Section 2: OSI Model

Open Systems Interconnect (OSI) Model

• Developed by the International Organization for Standardization (ISO)

• A reference model

  • Used to categorize functions of a network

  • Used for troubleshooting

• 7 Layers

Layers of the OSI Model (7)

• Layer 1 - Physical

• Layer 2 - Data Link

• Layer 3 - Network

• Layer 4 - Transport

• Layer 5 - Session

• Layer 6 - Presentation

• Layer 7 - Application

• Helpful Pneumonic

  • “Please Do Not Throw Sausage Pizza Away”

Names of Data as it flows through the OSI Model

• Layer 1 - Bits

• Layer 2 - Frames

• Layer 3 - Packets

• Layer 4 - Segments

• Layers 5-7 - Data

Physical Layer

• 1st Layer of the OSI Model

• Where transmission of bits across the network occurs and includes physical and electrical network characteristics

• Data Type : Bits

  • Binary bits represented as a series of 1s and 0s

Transition Modulation

Switching between levels to represent 1 or 0

• If it changes during the clock cycle, then a 1 is represented. otherwise, a 0 is represented

Transition Modulation Cables

• Copper Wire (Cat5/CAt6) - Uses voltage (0V for 0, +5V/-5V for 1)

• Fiber Optic Cable - Uses light (on for 1, off for 0)

Connector Standards

• RJ-45 Connector - Used in CAT5/CAT6 cables

  • Allows us to plug into the back of a computer or switch

Wiring Standards (2)

• TIA/EIA-568A

• TIA/EIA-568B

Crossover Cables

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

Straight-through cable

TIA/EIA-568B on both sides

Physical Topologies

• Bus

• Ring

• Star

• Hub-and-Spoke

• Full Mesh

• Partial-Mesh

Based on how cables are physically connected

Types of Synchronization (2)

• Asynchronous Communication

• Synchronous Communication

Asynchronous Communication

Start and stop bits for out-of-sync transmission

Ex. Voicemails

Synchronous Communication

Real-time communication using a common time source

Ex. Phone call

Bandwidth Utilization

• Broadband

  • Divides bandwidth into separate channels

  • Ex. cable TV

• Baseband

  • Uses all frequency of the cable all the time

  • Ex. telephone

  • Uses a reference clock (synchronous communication)

Multiplexing

Involves taking some limited amount of resource and using it more efficiently

• In our case:

  • It allows multiple people to use a baseband connection at the same time

Types of Multiplexing (3)

• Time Division Multiplexing (TDM)

• Statistical Time Division Multiplexing (STDM)

• Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

Each session takes a turn, using time slots, to share the medium between all users

Statistical Time Division Multiplexing (STDM)

Dynamically allocates the time slots based on when people need it

• If no one is using the time slot but it is not your time slot, you can use it

Frequency Division Multiplexing (FDM)

Divides the medium into channels based on frequencies and session is transmitted over a different channel

Layer 1 Devices

• Cables

• Wireless Media

• Infrastructure Devices

Layer 1 Device - Cable

• Fiber optic

• Ethernet

• Coaxial

Layer 1 Device - Wireless Media

• Bluetooth

• Wi-Fi

• Near field communication

Layer 1 Device - Infrastructure Devices

• Hubs

• Access Points

• Media Converters

Characteristics of a Layer 1 Device

• Simply repeat whatever they are told ; dumb devices

• No logic/ decision-making

Data Link Layer

• 2nd Layer of the OSI Model

• Packages bits from layer 1 into frames and transmits them on the network

• Data Type: Frames

• Performs:

  • Error Detection

  • Error Correction

  • Identifies devices using MAC addresses

  • Flow Control

Media Access Control (MAC) Address

A means for identifying a device physically and allowing it to operate on a logical topology

• A 48-bit physical addressing system is assigned to every network interface card (NIC) produced

  • Written in hexadecimal

  • First 24 bits - manufacturer ID

  • Last 24 bits - specific device ID

Logical Link Control (LLC)

• Provides connection services and allows acknowledgement of receipt of messages, ensuring controlled data flow

• Keeps receiver from being overwhelmed, limits sender data sent

• Uses checksum to detect corrupted data frames

Synchronous Methods at Layer 2 (3)

• Isochronous Mode

• Synchronous Method

• Asynchronous

Layer 2 Synchronization Methods - Isochronous Mode

• Common reference clock

• Time slots for transmissions

• Less overhead

Layer 2 Synchronization Methods - Synchronous Method

Devices agree on a common clocking method to indicate beginning and end frames, and control characters for synchronization

Layer 2 Synchronization Methods - Asynchronous

• Devices reference their own clock styles

• No control over communication timing

Layer 2 Devices

• Network Interface Cards (NICs)

• Bridges

• Switches

Switch Operation

• Switches use CAM tables with MAC addresses to identify physical ports connected to devices

