OSI Model Fundamentals

The OSI Model

• The OSI model is a 7-layered framework that helps understand how devices communicate over a network
• The 7 layers are:
  • Physical
  • Data Link
  • Network
  • Transport
  • Session
  • Presentation
  • Application

Communication Process

• When an application on one host wants to send data to an application on another host, it goes through the OSI model layers
• Each layer adds its own header and/or trailer to the data before passing it on to the next layer
• The process is reversed at the receiving end, with each layer removing its respective header and/or trailer

Layer 7: Application

• The application layer is where network APIs and apps that access the network live
• Examples include FTP, web browsing, and email
• This layer is concerned with how the application accesses the network

Layer 6: Presentation

• The presentation layer is responsible for converting data into a format that can be easily understood
• Examples include image and video files, encryption, and compression

Layer 5: Session

• The session layer tracks application processes and builds a session between a local application and a remote one
• Examples include remote procedure calls and service requests

Layer 4: Transport

• The transport layer is responsible for transporting traffic between endpoints
• Protocols like TCP and UDP are used at this layer
• Data is broken into manageable chunks (segments or datagrams) and headers are added with source and destination port numbers
• Multiplexing allows multiple applications to share the same connection

Layer 3: Network

• The network layer is responsible for routing data between hosts
• Protocols like IP are used at this layer
• Headers are added with source and destination IP addresses

Layer 2: Data Link

• The data link layer is responsible for getting data from one device to another on the same network segment
• Protocols like Ethernet are used at this layer
• Headers are added with source and destination MAC addresses
• Error correction information is also added

Layer 1: Physical

• The physical layer is concerned with the physical components of the network
• This includes radio frequencies, channels, pulsing light, pins, electrical signals, and copper cabling standards
• The physical layer is responsible for encoding information in a way that can be understood by the receiving device

The OSI Model

• A 7-layered framework to understand device communication over a network

OSI Layers

• The 7 layers are: Physical, Data Link, Network, Transport, Session, Presentation, and Application

Communication Process

• Data is sent through OSI layers, with each layer adding headers/trailers before passing to the next layer
• Receiving end reverses the process, removing headers/trailers at each layer

Layer 7: Application

• Network APIs and apps live here (FTP, web browsing, email)
• Concerned with application access to the network

Layer 6: Presentation

• Converts data into easily understood formats (image/video files, encryption, compression)

Layer 5: Session

• Tracks application processes, building sessions between local/remote apps (remote procedure calls, service requests)

Layer 4: Transport

• Transports traffic between endpoints using TCP/UDP
• Data is broken into chunks (segments/datagrams) with source/destination port numbers
• Multiplexing allows multiple apps to share connections

Layer 3: Network

• Routes data between hosts using protocols like IP
• Headers are added with source/destination IP addresses

Layer 2: Data Link

• Gets data between devices on the same network segment using Ethernet
• Headers are added with source/destination MAC addresses and error correction info

Layer 1: Physical

• Concerned with physical network components (radio frequencies, channels, pulsing light, etc.)
• Encodes information for receiving devices to understand

Cabling and Wi-Fi

• Wired connections use cables to connect devices, with copper cables being cheaper and suitable for short distances, and fiber cables being more expensive but better for longer distances.

Ethernet Protocol

• Ethernet is a protocol that devices agree on, comprising multiple parts that describe cabling types, speeds, and data formatting and transmission.
• Ethernet is layered, allowing devices with different cables and speeds to communicate.

Cabling Types

• Unshielded Twisted Pair (UTP) cables are the most common copper cables, with modern UTP cables having four twisted pairs of wires for sending and receiving.
• Category (Cat) 5, 6, and 7 cables have different numbers of pairs, wire thickness, and twisting tightness, with Cat 5e and higher supporting 1 Gb and 10 Gb speeds.
• UTP cables use RJ45 connectors with eight pins.

Straight-Through and Crossover Cables

• Straight-Through cables connect hosts to switches or routers, while Crossover cables connect hosts to hosts or switches to switches.
• Auto MDI-X technology allows devices to detect the type of cable used and adapt.

Fiber Cables

• Fiber cables use strands of glass and light to transmit data, and are used between networking devices like routers and switches.
• Fiber cables can be single mode or multi mode, with single mode using laser light and suitable for longer distances (up to 2 km), and multi mode using LED light and suitable for shorter distances (up to 500 meters).

Duplex and Bend Radius

• Duplex refers to a device's ability to send and receive data simultaneously, with full duplex devices capable of simultaneous transmission and reception, and half duplex devices only capable of one at a time.
• Bend radius is the minimum distance a fiber cable can be bent without causing signal degradation.

Wireless Networks

• Wireless networks use access points to connect devices, and use the 802.11 standard, which differs from the Ethernet standard.
• Wireless networks are useful for connecting end-user devices but not typically used for servers or routers.

Network Addresses

• Each device on a network has a MAC address and an IP address, with MAC addresses being permanently assigned to each network card and used for communication within a LAN segment.
• IP addresses are chosen by network administrators and allow devices to communicate with devices on different LAN segments.

Network Communication

• When sending a message, the sender's MAC address is added, and the router receives the message, strips off its own MAC address, and replaces it with the recipient's MAC address before forwarding the message.