Protocol Layering in Computer Network, Principles of Protocol Layering, Logical Connection of layering, Wireless Connection ,Types of wireless connection
PROTOCOL LAYERING :
In data communication and networking a protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively.
When communication is simple, we may need only one simple protocol when the communication is complex, we may need to divide the task between different layers, in which case we need a protocol at each layer, or protocol layering.
Scenarios :
Let us develop two simple scenarios to better understand the need for protocol layering.
First Scenario :
In the first scenario, communication is so simple that it can occur in only one layer. Assume Maria and Ann are neighbors with a lot of common ideas. Communication between Maria and Ann takes place in one layer, face to face, in the same language, as shown in Figure.
Even in this simple scenario, we can see that a set of rules needs to be followed.
- First, Maria and Ann know that they should greet each other when they meet.
- Second, they know that they should confine their vocabulary to the level of their friendship.
- Third, each party knows that she should refrain from speaking when the other party is speaking.
- Fourth, each party knows that the conversation should be a dialog, not a monolog: both should have the opportunity to talk about the issue.
- Fifth, they should exchange some nice words when they leave
Second Scenario:
In the second scenario, we assume that Ann is offered a higher -level position in her company, but
needs to move to another branch located in a city very far from Maria.
The two friends still want to continue their communication and exchange ideas because they have come up with an innovative project to start a new business when they both retire.
They decide to continue their conversation using regular mail through the post office.
Protocol layering enables us to divide a complex task into several smaller and simpler tasks
- For example : in Figure, we could have used only one machine to do the job of all three machines. However, if Maria and Ann decide that the encryption/ decryption done by the machine are not enough to protect their secrecy, they would have to change the whole machine. In the present situation, they need to change only the second layer machine the other two can remain the same. This is referred to as modularity. Modularity in this case means independent layers. A layer (module) can be defined as a black box with inputs and outputs, without concern about how inputs are changed to outputs. If two machines provide the same outputs when given the same inputs, they can replace each other. For example, Ann and Maria can buy the second layer machine from two different manufacturers. As long as the two machines create the same cipher text from the same plaintext and vice versa, they do the job.
Advantages of protocol layering :
- Protocol layering allows us to separate the services from the implementation. A layer needs to be able to receive a set of services from the lower layer and to give the services to the upper layer; we don’t care about how the layer is implemented.
- Protocol layering in the Internet, is that communication does not always use only two end systems; there are intermediate systems that need only some layers, but not all layers. If we did not use protocol layering, we would have to make each intermediate system as complex as the end systems, which makes the whole system more expensive.
Principles of Protocol Layering :
First Principle :
If we want bidirectional communication we need to make each layer so that it is able to
perform two opposite tasks, one in each direction.
For example, the third layer task is to listen (in one direction) and talk (in the other direction). The second layer needs to be able to encrypt
and decrypt. The first layer needs to send and receive mail.
Second Principle :
The two objects under each layer at both sites should be identical.
For example, the object under layer 3 at both sites should be a plaintext letter. The object under layer 2 at both sites
should be a ciphertext letter. The object under layer 1 at both sites should be a piece of mail
Logical Connections :
After following the above two principles, we can think about logical connection between each
layer as shown in Figure below
This means that we have layer-to layer communication
The concept of logical connection will help us better understand the task of layering we encounter in data communication and networking.
WIRELESS TRANSMISSION :
Wireless communications has many important applications besides providing connectivity to users from any
place.
The Electromagnetic Spectrum :
When electrons move, they create electromagnetic waves that can propagate through space. The number of oscillations per second of a wave is called its frequency, f, and is measured in Hz. The
distance between two consecutive maxima (or minima) is called the wavelength, represented by lambda.
In a vacuum, all electromagnetic waves travel at the same speed, no matter what their frequency. This
speed, usually called the speed of light, c, is approximately 3 × 108 m/sec, or about 1 foot (30 cm) per
nanosecond.
The radio, microwave, infrared, and visible light portions of the spectrum can all be used for transmitting
information by modulating the amplitude, frequency, or phase of the waves. Ultraviolet light, X-rays, and
gamma rays would be even better, due to their higher frequencies, but they are hard to produce and modulate,
do not propagate well. The terms LF, MF, and HF refer to Low, Medium, and High Frequency, respectively.
