Study this lesson and read chapters 0, 1 and 2 in your text.  Complete homework one (hw1) according to the schedule.

Layered Communications:
Communicating using signals is a complex process.  Creating electronic devices that can send and receive signals is extraordinarily complex.  There is a need to break this down into small tasks so that each one may be tackled by engineers with specialties in a particular task.

Some engineers may write software applications, while others may design a way to assign individual addresses to internetworked computers.  Still other engineers worry about how to design electrical systems to transmit binary digits. 

You might compose an E-mail message and send it out over your modem to the Internet, but are you aware of the intermediate steps that take place in preparing that E-mail for transmission?  Each of the many steps has to be designed to work together, without error from your computer all the way to the destination computer.  Fortunately, by breaking this complex process down into different layers, each layer dependent on a layer below for support, design becomes manageable.

At the lowest layers of this layered communication model are the physical signals.  There are many examples of signals in our world; smoke signals, semaphore flags, Morse code, a blinking flashlight, and a secret knock are good examples.  Each signal sent has some meaning and the combination of signals builds a total message.

Just as smoke signals send messages, so too, can electrical and optical signals.  In fact it is quite easy to send 1's and 0's with electrical and optical signals.  Simply turn on the voltage or light for a 1 and turn it off for a 0.  You could just as easily do this with a flashlight, just not as quickly.  You can't send bits any faster that you can turn the flashlight on and off.  Oh, another problem, your friend might have trouble understanding them if you sent them too fast.

How might you speed this up?

If you had a four color flashlight, you and your friend could memorize the following table:

00     blue
01     red
10     green
11     yellow

Now you can send two bits at a time by flashing a particular color.  You send one signal and your friend decodes it into two bits.  If you can flash the colors at the same speed you were turning the flash light on and off, you will cut the time to transmit your message in half.

To send 0110110001 you would flash a red, then a green, then a yellow, a blue and finally a red.  You send five signals and your friend gets 10 bits.  Your friend would decode the colors and discover the bits that you are trying to send, 01 ... 10 ... 11 ... 00 ... 01.

Now imagine trying to send 10 million bits this way.  Hmm, if we had plenty of time on our hands.  Still I would rather spend my time doing something else.  What we need are more signals so that each signal can represent more bits.  If we had 256 colors we could send 8 bits by flashing a single color.

The above example digs into the difference between signals and bits.  Transmission systems send signal of some type.  Each signal represents some number of bits.  In a simple system, one signal is just one bit.  You only need two different signals in this system. In other systems, there may be a great number of different signals, so that a single signal carries a large number of bits. 

As you can see, there are many complex issues even at the very lowest layer of the communications model.  The complex nature of designing communications has forced us to make a conceptual model to organize our efforts.  There are several models.  We'll study three of them. 

1. A simple 3 layer model
2. OSI model (created by ISO)
3. TCP/IP model

The three layer model:
This model simplifies communications into three jobs or duties.

Application Layer

Transport Layer

Network Access Layer

The Application Layer:
This layer interacts with the users and helps the user communicate with other application on the network  The application layer requires the assistance of both the transport and network layers to establish reliable communications with another application on the network.

The Transport Layer:
This layer makes sure that messages sent on the network arrive at the correct destination and makes sure messages are delivered to the application layer without errors.  The transport layer might have to ask for retransmission when information is received that has errors in it.  The transport layer relies on the network access layer for the actual signal transmissions.

The Network Access Layer:
This layer supports the higher layers by sending the actual signals.  This layer has to know all the rules about access to the network and is in charge of following them, so that all users have fair access to the network.  The network layer will actual create signals from the data handed to it from the transport layer and then send those signals on the transmission medium.  The lowest layer most likely sends those signals by electrical pulses, optical waves or electromagnetic waves.  It also must receive those signals and convert them back into bits.  The network layer will determine the data from the signals and then hands the data up to the transport layer.

These three layers interact.  The application layer takes user info (in bits) and requests that the transport layer deliver the info to another application layer within a computer or other device somewhere on the network.  The transport layer takes the application info and requests that the network layer transfer the info across a particular link to the next entity on the network.  The network layer takes the transport layer's info and converts the bits to signals.  The signals are then transmitted to the network layer of  the entity at the other end of the medium (wire, fiber or airspace).

At the far end of the network, the process is reversed.  The network layer views the incoming signals, checks them for some types of errors, converts the signals back to bits and hands the bits up to the transport layer.  The transport layer inspects the info to determine correctness, completeness and whether it has traveled to the right destination.  The information may be forwarded on several times before reaching the final destination in the network.  At the final destination, the transport layer hands the information up to the application layer where a user may finally gain access to it.

The OSI Model:
Communications are complex enough that the three layer model is nothing more than a simplified overview of the communications process.

The International Organization for Standardization (ISO) developed a more sophisticated model to organize communication system design efforts.  It is called the Open System Interconnection model, or OSI model.

This model has seven layers:

Application Layer:  Interacts with the User

Presentation Layer:  Formats data so application layer can understand it.

Session Layer:  Manages simultaneous communications processes

Transport Layer:  Handles reliability and end-to-end connectivity

Network Layer:  Path determination in the network

Data Link Layer:  Access to the link, follows the rules for access and prepares the final bit stream to hand to the physical layer

Physical Layer:  Encodes the bit stream into signals and sends them on the link

Information is passed down through the layers until is reaches the physical layer where it is converted to signals, which are sent to the next piece of equipment on the line (or fiber, etc.). 

The OSI model is the preeminent model used for comparison in the networking industry.  Vendors explain how their equipment functions by noting what layer it operates at.  The OSI model, a framework for a set of standards to be created by the ISO, never did become fully implemented.  As a model, however, it is indispensable.

The TCP/IP Model:
The Internet follows a similar conceptual organization as the OSI model.  The TCP/IP model has five layers.

Application Layer
Transport Layer
Internet Layer
Network Access Layer
Physical Layer

Again, bits are handed down through the layers.  Each layer adds some information that is sent to corresponding layers down the line.  Finally, signals are created at the physical layer to represent the bits and these signals are transmitted.

The main benefit of using models is that we have an opportunity to concentrate our research and design efforts within a single layer.  Others will work on other layers.  As long as a you follow the rules established for interaction with higher and lower layers, you are free to do anything you wish within your layer.

Standardization provides a solution to the many products that cannot communication with each other.  Vendors grope together to hammer out a set of rules that all will follow.  The benefits are that competing equipment can interconnect, vendors can compete on price, and increased market potential for the product.

Make no mistake, standards are critical in the computer industry (as well as other industries). 

For more information on standardization visit:

ISO - creator of the OSI model
ITU - standards at the international level, governments are members of this body
Committee T1 - US data link standards
IETF - Internet Standards

Used with permission of the Author; Copyright (C) Kevin A. Shaffer 1998 - 2018, all rights reserved.