Open Systems Interconnection (OSI) Model

• Objective 1.1 focuses on explaining the concepts of the Open Systems Interconnection (OSI) reference model.

Introduction to OSI Model

• The Open Systems Interconnect (OSI) model was developed in 1977 by the International Organization for Standardization.

• OSI serves as a reference model for understanding network functions and troubleshooting.

• Networks today commonly operate under the TCP/IP mode.

• Networks are structured to facilitate data flow.

• There are 7 Layers in the OSI model, including:

  • Physical, Data Link, Network, Transport, Session, Presentation, and Application.

• The OSI model dictates how data is named as it flows through the layers:

  • Layer 1 (Physical): Bits
  • Layer 2 (Data Link): Frames
  • Layer 3 (Network): Packets
  • Layer 4 (Transport): Segments
  • Layers 5, 6, 7 (Session, Presentation, Application): Data

Layer 1: Physical Layer

• The physical layer describes the network's physical and electrical characteristics for bit transmission.
• Data type occurs in this layer as bits, represented by binary 1s and 0s.
• Transition modulation occurs as the switching between voltage levels to represent 1 or 0.
• Copper wire uses voltage (0V for 0, +5V/-5V for 1).
• Fiber optic cables use light (on for 1, off for 0).
• RJ-45 connectors are used in CAT5/CAT6 cables.
• Wiring Standards are implemented
• ΤΙΑ/ΕΙΑ-568A and ΤΙΑ/ΕΙΑ-568B exist
• Crossover cables use TIA/EIA-568A on one end and TIA/EIA-568B on the other.
• Straight-through cables use TIA/EIA-568B on both ends.
• Physical topology relates to physical network layouts.
  • Network layouts include Bus, Ring, Star, Hub-and-Spoke, Full Mesh, and Partial Mesh.
• Physical topology is based on how cables are physically connected.
• Synchronization methods are used.
  • Asynchronous communication uses start and stop bits for out-of-sync data transmission.
  • Synchronous communication is real-time using a common time source.
• Bandwidth Utilization occurs
  • Broadband divides bandwidth into separate channels.
  • Baseband uses all cable frequency all the time.
• Multiplexing involves efficiently using limited resources.
  • It 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 need.
  • Frequency Division Multiplexing (FDM) divides the medium into channels.
• Layer 1 Devices include:
  • Cables (fiber optic, Ethernet, coaxial).
  • Wireless media (Bluetooth, Wi-Fi, near field communication).
  • Infrastructure devices (hubs, access points, media converters).
• Layer 1 devices simply repeat what they receive without logic or decision-making.

Layer 2: Data Link Layer

• The Data Link Layer (Layer 2) is responsible for packaging bits from Layer 1 into frames.
• This layer detects and corrects errors, identifies devices using MAC addresses, and provides flow control.
• A MAC Address (Media Access Control Address) identifies devices physically, enabling operation on a logical topology.
• Every Network Interface Card made has a unique 48-bit physical address
  • Written in hexadecimal numbers
  • The first 24 bits identify the manufacturer
  • Remaining 24 bits point to a specific device.
• Critical for logical topology in order to identify devices
• Logical Link Control (LLC) provides connection services and ensures controlled data flow with message receipt acknowledgements.
  • Flow control helps stop overwhelm
  • Checksums detect data corruption.
• Synchronization methods at Layer 2:
  • Isochronous Mode uses a common reference clock, time slots, and has less overhead.
  • Synchronous Method uses the same clock and control characters in frames.
  • Asynchronous references its own clock cycles without strict control on timing.
• Layer 2 devices include Network Interface Cards (NICs) and bridges.
  • Switches use logic to learn and send data to specific devices based on MAC addresses.
• Switch Operation occurs
  • Switches use CAM tables of MAC addresses to ID things
  • This enables data tramsission to specific areas

Layer 3: Network Layer

• The Network Layer transports data using logical addresses.
• IP variants are common logical addressing schemes.
  • IPv4 is written in dotted octet notation (e.g., 172.16.254.1).
  • IPv6 is an alternative IP addressing scheme.
  • Other protocols like AppleTalk remain, although IP is more common.
  • Internetwork Packet Exchange (IPX) also remains.
• Switching/Routing Methods:
  • Packet switching divides data into packets and forwards them.
  • Circuit switching establishes a dedicated communication link between devices.
  • Message switching divides data into messages that may be stored and then forwarded.
• Route Discovery and Selection:
  • Routers maintain routing tables and use dyanmic protocols to share info
  • Protocols (RIP, OSPF) dictate how data will flow.
• Connection Services at Layer 3:
  • Augments Layer 2 services
  • Prevents sender overwhelm
  • Packet reordering ensures correct end arrival
• Internet Control Message Protocol (ICMP) sends error messages related to an IP destination.
  • PING which is used to test response times and connectivity.
  • Traceroute traces a packet's route.
• Devices such as Routers combine Layer 2 switch and Layer 3 router features.

