OSI Model

OSI Layer 1

Physical

OSI Layer 2

Data Link

OSI Layer 3

Network

OSI Layer 4

Transport

OSI Layer 5

Session

OSI Layer 6

Presentation

OSI Layer 7

Application

Real World Applications: Layer 1 - Physical

Cables, fiber & the signal itself; Electrical signals

Real World Applications; Layer 2 - Data Link

Ethernet frame, MAC address, Extended Unique Identifier (EU-48, EU-64), switch; Ethernet

Real World Applications; Layer 3 - Network

IP Address, router, packet; IP encapsulation

Real World Applications; Layer 4 - Transport

TCP segment, UDP datagram; TCP encapsulation

Real World Applications; Layer 5 - Session

Control protocols, tunneling protocols; Link presentation to the transport

Real World Applications; Layer 6 - Presentation

Application encryption(SSL/TLS); SSL encryption

Real World Applications; Layer 7 - Application

Your eyes; The site you see

Function of the Physical layer?

Transmits raw bits over the physical medium (e.g., cables, hubs, radio frequencies).

Function of the Data Link layer?

Handles error detection, framing, and physical addressing using MAC addresses (e.g., switches).

Function of the Network layer?

Routes data between devices using logical addressing (e.g., IP addresses, routers).

Function of the Transport layer?

Ensures reliable data delivery with error checking, segmentation, and flow control (e.g., TCP/UDP).

Function of the Session layer?

Establishes, manages, and terminates communication sessions between devices.

Function of the Presentation layer?

Translates, encrypts, and compresses data so it’s readable for the application layer (e.g., encryption, file format translation).

Function of the Application layer?

Provides network services directly to the end-user (e.g., web browsers, email, file sharing).

Typical Problems; Layer 1 - Physical

Damaged cables, unplugged devices, loose connections, electrical interference, or power failures.

Typical Problems; Layer 2 - Data Link

MAC address conflicts, switch misconfigurations, frame errors, or VLAN tagging issues.

Typical Problems; Layer 3 - Network

Incorrect IP addresses, routing table errors, unreachable networks, IP conflicts, or misconfigured subnets.

Typical Problems; Layer 4 - Transport

Lost packets, high latency, excessive retransmissions, port blocking, or incorrect port numbers (e.g., firewall issues).

Typical Problems; Layer 5 - Session

Dropped or failed sessions, repeated disconnections, timeout errors, or inability to maintain a stable connection between devices.

Typical Problems; Layer 6 - Presentation

Data format incompatibility, encryption/decryption errors, and failed data compression or translation (e.g., mismatched character encoding).

Typical Problems; Layer 7 - Application

Misconfigured applications, software bugs, unsupported protocols, incorrect login credentials, or incompatible software versions.

Layer 1 - Troubleshooting

• Inspect cables, connectors, and ports

• Test with a known-good cable

• Ensure power is supplied to devices

• Use a cable tester or multimeter

Layer 2 - Troubleshooting

• Check switch configuration and VLAN settings

• Verify MAC address tables

• Look for duplicate MACs

• Replace or reseat network cables to rule out errors

Layer 3 - Troubleshooting

• Verify IP addresses and subnet masks

• Ping local and remote devices

• Check routing tables and default gateways

• Use traceroute to find where the path breaks

Layer 4 - Troubleshooting

• Test ports with tools like Telnet or Netcat

• Verify firewall and ACL configurations

• Check packet loss/latency with ping or traceroute

• Adjust TCP/UDP settings if needed

Layer 5 - Troubleshooting

• Restart the session or connection

• Check firewall/session timeout settings

• Test persistent connection options (keep-alive)

• Review authentication and session management logs

Layer 6 - Troubleshooting

• Confirm encryption/decryption keys match

• Test using standard formats (e.g., UTF-8 for text)

• Check compression or codec compatibility

• Reconfigure or disable encryption temporarily for testing

Layer 7 - Troubleshooting

• Verify app settings and credentials

• Check server status and logs

• Test with another application or account

• Reinstall or update software