U.S. patent application number 11/156537 was filed with the patent office on 2005-10-27 for fast initialization using seamless rate adaptation.
This patent application is currently assigned to Aware, Inc.. Invention is credited to Tzannes, Marcos C..
Application Number | 20050238091 11/156537 |
Document ID | / |
Family ID | 22996746 |
Filed Date | 2005-10-27 |
United States Patent
Application |
20050238091 |
Kind Code |
A1 |
Tzannes, Marcos C. |
October 27, 2005 |
Fast initialization using seamless rate adaptation
Abstract
A method for initializing modems in a multicarrier transmission
system to establish a communication link between the transmitter
and the receiver. An exemplary embodiment includes the steps of
providing a predetermined parameter value that approximates a
corresponding actual parameter value of the communication link,
establishing a data communication link between a first transceiver
and a second transceiver using the predetermined parameter value to
allow the transmission of data, determining the actual parameter
value, and seamlessly increasing the data rate of the established
data communication link by using the determined actual parameter
value to provide an steady state communication link with an updated
data rate.
Inventors: |
Tzannes, Marcos C.; (Orinda,
CA) |
Correspondence
Address: |
MILES & STOCKBRIDGE PC
1751 PINNACLE DRIVE
SUITE 500
MCLEAN
VA
22102-3833
US
|
Assignee: |
Aware, Inc.
Bedford
MA
|
Family ID: |
22996746 |
Appl. No.: |
11/156537 |
Filed: |
June 21, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11156537 |
Jun 21, 2005 |
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10760495 |
Jan 21, 2004 |
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10760495 |
Jan 21, 2004 |
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10459535 |
Jun 12, 2003 |
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10459535 |
Jun 12, 2003 |
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10046192 |
Jan 16, 2002 |
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6654410 |
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60262240 |
Jan 16, 2001 |
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Current U.S.
Class: |
375/222 |
Current CPC
Class: |
H04L 1/0009 20130101;
Y02D 30/50 20200801; Y02D 50/10 20180101; H04L 5/0007 20130101;
H04L 27/2608 20130101; H04L 5/0046 20130101; H04L 1/0015 20130101;
H04M 11/062 20130101; H04L 2012/5652 20130101; H04L 1/0071
20130101; H04L 1/0025 20130101; H04L 5/1438 20130101; H04L
2012/5665 20130101; H04L 1/0002 20130101; H04L 27/2601 20130101;
H04L 27/2655 20130101; H04L 1/0007 20130101; H04L 5/006 20130101;
H04L 1/004 20130101; H04L 5/0094 20130101; H04L 12/5601 20130101;
H04L 2012/5672 20130101; H04L 5/1446 20130101 |
Class at
Publication: |
375/222 |
International
Class: |
H04B 001/38 |
Claims
What is claimed:
1. A method to establish a communication link between the
transceivers comprising: providing at least one predetermined
parameter value, having an associated first data rate, that
approximates at least one corresponding actual parameter value of
the communication link; establishing a data communication link
between the transceivers using the at least one predetermined
parameter value as an approximation of the at least one actual
parameter value of the communication link to allow transmission of
data between the transceivers at the first data rate; determining
the actual parameter value, associated with a second data rate,
corresponding to the at least one predetermined parameter value
after establishing the data communication link using the
predetermined parameter value; and seamlessly adapting the first
data rate of the established communication link to the second data
rate.
2. A system for establishing a communication link comprising: means
for providing at least one predetermined parameter value, having an
associated first data rate, that approximates at least one
corresponding actual parameter value of the communication link;
means for establishing a data communication link between the
transceivers using the at least one predetermined parameter value
as an approximation of the at least one actual parameter value of
the communication link to allow transmission of data between the
transceivers at the first data rate; means for determining the
actual parameter value, associated with a second data rate,
corresponding to the at least one predetermined parameter value
after establishing the data communication link using the
predetermined parameter value; and means for seamlessly
transitioning from the first data rate of the established
communication link to the second data rate.
Description
RELATED APPLICATION DATA
[0001] This application is a divisional application of U.S.
application Ser. No. 10/459,535 entitled "Fast Initialization Using
Seamless Rate Adaptation," filed Jun. 12, 2003 which is a
Divisional of U.S. application Ser. No. 10/046,192, entitled "Fast
Initialization Using Seamless Rate Adaptation," filed Jan. 16, 2002
which claims the benefit of and priority to U.S. Provisional
Application Ser. No. 60/262,240, filed Jan. 16, 2001, entitled
"Fast Initialization Using Seamless Rate Adaptation," and is
related to U.S. patent application Ser. No. 09/522,870, filed Mar.
