The OSI Seven-Layer Network Model

Module NG08c

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Using the Internet - especially "surfing the Web" has become very easy. The process by which information arrives at your computer isn't magic - it's actually quite complex. In order to understand the core components of the Internet, we'll use the OSI (Open Systems Interconnection) 7-layer model.

The Open Systems Interconnection (OSI) Model

This is a standard which was developed primarily to help people understand and communicate about computer networking. It's not just about the Internet, but about networking in any context.

Image at TechTarget's WhatIs, http://whatis.techtarget.com/images/osi.gif

Each of the layers in the Model is associated with either hardware, software, or a combination of both. The specific details vary from one type of network to another.

Open Systems Interconnect (OSI) Model: Functions and Internet Examples in brief

OSI Layer
7
6
5
4
3
2
1
Name
Application
Presentation
Session
Transport
Network
Data Link
Physical
Function
 Interface to User, displaying information and interpreting input Client-server communication protocols Keeping track of which application is talking to which network node Packaging information for transmission Finding optimal routing for messages to reach their destination Getting digital messages into physical formats Transmitting signals between network nodes
Internet Examples
 Email
Web Browsers
File transfer
Newsgroups
Remote login 		POP
SMTP
HTTP
 "Ports" such as:
25 for incoming mail
80 for Web
20 and 21 for file transfer
23 for remote login		 TCP
UDP		 IP (versions 4 or 6)
IPX (Novell)		 Ethernet
SLIP
PPP		 Wire (CAT 1, CAT 5, ISDN, ADSL, ATM)
Fiber (FDDI)
Wireless (TDMA, CDMA, Bluetooth, IRDA)

 

There and Back Again

When a message used the Internet to go from one computer to another, it is passed through several several routers. Each router handles the lower four layers of the OSI model. To get back to the sender, a reply packet needs to go through the same process - though not necessarily through the same routers.

Packet hopping

Using the Internet: The OSI View

In order to understand how the Internet is related to the OSI Model, we'll start from the user perspective - something we're all familiar with. As we go, we'll discuss the protocols in use and where they fit into the Model. Let's assume that you want to surf the Web, either from home or from a workstation attached to a Local Area Network (LAN). For the sake of discussion, we'll assume you're using MS Windows, though the process is quite similar on a Macintosh.

Example: Browser running Windows requesting a Web page via modem

Part 1: Client initiates a request for a Web page

Layer
Application
Presentation
Session
Transport
Network
Data Link
Physical
  

Client

My computer at home

 
7
6
5
4
3
2
1
  
Browser: interprets my command: I clicked a link. Browser: creates HTTP request for the file, including IP address of server; passes request to session manager Windows session manager: opens a session on Port 80 associated with this request Windows TCP (Winsock): creates packet containing request; calculates checksum for accuracy; passes packet to Network Layer software. Windows IP (Winsock): looks up IP address of next node on route to server; creates packet address header; passes packet to Data Link software. Windows PPP: creates Point-to-Point Protocol header requesting ISP to forward this packet; adds it to the front of the packet. Modem driver converts packet to V.90 protocol analog waveforms; passes to V.90 modem, which transmits analog signals on the phone system.  

Example: Browser running Windows requesting a Web page via modem

Part 2: Internet Service Provider receives the request

Layer
Transport
Network
Data Link
Physical
  
7
6
5
4
3
2
1
  

ISP

My Internet Service Provider

       Transmission Control Protocol (TCP) computes the checksum for the packet and compares it with the checksum computed by the client TCP software. If they differ, TCP will send a request to the cleint to re-send the packet. Assuming it's OK, TCP temporarily saves a copy of the packet and sends the packet back down to layer 3 so it can be transmitted to the next destination. Internet Protocol (IP) software on the ISP's computer examines the IP header on the packet and determines that it is a TCP packet. TCP packets need to be sent to layer 4. PPP software identifies the packet's sender by reading the PPP header and verifies the sender's right to use the line. Strips off the PPP header and sends the packet up to Layer 3. ISP's modem receives analog waveform on a phone line, converts it using V.90 protocol to digital (ASCII) characters. Sends these to ISP Layer 2.

     

     

     

Network. IP determines the best route to send the packet toward its destination. An intermediate address is placed in the IP header. The packet is sent down to Layer 2, but not to the PPP software: this time, it goes to T1 software and hardware. T1 software codes a TI header for the packet and queues it up with other packets ready to be sent out. (It doesn't have to wait more than a few milliseconds.) The T1 hardware, including twisted-pair wire, send the signal to the next Internet router on the TI line.  
     

