An Overview of 56-kbps Modem Technology

Gary Kessler and Walter Goralski
February 1997

An edited version of this paper appeared with the title "Is 56Kbps in Your Customer's Future?" in the quarterly supplement to LAN Magazine and Network VAR, June 1997.

Over the last several years, the bandwidth of the network backbone has gotten faster and faster. Part of the reason has been the use of digital technology, but another part of the picture has been the use of optical fiber. Meanwhile, access speeds to the home or remote office has been relatively limited because it is cost-prohibitive to swap out today's copper pair for another medium; some estimates are that is a billion miles of local loop in the U.S. alone. But even higher speeds are becoming available on the local loop with the introduction of 56-kbps modems.

Many users will have similar recollections as one of the authors, who recalls clearly using his first modem. Actually, it was an acoustic coupler on a Texas Instruments Silent700 portable terminal, complete with thermal paper. The dial-up connection ran at up to 110 bits per second (bps), or 10 characters per second. The year was 1973. Meanwhile, his college (Humboldt State University, Arcata, CA) had two timesharing terminals — TTY Model 33s — with full-time, dedicated access to the California State College system's timesharing hub in Los Angeles, operating at the breakneck speed of 300 bps! When he bought his first personal computer in 1981, an Apple II+, it, too had a 300 bps modem because the 1200 bps modems were just too expensive; besides, he didn't dial in to anyplace that had one at the other end, anyway.

Modems first became commercially available in the 1960s and operated at woefully slow speeds by today's standards. Speed was not of the essence in early applications because modems were commonly used for simple text-based timesharing and the transfer of raw text files between computers. During the 1980s, of course, typical, inexpensive modem speeds increased to 1200 bps, then 2400 bps.

The 1980s also saw an increased use of digital technologies in the telephone network and a growing number of digital telecommunications services. T1 carriers, for example, carry twenty-four 64-kbps channels at a total bit rate of 1.544 Mbps over two of the same twisted pair as our ordinary analog telephone network. The Integrated Services Digital Network (ISDN) delivers two 64-kbps channels plus a 16-kbps channel over a single twisted pair, operating at a rate of 160 kbps (this channel rate includes 16 kbps of overhead information). These rates seemed to blow analog communication speeds right out of the water.

But ISDN service deployment was limited in the 1980s, and service and equipment costs were high. Advances in modem technology kept rolling, and the 1990s have seen 9.6, 19.2, and 28.8 kbps modems as the standard. In late-1995, 33.6 kbps became available, often as a free or inexpensive hardware or firmware upgrade to an existing modem. ISDN's 64 and 128 kbps are still much faster than even 33.6 kbps, but it doesn't knock the socks off the analog technologies as it did just a few years ago. (Whether the ISDN service providers and equipment vendors blew it in the 1980s and early-1990s by not being aggressive enough can be the subject of another article!)

Today, ISDN deployment is growing by leaps and bounds, fueled primarily as a tool for access to the Internet and/or for remote access to corporate telecommunications resources. But even as ISDN is becoming more available, modems continue to get faster and faster. In late-1996, modem upgrades to 56 kbps became available with the introduction of x2 technology.

But wait! Is 56 kbps a simple upgrade to an analog modem? Is this a sign that digital is dead and companies should continue to depend on "modem" technology? Or, is this merely extending the life of an ultimately dead-end technology?

This article will examine some of the reasons that anyone even cares about these very high-speed modems and explain 56-kbps modem technology.

In The Pursuit Of Speed

Most people recognize the main drivers for high-speed communications — faster processors on the desktop, multimedia applications, high-performance communications protocols, high-speed switches, etc. We tend, however, to associate this need with the backbone network infrastructure and the way in which we connect corporate LANs to public networks such as the Internet.

Increasingly, however, the issue of high-speed is being brought to the residence and small office. Remote access to the Internet and corporate LAN are important drivers for speed. Even just ten years, 2400 bps was adequate for uploading and downloading simple word processing and graphics files; today, users need access to documents, graphics, sound, image, and video files. Whereas batch-oriented access was adequate a few years ago, today people are using real-time applications. And the key to this access is speed, more often to reduce response time than because of bulk volume.

