CHAPTER 12 MODEMS

	Introduction :
	In a computer, information is stored in the form of binary digits, such as zeroes and ones. However, information transferred over the telephone or cable lines is transferred in the form of analog signals. Analog signals are similar to the waves because they vary continuously along with their length. When two computers communicate with each other over a telephone line, the binary information from the sending end needs to be converted into analog signals that can be sent over a telephone line. Again, at the receiving end, these analog signals need to be converted back to the binary information so that a computer can use the information. The process of converting the binary information into analog signals is called modulation and the conversion of analog signals back to the binary information is called demodulation. Modem is a communication device that enables a computer to transfer data over the telephone or cable lines. The word modem is the short form for Modulator Demodulator.
12.1 Features of a Modem
	Modems differ from each other depending upon their characteristics. Different modems have different data transmission speeds and are designed to work with several protocols. Again, different modems may support different types of communication such as Data, Fax, or Voice. Some of the most common features of a modem are: _ Transmission speed -The speed of the modem indicates how fast the modem can transmit and receive data. The data transmission speed is measured in terms of bits per second (bps). The speed differs from one modem to another. You can achieve higher data transmission rate by compressing the data. The faster the data transmission rate, the faster you can send and receive the data. However, note that you cannot receive the data faster than it has been actually sent. For example, a modem is sending data to the computer at 5,600 bps, you will receive the data at the same speed. _ Voice or Data communication - Many modems have a built-in switch that enables to change between the voice and data mode. In the data mode, modem operates like a standard modem whereas in the voice mode, modem operates like a standard telephone. Modems that allow voice communication have a built-in loudspeaker and microphone. _ Data compression - Some modems can compress the data before transmitting it over a telephone line. This helps to send the data at faster rates. However, the effectiveness of the compression differs from data to data. For example, if the modem is trying to compress the data that is already compressed, the compression will be less efficient. While compressing the data, ensure that the modem uses the same compression technique to decompress the data. _ Auto answering - Some modems enable computers to receive calls in your absence. This facility is useful when you are providing some type of computer service wherein people can call in to use. _ Fax capability - Some modems, also known as fax modems can communicate with facsimile machines apart from communicating with other computers. They have the capability to send and receive faxes.
12.2 Working of a Modem
	Modem is a device that is used for modulation and demodulation of information. The working of a modem includes the following steps: 1. Software such as Auto Dialers in the computer activates the modem, which listens for the dial tone on the phone line. 2. Once the dial tone is available, the modem dials a specified number. 3. The modem at the receiving end answers the call and sends out a signal to the modem at the sending end. This results in establishing a connection. 4. The modem at the sending end coverts digital signals from a computer to analog signals that can pass over the telephone line. 5. The modem at the receiving end converts analog signals back to digital signals that can be used by the computer. 6. The steps 4 and 5 keep repeating during the online session. 7. Once the online session is over, the computer issues a command to disconnect.
12.3 Types of Modems
	There are two types of modems, namely : _ Internal - An internal modem is a modem card that you can plug into an expansion slot on the motherboard. Internal modems do not take up the desk space because they are fixed on the motherboard. These modems use the power supply of the computer and therefore they are on whenever the computer is switched on. _ External - An external modem is connected to the PC through a cable, which is plugged into a port on the back of the system unit. External modems have their own power cords. Before establishing the connection, you must switch on the modem. Apart from this classification, the modems are further classified into X2, V90, ISDN, DSL, and Cable modems.
	12.3.1 X2 Modem
	U.S. Robotics developed a technology called X2 for transmitting data at the speed of 56 Kbps over plain old telephone service (POTS). X2 technology could transmit data at the speed of 56 Kbps because it took the advantage of the fact that the high-speed digital lines connect most phone switching stations. It does not perform the digital-to-analog conversion and directly sends the digital data over the telephone lines. Today, X2 modems use the V.90 modulation standard. They provide high-speed Internet access. However, X2 modems have the following limitations: _ Data transfer speed is high only when the data is sent to your computer. However, data transmission from your computer to the dial-up server is done using the normal techniques with a maximum speed of 40 Kbps. _ When you want to connect to the Internet using the X2 modem, the ISP modem must support the V.90 standard.
