OSI Model (Network+ 009)

Open Systems Interconnection (OSI)

Developed in 1977

- A reference model used to categorize the functions of a network.

- Useful for Troubleshooting

- Has 7 Layers

Layer 1: Physical

Data in layer 1 is bits.

Layer 2: Data Link

Data in Layer 2 is called Frames

Layer 3: Network

Data in Layer 3 is called Packets.

Layer 4: Transport

TCP: Segments

UDP: Datagrams

Layer 5: Session

Data in Layer 5 is called Data

Layer 6: Presentation

Data in Layer 6 is called Data

Layer 7: Application

Data in Layer 7 is called Data

What are the two types of transition modulation and how do they work?

Copper Wire: Uses voltage (0V for 0, +5V/-5V for 1)

Fiber Optic: Uses light (on for 1, off for 0)

What's the difference between crossover and straight-through cables?

Crossover: TIA/EIA-568A on one end, 568B on the other

Straight-through: TIA/EIA-568B on both ends

What's the difference between broadband and baseband?

Broadband: Divides bandwidth into separate channels (e.g., cable TV)

Baseband: Uses all frequency all the time (e.g., telephone)

What are the three types of multiplexing?

TDM (Time Division): Allocates dedicated time slots

StatTDM (Statistical TDM): Dynamically allocates time slots

FDM (Frequency Division): Divides medium into channels

Name 3 Layer 1 devices

Hubs, Access Points, Media Converters, Cables

What is the key characteristic of Layer 1 devices?

They simply repeat whatever they receive - no logic or decision-making

What is a MAC address and what is its structure?

48-bit physical address assigned to every NIC

Written in hexadecimal

First 24 bits: Manufacturer ID

Last 24 bits: Specific device ID

What are the two main functions of the Data Link Layer?

Logical Link Control (LLC) - connection services, flow control, error detection

MAC addressing - device identification

What are the three synchronization methods at Layer 2?

Isochronous: Common reference clock, time slots

Synchronous: Same clock, frames with control characters

Asynchronous: Own clock cycles, no strict timing

Name 3 Layer 2 devices

NICs, Bridges, Switches

How do switches operate?

Use CAM tables with MAC addresses to identify physical ports and selectively transmit data to specific devices

What does LLC provide?

Connection services, acknowledgment of messages, flow control, and checksum for error detection

What are the two main logical addressing schemes?

IPv4: Dotted octet notation (e.g., 172.16.254.1)

IPv6:

What are the three switching/routing methods?

Packet Switching: Data divided into packets (most common)

Circuit Switching: Dedicated communication link

Message Switching: Messages stored and forwarded

What does ICMP stand for and what are its two main uses?

Internet Control Message Protocol

PING: Tests connectivity and response times

Traceroute: Traces packet route through network

Name 3 Layer 3 devices/protocols

Routers, Multi-layer switches, IPv4, IPv6, ICMP

What is the difference between a regular switch and a multi-layer switch?

Regular switch: Layer 2 only

Multi-layer switch: Combines Layer 2 switch and Layer 3 router features

What is the Three-Way Handshake?

SYN (Synchronization)

SYN-ACK (Synchronization-Acknowledgement)

ACK (Acknowledgement)

TCP

Reliable

Connection-oriented

Uses three-way handshake

Acknowledgments and retransmission

Windowing and flow control

Sequencing

UDP

Unreliable

Connectionless

No handshake

No acknowledgments or retransmission

No windowing

No sequencing

When would you use TCP vs UDP?

TCP: Network data that must reach destination (web, email, file transfer)

UDP: Audio/visual streaming where speed matters more than reliability

What is windowing?

Allows clients to adjust the amount of data in each segment to optimize throughput; can open or close based on retransmissions

What is buffering at Layer 4?

Temporary memory storage for segments when bandwidth isn't readily available

Name 3 Layer 4 devices/protocols

TCP, UDP, WAN accelerators, Load balancers, Firewalls

What are the three main functions of the Session Layer?

Setting up sessions (credentials, session numbers)

Maintaining sessions (data transfer, acknowledgments)

Tearing down sessions (ending communication)

Name 2 Layer 5 protocols and their purposes

H.323: Sets up/maintains/tears down voice and video (uses RTP)

NetBIOS: File sharing over network (Windows file sharing)

What happens if a session breaks during maintenance?

It requires re-establishment of the connection

What does the Session Layer ensure?

Separate conversations to prevent data intermingling between different sessions

What are the two main functions of the Presentation Layer?

Data Formatting (compatibility between devices)

Encryption (data security)

Name 3 text format types

ASCII, Unicode, EBCDIC

Name 3 image formats

GIF, JPEG, PNG, TIFF, SVG

Name 3 movie formats

MP4, MPEG, MOV

Name 3 scripting languages at Layer 6

HTML, XML, PHP, JavaScript

What encryption protocols work at Layer 6?

TLS (Transport Layer Security), SSL (Secure Sockets Layer)

What does TLS do?

Creates an encrypted tunnel to protect sensitive information during transfer

What does the Application Layer provide?

Application-level services where users communicate with the computer

Name the email protocols (3)

POP3 (Post Office Protocol 3)

IMAP (Internet Message Access Protocol)

SMTP (Simple Mail Transfer Protocol)

Name the web browsing protocols (2)

HTTP, HTTPS

Name the file transfer protocols (3)

FTP, FTPS, SFTP

Name the remote access protocols (3)

Telnet, SSH, SNMP

What is DNS?

Domain Name Service - translates domain names to IP addresses

What is service advertisement?

Applications send announcements to advertise their services on the network (e.g., printers, file servers)

What is encapsulation?

The process of putting headers (and sometimes trailers) around data as it moves DOWN the OSI model (Layer 7→1)

What is decapsulation?

Removing headers/trailers to access original data as it moves UP the OSI model (Layer 1→7)

Name 6 TCP control flags

SYN: Synchronize connection

ACK: Acknowledgment

FIN: Tears down connection

RST: Reset when unexpected packet received

PSH: Prioritizes data

URG: Marks data as urgent

What information is in a TCP header? (Name 5 mandatory fields)

Source port

Destination port

Sequence number

Acknowledgment number

Control flags

Window size

Checksum

What information is in an IP header? (Name 5 fields)

Version

Source IP

Destination IP

TTL (Time to Live)

Protocol

Header checksum

What information is in an Ethernet header?

Source MAC address

Destination MAC address

EtherType field (IPv4/IPv6)

Optional VLAN tag

What is the Maximum Transmission Unit (MTU) for Ethernet? What are Jumbo Frames?

1500 bytes

Frames larger than 1500 bytes (requires reconfiguring MTU)

What is the minimum payload size?

46 bytes (no VLANs)

42 bytes (with VLANs)