OSI MODEL

The Open Systems Interconnection (OSI) model is a conceptual 7-layer framework developed by the ISO to standardize networking functions, allowing diverse systems to communicate. It maps the flow of data from software applications down to physical hardware, aiding in troubleshooting, security, and interoperability.

A	Layer	Protocols / Data	Data Unit	Functions	Hardware / Examples
A	Application	HTTP, HTTPS, FTP, TELNET, SMTP, DNS	Message	Provides services to end-users, interface for apps	Browsers (Chrome/Firefox), Email clients, Servers
P	Presentation	JPEG, PNG, GIF, MOV, SSL/TLS	Message	Data translation, encryption/decryption, compression	Handled by software, libraries (e.g., OpenSSL)
S	Session	RPC, NetBIOS, APIs	Message	Session establishment, management, termination, authentication	Software (API managers, session controllers)
T	Transport	TCP, UDP	Segment (Seq.Num +Port.Num +DataUnit)	Flow control, error detection/correction, sequencing, port addressing	Firewalls (stateful), load balancers
N	Network	IP, ICMP	Packet (Frame+IP)	Logical addressing, routing, packet forwarding	Routers, Layer 3 switches
D	Data Link	Ethernet, PPP, ARP	Frame (Bit+MAC)	Physical addressing (MAC), error detection, frame delimiting	Switches, bridges, NICs
P	Physical	Ethernet, DSL, USB	Bit	Transmission of raw bits over medium	Cables, hubs, repeaters, NICs

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Boolean Algebra

(A+0)//(A+A)=A=(A.1)//(A.A)
(A+1)//(A+A’)=1 (A.0)=0=(A.A’)

This Emplies,

Boolean Algebra – Key Points

1. Definition
   Mathematical system used in digital circuits.
   Variables have only two values: 0 (False) and 1 (True).

2. Basic Operations
   AND (·)
   1·1 = 1, otherwise 0

OR (+)
0+0 = 0, otherwise 1

NOT (')
0' = 1
1' = 0

3. Basic Laws

Identity Law
A + 0 = A
A · 1 = A

Null Law
A + 1 = 1
A · 0 = 0

Idempotent Law
A + A = A
A · A = A

Complement Law
A + A' = 1
A · A' = 0

Double Negation
(A')' = A

4. Commutative Law
   A + B = B + A
   A · B = B · A

5. Associative Law
   (A + B) + C = A + (B + C)
   (A · B) · C = A · (B · C)

6. Distributive Law
   A · (B + C) = AB + AC
   A + BC = (A + B)(A + C)

7. De Morgan’s Theorem
   (A + B)' = A'B'
   (AB)' = A' + B'

8. Absorption Law
   A + AB = A
   A(A + B) = A

9. Principle of Duality
   Replace + with · and 0 with 1 to get dual expression.

10. Boolean Functions
    Logical expressions made using variables and operators used to design digital circuits.

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