Black Box Corporation 1000 Park Drive Lawrence, PA Tech Support: V.

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1 200. All rights reserved. Black Box Corporation. Black Box Corporation 1000 Park Drive Lawrence, PA Tech Support: V.34 Adaptive intelligence makes this standard not only fast, but smart, too. What is V.34? The development of the V.34 modem recommendation, originally known as V.fast, began in October of The original objective was to develop a two-wire dial modem capable of operating speeds up to 19.2 kbps 33% faster than the 14.4-kbps V.32 bis modem standard. But leading contributors to the standard, such as Motorola, quickly demonstrated that the switched telephone network could support speeds of 24.0 kbps and above if several key new technologies were included in the V.34 standard. Eventually, the group decided to standardize on a single-carrier trellis-coded modulation technique based on Quadrature Amplitude Modulation (QAM), with several new enhancements, to achieve the 28.8-kbps top speed of the final draft standard. In February 1994, the ITU froze the draft specification and moved on to the internetworking phase the period when leading modem manufacturers actually build prototype modems and attempt to demonstrate the internetworking compatibility between vendors. Motorola successfully demonstrated V.34 internetworking between two independently developed V.34 platforms: one from Motorola Codex and one from Motorola UDS. Finally, in 1994, the ITU study committee approved the final standard. Almost immediately, manufacturers began shipping V.34- compatible modems. Adaptive intelligence is what really sets V.34 apart from all the standards before it. Unlike those older standards, V.34 employs multiple modulation methods and multiple impairment-compensation techniques. A V.34 modem can automatically and intelligently combine the optimum set of these modulation tools to adapt to any given telephone call. This tutorial will describe each of these technologies in simple terms, but first a brief description of V.34 modem features and how they compare to the features of a V.32 bis modem. V.34 vs. V.32 bis Like the V.32 bis specification, V.34 defines a two-wire, full-duplex dial-line and leased-line modem supporting both synchronous and asynchronous operations. Likewise, the specification calls for automatic fallback compatibility with lower-speed modems such as V.32 and V.22 bis. Now for a brief comparison of the differences: V.32 bis V.34 Modem Fixed Adaptive Type Modulation Intelligence Data 14.4 kbps to 28.8 kbps to Rates 7.2 kbps 2400 bps Bandwidth Fixed Variable Trellis 2-dimensional 4-dimensional Coding Adaptive Linear Precoding Equalization Mapping 2-D Shell 16-D Shell Mapping Mapping Auxiliary None 200 bps Channel Operating Full-Duplex Full-Duplex Modes Half-Duplex (Fax) Half-Duplex (Fax) Asymmetric 1 2/1999 #12290

