Network Models: OSI & TCP/IP

Network Models Overview

• Models like the OSI seven-layer model and the TCP/IP model aid technicians in understanding and troubleshooting computer networks
• The OSI seven-layer model is used to explain network hardware functions
• The TCP/IP model is used to describe network functions

Introduction

• The OSI seven-layer and TCP/IP models are powerful tools for diagnosing network problems
• Both models serve as a common language for describing networks

Biography of a Model

• A model is a simplified representation of something more complex
• Models retain all major functions of the real-world item

Network Models

• Understanding what functions are essential to all networks is important
• It's also important to know which details can be omitted when creating a model
• The International Organization for Standardization (ISO) proposed the OSI seven-layer model as a standard for network communication

The OSI Seven-Layer Model in Action

• Layer 7: Application
• Layer 6: Presentation
• Layer 5: Session
• Layer 4: Transport
• Layer 3: Network
• Layer 2: Data Link
• Layer 1: Physical

MHTechEd Scenario

• A worker needs to transfer a Word document to another worker for review through a network

Physical Layer (Layers 1-2)

• An employee can copy the file to a flash drive for transfer, or transfer the file using the network

Cabling

• Most networks use unshielded twisted pair (UTP) cable for data transfer

Central Box

• Each computer system has a cable that leads to a central box
• The central box sends data from one system to all connected systems

Network Interface Cards (NICs)

• NICs need to be installed in PCs for network cables to attach

NIC to Central Box Connections

• Cables run from the NIC in the PC to a jack on the wall
• Inside the wall, another cable runs back to the central box

The NIC

• Each system requires a unique identifier
• This identifier is called the Media Access Control (MAC) address
• The MAC address is burned into a ROM chip on the NIC
• Each MAC address consists of 12 hexidecimal characters or 48 bits
• They are typically represented using hexadecimal characters (12 hex digits = 48 bits)
  • A typical MAC address includes an Organizationally Unique Identifier followed by a device ID.
• No two MAC addresses should ever be the same
• Data moves along a wire inside the NIC
• NICs send and receive data in the form of ones and zeroes

Frames

• A frame is a container for data moving across a network
• Frames include the recipient's MAC address, the sender's MAC address, the data, and a frame check sequence (FCS) for error checking
• Different networks support different frame sizes
• Most frames hold at most 1500 bytes of data

Central Box - Getting Data to the Right Place

• When a system sends a frame, it goes into the central box
• The central box's technology determines the next steps for the frame

Hubs

• Early networks used hubs as central connection points
• Hubs made copies of a frame
• This frame was then sent to every other system on the network
• Every NIC recieved each frame sent on a network
• Only the NIC with a matching MAC address would process that frame

Switches

• Switches replaced hubs in modern networks
• They send a received frame only to the correct recipient MAC address

Getting Data on the Line

• Since the network cable is shared, only one device may transmit at a time
• Networks use frames to restrict the amount of data a NIC can send at once
• NICs handle data transmission issues automatically

Getting to Know You

• When two devices have communicated previously, they already know each other's MAC addresses
• Otherwise, a device will send out a broadcast message over the network
• The destination device will answer by sending its MAC address
• The MAC broadcast address is FF-FF-FF-FF-FF-FF

Frame Reception

• The receiving NIC uses the FCS to verify data validity
• Once verified, the NIC strips off framing information
• Afterwards, the data is sent the operating system for processing
• LLC and MAC are two parts of the Data Link layer

Network Software (Layers 3-7)

• Logical identifiers and rules enable data transfer in large networks

IP - Layer 3

• Packets are created and addressed
• The Internet Protocol is the primary logical addressing protocol for TCP/IP
• A router connects each subnet, forwarding data using the IP address
• Packets include a destination IP address, a source IP address, and the data

Packets

• The packet is enclosed within a frame
• The frame contains the sending and receiving MAC addresses
• There should ideally be a packet within a frame

