WO1998004060A1

WO1998004060A1 - Rate adaptive digital subscriber line ('radsl') modem

Info

Publication number: WO1998004060A1
Application number: PCT/US1997/009443
Authority: WO
Inventors: Ehud Landberg; William Henry Scholtz
Original assignee: Globespan Semiconductor, Inc.
Priority date: 1996-07-19
Filing date: 1997-06-02
Publication date: 1998-01-29
Also published as: TW362206B; US5751701A; US6167034A

Abstract

The invention provides a transceiver (12, 14) which is preferably used for high speed communications between a customer location and a central office over a subscriber line. The transceiver uses a peudorandom noise generator (38) both to provide a correlation for a complementary transceiver to use in achieving synchronization and as a way to transmit data in an extremely robust fashion. In addition, the transceiver uses frequency domain multiplexing so that downstream data and upstream data are transmitted by their respective transceivers in completely separate and distinct frequency bands. In order to achieve high data transfer rates, the transceivers can operate in an asymmetrical manner, whereby either (but, typically, the central office) transceiver can transmit data at a higher rate than the other transceiver.

Description

RATE ADAPTIVE DIGITAL SUBSCRIBER LINE ("RADSL") MODEM

Background of the Invention

The present invention relates to a high speed modem. In particular, the

present invention relates to a modem which is intended for use between a

subscriber's location and a telephone company central office.

Modems have been used for many years to modulate and demodulate

digital signals for transmission over telephone lines. As the demand for higher

and higher transmission speeds have increased, newer technologies have been

used to provide modems which are able to more effectively meet such demands.

With data transfer speeds ever on the increase, problems have developed

in achieving the highest possible data transmission speeds in specific situations.

Summary of the Invention

In accordance with the preferred embodiment of the invention, a modem

has been developed which is able to use Plain Old Telephone Service ("POTS")

to provide bit rates ranging from approximately 600 kilobits/second ("kb/s") to

speeds greater than 7 Megabits/second ("MB/s") from the network (central

office) to the customer premises, and from approximately 136 kb/s to at least 1

Mb/s from the customer premises to the network. The present invention is designed to operate on single twisted metallic

cable pairs which extend between the customers premises and the network.

Brief Description of the Drawing

In the Drawing:

FIG. 1 illustrates a schematic diagram of a typical system of the type used

in the present invention;

FIG. 2 illustrates the frequency domain multiplexing which is used by the

system in accordance with the present invention;

FIG. 3 is a diagram illustrating the synchronization signals received by the

receiver in response to the receiver's receipt of the PN signal transmitted by the

transmitting transceiver when communications are established.

FIG. 4 is a block diagram of the startup handshake/data signal generator of

the present invention;

FIG. 5 is a flow chart illustrating the startup procedure used by the

preferred embodiment of the transmitting transceiver of the present invention;

FIG. 6 is a flow chart illustrating the startup procedure used by the

preferred embodiment of the receiving transceiver of the present invention; FIG. 7 is a block diagram of the auto-baud/fixed baud state diagram in

accordance with the preferred embodiment of the present invention;

FIG. 8 is a flow chart illustrating the procedure used by the preferred

embodiment of the invention for performing auto-baud;

FIG. 9 illustrates the constellation used during the initial handshake; and

FIG. 10 is a high level depiction illustrating the startup signals on the

subscriber line.

Detailed Description of the Preferred Embodiment of the Invention

In accordance with the present invention, a system 10 employing Rate

Adaptive Digital Subscriber Line ("RADSL") transceivers 12, 14 based upon the

Carrierless AM/PM ("CAP") modulation scheme is illustrated in FIG. 1. As

shown, there is a RADSL modem, or transceiver 12, located at a customer's

location, and a complementary RADSL transceiver 14 located at the central

office of the local telephone company. The transceivers 12, 14 are connected

together over a communications channel 16 which generally comprises a standard

twisted pair of metal wires, of the type long used in Plain Old Telephone Systems

("POTS"). In accordance with the invention, the transceivers 12, 14 are able to

communicate digital data between their respective locations at extremely high

data rates. In particular, bit rates ranging from approximately 600 kb/s to

approximately 7 MB/s can be achieved for data transferred from the network

(central office) to the customer premises, and bit rates of approximately 200 kb/s

to 1 Mb/s can be achieved for data transferred from the customer premises to the

network. The foregoing is accomplished by use of a number of techniques,

described herein, which enable the data to be modulated and transmitted over the

POTS system.

