Network Applications
The computer networks form the main center of the systems of distribution of information. The meaning of the computer networks is erected on the bases of the connection of computers and other hardware equipments to each other for the possibility of exchanging information. A group of computers and other equipments connected to each other is called a network . The existing computers in a network can be shared of information , massages , software and hardwares among each other . Being shared in information , massages and softwares , almost is imaginable for all . In this process copy or copies of software is transferred from one computer to another one.
When we talk about sharing hardware , it means that equipments such as printer , or the modem devices can be connected to a computer , and can be used from anoter computer of the same network.
For example :usually , in an organization , the information about the sallary and personel payment , is kept in accouting department . If in this organization a computer network is used , the manager of the organization can reach those information from his own office and take a look at them . Sharing information and the sources of software and hardware , has many advantages . The computer networks can almost supply any kind of information to anyone who are reached to that network . This particular , provides discenteralization of information . In the past time , as far as the transmission of information were limited , So all things were done in a single place . The easiness and the speed of nowadays methods of exchanging information in relation with methods , such as disks or tapes transportation , couse that mankind communication plus to sonic talks , to find a new media of communication .
By the help of computer networks we can economize the expenditures related to expensive equipments of hardwares , such as hard disks , input information apparatus , and ect… . The computer networks , bring the needs of users in installation of hardware , removed or the least . The computer network can be used to standardize of applicant programs , such as word processing and page processing . An applicant program can be performed in central computer and the users without any need of keeping the real copy of program ,can get and use it by their own computer.
To standardize applicant programs , has the advantage , that all of the users are using the specific copy of it . This subject makes the supplier software company easy supporting of it,s product and the maintenance will be done efficiently . Another advantage of using the computer networks , is possibility of using the network for confirming communication Oonline by sending massage . For example , the managers can communicate with a number of employees by using electronic mail .
Network servers :
One of the most important part of computer network , is the server of the computer . Server is responsible to providing services , such as file transmission , print services , and ect … . Increasing the capaocity traffic of the network , it may cause problems in servers . In large networks , solving the problem the number of Computer Servers are increased .

These servers are called the particular Servers . Two kinds of these normal servers are as follows :
File print server
Application server
The first one meaning file and print server ,and is responsible to supply services such as saving the file , and changing the name of the file . which these requests are received from computer received servers . This server is also responsible to the printer affairs management . When a user issues an order to get a file existing in the server , the server computer sends a complete file copy for the user .
So , the user locally , meaning that from his own computer can process the file . The server printing computer is responsible to recive the user,s requests to print the documents . This server rows these requests and will send them to be printed one after the other one . This process is called "spooling". By spooling users without any waiting for the printing oeders , can do their other activities by their own omputer .The other kind of server is called Application Server . This server is responsible to Client/Server programs , and meet all of the data of Service resiever . Service suppliers , keep large capacity of information in them selves . For the possibility of quick and easy recalling the information , these data are stored in a specific structure .When a user sends a request to such server , the server transferes the result of the request to the user computer . For instance , consider a cammerial company This company tends to advertise a sum of its newly products . The company can only advertise for a group of products , in order to reduce the capacity of traffic , for a number of ostomers with specified income levels . Besides the mentioned servers , for services such as electronic mail , fax , the directory services and ect , in a network servers can also be allocated . But between File Servers and Applicaton Servers , there are many important dfferents underneat . A File Server in reply to user request , to reach a file, sends a complete copy of it , whereas , an Application Server send only the results of the user,s request .
The partition of netwoks
The partition of network on the base of geogrophy range :
The computer networks on the base of situation and installation position are of variouse kinds . One of the most important factor of determination of the network kind needed , is the communication distant between the network components. The computer networks have different geographical range , that is started from small distances about few meters , and in some cases is more than the distsnce between a number of countries . One of the computer networks is the Local Area Network or(LAN) . This kind of network includes short distances , such as the distances within a building or at most distances between a sum of buildings . For istance , suppose a network used at a company . In this network the most distsnces between computers are the distant between the building stories . In LAN networks , the computers are distributed on arlatively small scale , and are connected to each other by cables . That is why , the LAN sometimes negligently are called cable networks . The second kind of computer networks are Metropolitan Area Network or MAN . The distance in metropolitan networks is larger than the LAN networks . Also such networks distance are about a city dimensions . The MAN networks are usually combinated by two or a few LAN networks . For example , suppose a kind of MAN network that the LAN networks of a city , from head office in A city is connected to its agent office of the company in B city .
The third kind of computer networks , named WAN or Wide Area networks , or spread networks , the distances are more than the other kinds , and are about continental dimensions . The WAN networks are created from componet of So many LAN or MAN networks . The network connector of a company air ways office at the different cities of many countries , is a WAN network.
Partition on the bases of Node group