10, 2000, entitled "A Method for Seamlessly Changing Power Modes
and ADSL Systems," U.S. patent application Ser. No. 09/522,869,
filed Mar. 10, 2000, entitled "Seamless Rate Adapted Adaptive
Multicarrier Modulation System and Protocols," U.S. patent
application Ser. No. 09/523,086, filed Mar. 10, 2000, entitled "A
Method for Synchronizing Seamless Rate Adaptation," and U.S. patent
application Ser. No. 09/918,033, filed Aug. 1, 2001, entitled
"Systems and Methods for Transporting a Network Timing Reference in
an ADSL System," all of which are incorporated herein by reference
in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention generally relates to Digital Subscriber Line
(DSL) systems. In particular, this invention relates to a method of
initializing modems in a DSL system.
[0004] 2. Description of Related Art
[0005] Multicarrier modulation, or Discrete Multitone Modulation
(DMT), is a transmission method that is being widely used for
communication over media, and especially over difficult media.
Multicarrier modulation divides the transmission frequency band
into multiple subchannels, i.e., carriers, with each carrier
individually modulating a bit or a collection of bits. A
transmitter modulates an input data stream containing information
bits with one or more carriers and transmits the modulated
information. A receiver demodulates all of the carriers in order to
recover the transmitted information bits as an output data
stream.
[0006] Multicarrier modulation has many advantages over single
carrier modulation. These advantages include, for example, a higher
immunity to impulse noise, a lower complexity equalization
requirement in the presence of a multipath, a higher immunity to
narrow band interference, a higher data rate and bandwidth
flexibility. Multicarrier modulation is being used in many
applications to obtain these advantages, as well as for other
reasons. The applications include, for example, Asymmetric Digital
Subscriber Line (ADSL) systems, Wireless LAN systems, power line
communications systems, and other applications. ITU standards
G.992.1, G.992.2 and the ANSI T1.413 standard, each of which are
incorporated herein by reference in their entirety, specify
standard implementations for ADSL transceivers that use
multicarrier modulation.
[0007] FIG. 1 illustrates an exemplary standard compliant ADSL DMT
transmitter 100. In particular, the ADSL DMT transmitter 100
comprises three layers: the modulation layer 110, the
Framer/Forward Error Correction (FEC) layer 120, and the ATM TC
(Asynchronous Transfer Mode Transmission Convergence) layer
140.
[0008] The modulation layer 110 provides the functionality
associated with DMT modulation. In particular, DMT modulation is
implemented using an Inverse Discrete Fourier Transform (IDFT) 112.
The IDFT 112 modulates bits from the Quadrature Amplitude
Modulation (QAM) encoder 114 into the multicarrier subchannels. The
ADSL multicarrier transceiver modulates a number of bits on each
subchannel, the number of bits depending on the Signal to Noise
Ratio (SNR) of that subchannel and the Bit Error Rate (BER)
requirement of the communications link. For example, if the
required BER is 1.times.10.sup.-7, i.e., one bit in ten million is
received in error on average, and the SNR of a particular
subchannel is 21.5 dB, then that subchannel can modulate 4 bits,
since 21.6 dB is the required SNR to transmit 4 QAM bits with a
1.times.10.sup.-7 BER. Other subchannels can have a different SNR's
and therefore may have a different number of bits allocated to them
at the same BER. The current ITU and ANSI ADSL standards allow up
to 15 bits to be modulated on one carrier.
[0009] A table that specifies how many bits are allocated to each
subchannel for modulation in one DMT is called a Bit Allocation
Table (BAT). A DMT symbol is the collection of analog samples
generated at the output of the IDFT by modulating the carriers with
bits according to the BAT. The BAT is the main parameter used in
the modulation layer 110. The BAT is used by the QAM encoder 114
and the IDFT 112 for encoding and modulation. The following Table
illustrates an example of a BAT for an exemplary DMT system having
16 channels.
1 TABLE 1 Subchannel Bits per Number Subchannel 1 5 2 9 3 3 4 2 5 4
6 0 7 5 8 7 9 8 10 3 11 0 12 5 13 6 14 8 15 4 16 3 Total Bits Per
80 DMT Symbol
[0010] In ADSL systems, the typical DMT symbol rate is
approximately 4 kHz. This means that a new DMT symbol modulating a
new set of bits, using the modulation BAT, is transmitted every 250
microseconds. If the exemplary BAT in Table 1, which specifies 80
bits modulated in one DMT symbol, were used at a 4 kHz DMT symbol
rate, the bit rate of the system would be 4000*80=320 kbits per
second (kbps).