Referring to FIG. 2, one technique used by the present invention is to

separate the bandwidth of the line whereby the actual data being transmitted

"upstream", i.e., from the customer's location to the network, is transmitted in a

first band 18, having a frequency which runs upward from/y, while the data

which is transmitted "downstream", i.e., from the network to the customer's

location, is transmitted in a second, higher frequency band 20, which has a

frequency which runs upward from^. As illustrated in FIG. 2, the highest

frequency in the upstream band 18 is separated (i.e., lower than) from the lowest

in the downstream band 20. As further illustrated in FIG. 2, the lowest frequency,/;, in the upstream band 18 is separated from, and higher than,

the typical POTS frequency of 4 kHz.

In view of the above, it can be seen that in accordance with the invention,

the transceivers 12, 14 of FIG. 1 are each transmitting over the communications

channel 16 in different frequency ranges 18, 20, as illustrated in FIG. 2. Further,

in accordance with the present invention, as the ability to achieve the highest

possible data transfer rates is a particular objective of the invention, it is not

necessary to have both of the transceivers 12, 14 transmitting at the same rate.

Accordingly, it is within the scope of the invention to have either the customer's

transceiver 12, or (more typically) the network transceiver 14 transmitting at a

higher data rate. Thus, it is within the scope of the invention to have the

transceivers 12, 14 provide for asymmetrical data transmission rates.

An objective, therefore, of the present invention is to provide a means by

which the transceivers 12, 14 can establish a reliable connection with one another

while maximizing their ability to reliably transmit data at the highest possible

rates. With the foregoing objective in mind, the present invention provides a

means by which the transceivers 12, 14 can initialize communications and

establish the highest possible data transmission rates over the communications channel 16, which is comprised of a metal twisted pair which extends between

the customer's location and the network.

To accomplish this goal, the present invention requires a robust scheme

for providing the initial start-up of communication between the transceivers 12,

14. The intent of this start-up procedure is to allow the transceivers 12, 14 to

establish their initial handshake, perform rate negotiation, provide a symbol/baud

rate, and transfer such other information as fast, reliable communications require.

As is understood by those skilled in the art, the term "constellation" refers

to a mechanism by which a data bit stream can be converted into symbols for

transmission over the communications channel 16. The "size" of the

constellation is determined by the number of bits which can be encoded into each

symbol, while the largest size constellation which can be transferred over the

communications channel 16 is determined by the quality of the communications

channel 16.

From the perspective of bit stream transfer, the higher the symbol rate

which can be transferred over the communications channel, for a given number

of bits/symbol, the higher the data transfer rate over the channel. Alternatively,

the larger the constellation (meaning the greater the number of data bits which

are encoded, on a per symbol basis), the higher the data transfer rate over the channel. Thus, if a particular channel 16 is able to support both a high symbol

rate and a large constellation, a high data transfer rate will be supported on the

channel 16. Accordingly, it is an objective of the present invention to determine

both the highest symbol rate and the largest constellation which can be reliably

supported on the channel 16.

In order to accomplish these goals, a handshake/data signal generator 22,

illustrated in FIG. 4 is used to perform the rate negotiation of the RADSL

transceivers 12, 14 of the present invention. In particular, the invention employs

a RADSL transceiver which is able to support a predetermined, finite set of

constellations and baud rates, one of which will be selected for each startup. In

performing the startup sequence, there is an initial training sequence which uses

an extremely robust scheme to assure detection. This is needed, because at

startup the receiver and the timing recovery function are not yet operational. The

scheme selected for use in the preferred embodiment of the invention uses a

binary pseudorandom number ("PN") sequence which is spread over a large

number of symbols. In the preferred embodiment of the invention, the PN is 63

symbols long, although other values (preferably of the form, 2ⁿ -1, may be used).

