OSI and TCP/IP Models

Wednesday, 14 May 2025 11:49 PM

1. The OSI (Open Systems Interconnection) Model

The OSI model is a conceptual framework that describes how network communication should work. It's a 7-layer model, each layer
responsible for a specific part of the communication process. While it's a theoretical model, it's very helpful for understanding network
concepts.
• Layer 7: Application Layer: This is the layer closest to the end-user. It provides network services to applications, such as HTTP (for web
browsing), SMTP (for email), and FTP (for file transfer).
• Layer 6: Presentation Layer: This layer is responsible for data formatting, encryption, and compression. It ensures that data is in a
usable format for the application layer.
• Layer 5: Session Layer: This layer manages communication sessions between applications. It establishes, maintains, and terminates
connections.
• Layer 4: Transport Layer: This layer provides reliable or unreliable data delivery between applications. TCP (Transmission Control
Protocol) and UDP (User Datagram Protocol) operate at this layer. TCP provides reliable, connection-oriented communication,
while UDP provides unreliable, connectionless communication.
• Layer 3: Network Layer: This layer is responsible for routing data packets across the network. IP (Internet Protocol) operates at this
layer.
• Layer 2: Data Link Layer: This layer provides error-free transmission of data frames between two directly connected nodes. Ethernet is
a common Data Link Layer protocol.
• Layer 1: Physical Layer: This layer defines the physical characteristics of the network, such as cabling, voltage levels, and data rates.
2. The TCP/IP Model
The TCP/IP model is the practical model used in the Internet. It's a 4-layer model, which is a simplification of the OSI model.
• Layer 4: Application Layer: This layer combines the functions of the OSI model's Application, Presentation, and Session layers. It
provides network services to applications, such as HTTP, SMTP, and FTP.
• Layer 3: Transport Layer: This layer is the same as the OSI model's Transport Layer. It provides reliable or unreliable data delivery
between applications using TCP or UDP.
• Layer 2: Network Layer: This layer is the same as the OSI model's Network Layer. It's responsible for routing data packets across the
network using IP.
• Layer 1: Link Layer: This layer combines the functions of the OSI model's Data Link and Physical layers. It provides error-free
transmission of data frames over a physical link.
3. Comparison of OSI and TCP/IP Models

TCP/IP MODEL LAYER DIAGRAM

Network Edge vs. Network Core
Thursday, 15 May 2025 12:10 AM

1. Network Edge vs. Network Core

• Network Edge:
○ This is where the end systems (hosts) are located. Hosts are devices like computers, smartphones, servers, etc., that run network applications.
○ The network edge is where communication originates and terminates.
○ Examples of hosts:
§ Clients (e.g., your laptop, phone)
§ Servers (e.g., web servers, email servers)
• Access Networks:
○ Access networks connect end systems to the first router on the path from the end system to any other distant end system.
○ Different types of access networks exist:
§ DSL (Digital Subscriber Line): Uses existing telephone lines.
§ Cable networks: Uses cable television infrastructure.
§ FTTH (Fiber to the Home): Provides high-speed access using optical fiber.
§ Wireless access networks: Wi-Fi, cellular (e.g., 4G, 5G).
• Physical Media:
○ Physical media are the actual transmission media that carry the bits.
○ Examples:
§ Coaxial cable
§ Fiber optic cable
§ Twisted-pair copper wire
§ Radio waves
• Network Core:
○ The network core is the interconnected routers that forward data between end systems.
○ It's a mesh of routers connected by high-speed links.
○ The core's primary function is switching and routing.
2. Switching Techniques
• Circuit Switching:
○ Establishes a dedicated communication path (circuit) between two endpoints before data transmission begins.
○ Resources (bandwidth) are allocated exclusively to that circuit for the duration of the communication.
○ Common example: Traditional telephone networks.
○ Process:
1. Circuit establishment
2. Data transfer
3. Circuit teardown
○ Advantages:
§ Guaranteed bandwidth.
§ Low jitter (variation in delay).
○ Disadvantages:
§ Inefficient use of resources (bandwidth is reserved even if not used).
§ Setup time can be long.
§ Less flexible (fixed bandwidth).
• Packet Switching:
○ Data is divided into packets.
○ Each packet is transmitted independently through the network.
○ Packets can take different routes to the destination.
○ Routers forward packets based on destination addresses.
○ Common example: The Internet.
○ Process:
1. Data segmentation (into packets).
2. Packet transmission.
3. Packet reassembly at the destination.
○ Advantages:
§ Efficient use of resources (bandwidth is used only when needed).
§ More flexible (can handle varying data rates).
§ More robust (can handle link failures).
○ Disadvantages:
§ Can experience congestion and delays.
§ Can have jitter and variable delays.
§ Requires more complex protocols.
Trade-offs Between Switching Techniques
• The choice between circuit switching and packet switching involves trade-offs:
○ Circuit switching is suitable for real-time applications that require guaranteed bandwidth and low jitter (e.g., voice calls).
○ Packet switching is more efficient and flexible for data communication, which doesn't always require real-time delivery (e.g., web browsing, email).
• The Internet primarily uses packet switching because of its efficiency and flexibility.
Delay, Throughput, and Loss
Thursday, 15 May 2025 12:41 AM
Access Networks: Connecting End Systems to the ISP
Thursday, 15 May 2025 12:57 AM