OSI Model, Devices, Protocols and IEEE Standards

OSI Layer

OSI vs TCP/IP Layer Mapping

OSI Layer	TCP/IP Layer
7 - ==Application==	==Application==
6 - Presentation	Application
5 - Session	Application
4 - ==Transport==	==Transport==
3 - ==Network==	==Internet==
2 - ==Data Link==	==Network Access==
1 - Physical	Network Access

OSI Model – 7 Layers of Network Communication ⭐

7. Application Layer (Layer 7)

Function: Interface between user and network
Unit of Data: Data (Message)
Protocols: HTTP, HTTPS, FTP, SMTP, DNS, DHCP, SNMP, SSH
Services: File transfers, email, browsing, remote access
Port Numbers: Yes (e.g., HTTP – 80, HTTPS – 443)

6. Presentation Layer (Layer 6)

Function: Data translation, encryption, compression
Examples: SSL/TLS, JPEG, MPEG, ASCII, EBCDIC conversion
Encryption: SSL/TLS (also works with Layer 7 protocols)
Compression: e.g., MP3, ZIP
Note: Also merged into Application Layer in TCP/IP

5. Session Layer (Layer 5)

Function: Establish, maintain, terminate communication sessions
Examples: API calls, NetBIOS, RPC
Dialog Control: Half/full duplex session handling
Synchronization: Add checkpoints for recovery
Note: Merged into Application Layer in TCP/IP

4. Transport Layer (Layer 4)

Function: End-to-end communication==, ==reliability==, ==error checking
Unit of Data: Segments (TCP)== / ==Datagrams (UDP)
Devices: Gateway (in legacy context)
Protocols: TCP, UDP
Ports: Yes (Port Numbers like 80, 443, etc.)
Flow Control: TCP uses sliding window
Error Control: Acknowledgements, retransmission
Note: Gateway is Layer 3 to 7 Device

3. Network Layer (Layer 3)

Function: Routing, logical addressing, packet forwarding
Unit of Data: Packets
Devices: Router, Layer-3 Switch
Addresses: IP Address (32-bit IPv4 / 128-bit IPv6)
Protocols: IP, ICMP, IGMP, ARP, RARP
Routing Algorithms: OSPF, RIP, BGP

2. Data Link Layer (Layer 2)

Function: Node-to-node delivery, MAC addressing, framing
Unit of Data: Frames
Devices: Switch==, ==Bridge
Addresses: MAC Address (48-bit)
Protocols: Ethernet, PPP, ARP, STP
Sub-layers: LLC (Logical Link Control), MAC (Media Access Control)
Error Detection: Yes (CRC==, ==checksum)

1. Physical Layer (Layer 1)

Function: Transmits raw bits (0s and 1s) over physical medium
Unit of Data: Bits
Devices: Cables, Hubs==, ==Repeaters, Network Interface Cards (NIC)
Mediums: Copper wires, Fiber optics, Radio signals
Standards: IEEE 802.3 (Ethernet), RS-232, DSL

Layer	Data Unit	Device	Address Type	Special Features
Physical	Bits	==Hub, Repeater, Cable==	-	Signals, Voltages, Media
Data Link	Frames	==Switch, Bridge==	MAC Address	Framing, ==Error detection, Flow control==
Network	Packets	Router	IP Address	Routing, Logical addressing
Transport	Segments	-	Port Numbers	Reliability, ==Flow & Error control==
Session	Data	-	-	Session setup, checkpointing, Synchronisation
Presentation	Data	-	-	Encryption, Translation, Compression
Application	Data	-	-	End-user apps, Protocols like HTTP, DNS

Special concepts/features for each OSI layer

7. Application Layer (Layer 7)

Network Services: Email, file transfer, web
User authentication & authorization
Application Protocols: HTTP, FTP, DNS, SMTP, DHCP, etc.
Data presentation to end-user
More:
- Application Attacks: SQL Injection, Cross-site scripting (XSS)
- Cookies, Caching, Session management
- FTP Modes: Active vs Passive
- HTTP Methods: GET, POST, PUT, DELETE
- DNS Record Types: A, AAAA, MX, CNAME, NS

