Finding The Holy Grail Of Modem Connectivity By Chris Lewis   Dial-up modem connections running at 56 Kbps bear many similarities to the Holy Grail. Like the legendary cup, it's something that many people would like to get their hands on, but in reality, it's unattainable for most mortals. Don't get us wrong. The new 56-Kbps technologies available from U.S. Robotics, Lucent Technologies and Rockwell International Corp. are an improvement to what we have now, but many obstacles still exist before we can achieve the advertised connection rates. We'll describe how high-speed modems work, the environment you will need to have to deploy 56-Kbps technology and what the various vendor positions are in the marketplace. With V.34 modems tod ay, the 28.8-Kbps connection you get provides 28.8 Kbps in both directions. With the proposed 56-Kbps technologies, you get 56 Kbps in one direction only; the return path is limited t o 28.8 Kbps (if the modem is based on V.34+ technology, the 28.8 number is increased to 33.6 Kbps). To get 56 Kbps, one end of the connection must be digital (typically supplied on a T1 line); you cannot have two 56-Kbps modems connected to analog telephone lines and get a 56-Kbps connection. For anyone connecting to an Internet service provider (ISP) that claims to support 56 Kbps, the asymmetrical speed is probably not much of an issue, since the most heavily utilized bandwidth is typically the downstream direction. Where it does become an issue, however, is in corporate applications, such as distributed sales order systems that upload files on a daily basis; 56-Kbps technology will not buy much for these types of users. How We Got This Far To understand why you get 56 Kbps in only one direction, you need to und erstand how high-speed V.34 modems work when compared to the 300-baud modems we used to have. Back in the mists of time, the speed of a modem connection was measured in baud rate, so 300 baud meant that there were 300 events per second occurring on the line between the two communicating modems. An event, in this situation, meant a change in the frequency of the signal transmitted between the modems. With one frequency representing a binary 0 and a different frequency representing a binary 1, you could transmit 300 bits per second (bps). This old method of modulating binary numbers in a modem to modem connection (different frequencies representing binary 1 and 0) is termed Frequency Shift Keying (FSK). FSK maxed out at around 600 baud, so to get to higher bps rates, a different means of modulating the binary 0 and 1 bits on to the modem-to-modem connection was devised. To understand this more modern means of modulating bits on to a line, we need to delve into a bit of communications theory. If this ab ridged description whets your appetite for more information about how this all really works, see www.mot.com/mims/isg/papers/v34tutorial/v34_tutorial.html, or www.blackbox.com/ bb/refer/remo te/v34/v34.html. Now on to the communications theory. The newer modulation method that enables us to transfer 28,800 bps keeps the frequency constant, but modulates both the amplitude and the phase of the signal transmitted between the modems. Amplitude modulation is easy to understand; amplitude is simply the voltage level of the transmitted waveform. Phase is a bit more difficult to picture. The waveforms transmitted between modems are called sine waves (for more information, see "Modem Phase" graphic below). A sine wave is a repeating pattern that is measured from 0 to 360 degrees before the wave pattern repeats again. A sine wave shifted through 90 degrees would appear as shown in "90 Degree Shifted Sine Wave," and a sine wave shifted through 180 degrees would appear as shown in "180 Degree Shifted Sine Wave," s een in the chart below. Now comes the clever part. If you can transmit a waveform in one of four different phase states, then you can transfer two bits of binary information with every event. If you can modulate between two amplitude values, then you can transmit three bits for every event. The more amplitudes and phases you can use, the more bits of information you can transmit with each event (see "Four Phase State Transmissions" and "Four Phase State Transmissions with Two Amplitude Values" on page 117).

Multihoming Your Internet Service With BGP by Chris Lewis and Thanh Nguyen Updated July 10, 1997