It was just 1991 when the Consultive Committee for International Telephone and Telegraph (CCITT) ratified Recommendation V.32bis for full-duplex modem-to-modem communications—at the blazing speed of 14.4 kbps over the Public Switched Telephone Network (PSTN). Given the approximate 3-kHz bandwidth of the network, this seems an imposing task. But the public wasn’t satisfied: Because of the cost of using PSTN, telecommunication companies, which transfer large data files, demanded even faster modem speeds to cut costs. Many of these companies were forced to use expensive proprietary equipment to obtain higher data rates, which also speed remote access to data.

Almost immediately, the CCITT—now named the ITU-T (International Telecommunications Union)—established the TR-30 Study Group (formerly SG XVII) to work on V.fast for modems that operate faster than 14.4 kbps. The V.fast draft recommendation, to be renamed V.34 after ratification, has been three years in the making. In order to satisfy the needs of members in the study group, V.34 has undergone considerable design and extensive changes since its inception in 1991.

The current V.34 draft, expected for final balloting in June, supports multiple data rates ranging from 2.4 to 28.8 kbps. Because of the extensible time lapse between V.32bis ratification and the proposed final ratification of V.34, a number of chip-set and modem vendors that were working on the committee set out to find alternative means for achieving higher bit rates. Participants divide into two camps: proponents of a public-domain standard called V.32terbo and supporters of a proprietary standard called V.FC (V.Fast Class), which Rockwell International developed.

Placed in public domain in 1993, V.32terbo delivers data rates of 16.8 and 19.2 kbps. The V.32terbo-standard proposal is a simple extension of V.32bis to bridge the transition to V.34. V.32terbo is a de facto standard; the ITU-T has not sanctioned it. Because it is a V.32bis extension, V.32terbo is fully compatible with the V.32 and V.32bis standards. The handshake is completely compatible, although V.32terbo did use the last two available bits in the V.32 start-up sequence. The symbol rate remains the same at 2400 baud, and two more points are added to the signal constellation.

Stars point the way

To distinguish one transmitted symbol from another, high-speed modems employ a constellation of symbol points (to represent specific amplitude and phase combinations) plotted on a 2D I-Q (in-phase-and-quadrature) graph. For example, a point in a hypothetical constellation might represent a symbol transmitted at 50% of maximum amplitude and 1208 phase shift. The constellations contain the minimum number of points necessary to support a given data rate, which yields the maximum distance between symbol points.

The V.32bis-specification modification extends the 128-point constellation for 14.4 kbps to 256 points for 16.8-kbps and 512 points for 19.2-kbps transmissions. To accomplish the higher point density, the V.32terbo standard introduced nonlinear encoding, which improves distortion immunity near the perimeter of the signal constellation. More than 40 modem vendors have modems that support the V.32terbo standard. Most of these modems cost the same as a V.32bis modem or slightly higher ($50 or less).

Currently, the two most visible chip-set vendors for the V.32terbo standard are AT&T and Phylon. Phylon has a chip set for V.32terbo designs employing data, fax, and voice transmission. The $69 (1000) PHY1013 and $72 (1000) PHY2011 consist of a DSP chip and a mixed analog and digital front end. An EPROM retains the DSP software; RAM holds the program during operation. The DSP architecture integrates MNP 2-5 and V.42bis error correction and data compression. V.42bis compression enables a V.32terbo modem to attain a throughput as high as 155,200 bps. Because the chip set is based on a software architecture called Tru-

Speed, you can software-upgrade V.32bis products to V.32terbo or to V.34 when it becomes a standard. Depending on the implementation, you can upgrade the product over dial-up lines or floppy disk or by changing an EPROM.

Three angles of approach

AT&T has a 3-pronged approach for migrating to the higher modem speeds. AT&T offers three chip sets, called the DSP163x family, that integrate an analog codec onto the same silicon as a DSP1600 core. Each chip set comprises a DSP chip and a digital-interface device called VALV3x. All the devices share a common footprint. The DSP1632 ($55 (10,000)) supports V.32terbo, V.32bis, and V.17 standards and all fallback possibilities. The chip set doesn’t require external RAM. The DSP1633F ($95 (10,000)) employs the company’s V.flex technology, which lets you download upgradable DSP code into external RAM.

