OSI Model Flashcards
1.1
Open Systems Interconnect Model (OSI)
■ Developed in 1977 by the International Organization for Standardization
■ OSI is a reference model
● Used to categorize the functions of a network
● Useful for troubleshooting
○ Networks today operate under the TCP/IP mode
○ Layers
■ Physical - Layer 1
■ Data Link - Layer 2
■ Network - Layer 3
■ Transport - Layer 4
■ Session - Layer 5
■ Presentation - Layer 6
■ Application - Layer 7
○ Networks are designed to make data flow across networks
Physical Layer
(BITS)
■ First 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
○ Transition Modulation
■ 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)
○ Connector Standards
■ RJ-45 Connector – Used in CAT5/CAT6 cables
■ Wiring Standards
● TIA/EIA-568A
● TIA/EIA-568B
■ Crossover cables – TIA/EIA-568A on one end, and TIA/EIA-568B on the
other end
■ Straight-through cables – TIA/EIA-568B on both ends
○ Physical Topology
■ Different physical network layouts
● Bus
● Ring
● Star
● Hub-and-Spoke
● Full Mesh
● Partial Mesh
■ Based on how cables are physically connected
○ Synchronization
■ Asynchronous Communication
● Start and stop bits for out-of-sync data transmission
■ Synchronous Communication
● Real-time communication using a common time source
○ Bandwidth Utilization
■ Broadband
● Divides bandwidth into separate channels (e.g., cable TV)
■ Baseband
● Uses all frequency of the cable all the time (e.g., telephone)
Layer 1 Devices
■ Simply repeat whatever they receive
■ No logic or decision-making at Layer 1
■ Cables – media
● Fiber optic
● Ethernet
● Coaxial
■ Wireless Media
● Bluetooth
● Wi-Fi
● Near field communication
■ Infrastructure Devices
● Hubs
● Access points
● Media converters
Multiplexing
● Allows multiple people to use a baseband connection at the same
time
■ Time Division Multiplexing (TDM)
● Allocates dedicated time slots
■ Statistical Time Division Multiplexing (StatTDM)
● Dynamically allocates time slots based on when people need it
■ Frequency Division Multiplexing (FDM)
● Divides the medium into channels
Layer 2
Data Link
■ 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
MAC Address (Media Access Control Address)
■ A means for identifying a device physically and allowing it to operate on a
logical topology
■ 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 Control (LLC)
■ Provides connection services and acknowledges message receipt,
ensuring controlled data flow
■ Most basic form of flow control
● Limits data sent by a sender and prevents receiver overwhelm
■ Uses a checksum to detect corrupted data frames
■ 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 (layer 2)
● Devices reference own clock cycles
● No strict control over communication timing
Layer 2 Devices
■ Network Interface Cards (NICs)
■ Bridges
■ Switches
● Intelligent use of logic to learn and send data to specific devices
based on MAC addresses
○ 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.
Layer 3
Network Layer
■ Concerned with routing and forwarding traffic using logical addresses.
Logical Addressing
■ IP variants – common logical addressing schemes
● IPv4 – written in dotted octet notation which are four sets of
numbers separated by dots (e.g., 172.16.254.1)
● IPv6
■ Other protocols – these were replaced by IP (Internet Protocol)
● AppleTalk
● IPX (Internetwork Packet Exchange)
Packet Switching (Routing)
● Data is divided into packets and then forwarded
● Most commonly used method
Circuit Switching
● A dedicated communication link is established between two
devices
Message Switching
● Data is divided into messages which may be stored and then forwarded.
Route Discovery and Selection
■ Routers maintain routing tables for determining the best path
● Dynamic protocols (e.g., RIP , OSPF) enable routers to share and
update route information
■ 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
■ Augments Layer 2 services
■ Involves flow control
● Prevents sender from overwhelming the receiver
■ Packet reordering
● Ensures data packets arrive and are reassembled in the correct
order
Internet Control Message Protocol (ICMP)
■ 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
Layer 4
Dividing line between the upper layers and the lower layers of the OSI
model
■ Upper Layers
● Transport
● Session
● Presentation
● Application
Segments
■ Data Type in Transport Layer
TCP (Transmission Control Protocol)
Segment – data type for TCP
● Connection-oriented protocol that is a reliable way to transport
segments across the network
● With acknowledgement
● 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
● Reliable
○ Uses Three-way Handshake
● Connection-oriented
● Segment retransmission and flow control through windowing
● Sequencing
● Acknowledgment of segments
■ UDP (User Datagram Protocol)
Datagram – data type for UDP
● 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
● Unreliable
○ No Three-way Handshake
● Connectionless
● No retransmission and no windowing
● No sequencing
● No acknowledgment of datagrams
Windowing
■ 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
■ Occurs when devices allocate memory to store segments if bandwidth is
not readily available
Buffer
● Temporary storage for segments
■ Prevents overflow by clearing segments
Layer 4 Devices
Layer 4 Devices
■ Protocols
● TCP and UDP
■ Devices
● WAN accelerators
● Load balancers and firewalls
Layer 5
Session Layer
■ 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 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 and Protocols
○ Layer 5 Devices and Protocols
■ H.323
● Used for setting up, maintaining, and tearing down voice and
video connections
● Operates over the real-time transport protocol (RTP)
- Example is FaceTime.
■ NetBIOS
● Utilized by computers for file sharing over a network
● Commonly associated with Windows file sharing
Layer 6
Presentation layer
■ Responsible for formatting data for exchange and securing it through
encryption
Data Formatting
■ Formatting data 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
(Layer 7)
● GIFs – motion pictures
● JPEG – photographs
● PNG – Internet images
■ Formats enable compatibility between different devices
Encryption
■ 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
Scripting languages in Layer 6
■ Control how ASCII text is displayed on the screen
● HTML
● XML
● PHP
● JavaScript
Encryption Algorithms
■ Scrambles data to provide confidentiality and security during transit and
storage
● TLS
● SSL (Secure Sockets Layer)
■ Focus on Security
○ Application Layer (Layer 7)
■ Provides application-level services where users communicate with the
computer
■ Focus on lower-level applications
● File transfer
● Network transfer
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
Service Advertisement
■ Applications send announcements to other devices on the network
■ Devices advertise the services they offer
● Printers and file servers managed by Active Directory
● Self-advertising devices like wireless printers
Application Services
■ Unites 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
Encapsulation & Decapsulation
○ 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
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
○ 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)
TCP Header (Layer 4)
■ 10 mandatory fields, totalling 20 bytes of information
● Source port
● Destination port
● Sequence number
● Acknowledgment numbers
● TCP data offset
● Reserved data – always set to zero
● Control flags
○ SYN – synchronize connection in three-way handshake
○ ACK – acknowledgment 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 the total packet bytes
● Checksum – not mandatory
IP Header (Layer 3)
■ Contains various fields
● Version
● Length of IP header
● Type of service
● Total length of packet and header
● Identifier
● Flags
● Fragmented offset
● Time to live
● Protocol
● Header checksum
● Source IP Address
● Destination IP Address
● Options and Padding
Ethernet Header (Layer 2)
■ Features a few things
● Destination MAC Address
● Source MAC address
○ MAC Address
■ Physical address that is used to identify a network
card on a local area network
■ Processed by switches
EtherType field
○ Used to indicate which protocol is encapsulated in the
payload of a frame
■ IPv4 or IPv6
● Payload
A frame being sent at Layer 2 will also contain a payload
○ Data that 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 Frames
○ Frames larger than 1500 bytes
○ Require reconfiguring MTU
Data Transmission Travel
■ Encapsulation of data and adding 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
