If you are like many people, Dilbert® included, your old PC may have more than enough capabilities for the day-to-day applications you run. Outside of business uses, people typically have a spreadsheet to keep track of taxes, charge accounts, college fees, vacation costs, and mortgage repayments; a word-processing package for letters and a résumé; a web browser; and maybe some games.
So, you may ask, “What do I need with a new, fast PC that can perform all the things I need it to do within the first second after I turn it on? Does it have more power than really is necessary?”
One use for this excess capability is to feed the ever-expanding appetite of application programs and operating systems. Originally, a 486 processor was recommended for Microsoft Office, but a Pentium 75 now is the minimum CPU appropriate for Office 2000. For Windows 95, a 20-MHz 386 with 8 MB of memory was recommended, but for Windows 98, a full 24 MB and a 66-MHz Pentium are suggested.
A more recent trend is software replacement of hardware functions. Today, there is enough unused CPU power available in many PCs that the 50 MHz required by a software implementation of a V.90 modem accounts for only 10% to 15% of the total processor capacity.
Data Communications
The functionality of a modem, like that of a coder-decoder (codec), is well-described by its name. Modem is a contracted form of modulate-demodulate. Because phone lines traditionally have carried only analog signals, a modem was necessary at the user’s end to modulate an analog carrier with the data signal. At the receiving end, another modem demodulated the analog signal to recover the data.
A long succession of V.xx standards defines various combinations of speed, features, and modulation schemes. For example, V.17 and V.29 deal with fax modems and V.21, V.22, and V.23 relate to data modems. All modems require four key functional blocks:
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A DSP data pump, performed by a DSP chip in hardware and controllerless modems.
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A controller, performed by a microcontroller chip in hardware modems.
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A PC bus interface.
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A line codec.
A fifth block, the DAA, is a necessary part of a complete modem but may be separate from the chip set supplied by the modem chip developers. Because the DAA connects directly to the telephone line, it is subject to scrutiny by the relevant telecomm authority in each country where a modem is sold.
Generally, equipment connected to the line must not damage central-office devices nor interfere with other users. Consequently, an isolation transformer has been a necessary part of a DAA, but today there are 100% silicon DAAs that provide 4-kV protection and have been approved in many areas.
Obtaining the necessary approvals from all the countries within a modem’s market area is called homologation. Although it is still necessary for each modem manufacturer to obtain separate certification, in many cases, the modem chip set manufacturer already has done most of the work.
“Within the last six months, many countries in Europe formed CTR-21,” said Shawn Owens, senior marketing engineer at PC-TEL. “Rather than take our reference modem design to each country, we sent it to a CTR lab. The certification process took several months, but it means that life for our OEM manufacturing customer is easier, and certification of the products is almost assured.” PC-TEL provides a software utility to help customers adjust a modem’s characteristics to suit a country’s requirements.
Motorola also supports its customers with a homologation utility program. Mike Tramontano, director of marketing for Motorola’s Software Products Division, said, “We have developed a Windows program with a graphical user interface that helps our customers obtain approval for their modem. It offers the capability to reconfigure country-specific parameters in the modem, add a new country, or perform particular tests that the lab is requiring.”
To Control or to Replace
As microprocessors have become more powerful, it has been possible to perform some hardware functions in software without noticeably affecting a PC’s performance. The first step in this direction was the introduction of controllerless modems which constitute the major part of the PC-modem market today. As the name suggests, the role of the microcontroller has been assumed by the host microprocessor, running a layer of software corresponding to the controller functionality.
Software modems go further by replacing the DSP functionality with software algorithms executed by the host processor. The term software modem or soft modem emphasizes this functional replacement although hardware codecs and a DAA also are required. Still, the remaining hardware system, especially with the advent of silicon DAAs, is physically very small.
The new AMR form factor and the MDC equivalent for notebook PCs take advantage of the small size of highly integrated soft modem chip sets. Technically, placing a modem’s sensitive analog components on a separate riser card improves electrical isolation and reduces noise. This leads to better audio quality and easier modem certification as well as reduced cost. Figure 1 (below) shows the progression of modem architecture from hardware to software. The third step in the figure uses the new AC-Link technology that places modem and audio applications on a separate, dedicated bus. Core logic support for AC-Link in new PCs allows modem-specific hardware to be reduced to only the codec and DAA.
Figure 1.
Software-Assisted Modem Details
A typical V.90 software modem requires about 50 MHz of microprocessor capacity to perform well under all circumstances. This is the reason that it hasn’t been practical in the past to implement software modems on slower PCs.