	12.3.2 V.90 Modem
	Initially, modem standards were built on the assumption that both ends of a modem session have an analog connection to the telephone line. However, V.90 technology was built with an assumption that one end of a modem session has a pure digital connection to the telephone line. Because of the pure digital connection, telephone network acts a digital network. V.90 technology helps to increase the data transfer speed from the Internet to the computer. It supports data transmission speed of 56 Kbps. This helps to download the Web page with sound and video and other larger files. However, the data transmission speed from the computer to the Internet server is normal. Modems using the V.90 technology are called the V.90 modems.
	12.3.3 ISDN Modem
	Integrated Services Digital Network (ISDN) is a set of international communication standard for transmitting digital data over a telephone line. It enables to send data, voice, and video over digital telephone lines or over an ordinary telephone wire. It transmits data at the speed of 64 Kbps. ISDN needs equipment called ISDN adapter to perform conversion between the digital signals and analog signals. This ISDN adapter is physically and functionally similar to the modem. The ISDN modem can be an internal or external modem. ISDN modem provides high-speed Internet access. ISDN needs modems at both the transmission ends. Therefore, your ISDN access provider also needs an ISDN modem
	12.3.4 DSL Modem
	You can connect to the Internet using the standard modem, LAN connection, ISDN connection or the Digital Subscriber Line (DSL) connection. DSL connection is a high-speed connection that allows the phone wires to transmit modulated digital content at a high speed. DSL can carry both voice and data at the same time and therefore you do not need to install another phone line. DSL needs a device called DSL modem to convert digital signals to analog signals and analog signals back to the digital signals. A DSL modem provides high-speed access to the information through the telephone lines. DSL modems are also known as DSL transceiver or ATU-R. You can connect DSL modem to a PC through USB or Ethernet connection.
	12.3.5 Cable Modem
	A Cable Modem is a device that enables to connect a PC to the cable TV line. Its data transfer rate is much faster than the regular telephone modems, ISDN modems, and DSL modems. You can also add a cable modem to a set-top box that provides channels for Internet access on the TV set. Usually, a cable modem is provided when you subscribe for the cable access service. You cannot just purchase the cable modem and install it. While connecting the cable modem, you need to make two connections, one to the cable wall outlet and another to the PC or to the set-top box. A cable modem can be an external device or it can be integrated within a computer or a set-top box. Therefore, there are two types of cable modems: _ External Cable Modem - To connect the External Cable Modem to the computer, you need the Ethernet connection. Therefore, before connecting to the external cable modem, you need to add an Ethernet card to the computer. In addition, the external cable modems can be connected to the computer through the USB connection. However, the USB based cable modem can connect to only one PC at a time.
	External Cable Modem
	_ Internal Cable Modem - The Internal Cable modem is an add-in card that you can integrate within a PC or a set-top box. Cable modem communicates with a Cable Modem Termination System (CMTS) at the local cable TV company office. They can only communicate with a CMTS and not with any other cable modem on the cable TV line.
12.4 External Modem Controls
	The External Modem allows controlling and monitoring some functions on the device itself. For example, status lights on the modem indicate whether the modem is on or off. Some of the controls that are most commonly found on the external modem are : _ Power On Light - Indicates whether the modem is switched on or off _ Speed indicator - Indicates the speed at which the modem is currently operating _ Mode indicator - Indicates which mode is currently active. Some of the most commonly found modes are: _ Auto answer : Indicates whether the modem is set to answer an incoming call _ Dial : Indicates whether the modem is ready to dial _ Send : Indicates whether the modem is ready to dial and transmit the data _ Receive : Indicates whether the modem is ready to answer and download the data _ Error indicator - Indicates whether any error has occurred during the transmission
12.5 Modem Standards
	Modems use a modulation method to communicate with each other. This modulation method converts digital data from a computer to analog signals that can pass over a telephone line. The modulation methods are also known as modulation standards. The two modems communicating with each other must support the same modulation standard. Otherwise, the two modems cannot communicate with each other. Table 12.1 lists some of the most commonly used modulation standards.
