Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Enjoy The TCP/IP Guide? Get the complete PDF!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  The Open System Interconnection (OSI) Reference Model
      9  OSI Reference Model Layers

Previous Topic/Section
OSI Reference Model Layers
Previous Page
Pages in Current Topic/Section
1
2
Next Page
Data Link Layer (Layer 2)
Next Topic/Section

Physical Layer (Layer 1)
(Page 1 of 2)

The lowest layer of the OSI Reference Model is layer 1, the physical layer; it is commonly abbreviated “PHY”. The physical layer is special compared to the other layers of the model, because it is the only one where data is physically moved across the network interface. All of the other layers perform useful functions to create messages to be sent, but they must all be transmitted down the protocol stack to the physical layer, where they are actually sent out over the network.

Note: The physical layer is also “special” in that it is the only layer that really does not apply specifically to TCP/IP. Even in studying TCP/IP, however, it is still important to understand its significance and role in relation to the other layers where TCP/IP protocols reside.


Understanding the Role of the Physical Layer

The name “physical layer” can be a bit problematic. Because of that name, and because of what I just said about the physical layer actually transmitting data, many people who study networking get the impression that the physical layer is only about actual network hardware. Some people may say the physical layer is “the network interface cards and cables”. This is not actually the case, however. The physical layer defines a number of network functions, not just hardware cables and cards.

A related notion is that “all network hardware belongs to the physical layer”. Again, this isn't strictly accurate. All hardware must have some relation to the physical layer in order to send data over the network, but hardware devices generally implement multiple layers of the OSI model, including the physical layer but also others. For example, an Ethernet network interface card performs functions at both the physical layer and the data link layer.

Physical Layer Functions

The following are the main responsibilities of the physical layer in the OSI Reference Model:

  • Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer (although also partially the data link layer; see below).

  • Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.

  • Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it. Note that this applies equally to wired and wireless networks, even if there is no tangible cable in a wireless network!

  • Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology.

In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network. For example, when considering network interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no knowledge of the contents of a message. They just take input bits and send them as output. Devices like switches and routers operate at higher layers and look at the data they receive as being more than voltage or light pulses that represent one or zero.


Previous Topic/Section
OSI Reference Model Layers
Previous Page
Pages in Current Topic/Section
1
2
Next Page
Data Link Layer (Layer 2)
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.