There are a myriad of business environments where high-speed modems are useful, including corporate network access from a small or branch office, by a telecommuter, by a mobile user on the road, or by an employee working at home in the off hours. Internet access from a residence, school, municipality, or other small office could also use these high-speed modems. The specific applications include the usual: database access, electronic mail, surfing the Web (including the corporate intranet), file transfers, groupware access (e.g., central calendar). But the applications can also include the unusual, such as remote telemedicine or teleradiology for the physician or radiologist at home, home banking or stock trading, or remote editing of audio or video tapes.

These applications have a number of things in common. First, in almost all cases, the amount of information flowing from the server to the user exceeds that flowing in the opposite direction. Second, note that these applications are almost the same as those ostensibly driving the need for other high-speed services, such as ISDN, Asymmetric Digital Subscriber Line (ADSL), and other Digital Subscriber Line (xDSL) technologies.

Dubbed "midband" by some industry pundits, 56-kbps modems are seen by some as the most viable, affordable solution for faster Internet and remote network access. This technology provides faster access than "low-speed" 28.8/33.6 kbps modems, yet is less expensive for customers and service providers than ISDN and ADSL.

The 56-kbps technology is also exciting because speeds in the range of 33 kbps have long been thought of as an upper limit for modems on analog lines because of noise and other impairments. While digital loops operating at 56 or 64 kbps is no big deal, this breaks a major barrier for the analog loop.

Technology Review : Modems And Digital Voice

It is important that vendors and consumers alike understand the basics of 56-kbps modem technology in order to plan any sort of long-term strategy. In particular, it is critical to understand that 56-kbps modems are not purely analog devices as are today's V.34 and lower-speed modems. To really understand how these modems operate, it is necessary to understand both how today's analog modems work and how analog voice is carried on digital networks.

Let's start by defining some terms. Analog refers to information or signals that are continuously variable, such as sine waves and voice, while digital refers to information and signals that take on discrete values, such as square waves and bit streams. Another analogy may also help — the set of real numbers are to the set of integers what analog is to digital; real numbers can take on any imaginable value to any number of decimal places, while integers can only take on whole values.

Since human voice is analog in nature, the public telephone network has historically been optimized for analog signals. During the 1960s, people wanted to connect computers and terminals to the telephone network. Information within a computer, of course, is digital in nature, comprising digital bit streams of zeros and ones.

                 <==== 33.6 kbps FULL-DUPLEX ====>

             --------  AAA   (-------)   AAA  --------
------  DDD  |Analog|------>( Public  )------>|Analog|  DDD  ------
|Host|<----->|Modem |       (Telephone)       |Modem |<----->|Host|
------       |      |<------( Network )<------|      |       ------
             --------  AAA   (-------)   AAA  --------

FIGURE 1. Traditional analog modem scenario. The transmitting host sends a digital bit stream (DDD) to the modem. The modem, in turn, uses the digital data to modulate the analog carrier sent through the network. At the receiving side, the modem demodulat es the analog signal to regenerate the digital data to be delivered to the destination host. In reality, a number of PCM analog/digital conversions may occur within the network where use of digital carriers and switching facilities is widespread.

Modems were developed specifically for this purpose. Recall that the word "modem" is actually a contraction for modulation-demodulation. As shown in Figure 1, a modem accepts digital data from a computer and converts it to an analog signals for transmission over the public telephone network; the analog-to-digital conversion (ADC) is accomplished by modulating, or altering, the analog signal. At the receiving side, the modem converts the analog signal back into digital data for the destination computer; the digital-to-analog conversion (DAC) is accomplished by demodulating the analog signal to recover the digital data. This process, employed by all modems from the Bell 103 to today's V.34, allows voice and data to coexist on the analog public telephone network.

There are several factors, however, that limit the top speed that may be achieved on any given data call that do not negatively affect voice transmissions, such as local loop length and wire gauge, network signaling, loop termination card at the network switch, and the amount of echo and/or noise on the line. Several studies suggest that V.34 28.8/33.6 kbps modems achieve their maximum speed on less than half the calls.