 

  
     

Example: Browser running Windows requesting a Web page via modem

Part 3: Internet routers examine the request and pass it along to the next router

Layer
Transport
Network
Data Link
Physical
  
7
6
5
4
3
2
1
  

Internet routers

Computers belonging to telephone companies or other long-distance data forwarders

A packet may go through one or many Internet routers before reaching its final destination...

       Transmission Control Protocol (TCP) computes the checksum for the packet and compares it with the checksum computed by the client TCP software. If they differ, TCP will send a request to the cleint to re-send the packet. If it's OK, TCP sends a confirmation to the sending TCP software at the ISP, so it knows it can delete the temporary copy, and saves a temporary copy of its own. It then returns the packet back down to layer 3 so it can be transmitted to the next destination. Internet Protocol (IP) software in the router examines the IP header on the packet and determines that it is a TCP packet, and so sends it up to Layer 4. T1 software verifies that this packet arrived at the correct destination. If so, it passes it up to Layer 3. The T1 hardware receives the signal sent from the ISP and sends it up to Layer 2.

 

 
     

 
     

 

 

 
      IP determines the best route to send the packet on the next hop toward its destination. An intermediate address is placed in the IP header. The packet is sent down to Layer 2. This software formats the packet for the transmission medium, which may be fiber, microwave, wire, or any other technology. The transmission medium and its control hardware sends the packet as a series of physical signals to the next destination.  
       
      -

Example: Browser running Windows requesting a Web page via modem

Part 4: The server receives the packet, examines it, and attempts to find and return the requested file.

Layer
Application
Presen-
tation
 Session 
Transport
Network
Data Link
Physical
  
7
6
5
4
3
2
1
  

Server

The server is the computer with the information I want.

 Application. The request for a specific file is sent to the operating system (OS) file management system. If the file is found, it is returned to Layer 6. If it couldn't be found or accessed, an error message is returned to Layer 6 instead. The HTTP server software waits for incoming packets from Port 80, and interprets my browser's request for a specific file. It also makes a note of other HTTP information included in the packet, such as the type and version of browser, the operating system of my computer, the date and time the request was sent. The file path and name in my URL request are interpreted at this level, so that URL address format is converted as necessary to the specific file organization and naming conventions of the host server. Seeing that the Session Layer of the sending computer asked for a Port 80 connection, this Session Layer software checks to make sure there is an available Port 80, and if so assigns a session number for the interchange between the server and my browser. This session number will be included in Layer 5 headers of all packets dealing with this request for information, so each computer will know which data interchange session it relates to. TCP once again checks to make sure the packet arrived intact, and requests a re-send if not. We'll assume it's good, so TCP strips off the TCP info and passes the packet up. IP software verifies that the packet has arrived at its final destination. IP routing information is removed and the packet is send up to Layer 4. Since the packet came in on fiber, the data link layer software here interprets it using FDDI (Fiber Digital Data Interchange protocol). Any FDDI information is stripped off the packet before it's passed up to Layer 3. The transmission medium delivers the electronic signals to hardware at the server's location. Let's say it's a fiber line.

 

 

 

 

 
 

The file or error message is now returned to my (client) computer in the mirror image of the way it was sent: down through each layer of the server, through various routers and the appropriate layers on each, eventually ending up in my browser window. The miracle of it all is that it takes such a short time for a process of such complexity!

Note on Applications

The Applications Layer is not necessarily the actual programs we all use and love, but the network interface for them. Most of the Internet programs we do use include Applications Layer components to handle the first stages of networking. Such programs include...

About this document

Review:

 Click here for review questions and answers.
Audience:

This is for people who are studying for Basic Level Certification as an Internet Professional. 

Objectives:

 When you successfully complete this lesson, you will be able to:
  1. Explain the purpose of the OSI Seven-Layer Model;
  2. List the names of the seven layers in order;
  3. Explain the purpose of each of the seven layers;
  4. Trace the layers through which a network packet travels to reach its destination.
Module NG08c: The OSI Seven-Layer Network Model This document is part of a modular instruction series in Computer Instruction. For more information, see the overview or the list of modules in this series, NG: Networks (General). This document has been used in the following classes: INP 160/220
Author: Laurence J. Krieg
Institution:
Internet Professional Department, Washtenaw Community College
History: Original: 1 May 2000
Last modification:  Monday, 31-Aug-2009 11:47:59 EDT
Copyright: Copyright © 2005, Laurence J. Krieg, Washtenaw Community College.
Instructors: You may point to this file in your Web-based materials.
Students: you may make a copy for your personal use.
All other uses: contact the author, Laurence J. Krieg for permission. Email krieg@ieee.org