[0011] The BAT determines the data rate of the system and is
dependent on the transmission channel characteristics, i.e., the
SNR of each subchannel in the multicarrier system. A channel with
low noise, i.e., a high SNR on each subchannel, will have many bits
modulated on each DMT carrier and will thus have a high bit rate.
If the channel conditions are poor, e.g., high noise, the SNR will
be low and the bits modulated on each carrier will be few,
resulting in a low system bit rate. As can be seen in Table 1, some
subchannels may actually modulate zero bits. An example is the case
when a narrow band interferer, such as an AM broadcast, is present
at a subchannel's frequency and causes the SNR in that subchannel
to be too low to carry any information bits.
[0012] The ATM TC layer 140 comprises an Asynchronous Transfer Mode
Transmission Convergence (ATM TC) section 142 that transforms bits
and bytes in cells into frames.
[0013] The Framer/FEC layer 120 provides the functionality
associated with preparing a stream of bits for modulation. The
Framer/FEC layer 120 comprises an Interleaving (INT) portion 122, a
Forward Error Correction (FEC) portion 124, a scrambler (SCR)
portion 126, a Cyclic Redundancy Check (CRC) portion 128 and an
ADSL Framer portion 130. The Interleaving and FEC coding provide an
impulse immunity and a coding gain. The FEC portion 124 in the
standard ADSL system is a Reed-Solomon (R-S) code. The scrambler
126 is used to randomize the data bits. The CRC portion 128 is used
to provide error detection at the receiver. The ADSL Framer portion
130 frames the received bits from the ATM framer 142. The ADSL
framer 130 also inserts and extracts overhead bits from the module
132 for modem to modem overhead communication channels, which are
known as EOC and AOC channels in the ADSL standards.
[0014] The key parameters of the Framer/FEC layer 120 are the size
of the R-S codeword, the size, i.e., depth, of the interleaver,
which is measured in the number of R-S codewords, and the size of
the ADSL frame. As an example, a typical size for an R-S codeword
may be 216 bytes, a typical size for interleaver depth may be 64
codewords, and a typical size of the ADSL frame may be 200 bytes.
It is also possible to have an interleaving depth equal to one,
which is equivalent to no interleaving. In order to recover the
digital signal that was originally prepared for transmission using
a transmitter as discussed above, it is necessary to deinterleave
the codewords by using a deinterleaver that performs the inverse
process to that of the interleaver, with the same depth parameter.
In the current ADSL standards, there is a specific relationship
between all of these parameters in a DMT system. Specifically, the
BAT size, N.sub.BAT, i.e., the total number of bits in a DMT
symbol, is fixed to be an integer divisor of the R-S codeword size,
N.sub.FEC, as expressed in Equation 1:
N.sub.FEC=S*N.sub.BAT, (1)
[0015] where S is a positive integer greater than 0.
[0016] This constant can also be expressed as one R-S codeword
containing an integer number of DMT symbols. The R-S codeword
contains data bytes and parity, i.e, checkbytes. The checkbytes are
overhead bytes that are added by the R-S encoder and are used by
the R-S decoder to detect and correct bit errors. There are R
checkbytes in a R-S codeword. Typically, the number of checkbytes
is a small percentage of the overall codeword size, e.g., 8%. Most
channel coding methods are characterized by their coding gain,
which is defined as the system performance improvement, in dB,
provided by the code when compared to an uncoded system. The coding
gain of the R-S codeword depends on the number of checkbytes and
the R-S codeword size. A large R-S codeword, e.g., greater than 200
bytes in a DMT ADSL system, along with 16 checkbytes, i.e., 8% of
the 200 bytes, will provide close to the maximum coding gain of 4
dB. If the codeword size is smaller and/or the percentage of
checkbyte overhead is high, e.g., greater than 30%, the coding gain
may be very small or even negative. In general, it is best to have
the ADSL system operating with the largest possible R-S codeword,
with the current maximum being 255 bytes, and approximately 8%
redundancy.
[0017] There is also a specific relationship between the number of
bytes in an ADSL frame, N.sub.FRAME, and the R-S codeword size,
N.sub.FEC that is expressed in Equation (2):
N.sub.FEC=S.times.N.sub.FRAME+R, (2)
[0018] where R is the number of R-S checkbytes in a codeword and S
is the same positive integer as in Equation (1).