“Studies indicate that in nongraphics applications PC users don’t perceive a system slowdown until loading exceeds 65%. Users are less tolerant when using graphics applications; loading must be limited to 45% to avoid graphics-application performance degradation.”1 Today, most PCs feature CPU speed above 200 MHz, and the corresponding soft modem loading is below 25%.
“The first time there was enough CPU horsepower was probably in September 1998 when 233-MHz Pentiums became common,” said Rahul Shah, senior product marketing manager with Conexant’s personal computing division. “At 200 to 233 MHz, you started having enough CPU bandwidth to run a software modem as well as all the other wonderful things you wanted to do. That was when soft modems really took off.”
An additional consideration is the V.90 specification itself. Traditional modems with ADC and DAC interfaces to handle analog signals on the telephone lines are limited to the 33.6-kb/s rate of V.34. V.90 assumes that one end of the modem-to-modem link is a purely digital connection to the phone network. Downstream data is converted from digital to analog by a codec on a line card connected to the digital telephone network facilities. It is synchronously recovered by your V.90 modem. This approach allows speeds up to 56 kb/s and is a feature that ISPs exploit when downloading files from a large server to your PC.
V.90 is asymmetrical since upstream transmissions—in the internet case consisting mostly of keystroke and mouse commands from your PC to the server—are limited to 33.6 kb/s. Upstream transmission uses modulated, single-frequency carrier tones. In the downstream direction, the server modem uses PCM to send data.
Software modems require excess CPU capability and Windows or a similar multitasking operating system. Kapriel Karagozyan, vice president of sales at Smart Link Technologies, said, “Soft modem sales are being driven by three factors: increasing CPU speed, a maturing soft modem technology, and the market’s drive toward sub-$500 PCs. This is a price point where every dollar counts. People are willing to invest to integrate a soft modem, which is the lowest-cost way of providing a modem.”
Software Modem Test Issues
Testing divides into several parts. Development testing to verify product performance involves traditional modem tests as well as tests that address concurrency issues. Manufacturing test determines that a modem meets its specifications and is properly constructed. Because the PC software modem is a Windows application, it also should be certified to meet the requirements of Microsoft’s approval program.
According to Mr. Karagozyan, “Approval is more important to some modem manufacturers than others. The first approval for a soft modem was granted by Microsoft in early 1998. Until then, they felt uncomfortable. Since then, they’ve been granting it on a regular basis. It was more difficult two years ago when the CPU didn’t have enough horsepower, but right now it’s more a matter of perception.”
Traditional modem test equipment typically treated the modem as a hardware COM port with a physical UART. In a soft modem, the UART function is implemented entirely in software, so the tester you are using must account for this difference.
These test instruments simulate various parts of the telephone network (Figure 2 below). For example, “DFE provides programmable control of the loss and distortion associated with digital [loss] pads,” said Michael Pellegrini, product manager for modem test systems at Telecom Analysis Systems. “DFE also gives you the flexibility to model multiple PCM codec implementations to measure 56k modem performance over a range of typical network conditions. By programming the coding scheme and the transmit-and-receive filter characteristics used by the codec, the DFE can be customized to emulate virtually any type of codec.”2
Figure 2.
Similarly, there are emulators for the analog loop and the entire digital/analog telephone network. In addition to digital functions, the network emulator also includes analog transmission facility impairments such as transhybrid loss, attenuation, nonlinear distortion, and noise. The loop emulator recreates the characteristics of the local subscriber loop that interfaces to a user’s modem.3
“A total of 160 line connections is simulated for the network model,” according to Dave Jenkins, senior lab technician at Henderson Communications Laboratories. “A loop emulator provides various types of connection environments a modem would see in the United States—noise, analog phase jitter, round-trip delays, crosstalk, and intermodulation distortion as well as digital signal impairments.”
Henderson Labs stresses the importance of using both the U and I trunk models when testing modem performance in a typical POTS. The letter I identifies a single digital and a single analog link over a channelized digital link (usually T1) or over a clear-channel link (ISDN, either BRI or PRI).
The letter U is associated with multiple digital and analog channels over channelized or clear-channel links. For the U cases, the multiple links arise because the modeled connection uses a DLC system between the customer and the central office. Instead of maintaining a digital link as far as possible as the I model does, the DLC converts a two-wire connection to digital, digitally transports signals to the central office, changes the signal back to analog, connects to a standard POTS port in a switch, and lets the switch convert the signal back to digital.4
The capabilities of various types of simulators and emulators are grouped together in automatic modem test systems. Test sequences and result analysis can be programmed to suit your needs. Assuming that the test equipment is suited to soft modem testing, the types of tests required are the same for soft modems as for traditional hardware modems.