Layer 4: Transport Layer

• The Transport Layer divides between the upper-level and lower-level OSI layers.
• The upper Layers include Transport, Session, Presentation, and Application.
• The data type in the transport layer is Segments.
• TCP
  • Reliable method to transport segments across the network with acknowledgement
  • This acknowledgement uses a Three-Way Handshake
    • SYN for synchronization
    • SYN-ACK for synchronization-acknowledgement
    • ACK for just acknowledgement
  • Windowing for flow control
  • Used for network data requiring final destination needs
• UDP
  • An unreliable method to transport packets for streaming
  • No three-way handshake as it is not needed
• Layer 4 data types:
  • Segment for TCP
  • Datagram for UDP
• TCP is reliable, connection-oriented with segments and acknolwedgment
  • Uses 3 way handshake, sequencing and windows
• UDP is unreliable as it has no handshake
  • A connectionless protocol
  • There is no sequencing, windows or acknowledgement.
• Windowing allows the client to adjust the amount of data. It optimizes the throughput and retransmissions
• Buffering allocates memory to store segments
• Overflow is prevented by clearing segements

Layer 5: Session Layer

• Session Layer (Layer 5) manages separate conversations to halt intermingling
• Setting up the session
  • Users are checked and assinged numbers
• Maintaining session is also a key part
  • Parties transfer data
  • Includes acknowledgement
  • Re-establishment if there is a break
• Tearing down is to end communication when goals are achieved
  • This requires mutual agreement or one party simply d/c'ing
• Layer 5 protocols and software include:
  • H.323 which operates over the real-time protocol,
  • NetBIOS which shares files on Windows
• Issues involve protocols and software rather than devices

Layer 6: Presentation Layer

• Responsible formatting data for exchange and securing it through encryption
• Formats include:
  • ASCII for compatibility
    • Text, ensures readability and proper data structure via negotiated transfer syntax
  • Images
    • GIFs, JPEGs and PNG for different formats
• Layer 6 Formatting enables compatibility between different devices
• Encryption
  • Used to scramble data in transit with data confidentiality.
  • Uses Transport Layer Security (TLS) to create an encrypted tunnel and secure data transfer.
• Scripting languages
  • Scripts control how ASCII text is displayed on the screen. -HTML, XML, PHP, JavaScript
• Text formats also occur
  • Such as ASCII, Unicode, EBCDIC
• Graphical formatting
  • Such as GIFs, JPEGs, TIFFs
• Image formats
  • Such as SVGs, and PNGs
• Movie Files -1s and 0s watches on MP4s, MPEGs, MOVs
• Encryption Algorithms
  • These algorithms are key in security
  • TLS and SSL are common

Layer 7: Application Layer

• Provides the interfaces for user communication at an application level using file and network transfers on lower level.
• Application services unite components for more than one network app
• Low level protocols include:
  • POP3
  • IMAP
  • SMTP
• Remote Access, network maintenance and processes occur
• Applications send self-announcements of what they offer
• Devices advetise too via Active Directory
• Protocols here include
  • Email related POP3, IMAP, SMTP
  • And web ones like HTTP, and DNS
  • As well as transfer ones like FTP, FTPS and SFTP
  • Remote Access Telnet, and SNMP are included
• Data is put around headers and trailers for data encapsulation
• Data is taken about for data decapsulation
• Going down is encapsulation, going up is decapsulation

Protocol Data Units (PDUs) in OSI Model

• A single data unit is transmitted along a route.
• Terminology is written as L(layer number) PDU.
  • Example is : L7 for layer 7
• Special names for the PDUS are common for L1 to L4
  • Bits : Layer 1
  • Frame : Layer 2
  • Packets : Layer 3
  • Segements, Datagrams : Layer 4
• Mandatory elements can take up 20 bytes as an example, with:
  • Source Destination, Sequence number
  • Ackeoldgement, Offset -Reserved data for flags
• Control flags have SYN handshake options that can be reset, pushed and urgent
• With options size, checksum pointer and TCP
• Port and header need a lot of fields
• Headers include -MAC, Time To Live -Protocol -Checksum -IP Data, and padding to ensure switches know the protocols and such
• Layers include ports and IP Addresses, and they are transmitted as 1s and 0s
• A frame being sent at Layer 2 also contains a payload
• Payload
  • Data that is being sent across the network
  • 42 bytes using VLANs
  • 46 bytes no VLANs
  • MTU for payload which is 1500 bytes for ethernet
  • Jumbo frames can be up to 1500 bytes, reconfiguring MTU
• Encoding / decoding and formatting for headers and layers