56K or bust

Modem speeds have reached 33,600-baud rate from 9,600 in a short period of time as a result of manufacturers continually...

Modem speeds have reached 33,600-baud rate from 9,600 in a short period of time as a result of manufacturers continually improving compression techniques. Users, however, are still demanding faster transfer rates but how are manufacturers achieving greater data transfer speed?

V.90, a data transmission recommendation developed by Study Group 16 of the International Telecommunications Union (ITU), provides a specification for achieving line speeds of up to 56 Kbps. This paper explains V.90 in detail.

V.90 technology allows modems to receive data at up to 56 Kbps over the standard public switched telephone network (PSTN). V.90 overcomes the theoretical limitations imposed on standard analog modems by exploiting the digital server 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 the remote site to the telephone company's central office (CO). Over the past two decades, local 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 the home to the CO. That connection will likely remain analog for some years to come.

A software upgrade converts a service provider's Total Control remote access concentrator, SuperStack II Remote Access System 1500, with Universal Connect technology, NETServer I-modems or US Robotics MP I-modems to V.90 operation. 3Com calls modems that have direct digital connection to the PSTN V.90 "digital modems". Likewise, converting a US Robotics Courier V.Everything analog modem to a V.90 analog modem is as simple as downloading new software.

V.34 encoding in more detail

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.

V.34 modems are optimized for the situation where both ends connect by analog lines to the PSTN. 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).

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 pulse code modulation (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 can only approximate the original analog waveform. The difference between the original waveform and the reconstructed quantized waveform is called quantization noise. This limits modem speed.

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 is a 38 to 39 dB signal-to-noise ratio (the "noise floor") which limits practical V.34 speeds to 33.6 Kbps.

Upstream and Downstream Channels: Asymmetric Operation V.90 connections employ one bi-directional channel upstream and downstream. The V.90 analog modem's downstream (receive) channel is capable of higher speeds because no information is lost in the digital-to-analog conversion. The V.90 analog modem's upstream (send) channel goes through an analog-to-digital conversion, which limits it to V.34 speeds.

V.90 modem connections

During the training sequence, V.90 modems probe the line to determine whether any downstream analog-to-digital conversions have taken place. If the V.90 modems detect any analog-to-digital conversions, they will simply connect as V.34. The V.90 analog modem also attempts a V.34 connection if the remote modem does not support V.90.

V.90 encoding in more detail

Quantization noise limits the V.34 communications channel to about 35 Kbps. But quantization noise affects only analog-to-digital conversion-not digital-to-analog. This is the key to V.90: if there are no analog-to-digital conversions between the V.90 digital 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 analog modem's receiver and no information is lost in the conversion processes.

Here's how the process works:

The server connects, in effect, digitally to the telephone company trunk

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

These PCM codes are converted to corresponding discrete analog voltages and sent to the analog modem via an analog loop circuit, with no information loss

The client receiver reconstructs the discrete network PCM codes from the analog signals it received, decoding what the transmitter sent

Data is sent from the V.90 digital modem over the PSTN as binary numbers. But to meet the conditions of step 2 above, the V.90 digital modem transmits data (eight bits at 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.

The V.90 analog 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 that 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 digital-to-analog converter (DAC) equipment and the local loop service to the client's premises. This noise arises from various non-linear distortions and circuit crosstalk.

Second, network DACs are not linear converters, but follow a conversion. 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 V.90 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 and so on. Using fewer levels provides more robust operation, but at a lower data rate.

V.90 requirements

V.90 requires the following three conditions for full 56 Kbps transmission:

Digital at one end

Today, most service providers have digital connections to the PSTN. One end of a V.90 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 digital modem through a T1, PRI or BRI circuit.

V.90 support at both ends

V.90 must be supported on both ends of the connection, by the analog modem as well as by the remote access server or modem pool at the host end. Typically, the remote user will be using a 3Com Courier, US Robotics, Megahertz or other brand V.90 modem dialing into a 3Com US Robotics MP I-modem, NETServer I-modem, Courier I-modem, SuperStack II Remote Access System 1500, Total Control remote access concentrator or other brand V.90 digital modem.

One analog-to-digital conversion

There can be only one analog-to-digital conversion in the phone network along the path of the call between the V.90 digital modem and the analog 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 is typically an additional analog-to-digital conversion.

3Com x2 Technology vs. 3Com V.90 Technology

Until recently, proprietary implementations were the only options for 56 Kbps access. However, in February 1998, the ITU reached a determination for 56 Kbps technology, providing for one universally compatible solution-the V.90 standard. V.90 solution will remain compatible with proprietary transmission schemes for 56 Kbps access, x2 technology. All 3Com x2 modems, both client and server, will continue to support x2 technology when they are upgraded to V.90. Users who do not upgrade to the new standard will be able to connect to digital modems with 3Com's x2 technology for high-speed downloads. Client x2 modems that are not upgraded to the standard will receive a V.34 connection when they call a digital modem that was originally K56flex, even if it has been upgraded to the standard.

Paul Phillips

This was first published in July 1999



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