The Open Systems Interconnection (OSI) model was developed by the International Organisation for Standardisation (ISO) to allow systems with different platforms (hardware, software, or operating systems) to communicate with each other. The model divides network communication into seven layers, each providing specific functions.

The Seven Layers of the OSI Model

Functions of Each Layer

Physical Layer (Layer 1): Converts binary data (1s and 0s) into signals (electrical, light, or radio waves) for transmission over cables or wireless media. It defines network devices, interfaces, and mechanical, electrical, functional, and procedural characteristics.

Data Link Layer (Layer 2): Enhances reliability of the physical link. It performs error notification through ACK/NAK, manages network topology, and handles flow control. This layer uses MAC (Media Access Control) addresses (48-bit identifiers in hexadecimal format like 00:1A:2B:3C:4D:5E) for communication within local networks. It has two sublayers: LLC (Logical Link Control) and MAC.

Network Layer (Layer 3): Handles device addressing using IP addresses and determines the most efficient path for data transport. It divides messages into packets and adds headers containing source and destination IP addresses. Routers operate at this layer.

Transport Layer (Layer 4): Provides end-to-end communication services. It segments and reassembles data, handles multiplexing of upper-layer applications, and manages session establishment. Protocols include TCP (connection-oriented) and UDP (connectionless).

Session Layer (Layer 5): Manages establishment, maintenance, and termination of sessions between applications.

Presentation Layer (Layer 6): Handles data formatting, encryption, and compression to ensure data from application layer of one system is readable by application layer of another.

Application Layer (Layer 7): Provides network services to end-user applications. Protocols include HTTP, FTP, SMTP, DNS, and DHCP.

Advantages and Disadvantages of OSI Model

Advantages:

• Provides a clear, standardized framework for understanding network communication
• Modular design promotes interoperability between devices from different vendors
• Flexibility allows independent development of protocols within each layer
• Simplifies troubleshooting by isolating problems to specific layers

Disadvantages:

• Theoretical model that does not directly map to real-world protocols
• Seven layers may be overly complex for simple network setups
• Limited scope, focusing primarily on data communication

The Transmission Control Protocol/Internet Protocol (TCP/IP) model is the practical implementation used by the Internet. It organizes network functions into four layers:

TCP/IP Layers

Advantages of TCP/IP Model

• Practical and widely used: Directly corresponds to real-world protocol implementations
• Simpler structure: Four-layer design is easier to grasp than seven-layer OSI
• Flexibility: Accommodates new technologies without major changes
• Efficient data delivery: TCP provides reliable transmission with error correction and flow control

Disadvantages of TCP/IP Model

• Less emphasis on strict interoperability compared to OSI
• Some layers, particularly Internet layer, can be quite complex
• Limited troubleshooting guidance compared to OSI's structured approach

When data is transmitted from one device to another, it undergoes encapsulation—adding protocol information at each layer:

1. Application layer: Data is created by the application
2. Transport layer: Data is divided into segments with source and destination port numbers
3. Network layer: Segments become packets with source and destination IP addresses
4. Data Link layer: Packets become frames with source and destination MAC addresses
5. Physical layer: Frames are converted to bits for transmission

At the receiving device, this process reverses through de-encapsulation—each layer reads and removes its corresponding header before passing data to the layer above.

Transmission media are the pathways that carry data from source to destination. They are categorized as guided (wired) or unguided (wireless).

Guided Transmission Media

Twisted Pair Cable: Consists of two copper wires twisted together to minimize electromagnetic interference. Two types exist:

• UTP (Unshielded Twisted Pair): Most common, used in LANs and telephone systems. Categories include Cat5 (100 Mbps), Cat5e (1 Gbps), and Cat6 (10 Gbps). Uses RJ-45 connectors.
• STP (Shielded Twisted Pair): Has metal braiding for better protection against interference, used in environments with high EMI.

Coaxial Cable: Has copper core surrounded by insulating material, metallic shielding, and outer jacket. Used for cable TV and high-speed data transmission. Standards include RG-58 (thin Ethernet) and RG-59 (cable television).

Fibre Optic Cable: Transmits data as light pulses through glass or plastic strands. Advantages include highest bandwidth, immunity to EMI, longer distances, and better security. Disadvantages include higher cost and more difficult installation.

Unguided Transmission Media (Wireless)

Radio Waves: Frequency 3 kHz to 1 GHz. Omni-directional, can travel long distances, used in AM/FM radio, television, and Wi-Fi.

