OSI Model (Section 3)

OSI Model Layers

■ Physical - Layer 1  - Bits

■ Data Link - Layer 2 - Frames

■ Network - Layer 3 - Packets

■ Transport - Layer 4 - Segments

■ Session - Layer 5 - Data

■ Presentation - Layer 6 - Data

■ Application - Layer 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 occurs as bits (Binary bits represented as a series of 1s and 0s)

• Cables are also part of the physical layer.

Examples of Layer 1 Devices:
Layer 1 devices are essentially repeaters, passing along whatever is received. (Dumb devices)

• Cables (Fiber optic cable, Ethernet cable, Coaxial cable)

• Wireless (Bluetooth, Wi-Fi, Near Field Communication (NFC))

• Infrastructure Devices (Hubs, Access Points, Media Converters)

Transition Modulation

• Occurs in Layer 1 (Physical Layer)

• Switching between levels to represent 1 or 0

  • Copper Wire (Cat5/Cat6) – Uses voltage (0V for 0, +5V/-5V for 1) 

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

Synchronization (Layer 1)

• Asynchronous Communication: Start and stop bits for out-of-sync data transmission.

• Synchronous Communication: Real-time communication using a common time source 

Bandwidth Utilization (Layer 1)

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

• Baseband: Uses all frequencies of the cable all the time (e.g., telephone) 

Multiplexing (Layer 1)

Multiplexing enables simultaneous usage of a baseband connection by multiple users.

Multiplexing involves optimizing a limited amount of resources for more efficient utilization.

• Time division multiplexing (TDM): Each session takes a turn, using time slots, to share the medium between all users.

• Statistical time-division multiplexing (StatTDM): Dynamically allocates the time slots on an as-needed basis.

• Frequency division multiplexing (FDM): Divides the medium into channels based on frequencies and each session is transmitted over a different channel.

Data Link Layer

• 2nd layer of OSI Model

• Responsible for packaging bits from Layer 1 into frames and transmitting them across the network.

• Performs error detection and correction, identifies devices using MAC addresses, and provides flow control.

Examples of Layer 2 devices:

• Network Interface Cards

• Bridges

• Switches

  • Switch Operation:

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

    • Enables selective data transmission to specific areas in the network. 

MAC (Layer 2)

Media Access Control (MAC)

• A means for identifying a device physically and allowing it to operate on a logical topology (Ex. D2:51:F1:3A:34:65)

• A unique 48-bit physical addressing system is assigned to every network interface card (NIC) produced ● Written in hexadecimal numbers 

  • First 24 bits – identify the manufacturer 

  • Remaining 24 bits – identify the specific device 

  • Crucial for logical topology – identifying devices on the network 

Logical Link Layer (LLC) (Layer 2)

• Provides connection services and acknowledges message receipt, ensuring controlled data flow 

• LLC is the most basic form of flow control (Limits data sent by a sender and prevents receiver overwhelm)

• Provides basic error control functions

• Uses a checksum to detect corrupted data frames 

Synchronization Methods at Layer 2 

• Isochronous Mode

  • Common reference clock 

  • Time slots for transmissions 

  • Less overhead 

• Synchronous Method

  • Devices use the same clock, with beginning and ending frames, and control characters for synchronization 

• Asynchronous

  • Devices reference own clock cycles 

  • No strict control over communication timing 

Network Layer

• 3rd Layer of the OSI Model

• Concerned with routing and forwarding traffic using logical addresses 

Examples of Layer 3 devices:

• Routers

• Multi-layer switches (switches are always considered a layer 2 device unless it is multi-layered)
  

Layer 3 Protocols:

• IPv4

• IPv6

• Internet Control Message Protocol (ICMP)
  

(IP and routers are commonly encountered Layer 3 devices in exams)

How is data routed across a network? (Network Layer 3)

• Packet switching: Data is divided into packets and then forwarded. Most networks use packet switching.

• Circuit switching: Dedicated communication link is established between two devices. In circuit switching, a dedicated and constant communication path is maintained for the duration of a conversation. 

• Message switching: Data is divided into messages which may be stored and then forwarded.

Route Discovery & Selection (Network Layer 3)

• Routers maintain routing tables for determining the best path 

• Manually configured as a static route or dynamically through a routing protocol.