This kind of partition of networks is performed on the bases of the nature of Nodes or the point of connection lines of networks . In this parting groups , The networks rae divided into tow kinds . The difference of these two groups are existed into abilities of networks . These two main networks , are the kinds of networks , that are called (Peer To Peer ) , and the networks on the bases of Serever , are called (Server Based) .

In a Peer to peer network , there is no regularity between the groups of the networks , and all of computers in it are the same importance . Peer To Peer network is also named a Workgroup . In these kind of networks , non of any computers in the network , has the specific supplied function , such as servers . So , because of the cost of networks are low and the maitaning of it is relatively simple . In these networks , on the bases that which computer includes the information neded to other computers , that apparatus takes the role of server , and on the bases of the variation of situation , any of computers, any time , can be a sever , so the rest will be service receivers . To the reason that any computer with double functions , as sever and service reciver , each computer in the network is to decide two kinds of it,s functions . This process of decision making is called applicant station management or server .
The networks of the kind , Peer to Peer , are appropariate using in the ereas that the number of users should not be more than ten .
The agent systems such as.
Windows NT Workstation
Windows XP
Windows for work group
Are the kinds of operating systems with abilities of creation networks of Peer to Peer . In Peer to Ppeer networks , any of user is a determinent , so that on his own system , he can share any kinds of information in the netwok . This situation is as the same as any employee is responsible to keep and maintenance of his own documents .
In the second kind of computer networks , meaning the networks on the bases of servers , the limited number of computers are allowed to function as a server . In organizations that the users are more than ten , the usage of Peer to Peer network is not appropariate , and the networks on the bases of servers are preferred . In these networks for the vast capacity process of recieving server requests , the particular servers are used , and they are responsible to the security of the information . In the networks on the bases of server , the manager of the network , is responsible to the security managemet of the network,s information , and he manages to the determination of the level of users and who reach the sourses .
Because of the reason that information in such networks are only centeralized in the computer or server computers , suppling support copies from them are easier and determination of time table programs are appropariate for saving and support copies from the information is done easyly . In such networks the information can be kept on many servers , even though the saving of the first information damaged , (the first server computer) , the information still exist in the network , and the system can continue it,s working . These kind of systems are called .
Redundancy Systems or spare systems . For the usage preferences of these two kinds of networks , the organizations usually use a mixture of Peer to Peer networks and on the bases of server . These kind of networks , are named Combined network .

In Combined networks two kinds of operating systems are used to meet the needs of the network . For example , an organization can use operating system (Windows NT Server) to share with the information windows 95 . In this situation computers , plus of ability to reach the NT windows server information , they can share their own information to the other users .

