3.6.5. Modem

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The development PTT network, despite its technical inferiority to transfer data over the LAN, is used for communication between the PC and transfer data over longer distances device called MODEM (MODulator-DEModulator). The modem is based on converting a sequence of electrical serial digital signal into an analog signal suitable for transmission over a channel with PTT adjust the frame rate according to the principles discussed in Chapter 3.3.7. This kind of networking, which is used to communicate classical voice channel in the frequency range of 300 Hz to 4000 Hz is labeled PSTN (Public Switched Telephone Network).

Essentially modem represents the interface between the PC and phone environments. It has in it all the qualities of an integrated phone, the signal recognition switchboard, dial the number to the communication and interruption of communication.

Data are transmitted by PTT channel V.21-V.92 standards recommended by the CCITT, or by RS 232c specification between the modem and the computer. Most modems support the 'AT' command set used to manage the transfer parameters. For them to say that the Hayes-compatible to standard established by the firm of the same name.

 AT modem commands
Table 3.6.21 Part of the 'AT' commands of control the modem.

Command lines must start 'AT' prefix e.g. 'ATZ &F X3 L3 M1', a sequence of commands or the distance between them is of no importance, and command without a number implies a '0'. It is not advisable to mix uppercase and lowercase letters on the command line.

The development of technology has enabled the creation of a modem card that is inserted into the slot motherboards computer and very often with a communication function and integrated electronic support FAX transmission. The lack of an internal modem is lack of portability and use with only one kind of computer. The above disadvantage can be mitigated if you allow access to the modem via the network resources. Modem is inserted into the slot of the motherboard is an additional peripherals, and it should be defined during installation e.g. IRQ=3 and access the serial port to COM4, 'doubler' of serial port COM2.

The modem provides, along with related program support, the use of computers as terminals at greater distances from the host, the B.B.S. communicate and exchange files with sophisticated software and support, as Peter Norton pcAnywhere, 'intrusion' on the remote computer as a host, and remote work on it as a launching applications, data transfer or exchange messages.

As with communication in a local area network, data transmission using modems required for this purpose, a peculiar set of rules and ways of preparing data for shipping and receiving and transmission control. The most commonly used protocols and management are.

Example I

V RECOMMENDATIONS - A set of CCITT recommendations which prescribe how data telecommunication lines. The most commonly used are:

ASYNCHRONOUS data is based on identifying the beginning and end of the data during transmission, the next transmission of the next data is based on mutual 'understanding' of electronic circuits on receipt of the previous marks.

Example II

Selection of parameters modem for asynchronous communication:

 Parameters of modem communications

In front of each entry follows the START bit, then bits of data with parity bit (if specified), and stop bit, and again in the same order for the subsequent statement. So Any information added three more bits of management and control. Parameters must be set identically on both sides shall be set at such a value:


Baud rate determines the person who does transmit data during transmission. Transmission is slow, a lot is lost in the constant speed control with start / stop marks and mark parity. Usually the mark parity is not used with regard to the increase in speed, but then increased susceptibility to errors.

SYNCHRONOUS transmission is based on the transfer of blocks of data at a pre-set before the transfer speeds in both the transmitting and the receiving device. When the transfer begins, after the header of the data transmitted by the data without start / stop bits, and parity bits and CRC sum of all the data in the block and mark the end of the block and occasionally during the transfer blocks in the admission control and coordinate tact between the transmitting and receiving devices. If the CRC does not match requested the reopening of the transmission block. Synchronous transmission hardware device is more complex and faster.

Way to use the modem will determine the client software that uses it. The simplest way is to use a modem to the computer as a terminal linked to a remote host with a text mode. Such communication with B.B.S. servers. With one server is not possible to continue to connect with each other. It is a kind of communication points to the point (Point-to-Point).

Poverty is a cause of communication terminal development of global communications network - the Internet and the specific program elements to support graphics as WWW (World Wide Web) network service to the principles of client-server architecture.

WWW allows you to view documents on the server and their interconnection by linking the document, some of his words or pictures with another term on the same or another computer anywhere in the world. It's a huge e-book to multimedia elements that dynamically grows every day.

Such communication requires fast modems and is based on the use of advanced protocols like HTTP (HyperText Transfer Protocol). The documents are reviewed by a special computer program called BROWSER. Browser displays the content of the document and allows you to search for an appropriate new server side when the user selects one of the prominent fields offered (hypertext) to LINK on the page that is currently displayed. Each page is uniquely identified by its URL (Uniform Resource Locator). Typical communication between servers and users on the Internet is supported by the TCP / IP protocol suite.

To ensure numerical flow in each of the modem communication PTT server number is assigned to 'multiplexer', an electronic device that allows its electronic circuits in the temporal distribution of access to communication channel, i.e. the transmission of messages over the same communication channel. This approach is one PTT number increases. E.g. selecting the number of 9762 and its occupation of one user uses the first available time multiplexed and other user selecting the same number of free multiplexed busy next time. Only when all multiplexed busy roads will get a busy tone.