6. Presentation Layer (Layer 6)

Data Translation: ASCII ⇄ EBCDIC
Encryption/Decryption: SSL/TLS
Compression/Decompression: JPEG, ZIP
Serialization (e.g., JSON, XML)
More:
- Syntax Translation Examples: JSON ↔ Binary, XML ↔ Binary
- Secure Transmission Handling (TLS Handshake)
- Character Encoding Standards: UTF-8, ASCII, EBCDIC

5. Session Layer (Layer 5)

Session establishment, maintenance, termination
Dialog control: full-duplex/half-duplex
Synchronization: Add checkpoints for recovery
More:
- Token Management (used in some protocols)
- Dialog Management Protocols (not common now but exam relevant)
- Remote Procedure Call (RPC)

4. Transport Layer (Layer 4)

Segmentation and reassembly
Port numbers
Reliable transmission: TCP
Unreliable but fast: UDP
Flow Control: Sliding Window
Error Control: ACKs==, ==checksums
Multiplexing/Demultiplexing
Error control mechanism: ARQ
- Stop-and-Wait ARQ (Automatic Repeat reQuest):
- Go-Back-N ARQ (Automatic Repeat reQuest):
- Selective Repeat ARQ (Automatic Repeat reQuest):
More:
- TCP Header Fields: Sequence Number, ACK, Flags (SYN, FIN, RST, etc.)
- Connection Establishment: Three-Way Handshake
- Connection Termination: Four-step FIN process
- Congestion Control Algorithms:
  - Slow Start, Congestion Avoidance, Fast Retransmit, Fast Recovery
- Flow vs Congestion Control (difference)

3. Network Layer (Layer 3)

IP addressing (IPv4/IPv6)
Routing Algorithms: OSPF, RIP, BGP
Fragmentation/Reassembly
Packet forwarding
Error Messaging: ICMP
Multicast Handling: IGMP
Address Mapping: ARP/RARP
More:
- IP Header Fields (e.g., TTL, Version, Protocol)
- CIDR (Classless Inter-Domain Routing)
- NAT (Network Address Translation)
- Subnetting & Supernetting
- Private vs Public IPs
- ICMP Message Types (Echo, Destination Unreachable, etc.)

2. Data Link Layer (Layer 2)

Framing
MAC addressing (48-bit)
Error Detection: CRC==, ==Checksum
Error Detection & Correction: 2D Parity==, ==Hamming Code
Flow Control: Stop-and-wait ARQ==, ==sliding window== (optional), ==Go Back N ARQ== , ==Selective Repeat ARQ
Access control: CSMA/CD (Ethernet), CSMA/CA (Wi-Fi)
Sublayers: LLC & MAC
Protocols: Ethernet, PPP, ARP, STP
More:
- Framing Methods:
  - Character Count, Byte Stuffing, Bit Stuffing
- Logical Link Control (LLC) & Media Access Control (MAC) sublayers
- Access Control Methods:
  - ALOHA, CSMA/CD, CSMA/CA
- Switching Techniques: Circuit, Packet, Message (conceptual link)
- HDLC Protocol (High-Level Data Link Control)

1. Physical Layer (Layer 1)

Modulation/Demodulation
Signal encoding (analog/digital)
- Non-Return to Zero (NRZ): 1 = High, 0 = Low (no mid-bit transition)
- Return to Zero (RZ): 1 = High then zero in same bit, 0 = stays zero
- Manchester Encoding: 1 = Low to High transition, 0 = High to Low transition
Bit synchronization
Voltages, pin-outs, connectors
Media types: copper, fiber, wireless
Standards: IEEE 802.3, RS-232
More:
- Line Coding (Already Added): NRZ, RZ, Manchester
- Bit Rate vs Baud Rate
  - Bit Rate = bits/sec, Baud Rate = symbols/sec
- Multiplexing Techniques:
  - TDM (Time Division Multiplexing)
  - FDM (Frequency Division Multiplexing)
  - WDM (Wavelength Division Multiplexing – used in fiber)
- Attenuation, Noise, Signal-to-Noise Ratio (SNR)
- Transmission Modes: Simplex, Half Duplex, Full Duplex