[0019] It is apparent from equating the right-hand sides of
Equations (1) and (2) that the relationship expressed in Equation
(3) results in:
N.sub.BAT=N.sub.FRAME+R/S. (3)
[0020] The current ADSL standard requires that the ratio (R/S) is
an integer, i.e. there is an integer number of R-S checkbytes in
every DMT-symbol (N.sub.BAT). As described above, ADSL frames
contain overhead bytes, which are not part of the payload, that are
used for modem to modem communications. A byte in an ADSL frame
that is used for the overhead channel cannot be used for the actual
user data communication, and therefore the user data rate decreases
accordingly. The information content and format of these channels
is described in the ITU and ANSI standards. There are several
framing modes defined in ADSL standards. Depending on the framing
mode, the number of overhead bytes in one ADSL frame varies. For
example, standard Framing Mode 3 has 1 overhead byte per ADSL
frame.
[0021] Equations (1), (2) and (3) demonstrate that the parameter
restrictions imposed by the standards result in the following
conditions:
[0022] All DMT symbols have a fixed number of overhead framing
bytes that are added at the ADSL framer. For example, in Framing
Mode #3, there is 1 overhead framing byte per DMT symbol.
[0023] There is a minimum of one R-S checkbyte per DMT symbol.
[0024] The maximum number of checkbytes according to ITU Standard
G.992.2 (8) and ITU Standards G.992.2 and T1.413 (16) limits the
maximum codeword size to 8*N.sub.BAT for G.992.2, and to
16*N.sub.BAT for G.992.1 and T1.413.
[0025] An ADSL modem cannot change the number of bits in a DMT
symbol (N.sub.BAT) without making the appropriate changes to the
number of bytes in a R-S codeword (N.sub.FEC) and an ADSL frame
(N.sub.FRAME).
[0026] The above four restrictions cause performance limitations in
current ADSL systems. In particular, because of condition 1, every
DMT symbol has a fixed number of overhead framing bytes. This is a
problem when the data rate is low and the overhead framing bytes
consume a large percentage of the possible throughput, which
results in a lower payload. For example, if the date rate supported
by the line is 6.144 Mbps, this will result in a DMT symbol with
about 192 bytes per symbol (192*8*4000=6144 kbps). In this case,
one overhead framing byte would consume {fraction (1/192)} or about
0.5% of the available throughput. But if the date rate is 128 kbps,
or 4 bytes per symbol, the overhead framing byte will consume 1/4
or 25% of the available throughput. Clearly this is
undesirable.
[0027] Condition 2 will cause the same problems as condition 1. In
this case, the overhead framing byte is replaced by the R-S
checkbyte.
[0028] Condition 3 will not allow the construction of large
codewords when the data rate is low. The R-S codewords in ADSL can
have a maximum of 255 bytes. The maximum coding gain is achieved
when the codeword size is near the maximum 255 bytes. When the data
rate is low, e.g., 128 kbps or 4 bytes per symbol, the maximum
codeword size will be 8*4=32 bytes for G.992.2 systems and 16*4=64
bytes for G.992.1 and T1.413 systems. In this case the coding gain
will be substantially lower than for large codewords approaching
255 bytes.
[0029] In general, if the data rate is low, e.g., 128 kbps or 4
bytes per symbol, the above conditions will result in 1 byte being
used for overhead framing, and 1 byte being consumed by an R-S
checkbyte. Therefore 50% of the available throughput will not be
used for payload and the R-S codeword size will be at most 64
bytes, resulting in negligible coding gain.
[0030] Condition 4 affects the ability of the modem to adapt its
transmission parameters on-line in a dynamic manner.
[0031] G.992.1 and T1.413 specify a mechanism to do on-line rate
adaptation, called Dynamic Rate Adaptation (DRA), but it is clearly
stated in these standards that the change in data rate will not be
seamless. In general, current ADSL DMT modems use Bit Swapping and
dynamic rate adaptation (DRA) as methods for on-line adaptation to
channel changes. Bit swapping is specified in the ITU and ANSI
standards as a method for modifying the number of bits allocated to
a particular carrier. Bit Swapping is seamless, i.e., it does not
result in an interruption in data transmission and reception,
however, bit swapping does not allow a changing of data rates. Bit
Swapping only allows the changing of the number of bits allocated
to carriers while maintaining the same data rate. This is
equivalent to changing the entries in the BAT table without
allowing the total number of bits (N.sub.BAT) in the BAT to
increase or decrease.
[0032] DRA enables a change in data rate, but is not seamless. DRA
is also very slow because it requires the modem located in the
Central Office (CO) to make the final decision on the data rate
configuration. This model, where the CO being the master, is common
among ADSL modems that are designed to provide a service offered
and controller by the telephone company.
