Computer Networks

1. Basics of Computer Networks

Computer Network: A set of interconnected devices (computers, servers, phones, printers, etc.) that communicate and share resources/data over wired or wireless links.

Benefits

• Resource sharing – files, printers, storage, internet connection
• Fast communication – email, messaging, video calls
• Centralized data & management – easier backup, control, security
• Cost-effective – shared hardware/software reduces cost
• Scalability – devices can be added easily
• Reliability – data can be replicated; alternate paths on failure

Key Characteristics

• Performance – speed, throughput, latency
• Reliability – consistent operation, fault tolerance
• Scalability – handles growth in users/devices
• Security – protection from unauthorized access
• Availability – uptime and accessibility
• Cost – setup and maintenance

Essential Components

Component	Role
Nodes/Hosts	End devices (computers, servers, phones)
Network devices	Hub, repeater, bridge, switch, router, gateway
Transmission media	Wired (twisted pair, coaxial, fiber) / Wireless (radio, microwave, satellite)
Protocols	Rules for communication (TCP/IP, HTTP, etc.)
NIC	Hardware that connects a device to the network

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2. Network Devices

Device	OSI Layer	Function	Key Points
Hub	Physical (1)	Broadcasts incoming data to all ports	Multiport repeater; no filtering; causes collisions; half-duplex
Repeater	Physical (1)	Regenerates/amplifies signal to extend distance	Does not filter data; 2 ports
Bridge	Data Link (2)	Connects two LANs; filters by MAC address	Learns MAC addresses; reduces collisions; 2 ports
Switch	Data Link (2)	Forwards frames only to the intended port using MAC table	Multiport bridge; full-duplex; separate collision domain per port
Router	Network (3)	Forwards packets between different networks using IP address	Chooses best path (routing table); connects LAN to WAN/internet
Gateway	Any (up to 7)	Connects networks using different protocols/architectures	Protocol translator; entry/exit point of a network

Quick distinctions

• Hub = broadcasts to all; Switch = sends to the specific device
• Switch = works with MAC addresses; Router = works with IP addresses
• Router = connects similar networks; Gateway = connects dissimilar networks

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3. Types of Computer Networks

Type	Full Form	Coverage	Example	Notes
PAN	Personal Area Network	~1–10 m	Bluetooth headset, phone hotspot	Single person's devices
LAN	Local Area Network	Room/building/campus (up to ~1 km)	Home, office, school network	High speed, low cost, privately owned
CAN	Campus Area Network	Campus (multiple LANs)	University campus	Interconnected LANs of one organization
MAN	Metropolitan Area Network	City (~5–50 km)	Cable TV network, city-wide ISP	Larger than LAN, smaller than WAN
WAN	Wide Area Network	Country/continent/global	The Internet	Slowest of the four, highest cost, uses leased lines/satellite

Order by size: PAN < LAN < CAN < MAN < WAN

Network models are foundational frameworks that standardize communication in computer networks. They define a layered structure that organizes network functions for efficient data flow.

Covered models: OSI Model, TCP/IP Model

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1. OSI Model (Open Systems Interconnection)

A 7-layer conceptual framework (by ISO) that divides network communication into distinct layers. Each layer has a specific function and communicates with the layers directly above and below it.

#	Layer	Function	Data Unit	Examples
7	Application	Interface for user applications to access network services	Data	HTTP, FTP, SMTP, DNS
6	Presentation	Translation, encryption, compression	Data	SSL/TLS, JPEG, ASCII
5	Session	Establishes, manages, terminates sessions	Data	NetBIOS, RPC
4	Transport	End-to-end delivery, segmentation, flow & error control	Segment / Datagram	TCP, UDP
3	Network	Logical addressing, routing	Packet	IP, ICMP, Routers
2	Data Link	Physical addressing (MAC), framing, error detection	Frame	Ethernet, Switches
1	Physical	Transmission of raw bits over the medium	Bits	Cables, Hubs

Mnemonic (7 → 1): All People Seem To Need Data Processing

Key Points

• Top 3 layers (5-7): Software/user-oriented (Application, Presentation, Session).
• Bottom 4 layers (1-4): Data transport-oriented (Transport, Network, Data Link, Physical).
• Data is encapsulated going down the layers (sender) and de-encapsulated going up (receiver).
• A theoretical/reference model, not used directly in practice.

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2. TCP/IP Model

A practical, protocol-driven model used as the basis of the Internet. Originally a 4-layer model (also shown as 5-layer).

#	Layer	Function	Protocols
4	Application	User services, data formatting, session handling	HTTP, FTP, SMTP, DNS, SSH
3	Transport	End-to-end communication, reliability, flow control	TCP, UDP
2	Internet	Logical addressing, routing of packets	IP, ICMP, ARP
1	Network Access (Link)	Physical transmission and MAC addressing	Ethernet, Wi-Fi

Key Points

• Developed by DARPA for ARPANET.
• Protocol-oriented and implemented in real networks.
• TCP: connection-oriented, reliable. UDP: connectionless, faster.

