OSI model — 7 layers, responsibilities, PDU names

4.3.1 · Coding › Computer Networks

OSI model exist kyun karta hai?

Model ki zaroorat hi kyun hai?

Alag-alag vendors (Cisco, Intel, Microsoft) alag hardware/software banate hain. Agar shared blueprint na ho, toh kuch bhi interoperate nahi karta.
Separation of concerns: har layer ko independently design, debug, aur replace kiya ja sakta hai. Tum Wi-Fi ko Ethernet (Layer 1/2) se swap kar sakte ho bina web browser (Layer 7) ko chhue.

7 layers kya hain? (Top → Bottom)

#	Layer	Core responsibility	PDU (data unit)	Address used
7	Application	User/apps ke saath interface (HTTP, FTP, DNS)	Data	—
6	Presentation	Translation, encryption, compression	Data	—
5	Session	Sessions set up / maintain / tear down karna	Data	—
4	Transport	End-to-end delivery, reliability, ports	Segment (TCP) / Datagram (UDP)	Port number
3	Network	Logical addressing + networks ke across routing	Packet	IP address
2	Data Link	Same link par node-to-node delivery, framing, MAC	Frame	MAC address
1	Physical	Medium par raw bit transmission	Bit	—

Figure — OSI model — 7 layers, responsibilities, PDU names

Data actually kaise move karta hai: Encapsulation

Sender (stack mein NEECHE jaate hue): $Data + TCP header Segment + IP header Packet + MAC header/trailer Frame Bits$

Receiver (stack mein UPAR jaate hue): har header strip karo, names wapas Data tak reverse karo.

Layer-by-layer deep dive (har layer ki zaroorat kyun hai)

Flashcards

Order of OSI layers top to bottom

Application, Presentation, Session, Transport, Network, Data Link, Physical

Mnemonic for OSI top→bottom

All People Seem To Need Data Processing

PDU at the Transport layer

Segment (TCP) / Datagram (UDP)

PDU at the Network layer

Packet

PDU at the Data Link layer

Frame

PDU at the Physical layer

Bit

PDU at Application/Presentation/Session

Data

Which layer does a router operate at?

Layer 3 (Network) — uses IP addresses

Which layer does a switch operate at?

Layer 2 (Data Link) — uses MAC addresses

Which layer handles port numbers?

Layer 4 (Transport)

Which layer does encryption/compression/translation?

Layer 6 (Presentation)

Which layer manages sessions/checkpoints/dialog control?

Layer 5 (Session)

What is encapsulation?

Har layer ka upar se aaye data ko apne header (aur L2 par trailer) ke saath wrap karna

Why does the PDU name change at each layer?

Kyunki har layer apna header add karti hai, ek naya logical unit create hota hai

Which layer uses logical (IP) addresses?

Layer 3 (Network)

Which layer uses physical (MAC) addresses?

Layer 2 (Data Link)

Mnemonic for PDU chain bits→data

Big Fat Pandas Sleep Deeply (Bit, Frame, Packet, Segment, Data)

Which OSI layers map to TCP/IP's single Application layer?

Application + Presentation + Session

Recall Feynman: ek 12-saal ke bachche ko samjhao (click to reveal)

Socho tum apne ek dost ko doosre shehar mein birthday gift bhej rahe ho. Tum toy ko seedha road par nahi phek sakte aur umeed nahi kar sakte. Toh: tum toy wrap karte ho (tumhara message), use ek box mein daalo, uspar apne dost ka ghar number AUR gali ka naam likho, postman box ko apni cycle par post office tak le jaata hai, truck use highway par drive karke le jaata hai. Tumhare dost ki taraf, har koi ulta har step un-do karta hai jab tak tumhara dost toy haath mein nahi le leta. OSI model bas 7 helpers ek line mein hain, har ek bahar jaate waqt ek wrapper add karta hai, aur andar aate waqt ek wrapper hata deta hai. Alag wrapper = alag naam (Bit, Frame, Packet, Segment, Data).

Connections

TCP-IP Model — 4 vs 5 layers
Encapsulation and De-encapsulation
IP Addressing and Subnetting
MAC Address and ARP
TCP vs UDP
Routers vs Switches vs Hubs
Ports and Sockets