What is the OSI Model?
Conceptual/Reference Model (NOT a strict rulebook)
Seven-layer framework for network communication
Developed by: ISO (International Organization for Standardization) and ITU-T
Purpose: Create standardization for multivendor interoperability
Layer Groupings
Upper Layers (5-7) - Application Focused:
Layer 7: Application
Layer 6: Presentation
Layer 5: Session
Key Characteristic: Know NOTHING about networking
Focus: End-user communication
Lower Layers (1-4) - Data Transport Focused:
Layer 4: Transport
Layer 3: Network
Layer 2: Data Link
Layer 1: Physical
Key Characteristic: Handle data transmission through physical medium
Focus: Moving data from source to destination
Core Principle
Each layer serves the layer above it
Each layer is served by the layer below it
Processing is self-contained and transparent to other layers
Only requires knowledge of interfaces between adjacent layers
✓ Essential for troubleshooting methodology
✓ Helps understand protocol communication
✓ Critical even if you primarily work at Layers 2-3
✓ Enables systematic problem isolation
✓ Foundation for understanding multivendor networks
Standardization: Defines what occurs at each layer
Interoperability: Allows multivendor product compatibility
Simplification: Breaks complex processes into manageable components
Isolation: Problems in one layer typically stay in that layer
Development: Easier design and troubleshooting
Layer
Name
Function
Key Terms
7
Application
User interface, application services
End-user processes, application support
6
Presentation
Data formatting, translation
Encryption, compression, data representation
5
Session
Session management
Establishes, manages, terminates connections
4
Transport
Reliable/unreliable delivery
Segmentation, TCP/UDP, flow control, error recovery
3
Network
Routing and logical addressing
IP addressing, routing, packet sequencing
2
Data Link
Physical addressing, framing
MAC addresses, LLC, MAC sublayers, frames
1
Physical
Physical transmission
Bits, encoding, signaling, cables, voltages
Encapsulation: Process of adding layer-specific headers as data moves DOWN the stack
De-encapsulation: Process of removing headers as data moves UP the stack
LLC (Logical Link Control): Upper sublayer of Data Link layer
MAC (Media Access Control): Lower sublayer of Data Link layer; handles physical addressing
Segmentation: Breaking data into smaller pieces (Transport layer)
Logical Addressing: IP addresses (Layer 3)
Physical Addressing: MAC addresses (Layer 2)
End Units: Devices like PCs, servers, printers, IoT devices
Layer 7 = Application
Layer 6 = Presentation
Layer 5 = Session
Layer 4 = Transport
Layer 3 = Network
Layer 2 = Data Link
Layer 1 = Physical
Upper 3 Layers: 5, 6, 7 (Application-focused)
Lower 4 Layers: 1, 2, 3, 4 (Transport-focused)
Network Engineer Focus: Primarily Layers 2-3 (but must know all!)
Layer 2 has TWO sublayers: LLC and MAC
❌ WRONG: "Application is Layer 1"
✅ CORRECT: "Application is Layer 7, Physical is Layer 1"
❌ WRONG: "TCP operates at Layer 3"
✅ CORRECT: "TCP operates at Layer 4 (Transport)"
❌ WRONG: "IP operates at Layer 2"
✅ CORRECT: "IP operates at Layer 3 (Network)"
❌ WRONG: "Layer 7 handles routing"
✅ CORRECT: "Upper layers (5-7) know NOTHING about networking"
❌ WRONG: "Layer 2 is just one layer"
✅ CORRECT: "Layer 2 has LLC and MAC sublayers"
❌ WRONG: "OSI is the actual implementation used in networks"
✅ CORRECT: "OSI is a reference model; TCP/IP is the actual implementation"
❌ WRONG: "Layer 4 only does segmentation"
✅ CORRECT: "Layer 4 handles segmentation, acknowledgments, flow control, error recovery, and connection management"
Don't confuse which devices operate at which layers!
