Layers, functions, and protocols

1. Overview of the OSI and TCP/IP Models

• OSI Model (7 Layers): (Short Code – APSTNDP)
  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical
• TCP/IP Model (4 Layers): (Short Code – ATIN)
  • Application
  • Transport
  • Internet
  • Network Access (Link)

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2. Key Layers and Their Functions

A. Application Layer (OSI/TCP/IP)

• Functions: User interface, data generation, and communication.
• Protocols: HTTP, HTTPS, FTP, SMTP, POP3, IMAP, DNS, SNMP.

B. Transport Layer (OSI/TCP/IP)

• Functions: Reliable/unreliable delivery, error detection, segmentation, and flow control.
• Protocols: TCP (reliable), UDP (unreliable).
• Concepts: Port numbers (e.g., HTTP – 80, HTTPS – 443).

C. Network Layer (OSI) / Internet Layer (TCP/IP)

• Functions: Logical addressing, routing.
• Protocols: IP (IPv4, IPv6), ICMP, ARP, RARP, IGMP.

D. Data Link Layer (OSI) / Network Access Layer (TCP/IP)

• Functions: Framing, error correction, physical addressing (MAC).
• Protocols: Ethernet, PPP, Frame Relay, HDLC.

E. Physical Layer (OSI)

• Functions: Transmission of raw bits, hardware specifications.
• Examples: Cables, switches, hubs.

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3. Common Protocols and Ports

Protocol	Port(s)
FTP	20, 21
SSH	22
Telnet	23
SMTP	25
DNS	53
HTTP	80
HTTPS	443
SNMP -Simple Network Management Protocol is a network protocol that allows user to collect and organize information about devices on IP networks	161
DHCP -Dynamic Host Configuration Protocol is a client/server protocol that automatically provides an IP host with its IP address and other related configuration information such as the subnet mask and default gateway.	67, 68

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4. Popular Topics

• IP Addressing:
  • IPv4 vs. IPv6.
  • Subnetting and CIDR.
• Routing Protocols: OSPF, BGP, RIP, EIGRP.
• Error Detection Mechanisms:
  • Checksum, CRC.
• Flow Control Mechanisms: Stop-and-Wait, Sliding Window.

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5. Practical Scenarios

• Match protocols with their layers.
• Identify functions performed at each layer.
• Understand the flow of data through the layers.

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6. Study Tips

Here’s a simplified version to help remember each OSI layer and its purpose:

1. Physical Layer (The Roads)
   • Purpose: Moves raw data (1s and 0s).
   • Analogy: The roads for delivery. No roads = no data transfer.
   • Device/Role: Your device’s hardware (Wi-Fi card, cables, etc.) connects to the internet.
2. Data Link Layer (Traffic Rules)
   • Purpose: Organizes data and avoids errors.
   • Analogy: Traffic rules ensuring smooth flow.
   • Device/Role: Your device’s network adapter ensures data is sent correctly to the right hardware address (MAC).
3. Network Layer (GPS)
   • Purpose: Finds the best route (IP addressing).
   • Analogy: GPS guiding the delivery.
   • Device/Role: Your device’s IP address ensures data reaches the correct destination (like the restaurant’s server).
4. Transport Layer (Delivery Box)
   • Purpose: Ensures data arrives safely or quickly (TCP/UDP).
   • Analogy: The sturdy delivery box protecting your food.
   • Device/Role: Your app uses protocols like TCP (for reliability) or UDP (for speed).
5. Session Layer (The Conversation)
   • Purpose: Manages communication (start, maintain, end).
   • Analogy: Chat with the agent about your delivery.
   • Device/Role: Your app manages the connection with the server (starting, maintaining, and ending it).
6. Presentation Layer (Unboxing)
   • Purpose: Converts data into readable formats.
   • Analogy: Food unboxing, beautifully arranged.
   • Device/Role: Your app translates data formats (e.g., encryption, compression) into something you can read.
7. Application Layer (Enjoy the Meal)
   • Purpose: Interaction with apps like HTTP or browsers.
   • Analogy: Finally eating the food!
   • Device/Role: The app itself (like Zomato) interacts directly with you, the user.

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