YAGI-UDA ANTENNA

Yagi-Uda Antenna:

                                       It's the most simple type of antenna which basically uses the dipole antenna as its fundamental working element . It's an directional beam creating antenna which is mostly used in TV channel operations in olden days and even today in some parts of India.

Why The Name Yagi-Uda:

                                                   Basically this antenna was designed and developed by two scientists of japan named "Yagi and Uda " so in the honor of them it was named so . Here it should be noted that it has not got any technical meaning in it . most of antenna's were named on the basis of its inventor .

What is Use Of Yagi - Uda Antenna :

                                                                      AS we know that "dipole "  antenna is roughly an Omni directional antenna which radiate the electromagnetic waves in nearly 360 degree except at some points as illustrated in the below figure .

"dipole antenna "

                                                                   But the question here is that what is the necessary of this Yagi - uda  antenna to understand this we should first know some basic things about antenna Directivity . The problem with the dipole antenna is that as stated earlier it generates the radiation in all directions but it will cover a very small distance , which can't be tolerated in far distance communications like TV transmissions. 

                                                                Here the yagi-uda antenna came into picture which is capable of generating such an "radiation pattern which will cover a large distance ". That means the Directivity of the antenna is more .

Why the reflectors and directors are used in it :

                                                                                                Before we come to this point why these are used . we should know that a "inductor" basically "lag's the current in it " and the "capacitor lead's the current through it ". 

                                                 Now as we can see in figure that a dipole antenna is placed at the middle point . and an reflector is placed on left side of dipole and an directors are placed on right side of dipole antenna . here it is to be noted that we want to create the directive radiation on right hand side of the dipole . 

"Yagi-Uda Antenna "

             The" size of the reflector is made a little longer as compared to dipole antenna and that of director is made a little shorter why ?????.

                                                                             The reason behind is that "When an electromagnetic wave is created in both the directions we want the waves on right side only so we will keep an element longer than the length of dipole antenna which will be acting as the "inductor " and it will lag the current through it . so here when wave travel from dipole slit to reflector slit there will be change in the phase of electromagnetic wave so in order to correct that one the reflector acts as an inductor . 

                                                                                

                                           NOW coming to the directors as their name itself indicates "They are used to provide the direction to the electromagnetic waves " . as we know a smaller element will act as capacitor and the property of capacitor is that it tends to "lead the current through it " thus it will confine the radiation in one direction and also gives more strength to the radiation thus covering a larger distance . Here it should be noted that the length of directors could be kept in their decreasing order or as a same length as shown in figure . 

PN SEQUENCE

WHAT IS PN SEQUENCE :

                                                          

                                                        "It's a randomly created sequence to secure the information bits"

The PN sequence consists of number of bits depending on the application where it is used . The name i e: "pseudo-noise " sequence is due to the fact that it is created randomly during the transmission of information bits through the channel.

HOW RECEIVER RECEIVES THE SIGNALS:

                                                                                                 If you transmit some signals from your device say mobile, then how an receiver it may be any electronic device receives the signal ???? .

               Here is the answer , We all know that when we are transmitting the signal. In order to secure that signal, at transmitter we use a signal called as "PN sequence " which will be "mixed with the message signal " and transmitted through the channel. To generate the PN sequence we use a set of shift registers connected in series with each other and also an feedback circuitry in it .

                                                     

                                                                NOW ,   We  have transmitted the signal  in order to receive the signal properly "we should know the PN sequence which is used at the transmitter". The received signal is multiplied with the locally  generated PN sequence which is exactly same as the PN sequence used at the transmitter part of the system. 

                                                                                 Here the multiplication basically is the EX-OR of  the two sequences , so that if both the sequence are same then result is "1" otherwise it will be "0". Thus we will be getting the only "message signal" at the receiver end ,And then it is further processed by using various filters mostly the "Band pass filter" and amplified with the help of "Amplifiers" to get good strength signal. 

