Internet Connections With the 56Kbps Modems
Most of us are deeply concerned about the speed of our Internet connections. As we browse ever more complex and graphical pages on the World Wide Web, we often have time to read the newspaper while waiting for the desired information to appear on our screens. And we have all heard that our V.34 modems, which can communicate at up to 33.6 thousand bits per second (Kbps) are the final word in speed over our normal phone lines. The technological advances which used to double our connection speeds every few years have finally come to an end. So why are major modem players like U.S. Robotics and Rockwell International announcing new modems which can communicate at up to 56Kbps?
A major shift in how the Public Switched Telephone Network (PSTN) is constructed has taken place and has opened the door to this latest increase in speed. For over 100 years the telephone system was an incredibly complex analog switching system of circuits connecting telephones to other telephones. Recently, the PSTN was substantially upgraded to digital high-speed connections, often carried over fiber-optic cables. This modern telephone infrastructure can now be utilized for ever-higher data speeds.
Also, a new method of data encoding has been developed and will soon be available. This technology is called X2 by U.S. Robotics and K56Plus by Rockwell International. The systems are similar enough to be considered the same for the purpose of this discussion, and I refer to both as 56Kbps technology. It remains to be seen whether the implementations chosen by these two companies will be compatible.
In order to explore this new technology, a little background information on the public telephone system is in order. When you place a call to another person you probably don't think about how the connection is made. The telephone system is so reliable and easy to use that any 5-year-old can easily place and receive calls. Behind the scenes, however, is a very complex system of electronic equipment which is not widely understood. Most connections today are handled by digital switching equipment while almost all residential telephones are still analog. In fact, your residential line has changed little in the past 70 years. The changes have occurred inside and between the Central Offices (COs). Normal modems, including the latest 33.6Kbps models, assume they are communicating over a connection that is completely analog. This assumption is no longer always valid. There are still many analog switches in use in the United States, especially in rural areas, but they are being quickly upgraded to digital systems.
Your phone line is basically two copper wires which carry current between your telephone and your CO. Your voice is inherently analog, which means it is continuously variable and contains an infinite number of discrete levels. Your telephone converts your voice to fluctuations in the current carried over your phone line. Upon receipt at the CO, however, a major change occurs. Your phone line is connected to a piece of equipment which converts analog signals to digital and digital signals to analog. This equipment is called a codec, which stands for COder/DECoder. After your voice has been converted to a digital signal, it can be processed by the digital switching matrix in the CO and transported over digital lines to another CO or to another line in the same CO. Another codec converts this signal back to analog fluctuations in current on the copper wires which lead to another telephone. The other telephone then converts this current into an audio signal which can be understood by a listener. A similar process occurs going the other way, and you hear the other person's voice, no matter how far away from you he may be.
This digital-to-analog and analog-to-digital conversion is the area of interest to the designers of this new modem technology. A coding standard has evolved in the United States, and similar standards are in place in Europe and other parts of the world. The analog signal is sampled 8000 times per second by the codec and each sample generates an 8-bit byte of data, which can take on 256 distinct values from 0 to 255. These samples are taken continuously, resulting in a stream of data which is 8,000 X 8 = 64,000 bits per second or 64Kbps. It is this data stream which actually gets transported between COs and is usually combined (“multiplexed”) into another, higher-speed, connection. For example, 24 of these 64Kbps streams can be combined into one T1 line, which results in 1.544 million bits per second (Mbps). This rate is greater than 24 X 64,000 because additional bits are used for timing.
It is important to note that sampling is inherently imperfect. The codec attempts to quantify an infinitely variable signal into 256 discrete levels. Each of these levels is necessarily a best estimate of the actual value. The error introduced is called quantization noise and is measured in relation to the desired signal in decibels (dB). This is known as a signal-to-noise ratio. It is important to note that the corresponding conversion of digital to analog does not cause quantization noise since no error occurs: each of the 256 levels is correctly translated to a distinct voltage.
Because the telephone system was designed for voice, it was proper engineering practice to tailor the circuits to the characteristics of the human voice. Almost all of the energy in a human voice is contained in a band of frequencies from 300 Hertz to 3300 Hertz. (Hertz signifies cycles per second and is a measure of frequency or pitch.) Thus, the telephone circuits have been limited by design to this range of frequencies. This 3,000 Hertz range is known at the bandwidth of the telephone connection. It is this bandwidth which limits the speed at which data can be transferred.