Partition of networks on the bases of topology :
The way of adornment or the combination of network parts will be effective to management and ability to network development . For the best desiging network for meeting the needs , understanding the kinds of network adornments are the most important . Kinds of combining network , is evected of three kinds of topology .
These kinds are as follows :
Linear Topologe or Bus
Ring shaped Topology or RING
And Star Topology or STAR
Bus topology is the easiest topology using for networks in connection of the computers . In this adornment all of the computers are connected by a cable in a row . This cable in this adornment is named (Back Bone) or (Segment) . In this adornment each computer adds the address of he destination computer to its massage and sends on cable the information in the form of an electric signal . This signal is transmited by the cable to all of the computers within the network . The computers that the enclosured address coincidence with , will receive the massage . On the cables connecting the computers in the network , each electric signal after reaching the end side of the cable , react, and againe on the opposite side on cable will move . To prevent the reaction of the signal at the end side of the cables , there will be used a Terminator . Forgeting this small piece , sometimes will cause disablement of the whole network .

In this adornment of the network , in the case of disablement of any computer , it will not cause any damage to general function of the network . Opposite to this preference , the problem is that , all the existing omputers in the network will effect reversed and hard damage on the efficiancy of the network . In the case of many users the speed of the network notably is reduced . The problem is , whenever a computer are to send a massage , must wait a long time . The other important matter that should be taken into consideration is that of any damage to the network cable , communication will be cut off in the whole network . Second kind of adornment is "star", in this adornment all existing computers in the network are connected to a central piece named Hub . In this adornment the information before arrivel to destination pass through Hub . In this kind of network in the case of disabled a computer , or the cable is cut , disable all the network . On the other hand , in this kind of combination , a large capacity of cable is needed and as the HUB disabled , the whole of the network disable as well . The third one of topology is named ring shaped adornment . In this toplogy as the same as BUS , all computers are connected to each other by a cable . But in this kind , the top and bottom end of the cable are connected and form a ring shaped . So through this adornment there is no need for a piece of terminator . In tis kind of network also the communication signals pass through the cable and all of the existing computers towards destination . Meaning that all computers receive the signal . After relay the signal it will be sent to the other computer and so on . That is why this kind of topology is called active topology . Any computer has the function to receive signal , and so relay and send to the other computer . This signal is called (Token) , we must be aware not to make any mistake of this case with the case of sending a massage .
This case is something similar to relay of television broadcasting . By combination of star , ring shaped and HUB tpology , a combined topology called (Hy bird) is erected . Hybrid topology is used in very large networks. Hybrid topology itself includes two kinds . The first one is named HUB-STAR . As it comes from its name , in this network adornment as many STAR networks in a line form are connected with each other . In this circumstances any disorder in a computer performance , the rest of the network will not be effected . Although if HUB is disabled , a part of network will be disabled . In the case of causing damages in connective cable , the computers which are in different groups will be cut off , but the internal communication of the network is still working .
The scond kind also is named topology Star-Ring shaped In this topology HUB of many networks are of the kind ring – shaped form in a moded of star are connected to a central keyboard .
Security
Next to the exising of basic difficulties in design and performance of system process , application programs and existing protocols , the structure of computer networks , are not also without any fault , and are engaged with very many problems , and attackers penetrating to the lrss important existing of the network , and by ever spreading approach within the exising holes , have reached to all of the systems nd information that exist in the network . Network security includes many items , that most of them emphasise on keeping the network by Firewall from the penetration of Attakers . Although in commercial environment of today,s world , there are many examples and spieces that protection of sensitive information even for authorized users , that lawfully reach the internal networks is a necessity . This kind of protection of information is toled Encryption . Such a coding of information , will provide an out layer against people who try to penetrate into the management networks , having no authority .
Information security management systems
ISMS is a system of managerial security of information , that supply the ways of choosing the security standards which try to secure the networks from dangers of information , from worms and virouses upto threats , penetration and hacks . The standards which are provided in this case is BS7799 that specified the security policy and provide the appropartiate ways . By performance of such standard the organizations try to get BSI certificate . Khorasan Pajuhesh Rasa computer company as the agent of (SAMA) SA-IRAN communication industries on the basis of computer networks security and related actions , provides its services . Also on communication security direction , the Firewall eguipments and specific codings are provided by this company inside the state . These codings are as follows :

English

فارسی