• Routing protocols help us decided how data is going to flow across the network and how the routers are going to communicate that information

• Types of routing protocols: RIP, OSPF, EIGRP

ICMP

• Internet Control Message Protocol

• Occurs in Network Layer 3

• Used for sending error messages and operational 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 

Transport Layer

• 4th Layer of the OSI Model

• Dividing line between the upper layers and the lower layers of the OSI model 

• Deals with segments and datagrams

• 2 important protocols: TCP & UDP

• 2 reliability features: Windowing & Buffering 

• TCP works with segments

• UDP works with datagrams
  

Examples of Layer 4 Devices:

• TCP & UDP

• WAN accelerators

• Load balancers and firewalls

TCP

• Transmission Control Protocol

• Occurs in Transport Layer 4

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

• Uses Three-Way Handshake

  • SYN – synchronization 

  • SYN-ACK – synchronization - acknowledgement 

  • ACK – acknowledgement 

• Windowing for flow control

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

UDP

• User Datagram Protocol

• Occurs in Transport Layer 4

• A connectionless protocol that is an unreliable way to transport segments (datagram) 

• Used for audio and visual streaming 

• No three-way handshake and less overhead 

• No acknowledgment or retransmission 

TCP Vs. UDP

TCP:

• Reliable

  • Uses Three-way Handshake 

• Connection-oriented 

• Segment retransmission and flow control through windowing 

• Sequencing 

• Acknowledgment of segments 

UDP:

• Unreliable

  • No Three-way Handshake 

• Connectionless 

• No retransmission and no windowing 

• No sequencing 

• No acknowledgment of datagrams 

Windowing (Transport Layer 4)

• Allows clients to adjust the amount of data in each segments during transmission 

• Optimize throughput and bandwidth 

• Open or close window based on retransmissions 

Buffering (Transport Layer 4)

• Occurs when devices allocate memory to store segments if bandwidth is not readily available 

• Buffer: Temporary storage for segments 

• Prevents overflow by clearing segments 

Session Layer

5th Layer of the OSI Model

Manages sessions, ensuring separate conversations to prevent data intermingling 

• Setting Up Session

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

• Maintaining Session

  • Continuous data transfer between parties 

  • If the connection breaks, it will require re-establishment 

  • Includes acknowledgement of data 

• Tearing Down a Session

  • Ending a session once communication goals are achieved 

  • Mutual agreement or one party disconnects 

Layer 5 Devices & Protocols:

• H.323

  • Used for setting up, maintaining, and tearing down voice and video connections 

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

• NetBIOS

  • Utilized by computers for file sharing over a network 

  • Commonly associated with Windows file sharing 

Layer 5 issues involve protocols and software rather than specific devices

Presentation Layer

6th Layer of the OSI Model

Responsible for formatting data for exchange and securing it through encryption 

• Data Formatting: Formatting data by a computer to have compatibility between different devices 

• 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 (Layer 7) 

Examples of Devices at Layer 6:

• Scripting Languages (HTML, MXL, etc.)

• Standard Text (ASCII, Unicode,EBCDIC, etc.)

• Pictures (JPG, GIF, PNG, etc.)

• Movie Files (MOV, MPG, etc.)

• Encryption Algorithms (TLS, SSL…)

Encryption (Presentation Layer 6)

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

• Transport Layer Security (TLS)

  • Ensures secure data transfer 

  • Creates an encrypted tunnel, protecting sensitive information 

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 unify communication components from multiple network applications.

  • Service Advertisement: Sending out announcements to other devices on the network to state the services they offer.

Layer 7 Protocols:

• Email Applications

  • POP3 

  • IMAP 

  • SMTP 

• Web Browsing

  • HTTP

  • HTTPS

• Domain Name Services (DNS)

• File Transfer Protocols

  • FTP

  • FTPS

  • SFTP

• Remote Access

  • Telnet

  • SSH

  • SNMP

Encapsulation and Decapsulation 

Occurs in Application Layer 7

Encapsulation: Process of putting headers and sometimes trailers around data 

Decapsulation: Removing the applied encapsulation to access the original data 

OSI Model Layers

• Moving down from Layer 7 to 1 – encapsulation 

• Moving up from Layer 1 to 7 – decapsulation 
  

Data Transmission:

• Encapsulation of data and adding a header at each layer

  • Layer 4 – source/destination ports 

  • Layer 3 – source/destination IP addresses 

  • Layer 2 – soure/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 

Protocol Data Unit (PDU)

• A single unit of information transmitted in a computer network ●

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

    • ○ Example – 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)