News | May 7, 1999

Design Factors for Hard and Soft Modems

The convergence of International Telecommunications Union standards, emerging broadband communications services, and hot new communications semiconductors is impacting modem design. What are the engineering tradeoffs between various types of hardware and so-called "soft" software-only modems?

By: James Archibald, modem field applications manager, Lucent Technologies Microelectronics Group, Allentown, PA


•PC Modem Tradeoffs
•Set-top Boxes and Terminals Too
•Are They Design-compatible?
•Host Controller Tradeoffs
•Soft Modem Loading
•Similar But Different
•Gobbling Resources
•Walking the Last Mile
•DSL Screamers

The modem industry is rallying around the International Telecommunications Union (ITU) and its new V.90 recommendation, which delivers data transmission speeds much faster than the older V.34 technology's 33.6 kbits/s. The recent adoption of G.992.2 as a standard for splitter-less Asymmetrical Digital Subscriber Line (ADSL) services promises even greater performance over existing copper wires.

Simultaneously, a new generation of silicon has evolved with the computational power to perform analog and digital algorithms at ever-decreasing cost, size, and dissipation (power). Modems are once again in the industry spotlight, but important technology differences are often overlooked.

Personal computer (PC) modems are available today in three technology implementations: hardware microcontroller (so-called controller-based) modems which use digital signal processor (DSP) chips; host-controlled modems, which also use DSPs; and soft modems, which don't use DSPs.

This fact of PC life is largely a mystery to the buying public. End user buyers are unaware of the price versus performance tradeoffs. All three modem technology implementations have a place in the market. The best consumer is an informed consumer, and savvy engineers understand that.

PC Modem Tradeoffs

Let's examine the three classes of modems, starting with microcontroller-based modems (also known as controller-based modems). Some of the tradeoffs are shown graphically in this figure.

Controller-based modem designs use dedicated hardware to perform all functions. Special modem chip set silicon handles the signal processing algorithms needed to modulate and demodulate your digital data onto an analog phone signal.

This function is known as a "data pump," and when it's performed on dedicated silicon, the design is usually dubbed "DSP-accelerated." The modem chip set also performs the system control functions.


Set-top Boxes and Terminals Too

All external modems as well as many add-in boards and internal modems are of this DSP-accelerated type. Also, modems designed into non-PC devices such as set-top boxes and point-of-purchase terminals must have their own controller and data pump hardware.

Now let's look at so-called host-controlled modems (also called win modems). They use the PC processor to perform the control functions of the modem. The data pump functions, which are mathematically intensive, are still left to a programmable or non-programmable DSP.

Lastly, we have the so-called soft modems. They're the newest types of modem to come onto the modem market. Here, the PC processor performs both the control functions and the data pump functions. In other words, with a soft modem, the entire modem function itself is executed in "all-software" and there is no DSP acceleration.


Are They Design-compatible?

All three design approaches are compatible with the V.34 (33.6 kbit/s) and/or V.90 (56 kbit/s) Pulse Code Modulation (PCM) ITU modem standards?

So, what are the tradeoffs? What are the pluses and minuses of controller-based modem designs? The big advantage of controller-based modem designs is that the DSP and controller silicon remove all the processing load from the host, freeing the PC for higher-level applications such as word processing, graphics, imaging, Web browsing, games, or email.

Modem communications are performed reliably and quickly by subsystems designed for and dedicated to those specific tasks. Because all the modem functions are off-loaded from the host, DSP-accelerated designs are highly desirable for lower performance and legacy PC platforms. Even with higher performance PCs, DSP-accelerated modems provide important performance advantages when performing other tasks simultaneously with communications.

Controller-based designs are also suitable for standalone external modems, and for embedded devices where no PC may be present. On the other hand, controller-based modems also represent the highest hardware content and hence the highest incremental system costs. DSP and controller silicon costs are more significant when designing computer platforms for the very-low-price segment of the market.


Host Controller Tradeoffs

What are the pluses and minuses of host-controller modem designs? Well, host-controller-based modem designs take advantage of lower cost dedicated silicon while minimizing the load on the host PC processor.

Conventional modem designs used a dedicated DSP for data pump functions, and while these can be performed today on a general purpose CPU, doing so typically consumes too many host resources. Traditional modems also generally use a separate microcontroller to perform system set-up, call control, and other communications system functions. These control functions, however, represent a trivial load (approximately 1% to 3%) compared with today's high performance PCs. Hence, moving the modem controller functions to the CPU reduces system costs while having little impact on overall performance.