The V.flex modem comes with pre-V.34 release code that can be upgraded to the final V.34 standard when it becomes available. The ability to make software upgrades via floppy disk or dial-up lines eliminates the hassles of changing EPROMs, DSP chips, or daughtercards. The DSP1634 chip set will support the final V.34 standard as well as fallback specifications. The chip, which is ROM-based, can be used as a drop-in upgrade for the DSP1632 or for cost and power reduction when replacing a DSP1633F.

Rockwell International, a major modem chip-set vendor, chooses not to support V.32terbo. Instead, in late 1993, Rockwell introduced the V.FC proprietary standard. The V.FC family of single-chip devices offers speeds of 19.2, 24, and 28.8 kbps. The 19.2-kbps RC192AC/VFC costs $86; the 28.8-kbps RC288AC/VFC costs $123. Because V.FC now offers 28.8-kbps, full-duplex transmission speeds, more than 100 modem vendors already support the proprietary standard (according to Rockwell).

V.FC modems generally cost more than $500 each, but Microcom sells its V.FC Deskporte Fast and Travelporte Fast modems for $499. Microcom also recently introduced two low-cost versions of the Deskporte fast modem for $299 and $399. These modems don’t have all the bells and whistles of their higher-priced counterpart. V.FC provides many of the advantages that the upcoming V.34 standard will provide. And similar to the other modem standards, V.FC has fallback modes that are compatible with standards ranging from V.32bis to Bell 212A and 103.

Although information on the operations of the proprietary V.FC chip is scarce, evidence exists that the chip is a subset of the proposed V.34 standard. The reason V.FC doesn’t contain all of the V.34 features is that many V.34 features weren’t finalized until January of this year. Some of these features include the handshake sequence and the trellis- encoding options. According to Product Line Manager Angelo Stephano, Rockwell has been participating in all of the V.34 meetings and working on a chip that will be fully V.34 compatible when the standard is ratified. The new chip will be pin-for-pin compatible with the V.FC chip. However, a V.FC modem user must replace a socketed 68-pin PLCC to upgrade to the V.34 standard. When the chip becomes available, Microcom will offer a $39 upgrade kit that will include firmware changes to flash memory.

All of which brings us to the draft V.34 recommendation. The working group wrapped up work on the specification in March and is expecting to send the draft out to members for final ballot in June. Assuming a normal time span for ratification, a final standard should be available approximately 60 days thereafter. Thus, V.34 chip sets (and even modems) may begin to propagate by the fourth quarter of this year. Sierra Semiconductor is one company that’s working on a V.34 chip set to be ready by the fourth quarter.

Unfortunately, it may be a rocky road toward having a range of V.34 modems that routinely communicate at 28.8 kbps. The complexity of the V.34 specification is the reason. Perusing the V.34 draft recommendation seems a mathematician’s joy—and an engineer’s nightmare: The standard runs the gamut of complex mathematical operations. Essentially, the standard is a pool of many companies’ desires; 10 companies reserve intellectual property rights to parts of the specification they plan to make available in a "fair and impartial" manner.

Unlike other modem specifications, the V.34 standard specifies synchronous line transmission at selectable symbol rates. The mandatory rates are 2400, 3000, and 3200 symbols/sec. Optional rates are 2743, 2800, and 3429 symbols/sec. In addition, one of two carrier frequencies can be selected at each symbol rate. The carrier frequencies range from 1600 to 1959 Hz. The choice of symbol rate and carrier frequency depends on line conditions that exist at connection time. The synchronous, primary-channel data-signaling rates range from 2.4 to 28.8 kbps in steps of 2.4 kbps. Transmission is either half- or full-duplex. An optional auxiliary channel having a synchronous data rate of 200 bps can also multiplex into the data stream.

The variety of symbol rates gives rise to the possibility of having a fractional number of bits/symbol. For example, transmitting 28.8 kbps at a symbol rate of 3000 symbols/sec yields 9.6 kbits/symbol. V.34 uses a sequence of embedded frames to transmit fractional bits/symbol. Symbols are spread over multiple frames. In this example, if 10 frames contain 96 bits, then each frame would have 9.6 bits/frame, where 10 frames constitute 10 symbols. As many as 15 frames can represent 15 symbols. These symbols embed within a superframe for trellis-code-modulator encoding, which reliably stores the multiple bits for each symbol (signal transition).

