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COMPUTER SYSTEMS

Choosing and Buying Components
By: O'Reilly Media
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    2004-10-25

    Table of Contents:
  • Choosing and Buying Components
  • What You'll Need
  • Case
  • Power Supply
  • Processor
  • Heatsink/Fan Units (CPU Coolers)
  • Motherboard
  • Memory
  • Drives
  • Optical Drive
  • Video adapter
  • Display
  • FPD Monitors
  • Audio
  • Keyboards
  • Mice
  • Network adapters
  • Wireless Network Adapters
  • Modems
  • Buying Components
  • Recommended sources
  • Final Words

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    Choosing and Buying Components - Modems


    (Page 19 of 22 )

    A modem—a contraction of the term MOdulator/DEModulator—converts data from the digital form used by your computer to an analog form that can be transferred across an ordinary voice telephone line, and vice versa. A modem—also called a “dial-up modem” to differentiate it from cable modems and DSL modems—is typically used to connect via a POTS (Plain Old Telephone Service) line to an Internet Service Provider (ISP) or similar data service. Most current modems also support inbound and outbound faxing.

    Modems are a mature market segment. They reached their maximum possible speed—“56K,” which is actually limited by law to 53K—several years ago, and modem manufacturers have since devoted few development resources to them. Most current modems use one of a handful of chipsets, so the similarities between modems from different manufacturers are much greater than their differences. Most have very similar features and performance, so choosing the best model for your needs is usually a matter of deciding what type of modem you need and then balancing minor price differences against manufacturer reputations.

    Use the following criteria when choosing the type of modem appropriate for your needs:

    • All current modems support the so-called 56K data rate using V.90 or V.92 protocols. With perfect line and external network conditions, a V.90 modem can in theory transmit data at 31,200 bits/s and receive data at 56,000 bits/s (“56K”). In practice, regulatory limits on power levels limit the receive speed to 53,000 bits/s, and line conditions often limit it to even lower data rates. V.92 is a minor upgrade of the V.90 standard and maintains the 53,000 bits/s receive speed of V.90, but increases transmit speed to 48,000 bits/s. Again, the theoretical data rates are seldom achieved on real-world telephone lines. V.92 also extends the V.90 standard to add the following convenience features:

      QuickConnect

      Anyone who has used a dial-up modem is familiar with the series of tones that the sending and receiving modems exchange. During this “handshake” procedure, the two modems negotiate their fastest shared protocols and test the line to determine the fastest usable data rate. With V.90 modems, this handshake can require as much as 30 seconds to complete. The V.92 QuickConnect feature reduces the time necessary to establish the connection by up to 50% by storing information about modems it has previously connected to and the line conditions it experienced during earlier connections.

      Modem-on-Hold

      In the Bad Old Days, if you forgot to disable call-waiting on your modem line, your data connection was unceremoniously dropped if another call came in while you were using the modem. The V.92 Modem-on-Hold feature is designed to work with call-waiting. If you are using the modem and a voice call comes in on that line, the data connection is placed on hold. You can then pick up the phone and talk to the caller. When you complete the voice call, you hang up and re-establish the data connection where you left off. Note that using the Modem-on-Hold feature requires explicit support by the ISP, and many ISPs have not chosen to implement it.

      Although a few V.90 modems remain on the market, the vast majority of currently available modems support V.92. The differences between V.90 and V.92 are so minor that many existing V.90 modems can be upgraded to V.92 via a simple firmware update.

      Although V.92 modems ordinarily connect to V.90 modems without any problem, we have had reports from readers who upgraded their V.90 modems to V.92 and subsequently experienced problems connecting to their ISPs. Accordingly, before you upgrade to V.92 we suggest that you make sure you can later downgrade back to V.90 if necessary.

    • A true modem contains a CPU or controller that processes outbound data before delivering it to the telephone line interface and processes inbound data from the telephone interface before delivering it to the computer. Several years ago, various manufacturers began shipping controllerless modems, which are also referred to as soft modems, software modems, or Winmodems. These devices are not true modems at all, but are simple hardware interfaces between the computer and the telephone line. A soft modem has no internal CPU. Instead, it uses the main system CPU to do all the processing. The only advantage of a soft modem is low cost. The disadvantages of soft modems are that they consume CPU power, which may cause degraded performance; that they can be used only with an operating system that supports them, which typically means only recent versions of Windows; and that they frequently drop connections and crash the computer. Don’t use a soft modem, no matter how little it costs.

    • Modems are available in internal and external versions. External modems cost a bit more than comparable internal modems because they require a case and power supply, but it’s usually worth spending those few extra dollars for the additional flexibility an external modem provides. Most internal modems are installed in a PCI expansion slot. A few are designed to fit the dedicated AMR (audio/modem riser) or CNR (communications and networking riser) slot present on some motherboards. Internal modems may be controller-based or controllerless. External modems connect to either a serial port or a USB port. All external modems are controller-based. External modems provide status lights, which can be invaluable when you are troubleshooting connection problems. Also, external modems can be power-cycled independently of the PC if they lock up, which at times can keep you from losing all of the unsaved data in files that are open when the modem shoots craps.


    RECOMMENDATIONS

    Modems
    For general use, buy an external V.90 or V.92 controller-based modem. If you want the best modem available, bar none, buy the $250 U.S. Robotics Courier 56K V.92 Business Modem, which can establish and maintain connections under line conditions so bad that other modems don’t even attempt to connect. For a less expensive but still excellent external modem, buy the $50 U.S. Robotics USR5633A 56K USB Faxmodem. If for some reason you must have an internal modem, choose the $35 U.S. Robotics USR5699B 56K V.92 Internal Faxmodem PCI.

    Avoid internal modems, whether they are controllerbased or controllerless. Avoid no-name modems of any description.


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