[0033] Both Bit Swapping and DRA use a specific protocol that is
specified in the ANSI T1.413, G.992.1 and G.992.2 standards for
negotiating the change. This protocol negotiates the parameters
using messages that are sent via an AOC channel, which is an
embedded channel. This protocol is sensitive to impulse noise and
high noise levels. If the messages are corrupted, the transmitter
and receiver can enter a state where they are using different
transmission parameters, e.g., BAT, data rate, R-S codeword length,
interleaver depth, etc. When two communicating modems enter a state
of mismatched transmission parameters, data will be received in
error and the modems will eventually be required to take drastic
measures, such as full re-initialization, in order to restore error
free transmission. Drastic measures such as full reinitialization
will result in the service being dropped for approximately 10
seconds, which is the time required for the current standards
compliant ADSL modem to complete a full initialization.
[0034] A transceiver has both a transmitter and a receiver. The
receiver includes the receiver equivalent blocks of the transmitter
as shown in FIG. 1. The receiver has modules that include a
decoder, a deinterleaver and a demodulator. In operation, the
receiver accepts a signal in analog form that was transmitted by a
transmitter, optionally amplifies the signal in an amplifier,
filters the signal to remove noise components and to separate the
signal from other frequencies, converts the analog signal to a
digital signal through the use of an analog to digital converter,
demodulates the signal to generate the received bit stream from the
carrier subchannels by the use of a demodulator, deinterleaves the
bit stream by the use of a deinterleaver, performs the FEC decoding
to correct errors in the bit stream by use of an FEC decoder,
descrambles the bit stream by use of a descrambler, and detects bit
errors in the bit stream by use of a CRC. Various semiconductor
chip manufacturers supply hardware and software that can perform
the functions of a transmitter, a receiver, or both.
[0035] In addition, to establish communication between the
transceivers at the very onset, full initialization of the modems
of the transceivers must be completed. Conventional ADSL modems
will always go through an initialization procedure during which
known training signals are set between the transceivers.
Conventional ADSL modems utilize an initialization procedure as
specified in the 992.1 and 994.1 standards, as well as the
published but not yet adopted G.dmt.bis standard, which are
incorporated herein by reference.
[0036] The primary purpose of the initialization procedure is to
measure the line conditions and train all receiver functions of the
transceivers to optimize the ADSL transmission system to thereby
maximize the data rates.
[0037] During the initialization procedure various transmission
parameter values are determined. The parameters values include, for
example, bit error rate, bit allocation value, gain value, or such
parameter values that have been grouped such as in bit allocation
tables and gain tables as well as other parameters such as the
overhead bits of the EOC and AOC channels, size of the R-S
codeword, number of parity bits in the R-S codeword, depth of the
interleaver, size of the ADSL frame, and overhead framing bytes.
The parameter values may also be the signal to noise ratio (SNR) of
the channel that is accurately measured so that maximum possible
data rate can be attained, the time domain equalizer filter taps,
the frequency domain equalizer filter taps, the echo canceller
filter taps, and the like.
[0038] Typically, the full initialization procedure is attained in
a series of initialization steps where one or more of the above
noted parameter values that define the characteristics of the
communication link between the transceivers are determined in one
initialization step prior to proceeding to the next initialization
step. This standard initialization procedure is illustrated in the
functional block diagram of FIG. 2. Upon beginning the
intitialization of the modems of the transceivers in the ADSL
transmission system in step S20, a series of initialization steps
are taken in sequence: initialization step S22, initialization step
S24, and then initialization step S26. Each of these initialization
steps require one or more parameter values noted previously that
define the characteristics of the communication link between the
transceivers. In this regard, the actual parameter value A
indicated as 21 is needed to complete initialization step S22, the
actual parameter value B indicated as 23 is needed to complete
initialization step S24, and the actual parameter value C indicated
as 25 is needed to complete initialization step S26. Each of these
actual parameter values must be determined based on the type of
modem, the standards used, and the condition of the communication
channel in the standard initialization procedures.
[0039] Of course, these initialization steps are illustrated
generically since they depend on the particular initialization
standard followed. For instance, in initialization step S22, a
handshake procedure between the transceivers may be performed to
indicate that a communication link is desired between them. In
initialization step S24, a channel between the transceivers that is
available for use in establishing the communication link may be
discovered. The initializing step S26 may be the step in which the
transceivers are trained based on additional parameter values to
designate attributes of the discovered channel. For example, in a
multicarrier communication system step S26 may be used to measure
the SNR of every subchannel. Based on the measured SNR parameter
the transceiver would determine the bit allocation and gain tables.
