# OSI - Open Systems Interconnection

OSI standardizes how networks should communicate, so different vendors and systems can interoperate.

# OSI vs TCP/IP

osi vs tcpip

Physical: Binary transmission
- wires, connectos, voltages, data rates
Data Link: Access to media
- provides reliable transfer of data across media
- physical addressing, logical network topology, error notification, flow control
Network: address & best path
- provides connectivity & path selection between 2 end systems
- domain of routing
Transport: e2e connections
- concerned with transportation issues between hosts
- data transport reliability
- establish, maintain, terminate virtual circuits
- fault detection & recovery infor flow control
Session: Dialog control - interhost communication
- establish, maintain, terminate sessions between applications
- half duplex / full duplex
Presentation: Data representation
- ensure data is readable by receiving system
- format of data
- data structure
- negotiates data transfer syntax for app layer
Application: network processes to apps

encapsulation

https://afteracademy.com/blog/what-is-data-encapsulation-and-de-encapsulation-in-networking/

In real networks, protocols don’t strictly follow 7 layers.

For example:

TCP/IP stack (used in the Internet) combines some OSI layers:
- Network Layer → IP
- Transport Layer → TCP/UDP
- Application Layer → HTTP, FTP, etc.
Presentation and Session layers are often handled inside applications rather than as separate protocols.
Hardware (switches, routers, NICs) rarely implements OSI layers in a strict 1-to-1 way—they just handle whatever layers they need.

Layer 1	Layer 2
cannot organize streams of bit	uses framming to organize or group the bits
cannot identify computer	uses an addressing process to identify computer
cannot communicate with the upper-level layers	uses `LLC` to communicate with the upper-level laye
cannot decide which computer will transmit binary data	uses `MAC` to decide which computer will transmit