Protocols

Networking Protocols & OSI Model Layers

7. Application Layer (OSI Layer 7 / TCP Layer 4)

Protocol	Full Form	Function/Use	Transport Layer	Port Number
HTTP ⭐	HyperText Transfer Protocol	Web browsing (www), transfers web pages	TCP	==80==
HTTPS	HTTP Secure	Secure version of HTTP using SSL/TLS encryption	TCP	==443==
FTP ⭐	File Transfer Protocol	Transfer files between computers	TCP	20 (Data), 21 (Control) ⭐
SFTP	Secure FTP	Encrypted version of FTP (via SSH)	TCP	22
SMTP ⭐	Simple Mail Transfer Protocol	==Sends emails== (outgoing mail)	TCP	25 (or 587 for submission)
POP3 ⭐	Post Office Protocol v3	Downloads emails to local client (one-way)	TCP	110
IMAP ⭐	Internet Message Access Protocol	Email access & sync from server	TCP	143 (993 for IMAPS)
DNS ⭐	Domain Name System	Converts domain names to IP addresses	TCP/==UDP==	53
DHCP	Dynamic Host Configuration Protocol	Assigns IP addresses dynamically	==UDP==	67 (Server), 68 (Client)
SNMP	Simple Network Management Protocol	Monitors and manages network devices	==UDP==	161 (queries), 162 (traps)
Telnet	-	Remote terminal access (plain text, insecure)	TCP	23
SSH	Secure Shell	Secure remote login (replaces Telnet)	TCP	22

2. Transport Layer (OSI Layer 4 / TCP Layer 3)

Protocol	Full Form	Function/Use
TCP	Transmission Control Protocol	Reliable, connection-oriented, error-checked
UDP	User Datagram Protocol	Fast, connectionless, no guarantee of delivery

Note:

TCP is used in HTTP, FTP, SMTP, etc.
UDP is used in DNS, VoIP, video streaming, etc.

3. Network Layer (OSI Layer 3 / TCP Layer 2)

Protocol	Full Form	Function/Use
==IP==	Internet Protocol	==Assigns addresses== and routes packets
ICMP	Internet Control Message Protocol	Error & control messages (used in ping)
IGMP	Internet Group Management Protocol	Manages multicast group memberships
==ARP==	Address Resolution Protocol	==Resolves IP== → MAC address within LAN
RARP	Reverse ARP	Resolves MAC → IP address (obsolete)

IP Types:

IPv4: 32-bit
IPv6: 128-bit (modern, supports more devices)

4. Data Link Layer (OSI Layer 2 / TCP Layer 1)

Protocol	Full Form	Function/Use
Ethernet	-	LAN communication using MAC address
PPP	Point-to-Point Protocol	Connects two routers directly over serial link
HDLC	High-Level Data Link Control	Used in WANs
STP	Spanning Tree Protocol	Prevents loops in LAN (switch networks)

5. Physical Layer (OSI Layer 1 / TCP Layer 1)

Protocol/Tech	Function/Use
DSL	Digital Subscriber Line (broadband)
ISDN	Integrated Services Digital Network
Ethernet	Also works here (defines cables/speeds)
USB, RS-232	Serial transmission protocols

TCP/IP Protocol Suite with Real Life Examples | Why TCP/IP Used | Fundamentals of Networking

                                       ☁️ Server (eg. google)
                                       /
                                      /
These layer are manage by           🌐 www /Internet
OS in device automatically          /
┌---------┐                        /
| A Layer |                      📶 Internet Service Provider (ISP)
|---------|                      /
| T Layer |                     /
|---------|                    /
| I Layer |                  🛜 Router / Wifi Access Point
|---------|                 /
| L Layer |                /
└---------┘  ↰            /
             💻---------⬏
             Device

In every device, the way data is sent and received over the internet follows a specific structure of layers, which are managed by the operating system (OS). These layers ensure that data reaches its destination correctly and securely.

Imagine you want to visit a website like www.google.com on your browser. When you type the website address and press enter, the data flow through several layers in your device and the network, ultimately displaying the webpage on your screen. Let’s break down these layers:

7. Application Layer

When you open a website like www.google.com, your browser communicates using the http:// or https:// protocols. Here’s what happens at this layer:
Encryption & Decryption: If you’re using https://, your data is encrypted before being sent out, ensuring security. When the data is received, it’s decrypted so it can be understood by your browser.
Domain Name System (DNS): The website address you type, such as www.google.com, isn’t directly understood by computers. They need an IP address, which is like a phone number for servers. The DNS, working at this layer, translates the domain name (www.google.com) into an IP address of the nearest Google server so your request can be routed correctly.
For example, when you enter www.google.com, DNS might translate it to an IP address like 172.217.12.206, which your browser will use to reach Google’s server.
Note: Even if you’re using other applications like WhatsApp, which doesn’t use http or https, they still operate under the Application Layer. These applications might use proprietary protocols for encrypting and transferring data, but they still follow the same layering structure.

