U.S. Robotics x2 Technology: Technical Brief x2 is a new transmission scheme developed by U.S. Robotics that achieves line speeds of up to 56 kbps. This technology can take advantage of compression schemes such as V.42 bis, to further increase throughput. This paper explains x2 in detail. U.S. Robotics x2 technology allows modems to receive data at up to 56 Kbps over the standard, public switched telephone network (PSTN). x2 overcomes the theoretical limitations imposed on standard, analog modems by exploiting the digital connections that most Internet and online service providers use at their end to connect to the PSTN. Typically, the only analog portion of the phone network is the phone line that connects your home to the telephone company's central office (CO). Over the past two decades the telephone companies have been replacing portions of their original analog networks with digital circuits. But the slowest portion of the network to change has been the connection from your home to the CO. That connection will likely be analog for some years to come. x2 requires no changes to the wiring and equipment that are already in place. All that's required to convert a service provider's U.S. Robotics Total Control digital modems, NETServer I-modems or MP I- modems is a software upgrade. U.S. Robotics calls the modems that have a direct digital connection to the PSTN x2 server modems. Likewise, converting a Courier V.Everything analog modem to an x2 client modem is as simple as downloading new software. (In addition, some Sportster modems can be upgraded to x2 by swapping a memory chip.) Why V.34 Wasn't the Final Word In Throughput V.34 modems are optimized for the situation where both ends connect by analog lines to the PSTN. But today it makes sense to assume that service providers have digital connections to the PSTN. The PSTN's Optimization for Voice Hinders Data Communications The PSTN was designed for voice communications. By artificially limiting the sound spectrum to just those frequencies relevant to human speech, network engineers found they could reduce the bandwidth needed per call, increasing the number of potential simultaneous calls. While this works well for voice, it imposes limits on data communications. Remember that the PSTN was optimized for voice traffic. V.34 Modems Are Optimized for End-to-End Analog Connections Even though most of the network is digital, V.34 modems treat it as if it were entirely analog. V.34 modems are incredibly robust, but they cannot make the most of the bandwidth that becomes available when one end of the connection is completely digital. V.34 was built on the assumption that both ends of the connection suffer impairment due to quantization noise introduced by analog-to-digital converters (ADCs). Anatomy of a V.34 Connection Noise Introduced by Quantization of Analog Signals Analog information must be transformed to binary digits in order to be sent over the PSTN. The incoming analog waveform is sampled 8,000 times per second, and each time its amplitude is recorded as a PCM code. The sampling system uses 256 discrete 8-bit PCM codes. Because analog waveforms are continuous and binary numbers are discrete, the digits that are sent across the PSTN and reconstructed at the other end approximate the original analog waveform. The difference between the original waveform and the reconstructed quantized waveform is called quantization noise, which limits modem speed. Quantization noise limits the communications channel to about 35 Kbps. But quantization noise affects only analog-to-digital conversion -- not digital-to-analog. This is the key to x2: If there are no analog- to-digital conversions between the x2 server modem and the PSTN, and if this digitally connected transmitter uses only the 255 discrete signal levels available on the digital portion of the phone network, then this exact digital information reaches the client modem's receiver, and no information is lost in conversion processes. Signal-to-Noise Ratio (SNR) Signal-to-noise ratio is a measure of link performance arrived at by dividing signal power by noise power. The higher the ratio, the clearer the connection, and the more data can be passed across it. Even under the best conditions, when a signal undergoes analog-to- digital conversion, there's a 38 to 39 dB signal-to-noise ratio (the "noise floor") that limits practical V.34 speeds to 33.6 Kbps. Let's spell this out step by step: 1. The server connects, in effect, digitally to the telephone company trunk. 2. The server signaling is such that the encoding process uses only the 256 PCM codes used in the digital portion of the phone network. In other words, there is no quantization noise associated with converting analog-type signals to discrete valued PCM codes. 3. These PCM codes are converted to corresponding discrete analog voltages and sent to the client modem via an analog loop circuit - there is no information loss. 4. The client receiver reconstructs the discrete network PCM codes from the analog signals it received, decoding what the transmitter sent. Upstream and Downstream Channels: Asymmetric Operation x2 connections employ one bidirectional channel, upstream and downstream. The x2 client modem's downstream (receive) channel is capable of higher speeds because no information is lost in the digital- to-analog conversion. The x2 client modem's upstream (send) channel goes through an analog-to-digital conversion, which limits it to V.34 speeds. x2 Encoding in More Detail As discussed above, data is sent from the x2 server modem over the PSTN as binary numbers. But to meet the conditions of point two above, the x2 server modem transmits data (eight bits at a time) to the client's ADC at the same rate as the telephone network (8,000 Hz). This means the modem's symbol rate must equal the phone network's sample rate. An x2 Connection x2 Modem Connections During the training sequence, x2 modems probe the line to determine whether any downstream analog-to-digital conversions have taken place. If the x2 modems detect any analog-to-digital conversions, they will simply connect as V.34. The x2 client modem also attempts a V.34 connection if the remote modem does not support x2. The x2 client modem's task is to discriminate among the 256 potential voltages, to recover 8,000 PCM codes per second. If it could do this, then the download speed would be nearly 64 Kbps (8,000 x 8 bits per code). But, it turns out, several problems slow things down slightly. First, even though the