It seems by this that our Couriers can be made into these 56k modems

with a flash rom upgrade. (At least the US/Canada Domestic ones)

RRRrrripper..



=============================================================================

* Forwarded by Russell Brooks (3:640/305.55)

* Area : USR_MODEMS

* From : Clay Tinsley, 1:124/5125 (16-Oct-96  16:06:50)

* To   : All

* Subj : new USR x2 modems

=============================================================================

This is from a USR web page:





x2 is a new transmission scheme developed 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.)



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).



       --Graphic 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 (see the graphic, "An x2

     Connection" below).

  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.



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 end's DAC 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.



           -- Graphic: 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 (m-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.



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 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 converter (ADC) A device that samples incoming

analog voltage waveforms, rendering them as sequences of binary

digital numbers. Passing waveforms through an ADC introduces

quantization noise.



basic rate interface (BRI) An ISDN line that provides up to 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 signaling Operations associated with establishing and

tearing down virtual circuits through a network. For example,

dialing.



central office (CO) The facility at which individual telephone lines

in a limited geographic area are connected to the public telephone

network.



digital-to-analog converter (DAC) A device that reconstructs analog

voltage waveforms from an incoming sequence of binary digits. Does

not in itself introduce noise.



digital signal processor (DSP) A processor that is optimized for

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 interface (PRI) A four-wire ISDN line (or "trunk") 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 modulation (PCM) A technique for converting an 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 ratio (SNR) A measure of link performance 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.



--- Terminate 4.00/Pro