How Modems Work

Importance Of Modem Communications.

There is always going to be a high demand for PC users to communicate with each other through dialup Internet connections. The modem is a device that can facilitate the process of sending and receiving data communications across telephone lines ultimately adapting a digital machine to an analog technology. Dial up Internet access is preferable mainly for people in rural settings where high speed Internet access is unavailable, or for the more casual Internet user not requiring fast connections.

Short for modulator / demodulator, the modem converts a computer’s digital signal into analog frequencies for the telephone system and then converts these frequencies back into digital pulses so the computer can understand what is transmitted through the phone line.

Modem Speeds and Performance

A modem’s speed and performance is measured in the number of bits per second that it can transfer over a phone line. Typical speeds of old school modems range from 33,000 kilobits per second (kbps) to a maximum of 56,000 kbps. Additionally, in the computer industry eight bits is equal to one byte, so a transmission throughput of 33,000 kbps indicates a real transfer speed of approximately 0.039 megabytes per second (MBPS).

Although the most earliest types of modems manufactured were labeled asynchronous devices they were capable of operating at extremely low speeds typically with a total throughput of 14.4 kbps, barely enough juice to connect to a text based Bulletin Board System (BBS).

This maximum limit modems can transmit data is imposed by the technology itself; the hardware merely complies with the current technologies. While dialup modems can only utilize a tiny fraction of a telephone line, high speed DSL internet access requires the entire telephone signal to transmit data, which results in minimum speeds of up to 0.358 MBPS.

Modem Protocols

V.34 and V.90 are referred to as modem protocols and dictate how data should flow to and from communication devices. Dial up modems manufactured at present day are called synchronous modems and are capable of transferring data through dial up Internet connections at a more acceptable speed, or 56,600 kbps.

Modem Basic Terms And Definitons

BPS (Bits Per Second)
Bits Per Second is a term used to describe the speed at which a modem can send and receive data. At slower speeds modems are defined using the term baud rate. A faster modem however is measured in bits or the number of bits per second that it can transfer. The newest standard modems comply with can typically transfer data at speeds of up to 56,600 kilobits per second.

Internal Modem
An internal modem is an expansion card that plugs in directly to an expansion slot on the motherboard. Internal modems can either be PCI or ISA based.

External Modem
An external modem is a small box connected to a PC via a standard RS232 serial cable. External modems require a special 12 volt power adapter.

Communication Port
A communication port is an interface on a computer where a modem is connected to. On a PC there are typically 4 communication ports labeled appropriately COM1 through COM4. A modem is typically configured for use on COM1 or COM2.

Fax Modems
A fax modem is a special type of modem that has built in faxing capabilities in addition to standard data transmisson, allowing the user to send and receive facsimiles on a PC. It’s standard for newer modems to have faxing capabilities.

Voice Modems
A function on a modem that allows it to act like a standard voice answering machine and provides related functions, including voice mail configurable through a software based PC application.

Modem Interfaces

Modems can be either internal or external devices and are connected to a computer via communications port typically labeled COM1, COM2, COM3, or COM4. Internal modems are simple expansion cards that plug directly into expansion slots on the mainboard. These slots can either be ISA or PCI, with PCI modems being more common and easier to install.

ISA Modems

Older modems were manufactured using the Industry Standard Architecture (ISA) bus type. These types of modems worked just as well as newer standards but PCI based modems tend to offer easier installation.

PCI Modems

Newer modems use the PCI interface and typically support true Plug and Play technologies. Modems tend to have true plug and play capabilities built in, however, some modems may require device drivers be installed to function correctly and to provide additional features such as faxing capabilities.

External Modems and How They Work

External modems are small boxes that connect to the PC via serial cable links. There is a standard interface that external modems use called RS-232 also known as a serial interface. Any external modem supporting an RS-232 female connector can be attached to a PC's internal system through this connection, however this type of connection requires a separate power supply to give a modem the juice it needs to power correctly. By comparison, the internal modem gets a steady flow of power from the motherboard's PCI bus interface.

Modem Standards

Over the years, modem standards have developed in a somewhat disorderly manner. Standards were developed regarding the methods in which the modem compressed data, and the ways it performed error control. V.22, V.32, and V.32 bis were the first modem standards to develop which specified the speed at which modems could transmit data.