Trellis codes store information

A 4D trellis encoder generates the constellation points. Three optional convolutional encoders are available for three sets of trellis codes, each of which has a unique constellation pattern. The codes are called 4D 16-state Wei code, 4D 32-state code, and 4D 64-state code. The three trellis codes offer tradeoffs in decoder complexity vs immunity to Gaussian noise. For example, the 4D 16-state code should increase noise immunity over a V.32 2D 8-state code by approximately 1 dB, whereas the 4D 64-state code increases the immunity by 1.6 dB.

Therefore, a pair of modems using a 64-state code can operate 2400 bps faster than 16-state code. While two modems are negotiating a symbol rate, the modems probe the channel for Gaussian noise and determine whether the 16-state code is sufficient for optimal transmission or whether one of the more complex codes is required. All signal constellation points are a subset of a superconstellation that has 960 points; the superconstellation forms by rotating 240 points distributed throughout a 2D constellation by 0, 90, 180, and 2708.

In addition, the draft V.34 recommendation employs a variety of equalization and power-control techniques not used in current modem recommendations. The new equalization techniques include adaptive preemphasis, precoding, shaping, and nonlinear coding (sometimes called warping). Preemphasis normalizes the transmit spectrum and is defined as the ratio of f/S, where f is the transmit frequency and S the symbol rate. The linear increasing transmit frequency compensates for PSTN bandwidth roll-off characteristics. Perfect compensation means that a signal applied to the digital network’s backbone experiences a flat frequency response. Precoding is a nonlinear equalization method for reducing equalizer noise that causes amplitude distortion. The transmitter performs equalization using precoding coefficients the remote modem provides.

Trellis codes usually use equidistant points in the constellation. Some modem designers believe that you can improve noise immunity by concentrating the most frequently used points near the center of the constellation, which is called shaping. This technique can be of benefit when the connection is noisy. Shaping changes the distribution of points throughout the constellation. V.34 uses a shell mapper to shape the constellation points.

Nonlinear coding increases immunity to nonlinear distortion due to quantization noise. Think of this technique as another form of shaping, one that redistributes the points in the constellation (albeit in a different manner). Using nonlinear coding, the magnitudes of the low-amplitude signals reduce further, whereas the magnitudes of the high-amplitude signals increase. In this manner, it’s easier for the receiving modem to distinguish between points closer to the center of the constellation and those in the outer regions.

In addition to the new equalization techniques, V.34 will be able to adjust the modem’s transmit power to best match existing network conditions. High power can overload the receiver and introduce nonlinear distortion. A reduction in the power level eliminates this distortion and thereby improves the receiver’s accuracy. US modem standards to date have specified -9 dBm as the maximum transmission power, which the FCC set. Other countries have established even lower power levels for maximum power. V.34 will use -9 dBm as a starting point for maximum power and allow the modems to negotiate power levels as low as -17 dBm in 1-dBm increments.

The V.34 start-up sequence divides into four distinct phases. Phase 1 is a network-interaction phase, which identifies the transmitter as a V.34 modem and to establish the modem’s capabilities. Phase 2 is a probing and ranging sequence that establishes maximum attainable signaling rate, carrier frequency, optimum power level, and preemphasis filter shape. Phase 3 sets the operating characteristics of the echo cancelers and equalizers. Phase 4 is a final training phase, which refines the operating characteristics of the echo cancelers and equalizers under full-duplex operation. The precoding coefficients are also set during Phase 4. The entire start-up task sequence takes 5 sec, which is impressive, considering a V.32bis modem needs 8 to 10 sec for start-up.

The high data rates proposed for V.34 will challenge more than just the telephone connection. The widely used RS-232C serial interface is specified to operate only as fast as 20 kbps. However, modem vendors claim they routinely transfer 115.2 kbps through the serial port using improved drivers and receivers and V.42bis data compression. Microcom adds a parallel printer port to provide a data throughput as high as 300 kbps using V.42bis or MNP 5 data compression. In order for V.34 modems to be reliable over the digital interface, new approaches to designing the interface are requisite. Clearly, a good deal of interoperability work will be necessary for V.34 modems to proliferate.