Since the information in the PN is spread over many symbols, detection is very

reliable and robust, so can be made without the need for adaptive filters or timing recovery. The benefit of using the PN is that a synchronization detection device,

in the receiver, can use a correlator, which matches the synchronization signal,

and yields distinctive signal characteristics which can be used to differentiate the

synchronization signal from the data signal in noisy environments. In the present

invention, this feature is used to provide a particular training signal to the

receiver.

The handshake of the present invention is highly robust in that it is

preferably based upon a PN having a length of 63 symbols. Thus, while it is

relatively time consuming to send 63 symbols to represent a single "1" bit, the

long PN sequence allows the receiving transceiver 14 to use a correlator to

uniquely identify that the transmitting transceiver 12 has, in fact, sent a "1".

Similarly, in accordance with the preferred embodiment of the invention, in order

for the sending transceiver 12 to send a "0" bit, it will send 63 zero symbols. By

using this method, and by performing it at a low baud rate, there is a very low

error rate, irrespective of the quality of the communications channel 16.

This ability of the synchronization detection device in the receiving

transceiver 14 to differentiate the synchronization signal from the data signal in a

noisy environment is based upon the use of the PN sequence in the header of the

transmitted signal. When such a PN sequence is repeated at the transmitter and correlated at the receiver with a copy of the non-repeated pattern, the correlator

produces a high value (peak) when there is a match and low values (quiet zone)

otherwise. Since the sequence is repeated, information known, a priori, about the

periodicity and the width of the peaks and quiet zones can be used to enhance

detection reliability. Accordingly, the particular synchronization pattern used,

has distinctive properties in its autocorrelation function, so its use preferably

improves the synchronization reliability of the system. This feature of the

invention is the reason for using the PN in the header.

When such a sequence of length, N, with symbols assuming the values,

" 1 " and "- 1 " (which substitutes for "0"), is repeated at the transmitter and

correlated at the receiver, which uses a stored copy of the sequence with values,

"1" and "0", the correlator output yields a high peak when the sequence is

matched and a low value otherwise. It is also possible to use the sequences of

"1" and "0" values at the transmitter and correlate with a stored copy of a

sequence of "1" and "-1" (which substitutes for "0") values at the receiver.

With reference to FIG. 3, the "wake up" sequence used by the transmitter

is illustrated to be a continuous PN header. The correlator in the receiver

continuously looks for a "1", which corresponds to the recognition of the arrival

of the PN. Upon receipt of a valid PN signal, as illustrated at 102, 104, 106, the receiver continues to look for either a "1 ", corresponding to the receipt of another

PN signal following a time period indicated by the PN window 108. As there is

assumed to be no synchronization between the transmitter and the receiver, the

receiver looks for a high signal, representative of a "1" bit during a time period

represented by the peak detect windows 110, 112, 114, 116, 118. As illustrated,

these peak detect windows 110, 112, 114, 116, 118 are time periods at the end of

a PN window during which a bit is expected. Consequently, if the correlator

indicates the presence of a high signal, as shown at 102, 104, and during the peak

detect windows 110, 114, 116, 118, then a "1 " is recognized by the receiver.

Alternatively, if there is no high signal present, as illustrated in the peak detect

window 112, then a "0" bit is presumed to have been sent.