UWB( Ultra Wide Band) :
UWB sends a series of rapid pulses, varying their positions to communicate information. The rapid transitions
lead to a signal that is spread thinly over a very wide frequency band. UWB is defined as signals that have a
bandwidth of at least 500 MHz or at least 20% of the center frequency of their frequency band. It can tolerate a
substantial amount of relatively strong interference from other narrowband signals, because it is spread across
wide band of frequencies.
Radio Transmission :
Radio frequency (RF) waves are easy to generate, can travel long distances, and can penetrate buildings
easily, so they are widely used for communication, both indoors and outdoors. Radio waves also are omni
directional, meaning that they travel in all directions from the source, so the transmitter and receiver do not
have to be carefully aligned physically.
The properties of radio waves are frequency dependent. At low frequencies, radio waves pass through
obstacles well, but the power falls off sharply with distance from the source—at least as fast as 1/r 2 in
air—as the signal energy is spread more thinly over a larger surface. This attenuation is called path loss.
At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. Path loss still
reduces power, though the received signal can depend strongly on reflections as well. High- frequency radio
waves are also absorbed by rain and other obstacles to a larger extent than are low-frequency ones. At all
frequencies, radio waves are subject to interference from motors and other electrical equipment.
From fig(a): In the VLF, LF, and MF bands, radio waves follow the ground. These waves can be detected for
perhaps 1000 km at the lower frequencies, less at the higher ones. AM radio broadcasting uses the MF band.
Fig(b) In the HF and VHF bands, the ground waves tend to be absorbed by the earth. However, the waves that
reach the ionosphere, a layer of charged particles circling the earth at a height of 100 to 500 km, are refracted by
it and sent back to earth.
Microwave Transmission :
- Microwaves travel in a straight line. Thus, repeaters are needed periodically. The distance between repeaters is square root of the tower height. For 100- meter-high towers, repeaters can be 80 km apart. Microwaves do not pass through buildings well.
- Some waves may be refracted off low- lying atmospheric layers and may take slightly longer to arrive than the direct waves. The delayed waves may arrive out of phase with the direct wave and thus cancel the signal. This effect is called multipath fading.
One exciting development in the U.S. is the FCC decision in 2009 to allow unlicensed use of white
spaces around 700 MHz. White spaces are frequency bands that have been allocated but are not being used
locally. The only difficulty to use the white spaces, unlicensed devices must be able to detect any nearby licensed
transmitters, including wireless microphones.
- Bands up to 10 GHz are now in routine use. These waves are only a few centimetres long and are absorbed by rain. Microwave communication is so widely used for long-distance telephone communication, mobile phones, television distribution. It does not require to lay down cables. By buying a small plot of ground every 50 km and putting a microwave tower on it, one can bypass the telephone system entirely. Microwave is also relatively inexpensive.
InfraRed Transmission :
Unguided infrared waves are widely used for short-range communication. The remote controls used for
televisions, VCRs, and stereos all use infrared communication.
They are relatively directional, cheap, and easy to build but have a major drawback: they do not pass
through solid objects. It means that an infrared system in one room of a building will not interfere with a
similar system in adjacent rooms or buildings.
Infrared communication has a limited use on the desktop, for example, to connect notebook computers and
printers with the IrDA (Infrared Data Association) standard, but it is not a major player in the
communication game.
Light Transmission :
A more modern application is to connect the LANs in two buildings via lasers mounted on their rooftops. Optical
signaling using lasers is inherently unidirectional, so each end needs its own laser and its own photo detector.
To avoid unnecessary wiring for only limited days, the organizers placed the laser on the roof, and tested in the
night which worked perfectly. At 9 A.M. on a bright, sunny day, the link failed completely and stayed down all
day. The problem is that because heat from the sun during the daytime caused convection currents to rise up from
the roof of the building. This turbulent air diverted the beam and made it dance around the detector, much like a
shimmering road on a hot day. Communicating with visible light in this way is inherently safe and creates a low-
speed network in the immediate vicinity of the display.
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