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3. OSI vs TCP/IP

Aspect	OSI Model	TCP/IP Model
Layers	7	4
Nature	Theoretical, reference model	Practical, implemented
Approach	Generic, protocol-independent	Protocol-specific
Layer separation	Strict (Session, Presentation separate)	Combined into Application layer
Transport	Connection-oriented and connectionless	TCP (connection-oriented), UDP (connectionless)
Usage	Teaching/troubleshooting	Internet standard

Layer Mapping

OSI	TCP/IP
Application, Presentation, Session	Application
Transport	Transport
Network	Internet
Data Link, Physical	Network Access

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The foundation of network communication. Covers network topologies, transmission media, and transmission modes.

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1. Physical Layer (OSI Model)

• Layer 1 of the OSI model.
• Deals with the actual physical transmission of raw bits (0s and 1s) over a medium.
• Unit of data: Bit.
• Responsibilities:
  • Bit representation (encoding bits as electrical / optical / radio signals)
  • Data rate (transmission rate, bits per second)
  • Bit synchronization (sender and receiver clock sync)
  • Physical topology (arrangement of devices)
  • Transmission mode (simplex, half duplex, full duplex)
  • Physical interface and medium specification (cables, connectors, voltages)
• Devices: Hub, Repeater, Modem, Cables, Connectors.

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2. Types of Network Topology

Network topology = the arrangement of nodes (devices) and links in a network. Two kinds: Physical (actual layout) and Logical (how data flows).

Topology	Structure	Pros	Cons
Bus	All devices share a single backbone cable	Cheap, easy to set up, less cable	Backbone failure kills the network, low performance with load, hard to troubleshoot
Ring	Each device connects to two neighbours, forming a closed loop	Orderly data flow, no collisions (token-based)	One node/link failure breaks the ring (unless dual ring), adding nodes disrupts network
Star	All devices connect to a central hub/switch	Easy to add/remove nodes, one cable failure affects only that node, easy fault detection	Central device is a single point of failure, more cabling
Mesh	Every device connects to every other device (full) or some (partial)	Highly reliable, no traffic issues, robust	Very expensive, complex, heavy cabling (full mesh links = n(n-1)/2)
Tree	Hierarchy of star networks connected to a root (bus backbone)	Scalable, easy fault isolation, hierarchical management	Root/backbone failure splits network, complex cabling
Hybrid	Combination of two or more topologies	Flexible, scalable, reliable	Complex design, costly

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3. Modes of Transmission

Defines the direction of data flow between two devices.

Mode	Direction	Example
Simplex	One-way only (sender → receiver)	Keyboard, TV/radio broadcast
Half Duplex	Both directions, but one at a time	Walkie-talkie
Full Duplex	Both directions simultaneously	Telephone, mobile calls

• Simplex: uses the entire channel capacity for one direction.
• Half duplex: channel capacity shared; direction alternates.
• Full duplex: highest throughput, requires two channels or bandwidth split.

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4. Transmission Media

The physical path between sender and receiver. Two categories:

A. Guided Media (Wired / Bounded)

Signal travels through a physical conductor.

Type	Description	Key Points
Twisted Pair	Pairs of insulated copper wires twisted together to reduce interference	UTP (unshielded): cheap, common in LANs. STP (shielded): less noise, costlier
Coaxial Cable	Central copper core + insulation + metallic shield + outer jacket	Higher bandwidth and better noise immunity than twisted pair; used in cable TV, older Ethernet
Fiber Optic	Light signals through glass/plastic strands (core + cladding)	Highest speed and bandwidth, long distance, immune to EMI, secure; expensive and fragile

B. Unguided Media (Wireless / Unbounded)

Signal travels through air/space without a physical conductor.

Type	Description	Use
Radio Waves	Omnidirectional, pass through walls	Radio, Wi-Fi, Bluetooth, mobile networks
Microwaves	Line-of-sight, unidirectional	Satellite links, point-to-point communication
Infrared	Short range, cannot pass through walls	Remote controls, short-range device links

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Quick Summary

• Physical Layer → raw bit transmission over a medium (Layer 1).
• Topology → Bus, Ring, Star, Mesh, Tree, Hybrid.
• Modes → Simplex, Half Duplex, Full Duplex.
• Media → Guided (Twisted Pair, Coaxial, Fiber) and Unguided (Radio, Microwave, Infrared).

Layer 2 of the OSI model. Handles reliable data transfer between directly connected devices: framing, error detection & correction, flow control, and switching.

• PDU: Frame
• Addressing: MAC address
• Devices: Switch, Bridge, NIC
• Sub-layers: LLC (Logical Link Control) and MAC (Media Access Control)

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1. Framing

Divides the bit stream from the network layer into manageable units called frames, so the receiver can identify where each frame starts and ends.