The V.34 protocol was introduced at the end of 1999. It was capable of supporting a maximum transmission rate of 28.8 kilobits, very slow by current standards.

V.34 modems were manufactured backwards compatible with older standards to cope with noise in the telephone line. By 1996 V.34+ replaced the V.34 standard and permitted for a slightly faster data transfer rate supporting a 33.6 kilobits per second transmission rate. V.34+ remained backwards compatible with the older standard and is more able to adapt to line conditions to get the greatest amount of usable bandwidth over the copper telephone wiring.

This section highlights various modem standards and maximum speeds.

Bell 103M & 212A
Bell 103 transmits at 300 bps at 300 baud and 212A transmits at 1200 bps at 600 baud

V.21
Capable of only 300 bps - it is an international standard used mainly outside of the U.S.

V.22bis:
2400 bps at 600 baud. Used in the U.S.

V.23:
Allows the modem to send and receive data at the same time at 75 bps.Used mainly in Europe,

V.29:
Capable of 9600 bps. A one-way (half-duplex) standard that is used mostly for fax machines.

V.32:
A full-duplex standard capable of 9600 bps at 2400 baud. V.32 modems automatically adjust their transmission speeds based on the quality of the lines.

V.32bis:
it is capable of 14,400 bps. A second version of V.32,It will also fallback onto V.32 if the phone line is impaired.

V.32ter:
capable of 19,200 bps. The third version of V.32,

V.34:
Capable of 28,000 bps or fallback to 24,000 and 19,200. This standard is backwards compatible with V.32 and V.32bis.

V.34bis:
Capable of 33,600 bps or fallback to 31,200.   
 
V.42:   
Same transfer rate as V.34 but is more reliable because of error correction.

V.42bis:    
34,000 bps. A data compression protocol that can enable modems to achieve a data transfer rate higher than the what's reported by the standard.

V.44:          
Allows for compression of Web pages at the ISP end and decompression by the V.44-compliant modem, so transmitting the same information requires fewer data packets.

V.90:    
it is capable of 56,000 bps. The fastest transmissions standard available for analog modems.

V.92:          
Transmits at the same speed as V.90 but offers a reduced handshake time and an on-hold feature.

Fax Modem Classifications

Nearly all modems have faxing capability built into the modem itself. A fax modem enhances the performance of a PC by allowing it to become a standalone fax machine. Faxes can be manipulated immediately when received at the another end before it is printed. Additionally, a common fax utility in any faxing software is the ability to produce cover-sheets.

The following table summarizes the three differences between the three main classes of fax modems.

• Class 1 fax modems perform basic fax functions including sending and receiving faxes.
  
  
• Class 2 fax modems initiate and receive calls and perform various error checking functions.
  
  
• Class 3 fax modems provide an assorted amount of processing based upon their service class
  

Voice Modems

Voice modems have emerged as an excellent device for combining voice, data, and fax which is having an adverse affect on communications between personal computers.

The multipurpose voice modem can be configured as a basic answering messaging to a more complex voice mail system allowing the storage and retrieval of messages for literally hundreds of mailboxes. Most software applications configure space on a hard disk to store the messages for various mailboxes and allow for incoming data or fax calls to be automatically routed to a mailbox specified by a user, thus the size of a mailbox for a particular user is dependent on the amount of available hard drive space.

Installing an Internal Modem

This section will guide you through the process of installing an internal PCI modem into your computer. Although the process is not overly complex, this section will give you a basic understanding for a successful installation.

The following materials are required for the installation:

•   The PCI modem interface board.
•   Any manuals that accompanied your modem.
•   An open PCI slot on your motherboard.
•   One screw for securing the internal modem to the chassis.
•   Needle nose pliers for configuring jumpers and dip switches on the modem.
•   Phillips screwdriver.
  

Installation Instructions

1.     Shutdown Windows, turn off your computer, unplug the power connector (normally a thick black cable leading to the power supply unit (PSU) connector on the back of the computer), and unplug all other connectors including mouse, keyboard, and monitor.