                                         

                                       

EXAMPLE:

                         The below figure shows how a transmitted signal is received back at the receiver side with the help of PN sequence which is used at both transmitting as well as receiving ends of the of the system . In the particular example if we observe the message bits or "data bits " are : "10111" and the PN sequence bits are : "1010 ".  Here the PN sequence is "post padded" to the data bits then it becomes :"101111010 " which will be our whole signal at transmitting end . 

                                                                              

                                                                                                                           Now we have received the signal at the receiving end and we want to "extract our message signal" from the whole transmitted signal. in order to do so we have to multiply the received signal with the locally generated PN sequence.

                                                        

                                                                      101111010

                                               EX-OR           000001010

                                                                      101110000   

    Now we are having received signal as "101110000 " out of which the last 4 bits are padded bits which has got no significance so we leave these bits and we will be left with only bits:"10111" which is our transmitted message bits at the transmitter end .

                   Thus with the help of PN sequence we will be able to transmit the  message signal securely  and receive the same at receiver side .  

 The below figure shows how the signal is flowed from transmitter to the receiver .......

General communication model 

                                                        

   

                  

WHY IT IS NEEDED:

                                              The main application of the PN sequence is to secure the information bits i e : "To encrypt the message bits ". for example assume that you are operating an TV transmission channel named x at a frequency of around 2MHz (Assume). And you have got certain number of subscribers who paid for your service , so when you transmit the data that should be only received by the subscribed users . so the one who knows the "PN sequence" which is used at the transmitter will be able to receive the signal . otherwise if the non-subscribed users tune their receiver to 2MHz then it may possible that they also get access to your channel . This is just an one example where we will be using the PN sequence , there are  "n "  number of applications where it is used  mainly in "defense " applications where the data is very confident .

APPLICATION:

1) It is mainly used in "Defense" applications , where the transmitted signal should not be known to the enemy country soldiers . so it is very important to secure the message data .

2) In TV transmission where it is needed that only the subscribed members should get access to your channel , here we will be using the different length of PN sequences depending on the type of application .

3) In communication of "fighter planes " where the information being sent by our "pilot " will be tried to extract by enemy country also , so in order to avoid this we use PN sequences of  larger lengths as the priority for security is more here .

4) Not only these it can be every where there is need for the security of data .

HOW THE SEQUENCE IS GENERATED:

                                                                                       The generation PN sequence is simpler as it only needs some "flip-flops " and some feedback circuitry .

The below figure shows how to generate a PN sequence of length 7 bits .

                                

                                                      The circuit consists of 3 flip-flops (note: A flip-flop can store 1 bit at a time ) . so there is formula to find number of output bits created depending on the number of shift registers i e : 

Maximum Length Of PN Sequence is 

                                                              = 2^n - 1     => Formula 

                                                              =2^3 -1

                                                              =8-1

                                                              =7   bits 

where , 

             n= number of "flip-flops "

similarly ,

               If we use 4 flip-flops then length of the PN sequence generated will be equal to                                     

                                                                = 2^4 - 1

                                                                = 16-1

                                                                = 15   bits 

PN Sequence generator circuit 

PROPERTIES OF PN SEQUENCES :

1) Balance Property :

                                      This property states that the "number of 1's in the PN sequence will be always greater than the number of 0's ".

Example :

                 consider a PN sequence : 1101001

number of 1's   = 2^n/2        =>> Formula 

                         = 2^3 / 2

                         = 8/ 2 

                         = 4 bits          >>>>>>>>>>>1

where , 

n= number of flip-flops 

number of 0's   = 2^n / 2  - 1       =>>Formula

                         = 2^3 / 2  - 1

                         = 4 - 1

                         = 3 bits        >>>>>>>>>>>>2 

 n= number of flip - flops 

Thus by comparing equation 1 and 2  we come to know that number of 1's will be more than the number of 0's in any PN sequence .

2) Run Property :

                               Among the 1's and 0's in each cycle of the maximum length PN sequence . The number of run's is given by 

                                      =     N+1 / 2

                                      =    7+1 / 2

                                      =    8 / 2

                                      =      4        >> implies number of run's needed for a 7 bit PN sequence . 

