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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Over the years, the CCITT has set international standards for fax transmission. This has led to the grouping of faxes into one of four groups. Each group (I through IV) uses different technology and standards for transmitting and receiving faxes. Groups I and II are relatively slow and provide results that are unacceptable by today’s standards. Group III is the standard in use today by virtually all fax machines, including those combined with modems. Whereas Groups I through III are analog in nature (similar to modems), Group IV is digital and designed for use with ISDN or other digital networks. Because the telephone system has not been converted to a fully digital network yet, there are very few Group IV fax systems available.

Group III Fax
There are two general subdivisions within the Group III fax standard—Class 1 and Class 2. Many times you will hear about a FAXModem supporting Group III, Class 1 fax communications. This simply indicates which protocols the board is able to send and receive. If your FAXModem does this, it can communicate with most of the other fax machines in the world. In FAXModems, the Class 1 specification is implemented by an additional group of modem commands that the modem translates and acts upon.
Earlier you learned about the V.29 modulation standard. As stated in that section, this standard is used for Group III fax transmissions.
Modem Recommendations
Today the cost of 33.6Kbps modems has dropped to below $100, including fax capabilities. Many of these modems are upgradable to 56K although you should check with the vendor for details regarding the availability and procedure for getting the upgrade. The cost of 56K modems is in the $100-200 range. You need to be sure to buy the version that is compatible with the version that the number you use it with most (such as your internet service provider) and that it is flash upgradable to support any later versions of 56K standards. I would normally recommend that you purchase an internal modem if your computer has space for it; however, I prefer external modems myself, due to the additional troubleshooting capabilities possible by watching the LEDs that indicate the modem’s status. Internal modems usually ship with a high-speed UART on the modem card, thus eliminating the need to upgrade any older, slower UARTs you may have in your PC. If you don’t have an internal slot for a modem, be sure that you have the appropriate UART. Today, most modems come with multiple forms of error correction or data compression.

Integrated Services Digital Network (ISDN)

ISDN modems are the next step in telecommunications. ISDN modems make the break from the old technology of analog data transfer to the newer digital data transfer. Digital technology allows you to send voice, data, images, and faxes simultaneously over the same pair of wires at up to 128Kbps. ISDN modems required an ISDN service for connection, which is more readily available today. Prices for ISDN service vary widely, depending on your location. U.S. prices—on average—are approximately $130 to $150 for the initial installation and $50 to $100 a month. There is usually a connect-time charge as well that can range from 1 to 6 cents a minute, although many telcos are starting to drop the connect-time surcharge. These are all line charges paid to the telephone company. You will also have to purchase an ISDN modem, and there will also be an additional charge from your service provider for Internet Access at ISDN speeds.


Caution:  
When purchasing an ISDN modem, you will almost always want to purchase an internal version. An ISDN modem with compression can easily exceed a serial port’s ability to reliably send and receive data. Consider that even a moderate 2:1 compression ratio exceeds the maximum rated speed of 232Kbps that most high-speed COM ports support.

ISDN has become extremely popular in Europe, where leased lines are often prohibitively expensive. ISDN modems have also dropped considerably in price; what was once a $1,500 to $2,000 device can now be purchased for as little as $400. As demands for more bandwidth increase, and conventional asynchronous modem standards become more inadequate, ISDN should gain in popularity, causing prices to drop further. ISDN modems are far more complex than standard analog modems.

ISDN modems have three separate channels. Two of the channels are called B channels; these are the data carrying channels and are 64Kbps each. The third channel is the D channel, which is 16Kbps. The slower D channel is used for routing and handling information. It is this technology that is making it possible for more and more people to participate in video conferencing.

To be technically precise, ISDN devices are not “modems.” Modems modulate digital signals so they can be transmitted over an analog phone line and then demodulate the signal back to digital form for the computer. ISDN runs over an entirely digital telephone network (most of the telephone infrastructure in the United States is now digital), so there is no need for the modulation and demodulation processes. The most common type of ISDN device for a PC is called a terminal adapter. ISDN can be implemented as either a serial device or as a network interface. Using a network type interface eliminates the bottleneck at the computer’s serial port. This type of ISDN terminal adapter may be the preferred solution for reasons of performance, even if you have only one computer and don’t need the other services provided by a network.

ISDN requires additional telephone wiring and service from the telephone company. You will first need to check with your local telephone company to see if ISDN service is available in your area. (It is now available in most parts of the U.S.) In many cases, this can be the most difficult part of the installation. ISDN service, although available for many years, is only beginning to become a popular item. Often, the first hurdle to overcome is finding someone at telephone company who knows what ISDN is. Then, you will find that prices can vary widely, depending on the distance between your location and the nearest phone company POP (point of presence). You can have the telephone company install the wiring and jacks or install them yourself, if you are so inclined. Que’s Special Edition Using ISDN has excellent detailed coverage of the wiring process for the ISDN do-it-yourselfer.


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