Microwaves: Frequency 1 GHz to 300 GHz. Unidirectional, requires line-of-sight between antennas, used in point-to-point communications and satellite links.

Infrared Waves: Frequency 300 GHz to 400 THz. Cannot penetrate solid objects, used for short-range communication like TV remotes and Bluetooth.

Satellite Communication: Uses geostationary satellites to relay signals between ground stations. Provides wide coverage for television, telephone, and data services.

Each device on a TCP/IP network requires a unique IP address. IPv4 addresses are 32-bit numbers represented in dotted decimal notation (e.g., 192.168.3.24).

IP Address Classes

Private IP Addresses

Reserved for internal networks (not routable on Internet):

• 10.0.0.0/8 (Class A)
• 172.16.0.0/12 (Class B)
• 192.168.0.0/16 (Class C)

Subnetting Example

Problem: A company has network 192.168.1.0/24 and needs 4 subnets.

Solution:

• Borrow 2 bits from host portion: 2² = 4 subnets
• New subnet mask: 255.255.255.192 (/26)
• Subnets created:
  • Subnet 1: 192.168.1.0/26 (hosts: 192.168.1.1 - 192.168.1.62)
  • Subnet 2: 192.168.1.64/26 (hosts: 192.168.1.65 - 192.168.1.126)
  • Subnet 3: 192.168.1.128/26 (hosts: 192.168.1.129 - 192.168.1.190)
  • Subnet 4: 192.168.1.192/26 (hosts: 192.168.1.193 - 192.168.1.254)

TCP (Transmission Control Protocol)

Connection-oriented protocol providing reliable data transmission. Features include:

• Three-way handshake: SYN → SYN-ACK → ACK
• Flow control: Prevents overwhelming receiver using windowing
• Error correction: Uses acknowledgments and retransmission
• Sequencing: Ensures data arrives in correct order

UDP (User Datagram Protocol)

Connectionless protocol, faster but unreliable. Used for streaming audio/video, online gaming, and DNS queries where speed matters more than perfect delivery.

HTTP (Hypertext Transfer Protocol): Used for transferring web pages. Browser sends HTTP GET request, server responds with HTML content.

FTP (File Transfer Protocol): Transfers files between client and server. Uses separate control and data connections. Supports upload and download operations.

SMTP (Simple Mail Transfer Protocol): Sends emails between mail servers. Works with IMAP and POP3 for retrieval.

DNS (Domain Name System): Translates human-readable domain names (www.example.com) to IP addresses.

DHCP (Dynamic Host Configuration Protocol): Automatically assigns IP addresses to devices on a network.

PSTN is the traditional circuit-switched network for voice communication. Key components include:

• Local loop: Physical circuit connecting customer premises to central office
• Central Office (CO): Houses switching equipment
• Transmission media: Copper wires, fibre optics, microwave links

PSTN uses circuit-switching where a dedicated path is established for the entire call duration. Limitations include limited data transmission speed and scalability issues.

Mobile networks have evolved through generations:

• 1G: Analog voice only
• 2G (GSM): Digital voice, SMS, basic data
• 3G: Mobile Internet, video calls
• 4G (LTE): High-speed Internet, QoS for applications
• 5G: Ultra-fast speeds, low latency, IoT support

Key components include Mobile Stations (MS), Base Transceiver Stations (BTS), Base Station Controllers (BSC), and Mobile Switching Centers (MSC).

Security Goals

• Confidentiality: Data protected from unauthorized access
• Integrity: Data remains accurate and unaltered
• Availability: Resources accessible to authorized users

Common Threats

• Unauthorized access
• DDoS attacks
• Man-in-the-middle attacks
• Malware and viruses
• Phishing

Security Measures

• Firewalls (packet filtering, application gateways)
• Virtual Private Networks (VPN)
• Encryption (SSL/TLS, IPSec)
• Antivirus software
• Strong passwords and authentication

In Tanzania, understanding computer networks is essential for managing systems like the NMB mobile banking platform or the popular M-Pesa mobile money service. When a student in Dar es Salaam buys airtime via M-Pesa, the transaction involves TCP/IP protocols to securely transmit data between the phone, the mobile network, and the banking server—demonstrating how the transport layer ensures reliable communication, and how encryption (application layer security) protects sensitive financial information from unauthorized access.