• Enable selective data transmission to specific areas in the network

Network Layer

• 3rd Layer of the OSI Model

• Concerned with routing and forwarding traffic using logical addresses

• Data Type : Packets

Logical Addressing

IP Variants

• Common logical addressing schemes

• IPv4

  • written in dotted octet notation, four sets of numbers

  • ex. 172.16.254.1

• IPv6

Other protocols - replaced by IP

• Apple Talk

• IPX ( Internetwork Packet Exchange)

Switching/Routing Methods (3)

• Packet Switching (Routing)

• Circuit Switching

• Message Switching

Packet Switching (Routing)

Data is divided into packets and then forwarded

• Most common method

Circuit Switching

Dedicated and constant communication link is established between two devices

Message Switching

Data is divided into messages which can be stored and then forwarded

Route Discovery and Selection

Routers maintain routing tables for determining the best path

Routing Protocols

Help decide how data is going to flow across the network and how the routers are going to communicate that information

Connection Services at Layer 3

• Augment Layer 2 services to improve reliability

• Involves flow control

  • Prevents sender form overwhelming the receiver

• Packet reordering

  • Ensures data packet arrival and are reassembled in the correct order

Internet Control Message Protocol (ICMP)

Sends error messages and operation information to an IP destination

• PING

  • Most commonly used ICMP

  • Helps troubleshoot network issues by testing connectivity and response times

• Traceroute

  • Traces the route of a packet through the network

Layer 3 Devices and Protocols

• Routers

• Multi-layer switches

• IPv4

• IPv6

• ICMP

Multi-layer switches

• Combines layer 2 switch and layer 3 router features

• A switch will always be a layer 2 device unless it is mentioned that it is multi-layer switch which is a layer 3 device

Transport Layer

• 4th layer of the OSI Model

• The dividing layer between the upper and lower layers of the OSI model

• Data Type: Segments

Upper Layers of the OSI Model

• Transport

• Session

• Presentation

• Application

Lower Layers of the OSI Model

• Physical

• Data Link

• Network

Layer 4 Protocols

• TCP

• UDP

Transmission Control Protocol (TCP)

• Connection-oriented protocol that is a reliable way to transport segments across the network

• Uses acknowledgement

• Three-Way Handshake

  • SYN - Synchronization

  • SYN-ACK - Synchronization - Acknowledgement

  • ACK - Acknowledgement

• Used for all network data that needs to be assured to get to its final destination

User Datagram Protocol (UDP)

• Connectionless protocol that is an unreliable way to transport segments across the network

• No-three way handshake and less overhead

• No ACK

• Used for audio and visual streaming

• Data Type : Datagrams

TCP vs UDP

• TCP

  • Reliable

    • Three-way handshake

  • Connection-oriented

  • Segment retransmission and flow control through windowing

  • Segment sequencing

  • Acknowledges segments

• UDP

  • Unreliable

  • Connection-less

  • No retransmission and no windowing

  • No sequencing

  • No acknowledgements of datagrams

Windowing

• Allows clients to adjust the amount of data in each segment

• Optimizes throughput and bandwidth

• Opens or closes windows based on retransmissions

Buffering

• Occurs when devices allocate memory to store segments if bandwidth isn’t readily available

• Buffer

  • Temporary storage for segments

• Prevents overflow by clearing segments

Layer 4 Devices and Protocols

• Protocols

  • TCP

  • UDP

• Devices

  • WAN accelerators

  • Load balancers and firewalls

Session Layer

• 5th Layer of the OSI Model

• Manages sessions, ensuring separate conversations to prevent data intermingling

Components in the Session Layer

• Setting Up Session

• Maintaining Session

• Tearing Down a Session

Setting Up Session

Checking of user credentials and assigning numbers to sessions to help identify them

Maintaining Session

• Continuous data transfer between parties

• If connection breaks, it must be re-established

• Includes acknowledgement of data

Tearing Down a Session

Ending a session after the transfer is done or when the other party disconnects

Layer 5 Devices and Protocols

• H.323

  • Used to set up, maintain, and tear down voice and video connections

  • Operates over the real-time transport protocol (RTP)

    • ex. Facetime, Skype

• NetBIOS

  • Used to share files over a network

  • ex. Windows file sharing

• ○ Layer 5 issues involve protocols and software rather than specific devices

Presentation Layer

• 6th Layer of the OSI Model

• Formats the data to be exchanged and secures it with proper encryption

Data Formatting

• Formatting done by a computer to have compatibility between different devices

• Formats

  • American Standard Code for Information Interchange (ASCII)

    • Text-based language to use

    • Ensures data is readable by receiving system

    • Provides proper data structures

    • Negotiates data transfer syntax for the Application Layer

  • GIFs - motion pictures

  • JPEGs - photographs

  • PNGs - Internet images

Encryption

• Used to scramble data in transit to keep it secure and confidentiality

• Transport Layer Security (TLS)