In this regard, each of the initialization steps would likely
entail determination and/or use of one or more of the various
parameter values by one or both of the modems, depending on the
parameter value, to aid in the process of establishing the steady
state communication link.
[0040] Once the various parameter values are determined and the
receiver signal processors are trained in the initialization steps,
the initialization of the modems are complete as indicated by S27,
thus allowing the modems to establish a steady state communication
link as shown in S28. When such steady state communication link is
established as indicated by S28, the transmission system is
functional and is in a data transmission mode so that the user may
operate the communications system to transmit and receive data.
SUMMARY OF THE INVENTION
[0041] In the above described standard initialization procedure
shown in FIG. 2, the steady state communication link is only
established after the completion of all the initialization steps.
Time is required to determine the actual parameter values required
in the various initialization steps. In this regard, as noted
previously, the initialization of the modems of the transceivers
compliant to the current standards typically take approximately 10
seconds, during which time the user is precluded from using the
system. Thus, the user must wait for the completion of
initialization of the modems before the communication system
establishes a communication link that allows the user to utilize
the system to transmit and receive data. This delay of
approximately 10 seconds is viewed by many users, equipment
providers and service providers as a negative aspect of ADSL
service since it means that every time the ADSL link is established
or reconnected after a loss of synchronization, the user must wait
approximately 10 seconds for the complete initialization to finish
prior to using the system.
[0042] Moreover, as also noted previously, this initialization
period not only occurs during the initial powering of the system,
but also when two communicating modems enter a state of mismatched
transmission parameters which result in data being received, for
example, in error. Since full reinitialization is required to
restore error free transmission, the data service is dropped for
the duration of the reinitialization period so that the user is
again precluded from utilizing the system. This results in numerous
10 second delays if the two communicating modems are prone to
entering a state of mismatched transmission due to changes in line
quality, interference, or the like.
[0043] In view of the above, one aspect of an exemplary embodiment
of the present invention is that it provides a method for
initializing modems which reduces the duration in which the user is
precluded from utilizing the system as a communication link.
[0044] Another aspect of an exemplary embodiment of the present
invention is that it provides a method for initializing modems that
allows a rapid transition to a data communications state.
[0045] Still another aspect of an exemplary embodiment of the
present invention is that it provides such a method for
initializing modems that optimizes the communication link between
the modems while data is communicated therebetween.
[0046] In accordance with one embodiment of the present invention,
the above noted advantages are attained by a method for
initializing transceivers in a multicarrier transmission system to
establish a communication link between the transmitter and the
receiver. The method includes the steps of providing at least one
predetermined parameter value that approximates a corresponding
actual parameter value of the communication link between the
transmitter and the receiver, establishing a data communication
link between the transmitter and the receiver using the at least
one predetermined parameter value as an approximation of the actual
parameter value of the communication link, thus allowing the
multicarrier transmission system to transmit data between the
transmitter and the receiver, the data communication link
established using the at least one predetermined parameter value
having an associated data rate that may be different than a data
rate attained when the actual parameter value corresponding to the
at least one predetermined parameter value is used, determining the
actual parameter value corresponding to the at least one
predetermined parameter value after establishing the data
communication link using the predetermined parameter value, and
seamlessly adapting the data rate of the established communication
link by using the determined actual parameter value to provide a
steady state communication link with a different data rate.
[0047] In the above regard, the at least one predetermined
parameter value may be a plurality of predetermined parameter
values that approximate a plurality of actual parameter values
where the communication link is established using the plurality of
predetermined parameter values. The data rate of the communication
link established using the plurality of predetermined parameter
values may be different than a data rate attained when the
plurality of actual parameter values are used. Each of the
plurality of actual parameter values are determined and the data
rate of the communication link is seamlessly adapted using the
determined plurality of actual parameter values. In this regard,
the exemplary step of determining each of the plurality of actual
parameter values is attained iteratively in a manner that at least
one actual parameter value is determined in each iteration.
Preferably, the exemplary method further includes the step of
iteratively seamlessly adapting the data rate of the communication
link after each iteration as the at least one actual parameter
value is determined in each iteration.
