LAYER MODEL FOR OSI REFERENCE
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.