	Standard Description
	V.32bis Supports full-duplex transmission at a speed of up to 14,000 bps V.34 Supports full-duplex transmission at a speed of up to 28,800 bps V.42 Regulates error-detection for high-speed modems. This standard enables modems to work with both, digital and analog phone lines. V.42bis Used for data compression that enables to transmit data at a speed of up to 34,000 bps X2 Supports data transmission at a speed of up to 56 Kbps V.90 Integrates both K56Flex and X2 standards into one V.92 Provides features such as: _ Modem on hold - Enables to suspend the data call made by the user and answer to an inbound call _ Quick connect - Enables to reduce the connection time by remembering the line conditions to the dial-up server _ V.PCM-Upstream - Enables faster upstream communication at a speed of up to 48,000 bps. Table 12.1 -Modem Standards
12.6 Modem Protocols
	Modem protocols are used to transfer data over a telephone line. Most modems are designed to work with different protocols. When a modem sends a message to another modem to establish the connection, the handshaking process is conducted. The handshaking process negotiates to use the best speed, data compression standard, and error correction protocols. The protocols used are reported by the handshaking status message, which is added to the log file of the modem. Table 12.2 lists some of the most commonly used modem protocols :
	Protocol Description
	Automatic repeat request (ARQ) Enables the modem to automatically request to re-transmit any corrupted data. It helps to control error in data transmission. Link Access Procedure for Used to control error in data transmission. This protocol uses Modems (LAPM) cyclic redundancy checking and retransmits any corrupted data to ensure data reliability. Microcom Network Protocol Used to control error in data transmission and perform data (MNP) compression before transmission. The error control helps to resolve the changes that are caused due to the telephone line interference during data transmission. MNP offers various levels that are used for specific purpose. For example, MNP level 4 offers error correction, MNP level 5 provides data compression facility, and MNP level 6 provides error correction and data compression facility. K56Flex Is an interoperable high speed modem protocol
12.7 Installing an Internal Modem
	Internal modems are plug-in cards that can be inserted into an ISA or a PCI slot. Today, most modern modems use the PCI slot.
	Best practice
	1. Read the modem documentation before installing a modem.
	Precautions
	1. Wear an antistatic wristband. 2. Ensure the computer is switched off and unplug the power cord from the wall socket. 3. Disconnect all the peripheral devices attached to the computer.
	Hands on exercise
	To install the internal modem : 1. Remove the case cover. 2. Select a free PCI slot. 3. Remove the cover from the slot and keep the screw safely. 4. Hold the modem by its edges. Do not touch the connector pins of the modem or any chip on the motherboard. 5. Carefully, slide the internal modem into the slot. 6. Firmly, press on the top of the modem to roll the card into the slot. 7. Lock the modem to the system with the help of the screw. 8. Place the system cover back. 9. Locate the two phone jacks labeled Phone and Line on the back of the system case. 10. Plug one end of the phone cable into the Line jack and another end of the phone cable into the phone outlet. 11. Plug one end of another phone cable into the Phone jack and the other end of the phone cable into the telephone. This step is optional. 12. Reconnect all the cables and switch on the computer. Windows 2000 will automatically detect a new device and display a wizard to install the modem driver. 13. Follow the steps to complete the installation.
12.8 Installing an External Modem
	Installing an external modem is very easy and simple. You just need to connect the modem with a computer and a telephone line.
	Hands on exercise
	To install the external modem : 1. Place the external modem on the desk. 2. Locate the serial cable that comes with the modem. Note that the cable has a 25-pin male connector at one end and 25-pin female connector on the other end. 3. Plug the female connector of the serial cable into the 25-pin serial port on the back of the system unit. 4. Now plug the male connector of the serial cable into the modem. 5. Secure the connectors with the help of screws on both the sides. 6. Plug the power cord of the external modem into the power outlet. 7. Locate the two phone jacks labeled Phone and Line on the back of the external modem. 8. Plug one end of the phone cable into the Line jack and another end of the phone cable into the phone outlet. 9. Plug one end of another phone cable into the Phone jack and the other end of the phone cable into the telephone. This step is optional. 10. Switch on the computer. Windows 2000 will automatically detect a new device and display a wizard to install the modem driver. 11. Follow the steps to complete the installation.
12.9 Configuring the Installed Modem
	Once you have installed the modem and the modem driver, you can configure the modem. Configuring helps to modify the properties of the modem such as the communication port to which the modem is attached, or the maximum port speed. It also enables to modify the dialing rules.