3) Auto correlation property :  

                                                        The auto correlation property is basically is used to find the relation between the "present binary symbol and previous binary symbol ".

                                                                                                                         

                        Let us assume that the present binary symbol as Cn and the previous binary symbol as 

Cn-k . Where "k" is the delay of the correlation sequence .

 Then the auto correlation function is given by

                               N                                           Rc(k)=1 / N∑ Cn * Cn-k 

                              n=1                                      

 The value of

                         Rc(k) =  ❴ 1       if     k=0

                                          0       if     k ⧣ 0   ❵

DOMAIN NAME SYSTEM ( DNS)

Domain Name System:

                                             "It's a type of server which provides IP address of websites ".

So basically domain name system is a server which returns the IP address if we give the name of website to .

                   We all know that the computers only understands the number system i e : they don't understand the English or any other language . And it is impossible for us to remember the IP addresses of all the websites which we will be using in our daily life like Facebook, Instagram,whats app,twitter,You tube, OR any other website commercial or non commercial.

  So people started working on an issue to come with a solution such that , we will create a system which will give us with IP address of a website if we give its name to that system and thus a DNS (Domain Name System) is created to help human beings in their work.

NOW if we search Facebook in our device then it will send request to local DNS server to send the IP address of Facebook , once it receives the IP address then communication will be established easily between the server and end device willing to use it .

How Many DNS Servers Are There:

                                                                        If we consider in international level then there are totally "13"  DNS servers which contains huge data about different servers .

     Then on national level if we talk about INDIA then it contains around 140 but this number include both private and public servers.

                                                  below list shows some of important servers in India ....

How It Works:

                              If we see closely there are steps in servers as we can see in our below figure 

1)  The one at top is called as "Root name server" which contains the information of words like .com, .org, .nic, etc . 

2) The next one is "Top Level Domain Server" which contains the names of popular websites like Facebook , Instagram etc.

3) The last one in queue is the "Authoritative name server" which contains the host level names addresses . 

4) Below all these there is another server "local server" which is basically setup by the particular institution to handle there own traffic .

                                                          

                                                      

WORKING:

                     Assume you want to go to website "www. Facebook.com " the steps involved in this process are....

1) Initially the computer or any other device in which we are browsing the internet will ask the local server for IP address . It checks in its cache memory for address , if it finds address there then it is fine it returns it to computer. 

2) otherwise it will send a quarry to "root server" asking for IP address , but root does not contain the IP address , but it will tell the local server to whome it should ask to know the address. so it suggests to ask to "TLD server" .

3) Now the quarry is send to TLD server asking the same question ie IP address of Facebook , but here also it does not contain the address , it will also suggest to go to "Authoritative name server".

4) In its final step local server asks the "Authoritative name server" , now this contains the required IP address and send it to the local server . Then it will be given to computer or any other device which is requesting in our case we have taken it as computer.

                                                                                                  BUT the local server does an smart work to avoid all these steps of asking to all higher level servers , "It stores the IP address of most recently asked websites in its cache memory "  so that it will give the addresses immediately as and when required. 

  The below figure shows the steps involved in obtaining the IP address ...

THREE BASIC PROPAGATION MECHANISMS IN WIRELESS COMMUNICATION

PROPAGATION:

                                  We know that electromagnetic waves travel with the speed of light in free space or in air as the medium . The speed will be 3*10^8 m/sec .

But while travelling it suffers from three basic mechanisms which decreases its strength while travelling in space . 

                                 Those are the listed below..... 

1) Reflection 

2) Diffraction 

3) Scattering 

REFLECTION:

                                "Reflection occurs when a propagating electromagnetic wave impinges upon an object whose dimension is large , compared to wave length of travelling electromagnetic wave ".

So reflection can be seen on any object whose dimensions are greater as compared to the wavelength of electromagnetic wave which will be hitting that object.