Due to the fact that data is sent in bytes, in a so-called "trailer" 120, the

present invention uses a particular data format in the preferred embodiment so

that the receiver can both receive data of unspecified length, and remain

synchronized with the transmitter, using only the correlator, and without actual

synchronization. With continued reference to FIG. 3, the format of the trailer

information is preferably in the form of 8-bit bytes which follow the "wake up"

signal 100. The indication to the receiver that the "wake up" signal 100 has

ended and that trailer data is about to be sent is the presence of a "0" bit ("start bit"), as illustrated at 112. Following the transmission of the start bit, ST, the

transmitter will send data bits in 8-bit bytes. As the receiver has no way of

knowing (in advance) how many data bytes will be sent, in the preferred

embodiment of the invention, every eighth bit (i.e., bits I₇, 1₁₅, 1₂₃, 1₃j, ...) will be

used as a "continuation bit", and it will be set to either a "1", to indicate diat

another data byte is to be sent, or to a "0", to indicate that the byte just sent was

the last byte to be sent. Following the transmission of the last data byte, a parity

bit and two stop bits are transmitted in the preferred embodiment of the

invention, which happens to use odd parity.

A particular advantage of using a " 1 " as the continuation bit is that its

presence may be used by the receiver to keep the receiver in synchronization with

the transmitter by restarting the PN window, and thereby recentering the peak

detect window around the expected time of receipt of the continuation bit, upon

the receipt of each continuation bit.

While the foregoing explains the manner of performing both

synchronization and data transmission during the start up handshake in

accordance with the preferred embodiment of the invention, those skilled in the

art will recognize that other techniques could be used.

π In accordance with the invention, the handshake starts with a transmitter

operating at the low predetermined baud rate which has previously been

established. The low baud rate provides the maximum loop reach for the

transmitter. The use of a common, low baud rate guarantees the compatibility

with other RADSL products.

While other information can be negotiated, in accordance with the

preferred embodiment of the invention, the first thing which two transceivers 12,

14 negotiate is the symbol rate which they will use. Since the constellation size

(i.e., its bit/baud, or bit/symbol size) does not effect the frequency placement or

the bandwidth the transmitter is using, the selection is preferably negotiated at the

end of the startup using receive signal to noise ratio measurements.

Modems of the prior art, such as CCITT standard V.34 modems,

initialized communications with one another by first listing which the baud rates

they supported. Then they went into a test mode to determine which of the

supported baud rates was the highest one which could be used for each new

connection. They perform this testing anew each time a connection is made.

On the other hand, a transceiver 12 built in accordance with the present

invention negotiates its symbol rate by first sending a list of the symbol rates

which it can support. Then it sends a request to use a particular one of the supported baud rates, but no testing is performed. This can be accomplished by

the modems 12, 14 of the present invention, as they are always being used to

connect to each otiier over the same line 16. Due to the fixed line 16 between the

modems 12, 14, once testing has been accomplished, and the test results have

been stored, there is generally no need to retest on each new connection.

Another difference between the modems 12, 14 of the present invention,

and those known in the prior art is that during the initial exchange of all available

baud rates the modems 12, 14 each learn of the other's capabilities. Accordingly,

in the event that it becomes necessary to change baud rates due to changes in the

line condition, the other transceiver 14 will already know the baud rates which

the transceiver 12 can support. Thus, if the modems 12, 14 recognize that they

are experiencing a very low error rate over a suitable time period, they can agree

to go to a higher baud rate, if they are already aware that a higher, commonly

available baud rate is supported. Alternatively, they can go to a lower, commonly

supported baud rate if line conditions indicate that too many errors have occurred

during some period.

As training progresses, e.g., following the initial negotiation of baud rate,

the number of bits/symbol (i.e., bits/baud), as well as other parameters, including

Tomlinson precoder coefficients and forward error correction parameters, such as frame size and interleaving depth, which are used by Reed-Solomon ("RS")

codes, are negotiated, as will be understood by those skilled in the art. Other

data, such as the error count and the signal to noise ratio accumulated during the

last startup period in which auto-baud testing (see below) was used, can also be

transferred, and it is transferred in the preferred embodiment of the invention.