This design approach is optimal for internal add-in card modems and motherboard modems, where the connection between the host CPU and the DSP is a high speed one. Most PC modems today are of the host-controller design.

What are the pluses and minuses of soft modems? Because soft modems implement both the control and modulation functions as software algorithms running on the PC processor, the incremental costs for the modem design are small. The only hardware costs are those associated with the physical interface which must be present, of course, to plug in a telephone line.


Soft Modem Loading

While soft modems represent the lowest possible incremental system costs, they also represent the highest load on the host processor. Soft modems are a reasonable choice for end users who use analog phone connections only rarely, or who tend not to run multiple applications while connected on-line.

Knowing this, just what are the issues you, as a PC manufacturer, should consider when designing in a modem? While all modems today provide the same basic communications capabilities (V.34 and V.90), the different products still represent a wide price/performance range. Different modem design approaches will match different overall system design objectives.

High performance systems designers will find the incremental cost of a DSP-accelerated modem is well balanced by the advantages achieved in multi-tasking environments. End-users may find that a lower cost system meeting their performance needs can be configured by buying a lower speed host processor combined with a DSP-accelerated modem. This is particularly true in today's environment, where processors only slightly behind the leading edge of performance are selling at greatly discounted prices.


Similar, But Different

From a compatibility point of view, all three modem designs offer the same basic features and functions. All today's modems are compliant with ITU international standards and therefore promise users full interoperability with modems from multiple manufacturers and with equipment used by a wide variety of Internet Service Providers (ISPs) and corporate remote access servers.

In addition, many DSP-accelerated designs today are based on programmable chips. Programmable chip designs permit you to offer your customers standards upgrades and improved features that can be simply downloaded by the user. The advantage of DSP accelerated modems is that they off-load the host computer from the very-demanding data pump functions.


Gobbling Resources?

These cycle-stealing modulation/demodulation algorithms eat up to 50 MHz to 60 MHz on a Pentium II, for example. Losing 50 or 60 MHz of a host's resources means that Web browsers, e-mail programs, file transfer protocols, and common office software applications will run much slower if a software modem is also running. If Web browsing is going to be a major end-user application, modem choices can be critical. It's a function of the basic operations any modem has to perform.

As a designer, it pays to understand these functions. A PC modem acts as a communication device between local and remote users and enables the transmission of digital data over analog phone lines. The digital data could be a saved file, a representation of a fax page, or interactive gaming instructions. It could also be streaming audio or video, or even a voice message from a digital telephone answering machine. A user will run some application software to deal with incoming data to view the images, listen to the music and/or voice, display and/or store/retransmit the file, and so forth.


Walking the Last Mile

Because the last mile connecting most home or small office/home office (SOHO) users to the phone system is an analog phone line, digital data is imposed on the analog waveforms by a modulation process. The modem translates the Ones and Zeros of the computer into audio tones which can be carried by the telephone network. The receiving station then demodulates the signal to strip out the digital data to do something useful with it.

This process gives rise to the origin of the word modem (MOdulation-DEModulation). The specific techniques used to encode the digital bits into analog signals are called modulation protocols.

There are many standard protocols available covering data and fax transmissions. The exact modulation protocol used largely determines the "speed" of the particular call. Besides modulation protocols, which are often executed on special modem signal processor chips, a modem has to be concerned with command and control functions. The protocols govern modem-to-modem communications as well as the modem-to-PC interface and are often executed on the PC host processor.


DSL screamers

Increasingly popular ADSL services promise to be the next breakthrough in on-line connectivity. A wide range of ADSL services are being introduced today by both local and long distance phone companies and competitive local exchange carriers. This service will offer subscribers Internet and local area network access at speeds ten to fifty times faster than those possible with conventional analog telephone modems.

One version of DSL recently adopted as an international standard is G.Lite (designated G.992.2). Several PC manufacturers have already announced plans to ship DSL-equipped systems.

But, the modulation-demodulation algorithms for DSL are approximately 20 to 100 times more demanding than those for an ordinary telephone modem--and are beyond the processing power available with a Pentium host!

Fortunately, the current generation of DSP processors are sufficiently powerful, programmable, and flexible. You, as a modem designer, can produce communication products that provide both DSL and telephone modem functions on a single board.