network quantization noise floor problem is removed, a second, much lower noise floor is imposed by the network DAC equipment and the local loop service to the client's premises. This noise arises from various nonlinear distortions and circuit crosstalk. Second, network DACs are not linear converters, but follow a conversion rule (-law in North America and A-law in many other places). As a result, network PCM codes representing small voltages produce very small DAC output voltage steps whereas codes representing large voltages produce large voltage steps. These two problems make it impractical to use all 256 discrete codes, because the corresponding DAC output voltage levels near zero are just too closely spaced to accurately represent data on a noisy loop. (Note: Each network PCM code corresponds to a DAC voltage level.) Therefore, the x2 encoder uses various subsets of the 256 codes that eliminate DAC output signals most susceptible to noise. For example, the most robust 128 levels are used for 56 Kbps; 92 levels to send 52 Kbps, etc. Using fewer levels provides more robust operation, but at a lower data rate. Requirements x2 requires the following: 1. Digital at one end. One end of an x2 connection must terminate at a digital circuit, meaning a "trunk-side" channelized T1, ISDN PRI, or ISDN BRI. Line-side T1 will not work because additional analog-to-digital and digital-to-analog conversions are added. In a trunk-side configuration, once the user's analog call is converted to digital and sent through the carrier network, the call stays digital until it reaches a U.S. Robotics server modem through a T1, PRI or BRI circuit. 2. x2 support at both ends. x2 must be supported on both ends of the connection, by the client modem as well as by the remote access server or modem pool at the host end. Typically, the remote user will be using a U.S. Robotics Sportster, Courier, or Megahertz modem dialing into a U.S. Robotics MP I-modem, NETServer I-modem, Courier I-modem, or Total Control Enterprise Network Hub. 3. One Analog-to-Digital conversion. There can only be one analog-to-digital conversion in the phone network along the path of the call between the x2 server modem and the client modem. If the line is a channelized T1, it must be "trunk-side" and not "line-side." With line-side service from the phone company, there typically is an additional analog-to-digital conversion (this limitation is described below). U.S. Robotics Advantages U.S. Robotics Equipment Already Deployed Allows Digital Connections Digital modems, such as those in the Total Control Enterprise Network Hub, already process digital signals straight from digital lines. x2 drops cleanly into this configuration. U.S. Robotics server equipment can be software upgraded to x2. Companies that do not currently manufacture digital modems will need to invest considerable time and effort to develop them. U.S. Robotics Modems Being Shipped Today Are Upgradeable to x2 All U.S. Robotics products that currently support software downloads can be easily upgraded to x2. U.S. Robotics designs its own high- performance modem data pumps, using digital signal processors (DSPs). x2 upgrades are seamlessly integrated into these designs. Glossary amplitude A measure of the distance between the high and low points of a waveform. analog-to-digital A device that samples incoming converter (ADC) analog voltage waveforms, rendering them as sequences of binary digital numbers. Passing waveforms through an ADC introduces quantization noise. basic rate An ISDN line that provides up to interface (BRI) two 64-Kbps B-channels and one 16- Kbps D-channel over an ordinary two- wire telephone line. B-channels carry circuit-oriented data or voice traffic while D-channels carry call-control signals. call-control Operations associated with signaling establishing and tearing down virtual circuits through a network. For example, dialing. central office The facility at which individual (CO) telephone lines in a limited geographic area are connected to the public telephone network. digital-to-analog A device that reconstructs analog converter (DAC) voltage waveforms from an incoming sequence of binary digits. Does not in itself introduce noise. digital signal A processor that is optimized for processor (DSP) performing the complex mathematical calculations inherent in processing digital signals. A discrete DSP may be reprogrammed. A DSP integrated in a chipset typically contains its own ROM and cannot be reprogrammed. line-side T1 A T1 that undergoes at least one analog-to-digital conversion in the path between the x2 server modem and the PSTN. primary rate A four-wire ISDN line (or "trunk") interface (PRI) with the same capacity as a T1, 1.544 Mbps. PRIs contain 23 64-Kbps B-channels and one 64-Kbps D- channel. The D-channel carries call- control signaling for all the B- channels. pulse code A technique for converting an modulation (PCM) analog signal with an infinite number of possible values into discrete binary digital words that have a finite number of values. The waveform is sampled, then the sample is quantized into PCM codes. quantization The process of representing a voltage with a discrete binary digital number. Approximating an infinite valued signal with a finite number system introduces an error called quantization error. signal-to-noise A measure of link performance ratio (SNR) arrived at by dividing signal power by noise power. Typically measured in decibels. The higher the ratio, the clearer the connection. T1 A four-wire digital line (or "trunk") with the same capacity as a PRI line, 1.544 Mbps. T1s contain 24 DS0s, each of which carries 56 Kbps (call-control signaling is carried within the DS0). trunk-side T1 A T1 line that has a direct digital connection to the phone network, and therefore undergoes no analog conversions in the path between the x2 server modem and the PSTN. x2 client modem A modem equipped with x2 software that is attached to a standard analog telephone line. In order to connect at x2 speeds (32-56 Kbps), the device at the other end of the connection must be an x2 server modem that is attached to a trunk- side T1, BRI, or PRI line. x2 server modem A digital modem equipped with x2 software that is attached to a trunk-side T1, BRI, or PRI line. Client modems must be equipped with x2 software in order to connect at x2 speeds (32-56 Kbps). Current products that can act as x2 servers include the Total Control Enterprise Network Hub, NETServer I-modem, MP I-modem and Courier I- modem.