OSI Reference Model

The Open System Interconnect (OSI) reference model is a model, developed by the International Standards Organization (ISO), which describes how data from an application on one computer can be transferred to an application on another computer. The OSI reference model consists of seven conceptual layers which each specify different network functions. Each function of a network can be assigned to one, or perhaps a couple of adjacent layers, of these seven layers and is relatively independent of the other layers. This independence means that one layer does not need to be aware of what the implementation of an adjacent layer is, merely how to communicate with it. This is a major advantage of the OSI reference model and is one of the major reasons why it has become one of the most widely used architecture models for inter-computer communications.

The seven layers of the OSI reference model, as shown in Figure 1, are:

• Application
• Presentation
• Session
• Transport
• Network
• Data link
• Physical
  

Figure 1: Diagram of the OSI reference model layers,

Over the next few articles I will be discussing each layer of the model and the networking hardware which relates to that layer. This article, as you have probably guessed from the title, will discuss layer 1; the physical layer.

simply state that all networking hardware belongs exclusively in the physical layer, they are wrong. Many networking hardware devices can perform functions belonging to the higher layers as well. For example, a network router performs routing functions which belong in the network layer.

What does the physical layer include? Well, the physical layer involves the actual transmission of signals over a medium from one computer to another. This layer includes specifications for the electrical and mechanical characteristics such as: voltage levels, signal timing, data rate, maximum transmission length, and physical connectors, of networking equipment. For a device to operate solely in the physical layer, it will not have any knowledge of the data which it transmits. A physical layer device simply transmits or receives data.

There are four general functions which the physical layer is responsible for. These functions are:

• Definitions of hardware specifications
• Encoding and signaling
• Data transmission and reception
• Topology and physical network design

Definitions of hardware specifications

Each piece of hardware in a network will have numerous specifications. If you read my previous article titled Copper and Glass: A Guide to Network Cables [link this title to my previous article of that title], you will learn about some of the more common specifications which apply to network cables. These specifications include things like the maximum length of a cable, the width of the cable, the protection from electromagnetic interference, and even the flexibility.

Another area of hardware specifications are the physical connectors. This includes both the shape and size of the connectors as well as the pin count and layout, if appropriate.

Encoding and signaling

Encoding and signaling is a very important part of the physical layer. This process can get quite complicated. For example, let's look at Ethernet. Most people learn that signals are sent in '1's and '0's using a high voltage level and a low voltage level to represent the two states. While this is useful for some teaching purposes, it is not correct. Signals over Ethernet are sent using Manchester encoding. This means that '1's and '0's are transmitted as rises and falls in the signal. Let me explain.

to send signals over a cable where a high voltage level represents a '1' and a low voltage signal represents a '0' the receiver would also need to know when to sample that signal. This is usually done with a separate clock signal being transmitted. This method is called a Non-return to Zero (NRZ) encoding, and has some serious drawbacks. First, and to do include a separate clock signal you are basically transmitting two signals and doubling the work. If you don't want to transmit the clock signal, you could include an internal clock in the receiver but this must be in near perfect synchronization with the transmitter clock. Let's assume you can synchronize the clocks, which becomes much harder as the transmission speed increases, there is still the problem of keeping this synchronization when there is a long stretch of the same bit being transmitted; it is the transitions which help synchronize the clocks.

Data transmission and reception

Whether the network medium is an electrical cable, an optical cable, or radio frequency, there needs to be equipment that physically transmits the signal. Likewise, there also needs to be equipment that receives the signal. In the case of a wireless network, this transmission and reception is done by highly designed antennas which transmit, or receive, signals at predefined frequencies with predefined bandwidths .

Relationships between OSI Reference Model Layers

There are also certain OSI layers that have “natural” relationships to each other. The physical and data link layers, in particular, are closely related. For example, most people talk about Ethernet as being a “layer two technology”, but Ethernet specifications really deal with both layer 2 and layer 1. Similarly, layers three and four are often related; protocol suites are often designed.

 So that layer three and four protocols work together; examples being TCP and IP in the TCP/IP protocol suite and IPX and SPX in the Novell suite.

In some areas, the layers are so closely related that the lines between them become blurry. This is particularly the case when looking at the higher layers; many technologies implement two or even all three of these layers, which is another reason why I feel they best belong in a group together. One important reason why the distinctions between layers five through seven are blurry is that the TCP/IP protocols are based on the TCP/IP model, which combines the functions of layers five through seven in a single, thick layer.