Data Sending

• Data sent across a TCP/IP network traverses multiple routers
• Each router removes the incoming frame and creates a new one for the next connection
• When a packet reaches its destination subnet's router:
  • The router removes the incoming frame
  • The router assesses the destination address
  • The router adds a frame with the appropriate MAC address
• Next, the NIC removes the MAC header and hands off the frame to the network operating system

Layer 4

• Most data surpasses the available size of a single frame
• Therefore the transport protocol:
  • Divides data into segments or datagrams, depending on the specific protocol employed
  • Assigns each segment a sequence number

Transport Protocol

• Breaks data into segments and gives these segments a sequence number
• The sequence number indicates the total number of segments and describes how to organize them

Functions of the Transport Layer

• Includes segmentation/reassembly capabilities
• Initializes requests for packets that were not properly recieved

Session Layer

• One system can communicate with many others simultaneously
• Session software manages connecting applications to each other
• Layer 5 facilitates session initiation, acceptance, and termination

Presentation Layer

• Layer 6 translates data from lower layers into a format usable by the Application layer, and vice versa
• This is important because TCP/IP networks do not necessarily map directly to the OSI model

Application Layer

• Layer 7 enables users to exchange data on a network
• APIs at the Application layer are available in all operating systems
• This helps network applications function

Encapsulation and De-Encapsulation

• Encapsulation is the process of preparing data for network transit
• These steps happen from the application layer to the Data Link layer
• De-encapsulation is the reverse process of encapsulation
• This includes removing headers as data ascends the stack

TCP/IP Model

• The TCP/IP model is comprised of 4 layers:
  • Application
  • Transport
  • Internet
  • Link/Network Interface
• Unlike the OSI model, it does not have an exact standards body
• Version 1 (four layers) is used by major companies like Cisco and Microsoft

Link Layer

• Corresponds to OSI model Layers 1 and 2
• Handles physical elements like:
  • Cabling
  • Physical addresses
  • NICs
  • Switches
• Any part of the network that deals with complete frames is in the Link layer
• After the frame is stripped from an IP packet, the process moves to the Internet layer

Internet Layer

• The “IP-packet” layer
• This layer handles any device/ application that uses IP protocols, IP addressing, and/or IP routing
• Routers function at this layer
• This maps to the Network layer Layer 3 of the OSI model
• IP packets are created in this layer

Transport Layer

• Matches to OSI Transport/Session layers, as well as part of the Application Layer
• Manages segmentation/reassembly of data
• Provides both connection-oriented and connectionless communications

Connection-Oriented vs. Connectionless Communications

• Connection-oriented protocol: Transmission Control Protocol (TCP)
• Connectionless protocol: User Datagram Protocol (UDP)

TCP Segments

• These have fields that ensure connection-oriented communication

UDP Datagram

• Includes data from the Application layer
• There are added port and length numbers as well as a checksum
• Lacks most of the extra fields found in TCP segments
• Meaning that this process does not care if the data gets through

TCP/IP Application

• The TCP/IP Application layer maps to the top three layers of the OSI model
• Uses a port numbering system which has unique numbers between 1 and 65,535
• This enables Presentation layer formats, like MIME

Frames, Packets and Segments

• Application layer: Creates the data
• Transport layer: Divides the data into chunks
• Internet layer: Includes IP addressing, and creates IP packets
• Link layer: Wraps the packet into a frame, with MAC address and a frame check sequence (FCS)
• Therefore the datalink frame starts with frame header, then includes the header for the IP, then the segments from TCP

Troubleshooting

• When troubleshooting, use OSI model to methodically identify and resolve network issues from Layer 1 to Layer 7
• It applies to issues such as printer failures for example

Explore network models like OSI and TCP/IP, essential tools for technicians in understanding and troubleshooting computer networks. These models provide a structured approach to diagnosing network problems and serve as a common language for describing network functions. Learn about the OSI seven-layer model proposed by ISO.