2.     Pull off the back and side panels of the case by removing the screws from the the back and side of the chassis. There are normally four screws that need removal on most newly designed cases (two on the left back side, and two on the right back side of the chassis). Older cases might also have screws that secure the top of the unit to the chassis, so you will need to take a look and remove those screws accordingly if they appear to be an obstacle.

3.     Once you have successfully removed the back panel of the chassis, and top of the chassis (if necessary), place both panels and screws in a safe place as you will need to re-attach these items once you've finished the installation.

4.     You now have to choose an expansion slot on your motherboard to insert the modem board into. Make a note of the slot the modem will be inserted into.

5.     Discharge the electricity from your body by touching a part of the metal chassis.

6.     Remove the metal cover that is covering the slot on the back of your computer. Save the screw and metal bracket as they may become useful for any future installations.

7.     Before removing the modem from its packaging read the manual that came with your device. There are certain jumpers that may need to be set, so set them before you insert the card. Consult your manual for clarification if your modem has jumpers that need to be set.

8.     Remove the modem from its packaging and gently take hold of it by its sides.

9.     Gently insert the board into the free expansion slot and secure it to the metal bracket with the screw you removed in step #6.

10.     Attach the phone cables to your modem. There are typically two jacks on the modem itself; one for the telephone company’s line, and one for a standard telephone. Usually these are labeled either LINE and TEL, but it can be simple to get the combination wrong (i.e. inserting the telephone line cable into the jack used for the standard telephone). Most modems label these jacks, so consult your modem's documentation if you need assistance determining the proper connections. Connection to a standard telephone is done through the telephone jack on the modem (optional).

11. Once you have completed step ten attach any disconnected cables, close your system's panels, and power up your system.

If you are running Window XP and your modem is Plug and Play compatible, then Windows should detect and configured the modem automatically.

Once Windows XP detects your modem, a new Wizard screen should appear asking you for the appropriate drivers that accompanied your modem. If your modem is not Plug and Play compatible (i.e. your modem has not been automatically detected), then the configuration of your modem has to be done manually.

Windows XP has a built in Phone and Modem Options in which you can configure various communication related options for any modems and networking devices installed in the computer. This is also the best way to determine if your modem has been properly installed and recognized by the OS.

How To Access The Phone and Modem Options In Windows XP

1. Left click Start.

2. Left click Control Panel.

3. In the Control Panel window left click the Phone and Modem Options icon. The Phone and Modem Options window will load.

Windows XP Modem Diagnostics    

Once you've installed the drivers that accompanied your modem, Windows should automatically assign itself to an available communications port and other settings should be configured automatically. If Windows has succesfully detected and installed a driver for your modem, the device will be listed under the Modem label. It will also state the appropriate communications port, the modem has been assigned.

12. After the installation of all modem drivers, you might have to restart your computer for the settings to take full effect. If you have any communication software that accompanied your modem, install it so you can test your new modem.

Modem Troubleshooting

This section will outline how to effectively troubleshoot an internal modem.

PROBLEM: Windows does not recognize the modem.    

* Your modem may not be properly configured to use the COM port designated in the control panel of Windows.

* Check the BIOS settings to ensure the communications ports in the system are enabled.

PROBLEM: The modem will not initialize.   

* Before trying to establish a connection with an outside computer link, you have to be able to communicate locally with your modem. Not being able to communicate using the standard AT commands most likely means that you have configuration problems or resource conflicts within your system. Refer to the previous section on how to configure your modem in Windows 95. You can also check the settings for the communications port you are trying to use. If you are using a non-standard COM port (which is a COM port not specifically designed to use the selected COM port of your modem) ensure that is set up to use the proper IRQ setting.

* If your modem is external ensure you are using the proper type of serial cable; replace the cable and run modem diagnostics.

* For internal modems, check to see that the card is inserted properly into the expansion slot.

* Try replacing it with another modem, preferably the same make and model to see if the modem is defective.

PROBLEM: There’s No Dial Tone   

* Ensure that you have the telephone cable attached to the proper line jack on your modem. These jacks are typically labeled LINE or TELCO. Use the LINE connector to attach your telephone cable to the wall jack; the TELCO connector is used to connect a standard telephone.

* Ensure that nobody else is using the phone line you are using to dial out on.