Example: It can occur on the surface of "earth" , buildings and walls etc.

HOW IT HAPPENS:

                                          When the wave propagating in one medium impinges upon another medium having different electrical properties, Then the wave will be partially reflected and partially transmitted . Again reflection depends on the type of second medium that is if the medium is perfect "Dielectric" then a part of incident energy will be transmitted into the second medium and part of that will be reflected back to the first medium. Suppose if the second medium is the "conductor" then the incident wave will be completely reflected back to the first medium.In case of perfect conductor there is no absorption of energy .

                                          "Fresnel reflection coefficient " gives the information regarding the electric field intensity between the transmitted and reflected waves. The reflection coefficient is the function of material properties. and in general depends on "wave polarization, angel of incidence and also on frequency " of transmitted wave.

"Reflection"

DIFFRACTION :

                                    "Diffraction occurs when the path between the transmitter and receiver is obstructed by surfaces with sharp irregularities (sharp edges)".

So diffraction occurs due to the objects with sharp irregularities on them . 

Due to diffraction  there will be production of secondary waves which are present in space and they are also present "behind obstacles" Gives rise to the phenomena called as "Bending of waves" around the obstacle, even if there is no line of sight communication present .

                                                                       

                                                     At higher frequencies just like reflection the , diffraction also depends on geometry of object and also on amplitude , phase , polarization of the incident waves.

"Diffraction"

SCATTERING :

                                  " Scattering occurs when the medium through which electromagnetic wave travels contain's objects with dimensions lesser than wavelength of propagating wave".

                                         Consider a wave with wavelength 1mm is propagating in the medium then scattering only occurs if the objects which are present in medium are having the dimensions lesser than 1mm (wavelength < 1mm). And "number of obstacles per unit volume should be large" .

                              The scattered waves can be produced by rough surfaces, small objects , other irregularities present in transmission channel.

                               

"scattering"

DIPOLE ANTENNA

ANTENNA:
                    "Antenna is a any metallic structure which is used to transmit or receive the electromagnetic signals".
From definition we come to know that antenna can be used to transmit as well as receive the electromagnetic waves , but one thing which is important  is that the impedance should be matched between source side and load side .
When we pass an electric signal through a conductor then the electric signal will be reflected back to source end . but here the point is that the amount of electric current received after bounce back will be slightly less as compared to sent electric signal . Thus the loss here will be converted into radiations or electromagnetic waves which will be travelling in space to reach the receiver to form the communication between to ends.

TYPES OF ANTENNA:

                                               There are number of antennas based on their application in specific domain some of them are as listed below

1) Patch antenna 

2) mono pole antenna 

3) Dipole antenna 

4) Parabolic reflector antenna

5) yagi uda antenna 

6) loop antenna 

7) array antenna 

                                       Out of all these antennas the most important one is the Dipole Antenna which most basic one . The patch antenna is used in small electronic devices where space is very compact like mobile phones , wireless hand sets etc . The parabolic reflector antenna is used in the TV Transmission where it is used to receive the electromagnetic signals from the satellite . The name parabolic is based on the fact that the shape of reflector is made "parabolic"such that it concentrates most of signal into the receiving part of the antenna .

                                                                                     NOW coming to yagi uda antenna it is also used in TV Transmission itself but it was used in olden days , the disadvantage with this antenna is that it has got low gain due to this its efficiency will decrease.

we will be talking about the "Dipole Antenna " in details .

MONO POLE ANTENNA:

                                                       As its name itself suggests it a type of antenna with input at its" only one end ". these are most basic ones but the required radiation cannot be obtained from these type of antenna due to their mono pole structure.
                                                                            The improved version of these type of antenna is the dipole antenna which can be used to obtain convenient radiation pattern. which is discussed below .

DIPOLE ANTENNA:

Definition : "An antenna which contains two poles which are opposite in charge.  "

                          

                                                   From definition we come to know that a dipole antenna is one which contains two poles (i e : poles means basically two ends) which are opposite in charge , Hence the name to this antenna is the "dipole " meaning "two" poles .