[0048] In accordance with another exemplary embodiment, the
plurality of predetermined parameter values and the corresponding
actual parameter values may be indicative of at least one of a
signal to noise ratio, a bit error rate, a bit allocation value, a
bit allocation table, a gain value and a gain table. Alternatively,
or in addition, the plurality of predetermined parameter values and
the corresponding actual parameter values may be indicative of at
least one of overhead bits of EOC and AOC channels, codeword size,
number of parity bits in a codeword, depth of an interleaver, size
of an ADSL frame, and overhead framing bytes. Alternatively, or in
addition, the plurality of predetermined and the corresponding
actual parameter values may be indicative of the channel SNR, the
time domain equalizer filter taps, the frequency domain equalizer
filter taps and the echo canceller filter taps.
[0049] In accordance with another exemplary aspect of the present
invention, a modem initializing procedure is provided for
initializing modems in a multicarrier transmission system that
minimizes the amount of time in the initialization sequence before
transitioning to a data communication state. The modem initializing
procedure includes the steps of exchanging a message that indicates
that a communication link is desired between the transceivers,
determining a channel between the plurality of transceivers that is
available for use in establishing the communication link, accessing
at least one predetermined parameter value that approximates an
actual parameter value of the communication link between the
transmitter and the receiver, training the transceivers using the
at least one predetermined parameter value to designate attributes
of the determined channel, establishing a data communication link
through the determined channel using the at least one predetermined
parameter value to allow a user to use the multicarrier
transmission system to transmit and receive data between the
plurality of transceivers, the established data communication link
having a data rate that is generally, although not necessarily,
lower than a data rate attainable using the actual parameter value
that corresponds to the at least one predetermined parameter value,
analyzing the channel to determine the actual parameter value after
establishing the data communication link using the at least one
predetermined parameter value, and seamlessly increasing the data
rate of the established data communication link using the
determined actual parameter value to provide a steady state
communication link with an updated data rate.
[0050] According to another exemplary embodiment of the invention,
ADSL DMT systems and methods are provided that establish a data
communication link during initialization and that change the data
transmission bit rate parameters in a seamless manner during
initialization. The ADSL DMT systems and methods operate according
to protocols that allow the seamless change of transmission bit
rates during initialization and this seamless change of
transmission bit rates may be initiated by either the transmitter
or the receiver, e.g., the CO or the CPE modem.
[0051] These and other features and advantages of this invention
are described in, or are apparent from, the following detailed
description of the embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] The embodiments of the invention will be described in
detail, with reference to the following figures wherein:
[0053] FIG. 1 is a functional block diagram illustrating a standard
compliant ADSL DMT transmitter;
[0054] FIG. 2 is a functional block diagram illustrating a standard
initialization procedure;
[0055] FIG. 3 is a functional block diagram illustrating an
initialization procedure in accordance with one embodiment of the
present invention;
[0056] FIG. 4 is a flowchart outlining a method of initializing
modems in a multicarrier transmission system in accordance with one
embodiment of the present invention;
[0057] FIG. 5 is a flowchart outlining a method of initializing
modems in a multicarrier transmission system in accordance with
another embodiment of the present invention;
[0058] FIG. 6 illustrates an exemplary embodiment of an ADSL frame
and R-S codewords;
[0059] FIG. 7 is a functional block diagram illustrating an
exemplary dual latency ADSL DMT transmitter;
[0060] FIG. 8 is a flowchart outlining an exemplary method of a
seamless rate adaptive transmission;
[0061] FIG. 9 is a flowchart outlining a second exemplary method of
seamless rate adaptive transmissions;
[0062] FIG. 10 is a flowchart outlining an exemplary method of fast
seamless rate adaptive transmissions;
[0063] FIG. 11 is a flowchart outlining a second exemplary method
of fast seamless rate adaptive transmission; and
[0064] FIG. 12 is flowchart illustrating an exemplary method of
transporting a NTR.
DETAILED DESCRIPTION OF THE INVENTION
[0065] FIG. 3 is a functional block diagram illustrating an
initialization procedure in accordance with an exemplary embodiment
of the present invention. As can be seen, the full initialization
procedure is attained in a series of initialization steps where one
or more of the previously noted parameter values that define the
characteristics of the communication link between the transceivers
are determined and used for establishing the communication link.
However, in contrast to the standard initialization procedures such
as that shown in FIG. 2, rapid establishment of a data
communication link is made possible so that the user can quickly
utilize the transmission system to transmit and receive data.