Example III

Labeling of sites placed on the site and e-mail addresses on the Internet.

Common URL (Uniform Resource Locator) of the page specifies at site with the syntax:


          PROTOCOL - as a way of using HTTP, HTTPS, FTP ...
          sub_domain - mark WWW, which indicates the processing site
                       HTTP protocol or the name of the computer on
                       which the server is the seat, or path to
                       the desired content within the DOMAIN
          DOMAIN - unique name of server where the contents are
          main_domain - the state (for Croatia - tag 'hr')
          DOCUMENT.FORMAT - definition document which seeks

for example,


If the definition is not specified in the document, use the default name for the document that is marked: index.html or index.htm or index.php, or default.htm, or home.html, or something else that is set up by the server. The protocol essentially describe the user interface. The computer name is usually the host name entered in the table DNS (Domain Name System) server which is connected to its default IP address. Path, directories through which it passes to placement of the document is to be reviewed and that the type (format), HTML (HyperText Markup Language). The document will not be able to see if the browser does not recognize its format. Not necessarily specify the name and format of the document if it is to search the area that is indicated. Home domain or sub domain (DOCUMENT.FORMAT) need not be given if it is on the server recovered one of these default names. But it is not the rule. Uppercase or lowercase letters are of no significance to the notion DOCUMENT.FORMAT; Index.htm not the same document as index.htm. The name of the document, along with our graphemes, are not recommended even spacing between words in the title of the document, but the use of the character ' _ '. In addition, the HTTP protocol is the default if not specified otherwise.

Main_domain determined for each country (two letters), are defined and distinctive as major domains:

Also considered is the introduction of additional specific names. One of the services the Internet is e-mail (electronic mail) address for which the user indicates in the following way:

    user@DOMAIN.main_domain ...............usual e-amil address
    user@sub_domain.DOMAIN.main_domain ....extended e-mail address

      user - users title ('user_name' or 'Name.Surname')
      @ - sign of belonging and separation (at) and is part of
          the syntax of address
      sub_domain - place in the user part of the domain that
                   is used, or the name of the computer is
                   the server of e-mail
          DOMAIN - area where the user belongs (e.g. Spalato)
          main_domain - country (for Croatia, the label is 'hr')

and the term ' user ' refers to user_name selected to access the server or personal information in the form of ' Name.Surname ' user_name associated with, for example,


User_Name ' may not be consistent with the name and surname of the user, the data associated with your name. User_name is determined by the user in agreement with the administrator of the ISP's server, or ISP determined user_name using with an automatic mechanism. In the domain name in the electronic address, user_name and the name and surname of users, there is no grapheme localities. The reason is that the legacy of the servers at the Internet infancy remained until today.

User_name is the user identifier of a user on the system and it is password protected. User_name and password together together make up an ACCOUNT or otherwise called ACCESS (Log_IN). Thus, the system with regard to the use of its resources should be reported, and the useful work done by and check out. The password is good to use 8 or more characters, of which at least two should be numbers and the use of uppercase and lowercase letters. It is not good that the password contains elements of the user name. A good password is something long enough written in dialect, for example 'WreckWith1300Cubic'.


For security in network access through a modem, it is useful to use a modem that can send a callback user to verify his identity. Additionally, when exercising with B.B.S. or Internet server the user must log in and enter your password (not for the public network). Therefore, the password does not need to reveal to everyone because then the possible abuse to the detriment of users frivolous, especially if you are on a subscription basis and during the use of server resources.

Each modem, either external or built, has an interface with the LINE and PHONE jacks. Connects to the PTT central office by the following picture:

 Connect the phone to PTT connection through a modem
Figure 3.6.22 Diagram of a modem for PTT.

The modem is the first PTT terminal and if its a solidly built electronic circuits will turn off the 'phone' port at the moment to establish communication. Interface modem and the computer will be bus computers (built-in modem) or RS 232c serial channel (external modem). Communication cable to connect the modem to the phone jack and PTT uses RJ11 type connector with 4 or 6 of the pins, and to communicate using only two. Usually it takes a four-wire cable and RJ11 4/6 connector, which means that out of 6 of the connector 4 are used and only two active. The development of the Internet as a global network with a diversified services, largely driven by the development of other types of communication, as well as ISDN and ADSL, and very soon we can expect a declining share of modem communications. While the modem is losing the race with advanced technology, there are places where its use meets. For example, linking trade with accounting concerning the exchange of traffic data; short, fast enough and efficient communication.

Today, the PTT telephone number and fax devices, increasingly the bulletin pages of newspapers and TV commercials is the WWW address of advertisers and e-mail addresses to which to send the desired queries and find answers needy. Internet and its services inexorably penetrate, but the unwanted byproduct of abuse, and the security and reliability of data all the more questionable.


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