2 V.34 Modulation Methods So what are the technologies? Let s take a look at what s in the V.34 modulation toolbox: 1. V.8 negotiation handshake 2. Line Probing 3. Precoding 4. Adaptive Pre-Emphasis. Adaptive Power Control 6. Multi-dimensional Trellis Coding 7. Shell Mapping (or shaping ) 8. Warping 9. Fast Train & Training Recovery Procedures 10. High Speed DTE Interface How do these modulation technologies actually work? To say that they are complex is an understatement. Let s take a conceptual look at each. combinations to choose from, with 6 different symbol rates, each with 2 possible carrier frequencies. Pre-Emphasis Selection: The modems choose the optimum transmit pre-emphasis filter from a menu of 10 defined filters in the V.34 specification. (More on Adaptive Pre-Emphasis later.) Power Control Selection: The modems choose the optimum transmitter output power level with a range of selection of 14 db in 1-dB increments. (More on Adaptive Power Control later.) Line probing is performed on every new connection and whenever a full retrain occurs, which can be performed at any time during a connection. This allows V.34 modems not only to adapt to a broad range of different line types and distortions from call to call, but also to accommodate varying line conditions over long periods of time on any given connection. The latter is particularly important for leased-line operation. For example, leased lines routed over analog carrier systems (e.g. analog microwave or N-carrier links) can exhibit timevarying distortion, which over time could make the first choice of modem operating parameters less than optimal. With V.34 modems, as performance decays in the presence of these time-varying distortions, the modem can re-enter line probing at any time to adapt to the prevailing conditions. V.8 Mode-Negotiation Handshake Anew handshake start-up procedure developed specifically for V.34- based modems, V.8 includes backward compatibility to all lowerspeed modems, with provisions to recognize and interwork with the V.32-bis-defined Automode negotiation procedure. This is the first signal exchange that occurs between two V.34 modems when making a connection. Like other elements of the V.34 specification, V.8 is an intelligent procedure. With it, V.34 modems can perform feature and mode negotiation quickly, utilizing V.21 (300-bps FSK) modulation to exchange information. Negotiation parameters include such information as: Distinguishing V.34 modems from all other types Data mode or Text Phone operation Modulation modes available V.42 & V.42 bis support Wireline or Cellular operation V.8 is flexible enough to accommodate any V.34 application. More than that, it will be ready for future applications perhaps even applications that no one has thought of yet. Line Probing Line probing is the most significant enhancement in the V.34 standard. It allows a V.34 modem to intelligently choose the optimum operating parameters for any given telephone channel. Line probing is a bidirectional half-duplex exchange performed immediately after V.8 negotiation. It involves the transmission of complex signals that allow the distant receiver to analyze the characteristics of the telephone channel before entering data transmission. The modems use this line analysis to choose several key operating parameters, including: Carrier Frequency and Symbol Rate: This determines the optimum bandwidth and placement (center frequency) of the transmitted signal within the available channel bandwidth. The modems have 12 possible 2 Precoding Precoding is actually a modification of an adaptive equalizer technique developed in the 1970s, known as Decisions Feedback Equalizations or DFE. The trick is in combining DFE with Trellis coding. Like oil and water, they just don t go together easily. Decision Feedback equalizers have been proven to be the optimum receiver equalization technique for analog voice-grade modems, and can compensate for Intersymbol Interference (ISI) caused by severely distorted channels. This is essential for high-speed modems that need to utilize every ounce of the frequency spectrum available on the line. Precoding allows for the use of DFE techniques without compromising the Trellis coding. The basic idea is to split the DFE between the transmitter and the receiver. In so doing, the V.34 receiver calculates the optimum equalizer coefficients as it would for a normal DFE but relays them back to the transmitter, where the transmitted signal is equalized before transmission. The result is the best of both worlds: Decision Feedback Equalization with pre-equalization and Trellis Coding. Adaptive Pre-Emphasis This is another technology taken from the past (formerly known as compromise equalization or pre-emphasis ) and enhanced with adaptive intelligence. V.34 takes this old fixed technology and makes it adaptive based on actual line characteristics. With pre-emphasis, the transmitted signal is passed through a spectral shaping filter, which boosts signals in some parts of the transmitted spectrum while attenuating signals in other parts of the spectrum. Pre-emphasis is very effective against signal-dependent distortion. The idea is to pre-compensate for known channel distortions learned in Line Probing. If, for example, line probing detects that severe roll-off is present at the upper part of the chosen transmit spectrum, then an appropriate pre-emphasis filter can be introduced in the transmitter to compensate. Not only is the direct effect of the channel distortion compensated for, but the more severe side-effects of non-linear distortion are minimized as well.

3 The intelligence comes in with the selection of which pre-emphasis filter to use. The V.34 specification defines 10 different pre-emphasis filters to choose from. Which filter is chosen depends on the information obtained during line probing. The actual method of choosing is up to the implementor. Adaptive Power Control Proper selection of transmitter power is critical in high-speed echocanceling modems. With older four-wire modems or lower-speed V.22 bis modems, higher transmit power was always better. Not so with echo-canceling modems they need to strike a balance between high and low transmission power. High transmission power can improve signal-to-noise ratio for the distant receiver, but it can also introduce undesired echo distortion for the local receiver. On the other hand, too low of a transmitted signal compromises basic signal-to-noise ratio. Adaptive power control is an intelligent, adaptive scheme that automatically selects the optimum transmit level based on line-probing results. The idea is simple, but making it work requires a complex and delicate balance. Multi-Dimensional Trellis Coding Trellis coding, simply put, is a forward error-correction coding scheme. The value of the coding is expressed as a coding gain, which is a measure of the modem s error-rate improvement over an uncoded modem. The bar-graph below shows the effective coding gain of the three new codes employed in the V.34 specification as compared to the coding technique implemented in V.32 bis modems. V.34 employs three new four-dimensional coding schemes compared to the two-dimensional scheme employed in V.32 bis. Fourdimensional coding has been found to provide the best balance of performance, delay, and complexity of implementation. Each of the three new coding schemes has increasing performance benefits, but dramatically increased complexity. The V.34 standards body has approached the limits of diminishing returns to achieve the desired performance. 2D 8-State (V.32) 4D 16-State 4D 32-State 4D 64-State Effective Coding Gain (db) Relative Complexity The 4-D code is a good balance of performance, delay, and complexity. 3