During the 1960s, digital telecommunications carrier facilities were introduced into networks in North America, Europe, and Japan. While still optimized for voice, the digital nature of the transmission facilities brought several benefits to the carriers, such as reduced costs, cleaner (i.e., less noisy) lines, and more efficient multiplexing. As time went on, it was clear that digital carriers could also benefit the customer, as well, by allowing more intelligent communication devices on the customer premises and more advanced services, such as those associated with ISDN. In North America over the last 35 years, digital technology has replaced almost all analog technology in the switches and interswitch transmission lines (trunks) that make up the public telephone network; while some analog facilities still exist within the network, the bulk of the analog lines are those that connect our residences and small businesses to the local switch, or central office (CO).

It is easy to see how digital data can be sent on a digital carrier; the ones and zeros are merely translated to square waves. The new problem is to carry analog data, such as voice, over a digital carrier.

Analog voice is digitized by a process called pulse code modulation (PCM). While a complete discussion of PCM is well beyond the scope of this article, a basic understanding is necessary to appreciate how 56-kbps modems work (yes, really!).

The only portion of the human voice frequency spectrum that is actually carried on the analog voice network is that between about 300-3400 hertz (Hz, or cycles/second), although a 4000 Hz band is usually allocated to single voice channel. Nyquist's Theorem proves that if you sample a 4000 Hz analog signal at a rate of 8000 times per second, the set of samples are sufficient to completely reproduce the original signal. Thus, the PCM sampling rate is 8000 and each sample is represented by some voltage level.

PCM defines 256 different voltage (volume) levels with which to compare the volume of the voice samples. Thus, each sample is converted to an 8-bit value called a PCM word. Since we have 8000 8-bit PCM words each second, digital voice requires a bit rate of 64 kbps.

For data applications, however, 64 kbps is not yet achievable. The primary reason has to do with imperfections in the transmission facilities and noise, which effectively limits data transmission to 56 kbps. To understand why, we must return to PCM. The relationship between voltage level and digital encoding is non-linear, a scheme called companding (compression-expansion). With companding, we obtain a finer granularity at the low volumes, so that a small voltage change at softer volumes results in the same change in digital encoding as a large voltage change at louder volumes. Companding is employed because it actually results in a more efficient encoding than a linear scale and, in fact, the majority of useful spoken information is in the softer volumes. (In addition, when someone is whispering sweet nothings into your ear, you want to catch every subtlety and nuance, while it is easier to get all the information you need when someone is screaming at you!)

For data applications, it is extremely difficult to detect very small voltage changes accurately on a noisy loop. Therefore, the 56-kbps modem schemes use only half of the 256 PCM codes, eliminating those values most susceptible to noise. This means that 8000 7-bit samples are transmitted each second, yielding a 56 kbps data rate.

Note that conversion of the analog signal into a bit stream cannot be perfect; when an analog voice sample is converted to a digital value, it is converted to the closest digital value corresponding to the sample voltage (this is analogous to using integers to approximate real numbers). This error, indiscernible to the human ear, is called quantization noise.

56-KBPS Modem Technology Overview

The 56-kbps modem technology takes advantage of the widespread availability of digital carrier and switching facilities, and exploits some subtleties in the way in which PCM works. The technology requires that one modem be attached to a digital carrier and the other modem be attached to an analog line; it also requires that the internal network path between switches be fully digital. The modem connected to the digital carrier side is the server and the one at the analog side is the client. With this perspective, one can argue that "56-kbps modem" is somewhat of a misnomer; "modems" should be able to operate over analog networks and "56-kbps modems" cannot.

