OSI Reference Model: Layer 1 hardware

If you would like to read the next part in this article series please go to OSI Reference Model: Layer 2 Hardware

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, courtesy of catalyst.washington.edu

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.
While many people may 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.
If you were 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, if you 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.
The limitations of the NRZ encoding can be overcome by technology developed in the 1940s at the University of Manchester [link University of Manchester to http://www.manchester.ac.uk/], in Manchester, UK. Manchester encoding combines the clock signal with the data signal. While this does increase the bandwidth of the signal, it also makes the successful transmission of the data much easier and reliable.
A Manchester encoded signal, transmits data as a rising or falling edge. Which edge represents the ‘1’ and which represents the ‘0’ must be decided first, but both are considered Manchester encoded signals. Ethernet and IEEE standards use the rising edge as a logical ‘1’. The original Manchester encoding used the falling edge as a ‘1’.
One situation which you may be thinking about is that if you need to transmit two ‘1’s in a row the signal will already be high when you need to transmit the second ‘1’. This isn’t the case because the rising or falling edge which represents data is transmitted in the middle of the bit boundaries; the edge of the bit boundaries either contain a transition or do not, which puts the signal in the right position for the next bit to be transmitted. The end result is that at the center of every bit is a transition, the direction of the transition represents either a ‘1’ or a ‘0’ and the timing of the transition is the clock.
While there are many other encoding schemes, many of which are much more advanced than NRZ or Manchester encoding, the simplicity and reliability of Manchester encoding has kept it a valuable standard still widely in use.

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.
Optical transmission lines use equipment which can produce and receive pulses of light, the frequency of which is used to determine the logical value of the bit. Equipment such as amplifiers and repeaters, which are commonly employed in long-haul optical transmissions, are also included in the physical layer of the OSI reference model.

Topology and physical network design

The topology and design of your network is also included in the physical layer. Whether your network is a token ring [link token ring to http://en.wikipedia.org/wiki/Network_topology#Ring], star [link star to http://en.wikipedia.org/wiki/Network_topology#Star], mesh [link mesh to http://en.wikipedia.org/wiki/Network_topology#Mesh], or a hybrid topology [link hybrid topology to http://en.wikipedia.org/wiki/Network_topology#Hybrid_network_topologies], the decision of which topology to use was chosen with the physical layer in mind.
Also included in the physical layer is the layout of a high availability cluster, as described in my previous article titled High Assurance Strategies [link High Assurance Strategies to my previous article].
In general all you need to remember is that if a piece of hardware is not aware of the data being transmitted then it operates in the physical layer. In my next article I will discuss the Data link layer, what makes it different from it’s adjacent layers and what hardware is included in it. As always, if you have any questions or comments on what I have written in this article feel free to send me an email.

If you would like to read the next part in this article series please go to OSI Reference Model: Layer 2 Hardware

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