The recent development of V.34 modems makes maximum use of this bandwidth and approaches the theoretical maximum speed of 33.6Kbps. This limit is due to quantization noise as discussed above, and is determined by Shannon's Law. Shannon's Law defines the maximum data rate over a connection as a function of the connection's signal-to-noise ratio. This calculation is beyond the scope of this discussion but results in a theoretical upper limit of approximately 35Kbps for an analog phone line.
Now that we understand a little about the phone system, let's look at what happens to data as it travels from your computer to a local Bulletin Board System (BBS). Your data is born digital in your computer and translated to analog by your modem. At the phone company's Central Office, the analog signal is sampled and converted to a digital stream. This digital stream is switched and transported to the CO serving the BBS, where it is converted to analog for the local loop serving the BBS. At the BBS modem, the data is finally converted from analog to digital one last time, and it can now be understood by the BBS computer. If you were following closely, you noticed four separate conversions, two analog-to-digital and two digital-to-analog. Remember, when an analog-to-digital conversion takes place, quantization noise is introduced.
Now we start to understand why our bandwidth is limited. Recent technologies, such as ISDN (Integrated Services Digital Network), can bring 64Kbps, or even 128Kbps, to our computers. But ISDN requires expensive upgrades at the CO, and, therefore, much higher monthly charges when compared to normal analog phone service. 56Kbps technology requires no change to an already digital CO.
The good news, and the enabling factor for the new 56Kbps technology, is that most large service providers now use modems which skip two of the above-mentioned conversions. Because these service providers connect to the Central Office by way of digital lines such as T1s (1.5Mbps) or T3s (45Mbps) and ISDN, digital modems can be employed which do not need to translate data for transport over an analog local loop from the service provider to the CO. Here lies the secret behind 56Kbps technology. Recall that quantization noise results from an analog-to-digital conversion. If the service provider can avoid this conversion, the noise is not introduced. Theoretically, if this conversion step is eliminated, the entire 64Kbps data channel cad be exploited.
However, in the real world, only 128 of the possible 256 voltage levels will be used, resulting in 56Kbps transmission from the service provider to the home. Notice that the analog to digital conversion step from the home to the service provider has not been eliminated and, therefore, no increase in speed is afforded in that (upstream) direction, so standard V.34 33.6Kbps transmission continues to be employed. This indicates that the 56Kbps technology is asymmetrical in nature, resulting in different speeds in different directions. Also note, 56Kbps technology will only work when connecting to another modem which is served by a fully digital connection—you will not be able to enjoy 56Kbps transfer speed when calling your friend or a small bulletin board system. Normal data compression techniques will still be used as they are today to increase throughput. For example, a text file transmitted over a 56Kbps data link using V.42bis compression will result in a transfer speed of 230.4Kbps. That is quite an improvement over today's available transfer rates.
To wrap all this up, let's note a few important facts. A service provider must have a fully digital connection to the Internet, either T-carrier or ISDN. The service provider must have 56Kbps modems which are compatible with your modem. U.S. Robotics announced the availability of X2 upgrades in January 1997, and other modem manufacturers are expected to make upgrades available sometime in the first quarter. Some modems will require only software upgrades, while other modems will require hardware changes or will not be upgradable at all. In general, a late-model modem will be software upgradable if it uses FLASH ROM (the on-board firmware can be upgraded without physical replacement of ROM chips). As noted earlier, it is far from certain that the different manufacturers' implementations of 56Kbps will be compatible, so care must be taken to ensure your modem is using the same encoding scheme as your service provider's modems.
Further information on 56Kbps technology can be obtained on the U.S. Robotics web page at http://x2.usr.com/ and on the Rockwell International web page at http://www.nb.rockwell.com/mcd/K56Plus/.
Tony Williamitis can be contacted via e-mail at firstname.lastname@example.org. Your feedback concerning this article is welcome. Links to information on various telecommunications subjects can be found on the author's web page at http://www.dma.org/~twilliam/.