Network+ Lesson 2: OSI Model

What is a Protocol

- Set of rules for formatting and processing data
- A common language so different hardware/software can communicate
- Common examples: HTTP, HTTPS, SMTP, FTP, TCP, UDP

(OSI) Model Layers (overview and mnemonics)

- Open Systems Interconnection model: 7 layers
- 7 Application, 6 Presentation, 5 Session, 4 Transport
- 3 Network, 2 Data Link, 1 Physical
- Built for interoperability between different vendors' devices
- Top down (7-1): All People Seem To Need Data Processing
- Bottom up (1-7): Please Do Not Throw Sausage Pizza Away

(OSI) Model Layers (function of each layer)

- L7 Application: creates data to send, processes data received
- L6 Presentation: converts, encodes, compresses, encrypts data
- L5 Session: controls dialog; manages persistent connections
- L4 Transport: end-to-end delivery via TCP or UDP
- L3 Network: between networks, using IP addresses
- L2 Data Link: within the same network, using MAC addresses
- L1 Physical: bits as electrical signals, light, or radio frequency

Encapsulation / De-encapsulation

- Sender encapsulates (top down); receiver de-encapsulates (bottom up)
- L7-L5: Data (Don't)
- L4: Segment / Datagram (Stop)
- L3: Packet (Pouring)
- L2: Frame (Free)
- L1: Bits (Beer)

Layer 7 - Application

- Entry point to network services for applications
- Apps use it to send (TX) and receive (RX) data
- Handles user apps, network management, remote access
- Examples: SMTP 25, HTTP 80, FTP 21, Telnet 23
- Examples: TFTP 69, SNMP 161

Layer 6 - Presentation Layer

- Concerned with data format (apps need a common format)
- Services: data conversion, character code translation
- Services: compression, encryption and decryption
- Examples: ASCII, JPEG, MPEG

Layer 5 - Session

- Logical persistent connection between peer hosts
- Creates, monitors, and tears down sessions
- Modes: full duplex, half duplex, simplex
- Examples: SQL, RPC, NFS

Layer 4 - Transport

- End-to-end transport between peer hosts
- TCP: connection-oriented, reliable, full-duplex, uses flags
- UDP: connectionless, best effort
- Also here: SSL/TLS
- Ports: well-known 0-1023, registered 1024-49151
- Dynamic/private ports: 49152-65535

Layer 3 - Network

- Moves data between hosts on different networks
- Uses logical (IP) addresses and routing tables
- IP is connectionless: finds best route, no delivery guarantee
- Device: router
- Protocols: IP, ICMP, RIP, OSPF, BGP, IPSec, NAT

Layer 2 - Data Link Layer (functions)

- Takes packets from Layer 3 and sends frames
- Detects errors in frames
- Uses MAC (hardware) addresses
- Moves data to the next physically connected device
- Sublayers: LLC (upper) and MAC
- Devices: switches and bridges

Media Access Control layer

- Sublayer of Layer 2 (data link)
- Controls access to the physical network medium
- Handles addressing and channel access using MAC addresses

Logical Link Control layer

- Upper sublayer of Layer 2 (data link)
- Provides flow control and error control

Layer 2 - Data Link Layer (proto

- ARP (Address Resolution Protocol): resolves IP to MAC
- Ethernet (IEEE 802.3): the main Layer 2 standard in use

Layer 1 - Physical

- Receives bits from Layer 2
- Converts bits to electrical signals, or light pulses on fiber
- Includes physical topologies
- Devices: cables, connectors, hubs, modems, wireless access points
- Ethernet examples: 10BASE-T, 100BASE-TX, 1000BASE-T, 1000BASE-SX
- Note: Lesson 3 notes place access points at Layer 2 (data link)