In accordance with the preferred embodiment of the invention, the

transceivers 12, 14 can operate either in fixed mode, or in auto-baud mode. The

use of auto-baud mode is determined by an initial request made by either modem

12, 14 when communications are established. The determination of which mode

to select is included in the trailer data of the initial entry. As used herein, when a

transceiver 12 operates in fixed mode the transceiver 12 requests a single baud

rate to establish the connection, although, as always, all of the available baud

rates are preferably listed. The lowest common symbol rate which exists between

the transceivers 12, 14 will be used. In auto-baud mode, on the other hand, at the

end of the startup handshake, the transceivers 12, 14 will go into a timed testing

mode in which the signal quality and error rate are measured for all possible

configurations. The results are then stored and used to request a fixed baud rate

connection. A feature of the present invention is that optimum constellation, RS frame

size, and interleaving depth can be determined at the end of the receiver training

period and before startup is completed.

Yet another feature, which is of commercial interest to the network owner

is the ability to limit the maximum rate at which the transceiver 14 will operate

based upon subscription options selected by particular customers. For example,

if a customer's transceiver 12 is capable of receiving at 7 Mb/s, but the customer

has selected only 1 Mb/s service, then the network owner can program its

RADSL transceiver 14 to provide no more than 1 Mb/s service to the transceiver

12.

Referring now to FIGS. 4, 5, and 10, the handshake/data signal generator

22 of the present invention, and its operation, will be explained in greater detail.

The handshake/data signal generator 22 accepts serial data into a scrambler 24 on

a data line 26. The output of the scrambler 24, which is used to randomize the bit

stream before the bits are mapped into symbols, goes through a bit to symbol

mapping process 28. The specific bit to symbol mapping which is used is

determined by the startup state machine 30 which also controls various other

devices, such as the multiplexers 32, 34, and the symbol/baud and clock

generator 36. As explained above, during the initial handshake, a PN is used. The PN is

generated by a PN symbol generator, which provides one of the possible inputs

into a multiplexer 40. Also, as explained above, a zero symbol generator 42,

represented by the "0" symbol provides the other possible input into the

multiplexer 40. The input, whether a "1", i.e., the PN, or a "0", from the zero

symbol generator 42, is at least 63 symbols long in the preferred embodiment of

the invention. The data which is actually sent by the multiplexer 40 is

determined by the initial handshake and rate negotiation which the RADSL

transceiver performs. The output of the multiplexer 40 provides an input into the

multiplexer 32, whose output, in turn, is an input into the multiplexer 34.

As shown, the output of multiplexer 34 goes through a filter 44 and a

digital to analog ("D/A") converter 46, from which it exits as an analog transmit

signal on line 48.

The foregoing illustrated how the initial negotiation takes place. Once

that has been accomplished, the final handshake negotiation exchange, in which

Tomlinson precoder coefficients, forward error correction parameters (e.g., RS

codes), final baud rate, and constellation settings are transmitted, are

accomplished using the multiplexer 50, whose output also goes to multiplexer 32,

and then ultimately out onto the line 48 as an analog signal. The inputs to multiplexer signal, shown as "A", "B", "C", and "D", are representative of the

data which is sent during the final handshake. Note, that the initial handshake,

for purposes of maintaining a robust system, uses the constellation illustrated in

FIG. 9 for data exchange, as it is highly error resistant.

With reference to FIG. 5, a flowchart illustrating the initial handshake

procedure is shown. In order to accomplish the robustness of the invention, a

continuous PN CAP signal is sent. In accordance with the preferred embodiment

of the invention, a 63 symbol PN is sent to represent a data bit of "1", while a 63

symbol "0" symbol is sent to represent a data bit of "0". Thus, information is

transmitted for the initial handshake/rate negotiation by a relatively slow, but

highly robust procedure. As used in the preferred embodiment of the present

invention, and as illustrated in FIG. 5, data is transmitted using the coding

scheme described to transmit bits which make up bytes. When long pieces of

data (information bytes) must be transmitted, a scheme employing a continuation

bit (a "1") is sent as a so-called "continuation" bit, whose presence indicates that

additional trailer information is forthcoming. If it is not necessary to send trailer

information, then, at the completion of sending the current byte, a "0" bit,

followed by a parity bit and two stop bits are preferably sent (to indicate the end

of the trailer information) in lieu of another continuation bit . Of course, the particular encoding scheme, while preferred, could be changed without departing

from the overall scope of the invention.

