise, the maximum bit rate is determined by the bandwidth and the number of signal levels used.

Bit Rate = 2 * Bandwidth * log2(L)

• Bandwidth: The bandwidth of the channel in Hertz (Hz).
• L: The number of distinct signal levels used to represent data.

⇒16.2 Shannon Capacity (Noisy Channel)

In reality, every channel has some noise. Claude Shannon introduced a formula to determine the theoretical highest data rate for a noisy channel, regardless of signal levels.

Capacity = Bandwidth * log2(1 + SNR)

• SNR: Signal-to-Noise Ratio (often given in decibels, but must be converted to an absolute ratio for the formula).

Line coding is the process of converting digital data (a sequence of bits) into digital signals (voltage pulses) for transmission.

⇒17.1 Unipolar and Polar NRZ

• NRZ (Non-Return to Zero): The voltage level is constant throughout the bit interval. There is no return to zero voltage in the middle of the bit.
• Unipolar NRZ: Uses only one non-zero voltage level (e.g., +5V for '1', 0V for '0'). Prone to DC component problems.
• Polar NRZ: Uses two non-zero voltage levels (e.g., +V for '1', -V for '0'). NRZ-L (Level) and NRZ-I (Invert on one) are common variations.

⇒17.2 Manchester and Differential Manchester

These are biphase schemes that solve synchronization issues by introducing a transition in the middle of every bit interval.

• Manchester: A transition from high-to-low represents '0', and low-to-high represents '1' (or vice versa depending on the convention). Used in traditional Ethernet (10Base-T).
• Differential Manchester: A transition in the middle is guaranteed for clocking. The presence of a transition at the beginning of the bit indicates '0', while no transition at the beginning indicates '1'.

When digital data needs to be sent over an analog medium (like a telephone line or free space), it must be modulated onto an analog carrier wave.

⇒18.1 Key Modulation Techniques

• ASK (Amplitude Shift Keying): The amplitude of the carrier wave is varied to represent 1 or 0 (e.g., high amplitude = 1, zero amplitude = 0). Highly susceptible to noise.
• FSK (Frequency Shift Keying): The frequency of the carrier wave is varied. Often uses two distinct frequencies for 1 and 0. More resilient to noise than ASK.
• PSK (Phase Shift Keying): The phase of the carrier wave is shifted (e.g., by 180 degrees) to represent data changes. More robust and bandwidth-efficient.

⇒18.2 QAM (Quadrature Amplitude Modulation)

QAM is an advanced technique that combines ASK and PSK. By varying both the amplitude and the phase simultaneously, multiple bits can be transmitted per single signal change (baud). This is the foundation of high-speed modems and Wi-Fi.

To improve the performance of basic line coding schemes (like NRZ), network engineers introduce redundancy to ensure synchronization and error detection.

⇒19.1 Block Coding (4B/5B)

In 4B/5B encoding, every 4 bits of data are mapped into a 5-bit code. The 5-bit codes are mathematically selected to guarantee they do not contain more than three consecutive zeros.

• This prevents loss of synchronization in schemes like NRZ-I, which struggle with long strings of zeros.
• The trade-off is a 25% increase in required bandwidth (sending 5 bits for every 4 bits of payload).

⇒19.2 Scrambling (B8ZS and HDB3)

Used primarily in long-distance communication. Instead of adding redundant bits, scrambling intentionally replaces long sequences of zeros with intentional 'bipolar violations' (breaking the alternating voltage rule of AMI) to force synchronization pulses without increasing the bit rate.

In wireless communications, spread spectrum techniques deliberately spread the transmitted signal over a much wider frequency band than strictly necessary. This provides immunity against jamming, eavesdropping, and multi-path fading.

⇒20.1 FHSS (Frequency Hopping Spread Spectrum)

The transmitter dynamically changes the carrier frequency in a pseudorandom sequence known only to the sender and receiver. The signal 'hops' from channel to channel so quickly that an eavesdropper or jammer cannot lock onto it. (Originally used in Bluetooth).

⇒20.2 DSSS (Direct Sequence Spread Spectrum)

Each bit of original data is multiplied by a highly complex, pseudorandom 'chipping code'. If a 1-bit is multiplied by an 11-bit chip code, it effectively spreads the signal across 11 times the bandwidth. At the receiver, the exact same chipping code is used to mathematically reassemble the original bit out of the background noise. (Used in early Wi-Fi).