Because software modems are yet another application program running on the CPU, a number of concurrency tests should be run to prove system performance. These tests may include a mix of text-only applications, graphics plus text, spreadsheets, CD-ROM, and audio files together with printing and internet downloading. The object is to determine if your soft modem is a well-behaved Windows application or if it will drop the phone line connection or corrupt other applications when the host PC becomes heavily loaded. Suites of standard tests have been developed by testing laboratories, and modem manufacturers regularly use these facilities to benchmark their products against the competition.
CPU latency and PCI bus latency affect software modems much more than other types of modems. “There is no direct way to measure concurrency,” according to Conexant’s Rahul Shah. “But you can use the measure of tolerance to interrupt latency as a proxy for the capability to work concurrently with other applications inside a PC. The thrust of our entire development process was to deal with latencies because, if the process is not stable, there’s no point in developing and selling a software modem.
“We actually created a latency testing application utility to allow any soft modem to run against that utility on a PC,” he continued. “The program disables interrupts for a period of milliseconds and then enables them for so many milliseconds. The question is how much time can you disable interrupts before you fail.” Conexant has developed a similar application for PCI bus latency tolerance testing.
Figure 3
(below) shows test results for one controllerless and three software modems. In the figure, throughput drops off as the percentage of the network model increases. The network model emulates typical U.S. subscriber-loop conditions from short loops free of splices and bridged taps to long loops with several signal impairments. As the percentage of the network model increases, more of the very difficult loop conditions are included.
Figure 3.
The Future
So, where are soft modems and HSP headed? According to Shawn Owens of PC-TEL, “You won’t see more functionality made available as the price of PCs comes down. I don’t think people really want more functionality. They are buying low-cost PCs to get onto the internet.
“As ADSL technology is used in different regions, that’s where you’re going to see more functionality. The next generation will be modems that combine both V.90 and G.Lite capabilities. When G.Lite is finally deployed in your area, you will just turn it on,” he continued.
G.Lite also is presented by Conexant as the next logical performance improvement after V.90 modems. Its 1.5-Mb/s downstream speed is 20 to 30 times faster than analog modems. Interoperability in a G.Lite chip set-based modem will be ensured by including current V.90, K56flex, V.34, V.32bis, V.17, and V.29 standard data and fax modulation formats. The G.Lite/G.992.2 standard has been developed by the ITU Study Group 15.5
Motorola’s Mike Tramontano sees modems as just the beginning. “For us, it’s a whole software communications initiative that includes other transport modes like G.Lite, cable, and more broadband kinds of communications. We’re also using HSP technology on different platforms such as web appliances, game machines, and set top boxes—a variety of intelligent devices that have a need for connectivity.”
References
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SM56 PCI Software Modem: Product Information, Motorola Information Systems Group, 1998, pp. 1-3.
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Pellegrini, M., “Testing 56k Modems,” EE-Evaluation Engineering, January 1998, pp. 82-86.
NOTE: This article can be accessed at this web site. Select EE Archives and use the key word search. -
“V.90 Modem Testing Guidelines: TAS White Paper,” Telecom Analysis Systems, 1999, p. 5.
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“The PN 3857 “U” Model,” Henderson Communications Laboratories, www.henderson-labs.com/r-serv_b.htm.
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“ADSL/G.Lite: ITU Compliant Modems,” Conexant, www.conexant.com/products/modems.
Acknowledgments
The following companies provided information for this article:
Conexant (949) 483-1148
Henderson Communications Laboratory (909) 788-8849
Motorola Internet and Networking Group (508) 261-4200
NSTL (800) 220-6785
PC-TEL (408) 383-0452
Smart Link Technologies (617) 926-7322
Telecom Analysis Systems (732) 544-8700
Glossary
ADC
ADSL asymmetric digital subscriber line
AMR audio/modem riser
BRI basic rate interface
COM continuation of message
DAA data access arrangement
DAC digital-to-analog converter
DFE digital facility emulation
DLC digital link carrier
DSP digital signal processor
G.Lite a splitterless form of ADSL
HSP host signal processing
ISDN integrated services digital network
ISP internet service provider
ITU International Telecommunications Union
MDC mobile daughter card
NSTL National Software Testing Laboratories
PCM pulse code modulation
POTS plain old telephone system
PRI primary rate interface
PTT public telephone and telegraph
UART universal asynchronous receiver/transmitter
analog-to-digital connector
About the Author
Tom Lecklider is a Technical Editor with Evaluation Engineering.
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August 1999