	Hands on exercise
	To configure the installed modem : 1. Select Start _ Control Panel _ Phone and Modem Options. The Phone And Modem Options dialog box appears. The Phone And Modem Options dialog box consist of the following tabs: _ Dialing Rules - Specifies the location from which the modem must dial _ Modems - Lists all the installed modems _ Advanced - Lists all the telephony providers installed on the system 2. Click the Modems tab. The Modems tab appears. The options available in the Modems tab are: _ Modem - Displays the name of the installed modem _ Attached To - Displays the communication port to which the modem is attached _ Add - Enables to install a new modem _ Remove - Enables to remove the selected modem _ Properties - Enables to view or modify the properties of the selected modem 3. Click the Properties button. The Properties dialog box appears. The options available in the General tab of the Properties dialog box are: _ Port - Displays the communication port to which the selected modem is attached. _ Speaker volume - Enables to adjust the speaker volume of the selected modem . _ Maximum Port Speed - Specifies the maximum speed that programs can use to transfer data to the modem. _ Dial Control - Specifies the modem to wait for the dial tone before dialing. 4. Select the required options. 5. Click the Advanced tab. The Advanced tab appears. The options available in the Advanced tab are : _ Extra initialization commands - Enables to specify the commands that must be sent to the modem when it is initialized. _ Change Default Preferences - Enables to set the default call and data connection preferences such as the port speed, data protocol, and whether the compression is enabled or not. 6. Type the required commands in the extra initialization commands text box. 7. Click the OK button.
12.10 Testing Modems
	After installing and configuring the modem, you must test whether it is working properly using diagnostics. Diagnostics sends the standard AT commands to the modem and displays a report indicating how modem responds to each of these commands. Most modems support AT command set. AT commands are used to communicate directly with the modem. The modem documentation provides information about the commands supported by the modem.
	Hands on exercise
	To test the installed modem : 1. Open the Phone And Modem Options dialog box. 2. Click the Modemstab. The Modems tab appears. 3. Click the Properties button. The Properties dialog box appears. 4. Click the Diagnostics tab. The Diagnostics tab appears. The diagnostics tab enables you to identify the problems with the modem. This problem can either be a hardware conflict, a damaged modem, or an invalid device driver. The options available in the Diagnostic tab are: _ Modem Information -Displays information about the modem. It also displays the standard AT commands sent to the modem and how modem responds to each of these commands after you click the Query Modem button. _ Query Modem - Sends the standard AT commands to the modem and displays how modem responds to each of these commands. _ View log -Displays the log file for the modem. _ Append to Log - Adds session information to the log file. 5. Click the Query Modem button. The standard AT commands are sent to the modem and how modem responds to each of these commands is displayed.
12.11 Troubleshooting Modems
	Troubleshooting modem enables to solve the problems that might arise due to improper functioning of the modem. It also enables to solve the problems that could arise due to the communication software such as Auto Dialers or Dial-Up connection. This section helps you to locate some of these problems and fix them.
	12.11.1 The External Modem Is Not Working
	If your external modem is not working, check the following : 1. Ensure that the modem is switched on. 2. Ensure that the cable between the modem and the computer is properly connected. 3. Ensure that the power cord of the modem has been plugged into the power socket. 4. Ensure that the modem is set to the correct mode.
	12.11.2 The External Modem Is Not Receiving the Commands
	If your external modem is not receiving the commands, there may be some problem with the modem cables. Try to connect the modem to the computer using a new cable. If the cable is good and still the modem is not able to receive the commands, you may have not installed the modem correctly. Check the documentation to verify whether the modem has been installed correctly.
	12.11.3 The Modem Dials but Does not Connect
	If the modem dials but you cannot make a connection, check the following : 1. Ensure that the hardware settings for both, the sending modem and the receiving modem are identical. 2. Ensure that the modem is dialing the correct number. 3. Ensure that there is no interference on the telephone line. Because if somebody picks up the extension phone or even if there is a cross connection, the call is cancelled.
	12.11.4 The Modem Does Not Get a Dial Tone
	If your modem does not get a dial tone, check the following : 1. Ensure that the telephone is properly working. 2. Ensure that the phone cables are properly connected. You may have wrongly placed the Line jack to the telephone and the Phone jack to the phone outlet. 3. Ensure your communication software has been installed correctly.
	12.11.5 Modem is Busy or Not Responding
	If you get an error that, the modem is busy or not responding, check the following: 1. Ensure that the modem connection has been disconnected properly. 2. Ensure that the modem is not been used by any other software such as the Fax software. 3. Ensure that the modem has been installed properly.