With reference to FIG. 6, the operation of the receiving transceiver 14 is

illustrated in a flow chart. As illustrated, the transceiver 14 continuously

searches for a sequence corresponding to a PN. Upon detecting the wake up

sequence using the PN correlator, the transceiver 14 looks for a start bit, and

follows a procedure which is complementary to that set forth above, the details of

which are set forth in the flow chart of FIG. 6.

Referring now to FIGS. 7 and 8, the auto-baud operation of a transceiver

12 (at the customer's location, CP) and a transceiver 14 (at the central office,

CO) are shown. The transceivers 12, 14, are controlled by state machines 60

which control data transmission and reception through multiplexers 62. Test

generators 64 which supply inputs to the multiplexers 62, are used, as will be

explained with reference to the flow chart of FIG. 8, to generate data which is

used by the other transceiver to detect errors using an error rate detector 66. The

state machines 60 are able to collect data from the error rate detectors 66 and

store that data in an appropriate memory storage means 68.

With reference to the flow chart of FIG. 8 the auto-baud test, in

accordance with the preferred embodiment of the invention, is described. As illustrated in the flow chart, if an auto-baud test is requested, information about

the length of the test will be transmitted. Assuming that the test is successfully

completed, the highest allowed baud and optimum constellation for the user

programmable noise margin will be selected. As illustrated in the flow chart, if

no auto-baud test is selected, the results which were stored when the last, prior

auto-baud test will be used. Alternatively, if no auto-baud test was ever

completely performed, then the transceivers will start using the lowest baud, or,

alternatively, by using a user programmed baud rate.

While the present invention has been described with reference to the use

of a CAP system, those skilled in the art will recognize that a quadrature

amplitude modulation ("QAM") system could also be used without departing

from the spirit or scope of the invention. As explained, the PN is used in the

present invention to actually send data, as well as for the initial synchronization.

This encoding of actual data onto the PN, as explained, is relatively slow, as it

takes 63 symbols (in the preferred embodiment) to send a single bit. However,

this makes the current system extremely robust. To further enhance the operation

of the present invention, during the initial negotiation, the transceivers exchange

information about themselves, including both the desired symbol rates, and the list of available symbol rates. This negotiation, is based entirely upon the trailer,

rather than on line testing.

Yet another feature of the present invention, not heretofore used, is that

the transceivers use frequency division multiplexing, to separate the upstream and

downstream data, together with symbol negotiation.

Claims

I Claim:

1. A rate adaptive digital subscriber line ("RADSL") transceiver

system for use between a customer location and a central office,

comprising:

(a) frequency division multiplexer means for providing

frequency division multiplexing, whereby upstream data, from the

customer to the central office is all placed within a first frequency range,

and downstream data, from the central office to the customer location, is

placed into a second frequency range, there being no overlap between

said first frequency range and said second frequency range; and

(b) rate negotiation means for providing rate negotiation

between a customer transceiver, located at the customer location, and a

central office transceiver, located at the central office, said rate

negotiation means utilizing a pseudorandom noise (("PN") generator for

generating a PN, said PN being used for both correlation and data

transmission.

2. The transceiver system as defined in claim 1, wherein the first and

second frequency range are each defined by lower and upper frequency

limits.

3. The transceiver system as defined in claim 2, wherein the lower

frequency limit of the first frequency range is greater than four kilohertz.

4. The transceiver system as defined in claim 2, wherein the lower

frequency limit of the second frequency range is greater than the upper

frequency limit of the first frequency range.

5. The transceiver system as defined in claim 1, wherein the first and

second frequency ranges are dynamically definable, and are established

in response to the rate negotiation means.

6. The transceiver system as defined in claim 1 , wherein the pseudo¬

random noise sequence utilizes sixty-three symbols to represent a single

bit.