  • Ensures secure data transfer

  • Creates an encrypted tunnel, protecting sensitive information

Scripting Languages in Layer 6

• All control how ASCII text is displayed on-screen

  • HTML

  • XML

  • PHP

  • JavaScript

Standard Text Formats

• Different ways for displaying text using ones and zeros

  • ASCII

  • Unicode

  • EBCDIC

Image Formats

• Graphical representations of ones and zeros

  • GIFs

  • JPEGs

  • TIFFs

  • SVGs

  • PNGs

Movie Files

• Ones and zeros formatted to create watchable videos

  • MP4s

  • MPEGs

  • MOV

Encryption Algorithms

• Scrambles data to provide confidentiality and security during transit and storage

  • TLS

  • SSL (Secure Sockets Layer)

Application Layer

• 7th Layer of the OSI Model

• Provides application-level services where users communicate with the computer

• Focus on lower-level applications

  • File Transfer

  • Network Transfer

Application Services

• Unites communicating components for more than one network application

  • File transfer

  • File sharing

  • Email

    • Low-level protocols

      • POP3 (Post Office Protocol 3)

      • IMAP (Internet Message Access Protocol

      • SMTP ( Simple Mail Transfer Protocol)

  • Remote access

  • Network Management

  • Client-server processes

Service Advertisement

• Applications send announcements to other devices on the network

• Devices advertise the services they offer

  • Printer and file servers managed by Active Directory

  • Self-advertising devices like wireless printers

Layer 7 Protocols

• Email Applications

  • POP3

  • IMAP

  • SMTP

• Web Browsing

  • HTTP

  • HTTPS

• Domain Name Service (DNS)

• File Transfer Protocols

  • FTP

  • FTPS

  • SFTP

• Remote Access

  • Telnet

  • SSH

  • SNMP

Encapsulation

The process of putting headers( and trailers) around some data

• Moving from Application Layer (7) to Physical Layer (1) - Encapsulation

Decapsulation

Removing the applied encapsulation to access the original data

• Moving from Physical Layer (1) to Application Layer (7) - Decapsulation

Protocol Data Units (PDUs) in OSI Model

• A single unit of information transmitted in a computer network

  • Terminology used for each layer is written as L(layer number) PDU

    • Ex. L7 PDU for Layer 7

• There are special names for the PDUs for layers 1,2,3, and 4

  • Layer 1 - Bits

  • Layer 2 - Frames

  • Layer 3 - Packets

  • Layer 4 - Segments (TCP) or Datagrams (UDP)

TCP Header (Layer 4)

• 10 Fields, 20 bytes of information

  • Source port

  • Destination port

  • Sequence number

  • Acknowledgment numbers

  • TCP data offset

  • Reserved data - always set to zero

  • Control flags

    • SYN - synchronizes connection in three-way handshake

    • ACK - acknowledgement of the successful receipt of data

    • FIN (Finished) - tears down connections created by three-way handshake

    • RST (Reset) - used when an unexpected packet is received

    • PSH (Push) - ensures data is given priority

    • URG (Urgent) - identifies incoming data as urgent

  • Window size

  • TCP checksum

  • Urgent pointer

  • mTCP * Optional

UDP Header (Layer 4)

• 8-byte header

  • Source port

  • Destination port

  • Length - indicates total packet bytes

  • Checksum - not mandatory

IP Header (Layer 3)

• Various Fields

  • Version

  • Length

  • Type of service

  • Total packet and header length

  • Identifier

  • Flags

  • Fragmented offset

  • Time to Live

  • Protocol

  • Header checksum

  • Source IP

  • Destination IP

  • Options and Padding

Ethernet Header (Layer 2)

• Source and Destination MAC Address

  • MAC Address

    • Physical address that is used to identify a network card on a LAN

    • Processed by switches

• EtherType field

  • Indicates which protocol is encapsulated in the payload of a frame

• VLAN Tag - optional

  • IEEE 802.1Q

  • IEEE 802.1AD

• A frame sent at Layer 2 will also contain a payload

  • Payload

    • Data that is being sent across the network

    • 42 bytes - using VLANs

    • 46 bytes - no VLANs

    • Maximum Transmission Unit (MTU)

      • Maximum size for payload

        • 1500 bytes for Ethernet

  • Jumbo Frame

    • Frames larger than 1500 bytes

    • Require reconfiguring of MTU

Data Transmission (Encapsulation/Decapsulation)

• Encapsulation of data and adding header at each layer

  • Layer 4 - Source/Destination Ports

  • Layer 3 - Source/Destination IP addresses

  • Layer 2 - Source/Destination MAC addresses

  • Layer 1 - data transmitted as 1s and 0s

• Decapsulation at each intermediate device until the final host is reached

• Final host decapsulates to Layer 7 for application understanding