Embedded communications ap-plications, such as credit-card verification, transferring medical data (biometrics), payphones, and cable and satellite television, connect either directly to another modem or else through the Internet. For such embedded modem applications, does the designer use a modem module or card, which can sometimes cost more than the modem in your PC, or else build a custom modem from the ICs presently available at a reasonable cost?
If more than a few modems are necessary, it's usually worth designing the modem as part of the product—either as a subassembly for use in a number of different applications, or embedded in the design itself. The cost of obtaining certification for FCC Parts 68 and 15 is quickly amortized if any significant volume production is planned. The modem data-pump functions, or the part of the modem that does the modulation and demodulation, is taken care of by the modem IC. Even the firmware for the modem command interface and control can be part of the modem IC that's employed.
These days, in nearly all cases, the only real challenge in modem design is the design of the direct access arrangement (DAA). This is the piece of the design that couples the telephone network to the analog connections of the modem and provides protection to the network. Prepackaged DAAs are available. Although relatively easy to use, the cost can be quite high because these are really hybrids or modules containing many components. They can easily exceed the cost of the rest of the modem and negate any advantage to including a modem as part of an embedded design. The most economical way is to make the DAA a part of the total modem design.
Currently, three standards exist that cover the requirements for DAAs in most countries worldwide. In North America, FCC Part 68 is the standard. It also is harmonized (compatible) with most Canadian and Mexican regulations as a result of NAFTA. In most of Europe, the European Telecommunications Standards Institute (ETSI) has defined a standard called CTR-21. All EC members have committed to adopt it to unify European telecom standards (with some minor exceptions for certain countries). JATE defines the standards for Japan. Most other countries have standards falling within one of these groups. If a product can meet all of these standards, then it probably will work anywhere.
Originally, the Public Switched Telephone Network (PSTN) was designed for voice telephones. As new devices arrived on the scene, such as answering machines, faxes, and modems, they were (and still are) required to have electrical characteristics like a telephone and compatibility with the network. Most devices that use the PSTN derive power from some source other than the telephone network, so the network providers want to make sure that any device connected won't interfere with normal network operation. The purpose of the DAA is to give the same benign connection as a telephone provides.
DAA isolation design criteria: Several functions must be provided for in the DAA design. Arguably, the most critical function is the high-voltage isolation of the modem circuitry from the telephone network. The designer's selection of the voltage-isolation method will often impact how most of the other DAA functions will be supported. Now, several techniques are used to achieve the high-voltage isolation. Traditionally, this isolation was the main function of a telecom-coupling transformer, but capacitors and optocoupler-based techniques are also becoming popular.
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