7. The transceiver system as defined in claim 1 , wherein the

communication between the customer location and the central office is

performed in accordance with a carrierless AM/PM ("CAP") modulation

scheme.

8. The transceiver system as defined in claim 1 , wherein the

communication between the customer location and the central office is

performed in accordance with a quadrature amplitude modulation

("QAM") scheme.

9. A method for establishing a high-speed data communication

connection over a plain old telephone service ("POTS") between a

central office transceiver and a remote transceiver by executing an initial

training sequence comprising the steps of: utilizing a common, relatively low symbol rate;

negotiating a symbol rate by first sending from the remote

transceiver to the central office transceiver a list of symbol rates

supported by the remote transceiver, then receiving from the central

office transceiver a list of symbol rates supported by the central office

transceiver, then sending a request for a specific symbol rate;

negotiating additional communicating parameters;

determining the optimum constellation and interleaving depth; and

setting the constellation and interleaving depth to the determined,

optimum values.

10. The method as defined in claim 9, wherein the additional

communicating includes one or more parameters selected from the group

consisting of:

Tomlinson precoder coefficients;

Reel-Solomon forward error correcting codes.

11. The method as defined in claim 9, further including the step of

ascertaining subscription options selected by particular customers.

12. The method as defined in claim 1 1, further including the step of

limiting the maximum rate at which the central office transceiver

operates based upon the selected subscription options.

13. The method as defined in claim 9, wherein the communication

between the transceivers is performed in accordance with a carrierless

AM/PM ("CAP") modulation scheme.

14. The method as defined in claim 9, wherein the communication

between the transceivers is performed in accordance with a quadrature

amplitude modulation ("QAM") scheme.

15. The method as defined in claim 9, further including the step of

negotiating a bit-per-symbol quantity.

16. A method for establishing a high-speed data communication

connection over a plain old telephone service ("POTS") between a

central office transceiver and a remote transceiver comprising the steps

of:

executing an initial training sequence;

executing an automatic symbol rate startup sequence, including the

step of running a bit error rate test and examining a signal quality

parameter from the transceiver, to establish the maximum supportable

symbol rate that the transceivers can support;

controlling the transceivers, in response to the automatic symbol

rate sequence, to communicate at the maximum supportable symbol rate;

executing an automatic constellation startup sequence at the

controlled, highest symbol rate, to identify the largest signal constellation that sustains a predetermined signal quality parameter

margin; and

setting the constellation size to the identified largest constellation.

17. The method as defined in claim 16, wherein the step of executing

an initial training sequence further comprises the steps of:

utilizing a common, relatively low symbol rate;

negotiating a symbol rate by first sending from the remote

transceiver to the central office transceiver a list of symbol rates

supported by the remote transceiver, then receiving from the central

office transceiver a list of symbol rates supported by the central office

transceiver, then sending a request for a specific symbol rate;

negotiating additional communicating parameters; and

determining and setting the optimum constellation and interleaving

depth,

18. The method as defined in claim 16, wherein signal quality

parameter includes the signal-to-interference ratio.

19. The method as defined in claim 18, wherein the interference

includes one or more items from the group consisting of: background

noise, crosstalk, residual intersymbol interference, residual echo from a

neighboring channel, and signal distortion.

20. A method of transmitting a message of variable and undefined

length over a plain old telephone service ("POTS") between a central

office and a remote transceiver comprising the steps of:

(a) transmitting a header, including a pseudo-random noise

sequence generator, to provide an initial synchronization signal;

(b) transmitting a start bit to signal the beginning of the

message transmission;

(c) transmitting the message one bit at a time;

(d) transmitting a "1" at periodic intervals throughout the

message, the "1" indicating a continuation bit, the "1" being transmitted

for a duration equal in length to the pseudo-random noise sequence;

(e) resynchronizing a receiver upon receipt of each " 1 "

received; and

(f) indicating the end of the message transmission by

transmitting a "0", the "0" being transmitting for a duration equal in

length to the pseudo-random noise sequence.

21. The method as defined in claim 20, wherein the periodic interval is

every eighth bit.