OSI Model Overview

Application Layer (Layer 7)
- Functions: Manages user interaction, application services, and protocols.
- Common Protocols: FTP, SNMP, TELNET, HTTP, SMTP.
- Technologies: Gateways.
- Security: Involves encryption technologies like SSL, S-HTTP.
Presentation Layer (Layer 6)
- Functions: Translates data between the application layer and the network, handles encryption, compression, and data formatting.
- Common Standards: JPEG, TIFF, HTML.
- Technologies: Gateways.
- Security: Encryption and decryption.
Session Layer (Layer 5)
- Functions: Manages sessions between applications, including setting up, coordinating, and terminating connections.
- Common Protocols: NFS, SQL, RADIUS, RPC.
- Technologies: Gateways.
Transport Layer (Layer 4)
- Functions: Ensures reliable data transfer, error checking, flow control, and segmentation.
- Common Protocols: TCP, UDP, SSL, SSH.
- Security: Confidentiality, authentication, integrity.
- Technologies: Gateways.
Network Layer (Layer 3)
- Functions: Manages logical addressing, routing, and packet forwarding.
- Common Protocols: IP, ICMP, BGP, OSPF, ARP.
- Security: Path selection, logical addressing.
- Technologies: Routers, virtual circuits.
Data Link Layer (Layer 2)
- Functions: Handles physical addressing, error detection, and frame formatting.
- Common Protocols: ARP, PPP, Ethernet, Token Ring.
- Technologies: Switches, bridges.
- Security: Physical hardware addressing.
Physical Layer (Layer 1)
- Functions: Deals with the physical transmission of data, converting bits into electrical or light signals.
- Technologies: Cables, hubs, repeaters, USB, DSL, ISDN.
- Physical Topologies: BUS, MESH, STAR, RING.

Layer	Name	Layer Number	Functions	Common Protocols/Standards	Technologies	Security Focus	Examples
7	Application	7	User interaction, application services	FTP, SNMP, TELNET, HTTP, SMTP, TFTP, LDAP	Gateways	User data security	Email, file transfer, remote login
6	Presentation	6	Data translation, encryption, compression	JPEG, TIFF, GIF, HTML, MIDI, ASCII, EBCDIC	Gateways	Data encryption and decryption	Data formatting, character encoding, multimedia processing
5	Session	5	Session establishment, maintenance, termination	NFS, SQL, RADIUS, RPC, PPTP	Gateways	Session management and security	Remote procedure calls, session management
4	Transport	4	End-to-end data transfer, error correction, flow control	TCP, UDP, SSL, SSH, SPX	Gateways, firewalls	Confidentiality, integrity, authentication	Segmentation, sequencing, acknowledgment
3	Network	3	Routing, logical addressing, packet forwarding	IP, ICMP, BGP, OSPF, ARP, RIP, IPX, X.25, NAT	Routers, virtual circuits	Path selection, error detection	IP addressing, routing decisions, packet forwarding
2	Data Link	2	Physical addressing, error detection, frame formatting	Ethernet, Token Ring, PPP, ARP, HDLC, Frame Relay, MAC, LLC	Switches, bridges, NICs	Hardware addressing, data framing	MAC address management, error checking, flow control
1	Physical	1	Physical signaling, bit transmission, electrical/optical characteristics	Ethernet (cabling), DSL, ISDN, USB, Fiber, Wireless	Cables, hubs, repeaters, transceivers, connectors	Physical media security	Voltage levels, timing of signals, transmission media

Additional Information

Mnemonic: “All People Seem to Need Data Processing” for remembering the layers in order.
Encapsulation Process: Data is encapsulated with protocol information as it moves down the OSI layers, forming a data packet ready for transmission.
Topologies at Physical Layer: BUS, MESH, STAR, TREE, RING.
Session Layer Communication Modes: Simplex, Half-duplex, Full-duplex.
Transport Layer Communication: Connection-oriented (TCP) vs. connectionless (UDP).
Network Layer Addressing: Logical addressing (IP addresses).
Data Link Layer Addressing: Physical addressing (MAC addresses).