                 ====== UPSTREAM DIRECTION (33.6/40 kbps) =====>

             ---------  AAA  -----  DDD  (-------)   DDD  ---------
------  DDD  |56-kbps|------>|ADC|----->( Digital )------>|56-kbps|  DDD  ------
|Host|<----->|Client |       ----       (Telephone)       |Server |<----->|Host|
------       | Modem |<------|DAC|<-----( Network )<------| Modem |       ------
             ---------  AAA  -----  DDD  (-------)   DDD  ---------

                 <===== DOWNSTREAM DIRECTION (56 kbps) =======

FIGURE 2. Scenario using 56-kbps modem technology. The client host sends a digital bit stream (DDD) to the 56-kbps client modem, which in turn creates an analog signal (AAA). When the analog signal reaches the digital portion of the network, the network performs a PCM analog-to-digital conversion (ADC); the resulting digital signal is transmitted to the 56-kbps server modem and host. Since the ADC is affected by quantization noise, this upstream path is limited to about 33.6 or 40 kbps. When the server host transmits, the 56-kbps server modem merely forwards the digital data. When the signal reaches the last analog leg of the connection, the network performs a digital-to-analog conversion (DAC) to create the analog signal to be delivered to the 56-kbps client modem, and reconverted to digital for the client host. Since this downstream direction is not affected by quantization noise, 56 kbps can be achieved.

As figure 2 shows, the client computer's digital data is modulated onto an analog carrier signal by the client modem, converted to a digital signal somewhere in the public network, and delivered to the server modem and server's computer in digital form; the reverse operation occurs in the server-to-client direction. The key to achieving 56 kbps in the server-to-client direction using a combination of analog and digital carrier facilities by taking advantage of the fact that quantization noise only affects analog-to-digital conversion and not digital-to-analog conversion.

This is a critically important observation and the main point to understanding 56-kbps modems. Consider the following example. If you have a string of real numbers, you can approximate them with integers, but doing so will introduce some "approximation error;" this is analogous to ADC and quantization error. But if you are converting a string of integers into real numbers, the conversion will be exact; this is analogous to DAC and the absence of quantization error.

New modem technologies, then, take advantage of the fact that we can send data at a rate of 56 kbps on a digital carrier and that there will be no data loss due to a DAC step in the downstream direction. Since ADC quantization error causes data loss, the upstream transmission rate is limited to today's analog speed of about 33.6 kbps (although there are claims that 40 kbps can be achieved). This is still advantageous for most applications, where the user usually sends less information to the server than the server sends to the user.

With this understanding, it is a little easier to separate some of the hype from the reality of 56-kbps modems.

First and foremost, 56-kbps modems are not a simple replacement for today's V.34 modems. That is, it is not possible to merely throw out a pair of V.34 modems and replace them with a pair of 56-kbps modems. A 56-kbps client modem must communicate with a 56-kbps server modem, and they must be connected to an analog and digital line, respectively.

Second, 56-kbps modems do not operate at 56 kbps bi-directionally. Communication in the server-to-client direction (downstream) operates at 56 kbps, but communication in the client-to-server direction (upstream) is limited to something less than that, such as 33.6 or 40 kbps. This is useful, of course, because you are usually downloading more data from the server than you are uploading; you do not gain much by putting your Web server on the analog side of the connection.

Third, it is true that no rewiring needs to be done within the network or on the premises, as is the case with ISDN and ADSL. However, the client-side local loop and any other analog components that are present must be relatively noise-free for 56-kbps modems to work.

The requirements for 56-kbps modems, then, are:

1. One end of the connection must connect to a digital line, such as a Primary Rate Interface (PRI) or Basic Rate Interface (BRI) ISDN, or "trunk-side" T1 ("line-side" T1 connections incur additional ADC and DAC steps).
2. Both ends of the connection must support the same 56-kbps modem technology.
3. There can only be a single analog-to-digital conversion in the network path between the server and client. This avoids introducing another set of ADC quantization noise errors that would otherwise limit the speed.

THE FUTURE OF 56-KBPS MODEM TECHNOLOGY

There are a number of competing standards for 56-kbps modems. The first company to ship products was U.S. Robotics (Skokie, IL, http://www.usr.com), the developer of x2 technology. The x2 specification has been submitted to the International Telecommunication Union (ITU) for adoption as an international recommendation. In addition, a large number of Internet service providers have indicated their support for x2.