4. Transport Layer

In your device, you might have several applications running simultaneously, like your browser, email client, or messaging apps. The Transport Layer is responsible for managing how data is sent and received by these different applications. It does this by assigning a port number to each application, ensuring that data intended for your web browser doesn’t accidentally end up in your email client.
TCP (Transmission Control Protocol): This is used when you need a reliable connection. For example, when you’re loading a webpage, TCP ensures that all data packets arrive correctly and in the right order.
UDP (User Datagram Protocol): This is used for faster, but less reliable connections. It’s often used for live streaming or online gaming, where speed is more critical than perfect accuracy.
For instance, when you load a website, TCP might use port 80 for http or port 443 for https. Meanwhile, your email application might be using port 25 for sending emails.

3. Internet Layer

This layer deals with IP addresses, which are like the virtual addresses for devices on the internet. When you connect to a Wi-Fi network, your Internet Service Provider (ISP) assigns an IP address to your device through the router. This IP address is used to identify your device on the internet.
IP Address: Think of it as your device’s home address on the internet. Just as a letter needs your home address to reach you, data packets need your IP address to find your device.
For example, when your browser sends a request to Google’s server, it uses your IP address as the return address so Google knows where to send the webpage data back.
Note: To send data, your device needs the IP address of the destination server (like Google’s server). Similarly, the server needs your IP address to send data back to you.

2. Link Layer

This layer is responsible for the physical connection between your device and the network, usually through a MAC address. The MAC address is unique to each network device, like your computer’s Wi-Fi card or the router. It ensures that data packets can be directed to the correct hardware.
MAC Address: Think of it as the physical address of a device on a local network. It’s like the unique serial number of your device’s network interface.
When you send data, the data packet first travels to the router using the MAC address. The router then forwards the data to the ISP, and it continues on to the internet. When the response comes back, it follows the same path in reverse.
For instance, when you send a request from your laptop to the router, your laptop’s MAC address ensures the data packet reaches the correct device (the router) before it’s sent out to the wider internet.
Note: The MAC address is only needed for communication between devices on the same local network, like between your device and the router. Once the data packet leaves your local network, the MAC address isn’t used anymore.

Summary of Addresses in Your System:

Port Number: Used at the Transport Layer to identify the specific application sending or receiving data.
IP Address: Used at the Internet Layer to identify the device on the internet.
MAC Address: Used at the Link Layer to identify the physical device on a local network.

Devices

Network Devices and Their OSI Layers

Layer 7: Application Layer Device	Function	Example(s)
Proxy Server	Acts as intermediary; filters content, caches requests	Squid Proxy, Web Proxy
Gateway ⭐	Translates between different protocols (e.g., HTTP to FTP, VoIP)	Cloud Gateway, IoT Gateway
Load Balancer	==Distributes HTTP/HTTPS traffic==; can perform SSL termination	AWS ELB, HAProxy

Layer 4: Transport Layer Device	Function	Example(s)
Firewall	==Inspects port numbers and transport protocols (TCP/UDP)==	==Stateful Firewall==
Load Balancer	==Distributes TCP/UDP traffic== across servers	F5, NGINX Load Balancer

Layer 3 : Network Layer Devices	Function	Example(s)
Router ⭐	Connects different networks; routes packets using IP addresses	Home Router, Cisco Router
Layer 3 Switch ⭐	Combines switching and routing at high speed	Core Switch, Distribution Switch
Firewall (partially)	==Filters packets based on IP addresses and protocols==	==Network Firewall==
Brouter	Works as both Bridge (Layer 2) and Router (Layer 3)	Hybrid Networking Device

Layer 2 :Data Link Layer Devices	Function	Example(s)
Switch ⭐	Forwards frames to correct MAC address; avoids unnecessary traffic	Ethernet Switch
Bridge ⭐	Connects two LAN segments and filters traffic using MAC addresses	Transparent Bridge
Access Point	Provides wireless connectivity to wired LAN; uses MAC address	Wi-Fi Access Point
NIC	Provides MAC addressing; handles framing and error detection	Ethernet Card, Wi-Fi Adapter