CONSTRUCTION:

                                      The construction of dipole antenna is very easy compared to other types of antennas as it contains only two poles which are center fed by suitable voltage .

                                      consider a metal of length L , now bend the metal in two equal parts as shown in below figure and in between these two metallic parts give source may be current or voltage depending on the application we may use either of them. and that's it our dipole antenna is ready .

"dipole antenna with center feed "

HOW IT WORKS:

                                      Until now we have seen how to construct an dipole antenna , now lets see how  it works actually.

As stated earlier the antenna contains two poles "so when we apply current between the two poles " due to difference in voltage "positive and negative charges will be stored on opposite  sides to each other ". As current varies in the metallic conductor   the charges will move in opposite direction to each other , but at one point in between they will be meeting with each where they will be moving with "maximum velocity". Thus again for other cycle of current the charges will again reverse their direction and move in opposite direction to each to other. now they will be again meeting at a point with maximum velocity and creating another wave (electromagnetic wave). 

                                                         Thus as the amount of current through the conductor (antenna) increases the back and forth motion of charges increases and amount on radiation also increases .

below figure shows the process of radiation production.

"dipole antenna radiation pattern"

APPLICATIONS:

 1)  It is used as a transmitter and receiver (Trans-receiver) in Radio frequency communication .

2)   It is used in military applications to form communication between solders and the base station(     army head quaters).

3) It is used in radio stations to transmit data either by AM or FM  techniques.

4) It can also be used to form mutual communication between trains.

5) Apart from this it can be used every where there is use of RF waves for communication.

HALF WAVE DIPOLE ANTENNA:

                                                                          As name it self suggests it is half wavelength long  (L/2) . This is the shortest length of antenna which can be used to get the convenient radiation pattern. It is most widely used type of antenna due to production of most suitable pattern of radiation 

                                       

                                           It is normally fed at a point which is equidistant from both the the ends means at a length of "quarter" from both the sides of antenna. It is to be noted that the length of the antenna will be equal to the half of wavelength of signal which is flowing through the antenna conductor or metallic tube of antenna depending on the fact that the material which is used to construct the antenna.

The below figure shows the half wave length dipole antenna ..

"half wave length dipole antenna"

=

                           

                                    

                                      

    

HOW TO IMPROVE CELL CAPACITY AND COVERAGE AREA

CELL:

            We all know that in wireless communication hexagonal cells are used to provide communication. But the problem is that each hexagonal cell is having specific limit on number of users it call handle.

                               Suppose consider a situation where the users are huge in small amount of area, Then the conventional cell design will not work properly there the users will face problems like call connection delay, call drop,queuing of call requests etc. So to avoid these kind of problems scientists created some cell designing techniques those are 

1)Cell splitting

2) sectoring

3) Micro cell zone concept

The cell splitting mainly focus on the capacity of the system, where as other two techniques will focus on reduction of channel interference and frequency reuse  and also on reducing number of handoff's performed.   

CELL SPLITTING:

                                As we stated earlier the main goal of cell splitting is to increase the "capacity" of the system. So in order to do that  in splitting technique one big cell is replaced by number of small cells with individual base station for each small cell.

                                                                                   Thus number of antennas will increase in the same area which will also increase number of channels allocated for communication. Imagine we have one big cell of radious R , now we have replaced that cell by 4 small cells with each antenna for all . let radious of the small cells be R/2 . If we add all the small cells radious then it will be equal to radious of bigger cell.

                                   BUT the number of users handled by smaller cells added together will be greater as compared to the users handled by single big cell due to increase in number of channels .

The below figure will illustrate the cell splitting concept.

"cell splitting "

Here the cell A is having huge traffic so it has to be splitted . so it is now split into 6 cells each having antenna at its edge. 

It is to be noted that the cells which use "same frequency " are separated by enough distance to avoid co-channel interference. If we observe G we will come to know this.