4 Shell Mapping (Shaping) In high-speed modems, each symbol transmitted contains a number of user data bits and coding bits. These bits are grouped into symbols, and then mapped into a two-dimensional signal constellation (as shown in the illustration). The resulting signal point is then transformed to its analog signal equivalent for transmission over the analog voice channel. Shell mapping is a signal-constellation mapping technique that attempts to distribute these signal points in the two-dimensional space in such a way as to improve the resultant noise immunity by approximately 1 db. An optimum constellation would be a disk shape, but a perfect disk is not possible. Shell mapping approximates the disk shape by mapping a square grid constellation to a near-disk shape with Gaussian distribution of the signal points in the two-dimensional space. The net effect is that the constellation is expanded, and the signal-to-noise ratio is improved by approximately 1 db. The V.34 specification supports two levels of shell mapping. The first expands the constellation by 12.%; the second expands it by 2%. Shaping 12.% Shaping 2% 6 2D 8-State (V.32) 4D 16-State Shell Mapping Gain (db) Trellis shaping provides close to 1 db gain with small complexity. Warping (Nonlinear Encoding) Warping is another form of signal-space coding specifically designed to combat the effects of signal-dependent channel distortion, also known as nonlinear distortion or harmonic distortion. Nonlinear distortion is present in all types of telephone channels; it s mostly due to the PCM digital encoding of the analog signals. The nonlinear nature of PCM coding, compounded by the nonlinear distortion introduced by analog components such as transformers and loading coils, can make high-speed data signals difficult or impossible to transmit. Warping is a means of trading off signal-to-noise immunity for improvement in signal-dependent-distortion immunity. Since nonlinear distortion affects the outer constellation points more severely than the inner ones, warping compromises the noise immunity of the inner points in favor of the more susceptible outer points. The result is that the mean distance between points is increased in the outer fringe of the constellation (improving the immunity to all types of distortion, but particularly nonlinear distortion) while the mean distance between the inner points is reduced. Warping 4

5 Fast Training and Training Recovery Procedures Training is a general term used to describe the process by which the modem s receiver synchronizes on the remote modem s transmit signal at initial connection establishment. Training is also sometimes performed after a connection is up and running to recover from extreme disruptions (line outages or bursts of noise) that may periodically occur during a connection. The training process involves many stages of receiver acquisition: automatic-gain-control adjustment, receiver-timing acquisition, halfduplex equalizer convergence, echo-canceler convergence, fullduplex echo-canceler and equalizer convergence, this and so on. Training is a time-consuming process, but a necessity in analog modems. During this training time, user data cannot be sent, so from the user s perspective it s lost time. The faster a modem can train, the sooner it can get to data mode and begin user data transmission. However, the downside is that, in general, the higher the raw data rate, the longer the training time required for the modem to successfully acquire synchronization. To make the higher speeds of V.34 practical, the training process had to be sped up. Earlier modem training sequences were transmittertimer based, and were therefore fixed in duration. The V.34 training sequences are receiver-timing based, so the time to complete a train can be minimized. Another important new feature of the V.34 training procedure is in the intelligent fault-recovery mechanisms. Earlier modem standards either did not define recovery procedures or required complete restart from the beginning should a fault occur during training. Telephone-line clicks and pops are a fact of life and can cause faults during training, resulting in a bad train. V.34 minimizes the time to recovery. The net result is that although V.34 represents a doubling of the raw data rate relative to V.32 bis modems, the V.34 fast train is shorter than V.32 bis by approximately 3 seconds. A nominal V.32 bis connectionestablishment sequence takes about 10 seconds, while a V.34 will connect at 7 seconds at double the bit rate. High-Speed DTE Interface The V.34 specification defines two new DTE interfaces, a preferred interface and an alternative interface. Both are defined so as to allow for reliable operation up to the speeds supported by the V.34 specification. The old EIA-232 (V.24/V.28) interface was originally designed for operation up to 20 kbps at distances to approximately 0 ft. (1 m). V.34, on the other hand, supports synchronous operating speeds well above 20 kbps and asynchronous speeds well above 100 kbps (when combined with V.42 bis data compression). Basically, the ITU has defined two interfaces: 1) The preferred interface is a 2- or 26-pin physical interface utilizing a mix of V.10 (TIA-423-B) and V.11 (TIA-422-B) electrical standards. This interface calls for balanced electrical signals (V.11) on all data and clock signals while all control signals operate in the unbalanced V.10 mode. This is very similar to the interface commonly referred to in Europe as X.21, and is likely to become the preferred interface for V.34 modems targeted at synchronous applications. It is unlikely that this interface will gain much acceptance in the asynchronous PC user market, since it is not backward-compatible with the huge installed base of EIA-232 interfaces. 2) The alternative interface, on the other hand, is backwardcompatible with the EIA-232 installed based. It calls for a 2- or 26-pin physical interface and V.10 electrical interface, which both is backward-compatible to 232 and provides for reliable highspeed operation. The cable lengths supported by the V.10 interface at these higher speeds are considerably longer when interconnected with another V.10 electrical interface, but are much more restricted with interfacing with an existing 232 device. Nevertheless, because so much EIA-232 equipment is already installed, it is reasonable to assume that most consumer and desktop V.34 modems will employ the V.10 alternative interface. It is also very possible that the semi-conductor manufacturers and PC manufacturers will migrate to the V.10 standard in place of 232 in an effort to interwork reliably at the high speeds of V.34 and successor technologies.