Layer 1 : Physical Layer Devices	Function	Example(s)
Hub ⭐	Broadcasts signals to all ports; no filtering	Passive Hub, Active Hub
Repeater	Regenerates and amplifies signals to extend network range	Wi-Fi Repeater, Signal Booster
Modem (partially)	Converts analog signals to digital and vice versa (modulation/demodulation)	DSL Modem, Cable Modem
NIC (partially)	Transmits raw bits through physical medium	Ethernet Card, Wi-Fi Adapter

Layer 5 & 6 Doesn’t have Device

OSI Layer 5 (Session) and Layer 6 (Presentation) do not have dedicated hardware devices.
These layers are handled by software within end systems (like applications or OS protocols), not by network hardware.

OSI Layer	Purpose	Handled By
Layer 5 – Session	Manages sessions (start, control, end of communication)	Software (APIs, sockets, OS)
Layer 6 – Presentation	Translates data formats (encryption, compression, encoding)	Software (codecs, TLS/SSL libs)

Gateway vs Switch vs Bridge vs Hub vs Repeater

Gateway

A gateway can be any layer of the OSI model, from the network layer (Layer 3) up to the application layer (Layer 7). Unlike a router, which primarily works at the network layer, a gateway’s defining
Explanation:
Basic gateway (router-like) → Works at Layer 3 (Network Layer) to connect networks using different protocols (e.g., IPv4 ↔ IPv6).
Application gateway (proxy, firewall, etc.) → Works at Layer 7 (Application Layer) to translate or filter application data.

Switch

Layer: Works at Data Link Layer (Layer 2)
Function: Connects multiple computers in a LAN== and ==sends data only to the correct device using its MAC address
Smart (It learns MAC addresses and reduces unwanted traffic)
Collision Domain: Each port has its own → No collisions
Broadcast Domain: One (can be divided using VLAN)
Use: Used in offices, colleges for fast and efficient communication
Extra: Supports full-duplex (send + receive together), very fast

Bridge

Layer: Also at Layer 2
Function: Connects two LANs or two segments== of a LAN, and ==controls traffic using MAC addresses
Semi-smart (Not as fast as switch, but can filter traffic)
Collision Domain: Divides collision domain (each side separate)
Broadcast Domain: One
Use: Used to reduce traffic between two departments/floors
Extra: Old device, now mostly replaced by switch

Hub

Layer: Works at Physical Layer (Layer 1)
Function: Blindly broadcasts data to all devices, not just one
Dumb (Doesn’t care who the data is for)
Collision Domain: One big shared → More collisions
Broadcast Domain: One
Use: Very small networks (now outdated)
Extra: Works in half-duplex (send or receive, not both)

Repeater

Layer: Works at Layer 1
Function: Boosts weak signals so that data can travel longer distances
Not smart (No address checking)
Use: Extend range of LAN cable or Wi-Fi signal
Extra: Used in large buildings or long cable runs

Device	Layer	Smart?	Filters Data?	Use
Switch	L2	Yes	Yes (MAC)	Efficient LAN communication
Bridge	L2	Medium	Yes (MAC)	Connect 2 LANs or segments
Hub	L1	No	No	Small/simple network (obsolete)
Repeater	L1	No	No	Boost signal over long distance

IEEE 802 Networking Standards

IEEE Std	Technology	Description / Use	Common Devices
802.1 ⭐	Bridging & VLAN	LAN switching, VLANs, QoS	Switches
802.2 ⭐	LLC	Logical Link Control (Data Link sublayer)	NICs, Switches
802.3 ⭐	==Ethernet==	Wired LAN using ==CSMA/CD==	NICs, Switches
802.4	==Token Bus==	Token passing on bus topology (obsolete)	Industrial networks
802.5	==Token Ring==	Token passing on ring topology (obsolete)	MAUs, NICs
802.6	DQDB	MAN using dual bus (rare)	MAN switches
802.11 ⭐	==Wi-Fi (Wireless LAN)==	Wireless LAN (a/b/g/n/ac/ax)	Routers, Access Points
802.15	==Bluetooth==, WPAN	Wireless Personal Area Network	Bluetooth Devices
802.16	WiMAX	Broadband Wireless MAN	WiMAX Devices
802.17	RPR	High-speed metro networks	Metro Rings
802.20	Mobile Broadband	Mobile broadband wireless access	Base stations