SECTORING:

                       THE one way to increase the capacity is cell splitting which uses several cells with reduced radious R. Now in second approach the radious is kept constant instead the SIR (signal to noise ratio) is increased which in turn allow reduction of "cluster size" so in the same area we will be having more number of clusters as compare to previous one cell method.

HOW IT IS DONE:

                                In normal design we will be using a single"omnidirectional" antenna , but in this method we will use several "directional" antennas to cover the area. As we know omnidirectional antenna will be radiating the signal  in all directions which will lead to channel interference.

but directional antennas will be radiating the signal only in selected or interested direction. so unwanted channel interference can be avoided, thus increasing SNR which will allow reduction in cluster size thus increasing channel capacity.

The below figure shows sectoring 

It is to be noted that the cells named 5 will be using the same channels so in order to avoid interference they will be placed apart and if it comes in direction also the strength of signal will be controlled so that it wont reach to next cell which is using same channels.

   

"sectoring  with 120 degree radiating antenna"

MICROCELL ZONE CONCEPT:

                                                      This concept is mainly used to "reduce number of handoff's" .

deff: 

        In this technique a single base station is used by number of small cells to avoid the handoffs and channel interference .

In sectoring we have used number of cells , So when a mobile user travel from one area to other area the handoff will be needed . thus there will be extra work for MSC (moblile switching center) and there are chances of call drop during the handoff due to speed of the device.

So to avoid these type of things the microcell zone concept came into picture.

HOW IT WORKS:

                              Consider there is a big cell which will be consisting of 3 microcells with each having its own antenna at its corner . All the microcells will be connected to each other either by "electric cables , optic fibers ". and all cells will be using the same base station to serve the user.

suppose a user is currently in cell A and now after some time he will move to cell B , now the channel which is used by user will not be changed instead the "zone selector " connected to the base station will just assign same channel to user as in cell A . Thus there is no need for handoff in the microcell zone concept.

The below figure illustrates the microcell zone concept .

"micro cell zone concept"

ALSO READ THE FOLLOWING TOPICS


1) TYPES OF NETWORK                             >>>>>>>>>>>>>>>>>>>       click here

2)  HAND OFF                                                >>>>>>>>>>>>>>>>>>>       click here

3)  FREQUENCY REUSE                              >>>>>>>>>>>>>>>>>>>        click here

4)  WHY HEXAGONAL CELLS ARE USED 
      IN WIRELESS COMMUNICATION       >>>>>>>>>>>>>>>>>>          click here

WIRELESS ANTENNA

WHAT IS ANTENNA :

                                             "It's a electromechanical structure which is used to transmit and receive  electromagnetic signals into space" .

HOW THE NAME ANTENNA COINED:

                              We know that in communication we require a electrical signal to be transmitted in the wire. But we cannot link long distances due to requirement of high infrastructure . And maintaing of these electrical cables is very difficult or we can say almost impossible .

                     So avoid this problem the scientists decided to use free space as the medium . But we all know that free space cannot be used to transmit electrical signals .

         NOW Scientists thought of something which will convert the electrical signal to electromagnetic signals and radiate it in free space 

And this is how "Antenna" was come into picture.

HOW ANTENNA WORKS:

                                             According to source theorem we know that "Maximum power will be transfered between source and destination if there is equal load on both sides or there should be matching in impedance ".

                        So what if there is mismatch in impedances , Suppose we have sent 1 watt power and the power reached to destination is 0.9 watt . 

          The question arises that where that 0.1 watt of power has gone. The Answer to this question is that " LEAKAGE CURRENT" .

The 0.1 watt will be converted to leakage power and it will be in terms of "RADIATION". Which is basically a electromagnetic waves.

The below figure shows "reflection signal and radiation"

"Production of Radiation by using waveguide "

                                                                                 

But the problem is that it is only small amount of radiation which can cover very small distance of about 100 meters .

CHALLENGES:

1) The radiation cover very small distance .