Hub: Layer 1
Switch: Layer 2
Router: Layer 3
"All People Seem To Need Data Processing"
Application (7)
Presentation (6)
Session (5)
Transport (4)
Network (3)
Data Link (2)
Physical (1)
"Please Do Not Throw Sausage Pizza Away"
Think of...
Key Word
What the USER sees
USER
How data is FORMATTED
FORMAT
CONVERSATION setup
SESSIONS
Package DELIVERY method
DELIVERY
Street ADDRESS
ROUTING
Local SWITCH
SWITCHING
Physical CABLE
CABLE
What Happens
7 - Application
Browser sends HTTP GET request
6 - Presentation
Data encoded (HTML, SSL/TLS encryption)
5 - Session
Session established with web server
4 - Transport
TCP ensures reliable delivery, creates segments
3 - Network
IP packet created with source/destination IP
2 - Data Link
Frame created with source/destination MAC
1 - Physical
Bits transmitted over Ethernet cable
Layer 7: Click "Send" in email client (SMTP protocol)
Layer 6: Email formatted (MIME encoding), possibly encrypted
Layer 5: Session with mail server established
Layer 4: TCP connection (port 25, 587, or 465)
Layer 3: Routed using IP addresses
Layer 2: Switched using MAC addresses
Layer 1: Electrical signals on wire
Device
Hub
Repeats electrical signals (no intelligence)
Switch
Forwards frames using MAC addresses
Router
Routes packets using IP addresses
Multilayer Switch
2-3
Switching + routing capabilities
Firewall
3-7
Filters based on various criteria
Load Balancer
4-7
Distributes traffic across servers
Common Protocols/Technologies
HTTP, FTP, SMTP, DNS, DHCP, Telnet, SSH
SSL/TLS, JPEG, GIF, MPEG, ASCII
NetBIOS, RPC, SQL
TCP, UDP, Port numbers
IP, ICMP, OSPF, EIGRP, BGP
Ethernet, PPP, Frame Relay, MAC addresses
Cables, voltages, hubs, repeaters
Data Unit Name
7-5
Data
Segment (TCP) or Datagram (UDP)
Packet
Frame
Bits
What does OSI stand for?
Open Systems Interconnection
Which organizations developed the OSI model?
ISO (International Organization for Standardization) and ITU-T
How many layers are in the OSI model?
7 layers
List all OSI layers from Layer 7 to Layer 1.
Application, Presentation, Session, Transport, Network, Data Link, Physical
Is the OSI model a mandatory set of rules?
No, it's a conceptual reference guide for vendors
What are the upper three layers of the OSI model?
Application (7), Presentation (6), Session (5)
What are the lower four layers of the OSI model?
Transport (4), Network (3), Data Link (2), Physical (1)
Which layers "know nothing about networking"?
Upper three layers (Session, Presentation, Application)
Which layers handle data transmission from source to destination?
Lower four layers (Physical, Data Link, Network, Transport)
What is the main function of Layer 7 (Application)?
Support for application and end-user processes; user interface
What is the main function of Layer 6 (Presentation)?
Data representation, translation, encryption, and compression
What is the main function of Layer 5 (Session)?
Establishes, manages, and terminates sessions between applications
What is the main function of Layer 4 (Transport)?
Data transfer between end systems; segmentation/reassembly; acknowledgments; flow control; error recovery
What is the main function of Layer 3 (Network)?
Switching and routing; logical addressing (IP); error handling; packet sequencing
What is the main function of Layer 2 (Data Link)?
Physical addressing (MAC); framing; LLC and MAC sublayers; error detection from physical layer
What is the main function of Layer 1 (Physical)?
Encoding and signaling; physical transmission of data; defining medium specifications
What are the two sublayers of the Data Link layer?
LLC (Logical Link Control) and MAC (Media Access Control)
Which layer uses IP addresses?
Layer 3 (Network)
Which layer uses MAC addresses?