Another proposal is from a consortium led by Lucent Technologies (formerly AT&T Bell Labs; Allentown, PA, http://www.lucent.com/micro) and Rockwell. Their scheme, called V.flex2 by Lucent and K56flex by Rockwell, has been submitted to the Telecommunications Industry Association (TIA) for adoption as a U.S. standard; the TIA forwards U.S. contributions to the ITU. In addition, a large number of vendors have indicated support for this plan, as well.

Both 56-kbps modem schemes operate at 56 kbps in the downstream (server-to-client) direction. In the upstream (client-to-server) direction, x2 operates at 33.6 kbps and the AT&T/Rockwell scheme operates at 40 kbps. It remains to be seen which specification will be adopted by the standards organizations.

This midband technology is expected to become incredibly important within the next few years as 56-kbps modems grab an increasing share of a growing market. Jupiter Communications (New York, NY, http://jup.com), for example, has released a study predicting that 56-kbps modems will control 50% of the Internet access market by 1998 and 65% by 2000, particularly important numbers since consumer access to the Internet will remain predominately dial-up for the foreseeable future. Coupled with V.42bis compression, 56-kbps modems can transfer at rates up to 230.4 kbps; Internet sites employing digital video disk (DVD) technology will have even more worth looking at and affordable dial-up access will be critical to distributing this information.

So, does this suggest that vendors should abandon their ISDN and xDSL products? As the first 56-kbps modem offering, is it yet another nail in ISDN's coffin?

Our answer would be a cautious no to both questions. There is no question that 56-kbps modem technology extends the viable life of some modem-based products. But nearly all such applications could operate over ISDN or ADSL; Lucent, Rockwell, and U.S. Robotics all have ISDN products. And in any case, there are still some significant roadblocks to ubiquitous availability of 56-kbps modems.

First of all, 56-kbps modems require a digital path with only a single digital-to-analog conversion and a relatively noise-free analog local loop. If these criteria are not met on a particular call, the modem will automatically fallback to V.34-type operation. For some users in some areas, all (or nearly all) calls will meet the criteria; for others, many or most calls will not. Use of this technology, then, will depend significantly on where the analog site is located, where the digital site is located, and what facilities are in-between.

Second, this technology assumes that the corporate office or ISP has digital connectivity. While this is increasingly true, it is not anywhere near always the case.

Finally, there are still many unanswered questions. The x2 technology itself, while a simple hardware or software upgrade to most of U.S. Robotics' products, is still a young technology and not well-tested in the field. The cost of 56-kbps modems is still relatively high. And, there is no way to test nor guarantee that it will always — or ever — work in a given customer's environment.

This new modem technology is, as best, a stopgap between today's actual deployment of digital local loops and the deployment that we will eventually see in the future. One can argue that it gives service providers yet another excuse to not roll out ISDN. But as digital services and technologies continue to become available, savvy customers will still demand ISDN since ISDN is designed as an anywhere-to-anywhere switched network service that can support many types of applications. Although remote network access, telecommuting, and high-speed Internet access are driving ISDN today — as well as x2 and xDSL technologies — only ISDN was designed to allow logical integration of voice and data into the same application using a common set of standard protocols. ISDN will also provide access to future broadband services.

Additional information on x2 technology can be found at U.S. Robotics' x2 Web site at http://x2.usr.com; the technology white paper at that site gives an excellent overview. But look elsewhere for information, as well; white papers from MultiTech Systems (Mounds View, MN, http://www.multitech.com) and other sites point out that 56-kbps modems represent a very promising technology but acceptance by customers and the affect on, and by, other technologies is still unknown.

The emerging 56-kbps technology clearly extends the speed of analog modems by exploiting the widespread use of digital facilities in today's networks. But it cannot be used by all of the customer's who can today use V.34 analog modems. In addition, it will not be an appropriate solution in the fully-digital environment, where xDSL and ISDN would be better, more appropriate choices. Vendors -- and customers -- would be wise to consider 56-kbps modems as another tool in the tool kit but probably not, in and of itself, a long-term solution to high-speed communication.