2) To increase radiation we may think of increasing power but it should be kept in mind that the human and other living cells will be affected due to more powerful radiation so we can no more increase power level.

USE OF ANTENNA TO SOLVE THIS:

Now the concept of antenna came that , If we increase the width of waveguide at destination side we can get more radiation's  with the same input power of 1 watt ( just for example purpose) .

HOW RADIATION INCREASE WITH INCREASE IN WIDTH OF WAVEGUIDES END:

                                   Basically radiation is nothing but leakage current . so as conducting medium increase the amount of leakage current will increase and automatically the amount of radiation will increase.

The below figures shows amount of radiation for various antenna widths .

Note: It is to be pointed that the width is increased in small amounts for comparison purpose.

"small amount of radiation for small antenna width"

"Amount of radiation increase with increase in antenna width"

"Radiation further increase with antenna width"

  BUT up to what extent we can increase the width of antenna .

The limit for that is " not more than 90 degree".

                    Because anything beyond this will lead to same results as original width of waveguide . it is shown in below figure .

                                          

"It is maximum amount of tapering limit for antenna beyond which the results will be similar to original waveguide".

ALSO READ HAND OFF IN WIRELESS COMMUNICATION
                                                 click here

TRUNKING AND GRADE OF SERVICE (GOS)

TRUNKING: 

                   A radio system where the channel is allocated on demand by user is called "trunked" system.

           In this concept large number of users will be sharing limited number of resources provided by system. 

HOW IT WAS DONE: 

                                       There will be pool of channels available for users . When user request for channel then on the basis of channel per cell concept a channel is allocated to him. 

when user finish his conversation and cut the call the occupied channel is again returned to the pool of available channels .

Thus limited number of channels can be used to accommodate large number of users .

PROBLEM WITH THIS CONCEPT:

All though it's a good system to accommodate large users but it has got some problems with it ...

1) Suppose we have got a pool of 50 channels , numbers of customers at this time requesting for call or other service are more than 50 .

2) In this situation a channel cannot be allocated to user or the call will be in blocked state.

3) The user has to wait untill a channel become free and it is again returned to the pool of available channels.

4) Some times the calls are queued to keep the users on waiting.

GRADE OF SERVICE(GOS):

                                                   To measure the efficiency of trunked system a term is coined i e GOS .

Its like a "benchmark" for trunked system.

" It's a measure of the ability ot a user to access a trunked system during the busiest hour".

                 The busiest hour may be based on end of year , month, week,  and also based on timings i e between 4 p.m and 6 p.m .

It is duty of system designer to consider all these concepts and create a good system.

TYPES OF NETWORKS

       

       We all know about networks , There are two types of networks
1) Wired networks
2) Wireless networks

So in networks we have sub division again as
1) LAN
2) WAN
3) MAN

LAN:( Local Area Network)

                       Its the type of network which is limited in coverage area . 

It will be covering area in terms of few meters .

example : It is used to cover the building , offices , school  / colleges lab connections / hospitals etc 

The speed of the network will be higher 

LAN's generally consists of wired network with switches between them . It may also contain other networks such as load balancers , network intrusion detection and also firewalls .

Disadvantages :

1) Limited coverage area .

2) Limited number of elements are supported by this network 

ex: In a star connected network we can connect a maximum of 40 computers .

WAN:(Wide Area Network)

            Its a type of network which covers larger distance as compared to the LAN . It is used to connect LANs . Its speed will be less compared to LAN .

example: It is used to connect two places which are few kilometers apart .

The connection may be wired or it may  be wireless depending on the requirements . 

MAN:(Metropolitan Area Network )

                 Its a type of network which is used to connect the metropolitan cities . i e : its coverage area will be higher as compared to "WAN AND LAN " 

Generally it prefers the wireless connectivity between its nodes         ( computers ).

Advantages :

1) coverage area is higher .

2) possibility of error while data transfer is less .

Internet :

                   This is the network which can handel huge amount of traffic . Coverage area of this network is so high that it can cover whole world or it is used to establish the connection between two countries .