Layer 2 (Data Link)
Which layer handles TCP and UDP?
Layer 4 (Transport)
At which layer does a hub operate?
Layer 1 (Physical)
At which layer does a switch operate?
At which layer does a router operate?
What is encapsulation in the OSI model?
The process of adding layer-specific headers as data moves down the OSI stack
What is segmentation?
The process of breaking data into smaller units at the Transport layer
What does LLC stand for?
Logical Link Control
What does MAC stand for in Layer 2 context?
Media Access Control
Which layer decides between TCP and UDP?
What is the purpose of the OSI model?
To allow multivendor networks to interoperate by standardizing communication functions
Do problems in one layer typically affect other layers?
No, processing is self-contained; problems are usually isolated to one layer
What are examples of end units on a network?
PCs, servers, printers, scanners, IoT devices (appliances, lightbulbs)
Each layer supports which layer?
The layer above it
Each layer is served by which layer?
The layer below it
What is the CEO letter analogy for Layer 4 (Transport)?
Mail room deciding delivery method and whether to get delivery receipt (TCP vs UDP)
What is the CEO letter analogy for Layer 3 (Network)?
Courier company determining routing path (encapsulation and route planning)
What are the four advantages of the OSI model?
1) Standardization, 2) Multivendor interoperability, 3) Simplification of complex processes, 4) Problem isolation
What is logical addressing?
IP addressing used at Layer 3 (Network layer)
What is physical addressing?
MAC addressing used at Layer 2 (Data Link layer)
What is the data unit at Layer 4 called?
What is the data unit at Layer 3 called?
What is the data unit at Layer 2 called?
What is the data unit at Layer 1 called?
Which layers are most important for network engineers?
Layers 2 and 3, but understanding all layers is essential
Why must network engineers understand all OSI layers?
For comprehensive troubleshooting, understanding applications, and creating effective troubleshooting methodologies
Is processing at each OSI layer transparent to other layers?
Yes, each layer's processing is self-contained and transparent to other layers
What knowledge is required between OSI layers?
Only knowledge of interfaces between adjacent layers
Name three protocols that operate at Layer 7.
HTTP, FTP, SMTP, DNS, DHCP (any three)
Name the transport layer protocols.
TCP and UDP
What layer handles flow control?
What layer handles routing?
What layer handles switching based on MAC addresses?
In the OSI model, what moves DOWN the stack during transmission?
Data being encapsulated with headers from each layer
In the OSI model, what moves UP the stack during reception?
Data being de-encapsulated as headers are removed
"At which layer does a router operate?" → Layer 3
"Which layer is responsible for logical addressing?" → Layer 3
"What are the two sublayers of the Data Link layer?" → LLC and MAC
"Which layer handles segmentation?" → Layer 4
"Which layers know nothing about networking?" → 5, 6, 7
Write it down: As soon as exam starts, write "APSTNDP" (layers 7-1) on your whiteboard
Eliminate wrong answers: If question asks about routing, eliminate all non-Layer 3 answers
Look for keywords:
"IP address" = Layer 3
"MAC address" = Layer 2
"Reliable delivery" = TCP = Layer 4
"Encryption" = Layer 6
Watch for questions trying to confuse TCP/IP model with OSI model
Be careful with "Application layer" - could mean Layer 7 OR TCP/IP Application layer
Don't assume a protocol only operates at one layer (some span multiple)
LAYER | NAME | KEYWORDS | DEVICES | DATA UNIT
------|--------------|-------------------------|---------------|----------
7 | Application | User, HTTP, FTP, DNS | - | Data
6 | Presentation | Encryption, Format | - | Data
5 | Session | Dialog, Connection | - | Data
4 | Transport | TCP, UDP, Port, Segment | - | Segment
3 | Network | IP, Routing, Packet | Router | Packet
2 | Data Link | MAC, Switch, Frame | Switch | Frame
1 | Physical | Cable, Hub, Bits | Hub, Repeater | Bits
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