Frame structure: Header | Payload | Trailer

Type	Description
Fixed-size	Frame length is constant; no delimiter needed (e.g., ATM cells)
Variable-size	Needs a method to mark frame boundaries

Methods for variable-size framing:

Method	Idea
Character count	Header field stores the number of characters in the frame
Flag bytes + byte stuffing	Special flag byte marks start/end; an escape byte is inserted if the flag appears in data
Flag bits + bit stuffing	Flag 01111110; a 0 is inserted after five consecutive 1s in data
Physical layer coding violations	Uses invalid signal patterns to mark boundaries

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2. Error Control

Detects and corrects errors in frames during transmission.

• Error types: Single-bit error, Burst error
• Approach: Detect the error → correct it (by retransmission or by correcting codes)

2.1 Error Detection

Adds redundant bits to the data so the receiver can detect corruption.

Technique	Key Points
Parity check	Adds 1 parity bit (even/odd); detects single-bit errors only
Two-dimensional parity	Parity on rows and columns; detects burst errors and locates single-bit errors
Checksum	Sender sums data segments and sends the complement; receiver verifies the sum
CRC (Cyclic Redundancy Check)	Polynomial division; remainder appended to data; widely used, detects burst errors

2.2 Error Correction

Approach	Description
Backward error correction	Receiver detects the error and requests retransmission (ARQ)
Forward error correction (FEC)	Receiver corrects errors itself using redundant bits (e.g., Hamming code)

Hamming code: Redundant bits placed at positions that are powers of 2; formula 2^r ≥ m + r + 1 (r = redundant bits, m = data bits).

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3. Flow Control

Prevents a fast sender from overwhelming a slow receiver.

Type	Description
Stop-and-Wait	Sender sends one frame and waits for its ACK
Sliding Window	Sender can send multiple frames before needing an ACK

3.1 Stop-and-Wait ARQ

• Sender sends 1 frame, then waits for ACK
• Timer starts after sending; if no ACK before timeout → retransmit
• Window size: sender = 1, receiver = 1
• Simple, but low efficiency (channel idle while waiting)
• Efficiency = 1 / (1 + 2a), where a = Tp / Tt

3.2 Sliding Window – Go-Back-N

• Sender window size = N, receiver window size = 1
• Receiver accepts frames only in order; out-of-order frames are discarded
• On error/timeout, sender retransmits the lost frame and all frames after it
• Uses cumulative acknowledgement
• Sequence numbers needed: N + 1
• Simple receiver, but wastes bandwidth on retransmission

3.3 Sliding Window – Selective Repeat

• Sender window size = N, receiver window size = N
• Receiver buffers out-of-order frames and reorders them
• Only the lost/damaged frame is retransmitted
• Uses individual acknowledgement
• Sequence numbers needed: 2N
• More efficient, but complex receiver (sorting & buffering)

Feature	Stop-and-Wait	Go-Back-N	Selective Repeat
Sender window	1	N	N
Receiver window	1	1	N
Retransmission	Lost frame	Lost frame + all after	Only lost frame
Receiver buffering	No	No	Yes
Efficiency	Low	Medium	High
Complexity	Low	Medium	High

3.4 Piggybacking

• In bidirectional (full-duplex) communication, the ACK for received data is attached to an outgoing data frame instead of sent as a separate frame
• Benefit: Reduces the number of frames and improves bandwidth utilization
• If no data is ready to send, the receiver waits briefly, then sends a standalone ACK

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4. Switching Techniques

Methods of transferring data from source to destination across a network.

Technique	Description	Key Points
Circuit switching	Dedicated path established before transmission	Fixed bandwidth, constant delay, setup time, resources wasted when idle (e.g., telephone)
Message switching	Entire message stored and forwarded hop by hop	Store-and-forward, no dedicated path, high delay, needs large buffers
Packet switching	Message split into packets sent independently	Efficient bandwidth use, packets may take different routes (e.g., Internet)

Packet switching approaches:

Approach	Description
Datagram	Connectionless; each packet routed independently; may arrive out of order
Virtual circuit	Connection-oriented; fixed route set up first; packets arrive in order

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5. Virtual LAN (VLAN)

A logical grouping of devices into a separate broadcast domain, regardless of physical location.

• Configured on switches
• Each VLAN = one broadcast domain; traffic is isolated between VLANs
• Inter-VLAN communication requires a router / Layer 3 switch
• Standard: IEEE 802.1Q (adds a VLAN tag to the Ethernet frame)

Benefits: Better security, reduced broadcast traffic, flexible network management, improved performance.

Port Type	Description
Access port	Belongs to a single VLAN; connects end devices
Trunk port	Carries traffic of multiple VLANs between switches

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6. Link Aggregation

Combines multiple physical links into one logical link to increase bandwidth and provide redundancy.

• Protocol: LACP (Link Aggregation Control Protocol), IEEE 802.3ad / 802.1AX
• Also called port channel / EtherChannel / NIC teaming

Benefits:

• Higher throughput
• Fault tolerance (traffic continues if one link fails)
• Load balancing across links