[0066] In particular, as can be seen in FIG. 3, the intitialization
of the transceivers in the transmission system is started in step
S30. However, unlike the standard initialization procedure in which
the actual parameter values are determined and initialization steps
are executed in sequence using these actual parameter values, at
least one predetermined parameter value such as the predetermined
values A, B, and C, indicated by 31, are provided in accordance
with the initialization procedure of an exemplary embodiment of the
present invention. These predetermined values may be stored on one
or more of the transceivers or other storage devices. These
predetermined parameter values A, B, and C are used for the various
initialization steps S32, S33, and S34, respectively, to allow
quick establishment of a data communication link in step S35. Of
course, in other examples, there may be fewer or additional steps,
three steps being shown here as an example only. When such data
communication link is established as indicated by step S35, the
communication system is functional and the user may utilize the
transmission system to transmit and receive data.
[0067] Moreover, it should again be noted that these initialization
steps are illustrated generically since they depend on the
particular initialization standard followed. For instance, in
initialization step S32, an information exchange procedure between
the transceivers may be performed to indicate that a communication
link is desired between the transceivers. In initialization step
S33, a channel between the transceivers that is available for use
in establishing the communication link may be determined. The
initializing step S34 may be the step in which the transceivers are
trained. Of course, in other embodiments, these initialization
steps may entail different, additional or a lesser number of
specific steps.
[0068] It should be appreciated that because the data communication
link established in S35 in accordance with an exemplary embodiment
of the present invention utilizes predetermined parameter values 31
instead of the actual parameter values, the data rate or capacity
of the communication link is not necessarily optimized. Therefore,
the data rate of the communication link established using the
predetermined parameter values could be higher or lower than the
data rate attained when the actual parameter values are used. In
the case when the data rate is higher than can be attained when the
actual parameters are used, the connection can be established at a
bit error rate that is higher than expected. For example, if the
BER is expected to be 1E-7, as is in most conventional ADSL
systems, connection at a higher data rate could result in a BER of
1E-5. This means that the received data will have, on average, 1
bit out of every 10000 bits in error, whereas it is desired to have
1 bit out of every 10,000,000 bits in error. Obviously operating at
a data rate without achieving the required BER is a sub-optimum
mode of operation. Nonetheless, in either case, whether the data
rate is too high or too low, the user is allowed to use the
transmission system to transmit and receive data, although possibly
at a sub-optimum data rate.
[0069] To optimize the data rate of the communication link,
subsequent to the establishment of the data communication link in
step S35, the actual parameter values 36 corresponding to the
predetermined values 31 are determined and the data rate of the
established data communication link is seamlessly updated using the
determined actual parameter values 36. The seamless updating of the
data rate is attained, for example, utilizing Seamless Rate
Adaptation techniques as described in further detail herein below.
In this regard, an exemplary embodiment of the present invention
allows, for example, such seamless adaptation of the data rate to
be attained even while the user is transmitting and receiving data
over the communications system. Once all the actual parameter
values 36 are determined, and the data rate of the data
communication link established in step S35 is seamlessly adapted
using the determined actual parameter values 36, the initialization
procedure between the modems is completed in step S37 and the
modems enter steady state communication.
[0070] Again, the exemplary predetermined parameter values 31 and
the corresponding actual paramete values 36 include those parameter
values that define the characteristics of the communication link
between the transceivers of the transmission system, and are
preferably required for the establishment of the data communication
link. In this regard, the predetermined parameter value 31 may be
bit error rate, bit allocation value, a gain value, or such
parameter value(s) that have been grouped together such as in bit
allocation tables and/or gain tables as well as other parameters
including a signal to noise ratio (SNR). Moreover, the
predetermined parameter value 31 may also be overhead bits of EOC
and AOC channels, a codeword size, number of parity bits in a
codeword, depth of an interleaver, size of an ADSL frame, and
overhead framing bytes. Alternatively, or in addition, the
predetermined parameter values 31 may be the channel SNR, the time
domain equalizer filter taps, the frequency domain equalizer filter
taps and the echo canceller filter taps.
[0071] Of course, these are merely examples of parameter values and
are not exhaustive. In this regard, in other exemplary embodiments,
the predetermined parameter values may also merely be pointers that
point to a particular predetermined parameter value or set of
values to be used, the predetermined parameter(s) being stored in a
storage device accessible by one or more of the modems of the
communications system. Moreover, in yet other exemplary
embodiments, the predetermined parameter values may even be
functions or equations that provide estimates or approximations of
the actual parameter values based on various known or determinable
actual parameter values. As to which parameter values are provided
in the predetermined parameter value 31 is largely dependent on the
design of the transmission system and the standard used, such
design and standard determining the parameter values required to
establish the data and steady state communication link.
[0072] Thus, for the transceivers in the ADSL transmission system
that utilize the initializing procedure in accordance with the
present invention as shown in FIG. 3, a data communication link is
established much faster than conventional modems that utilize
standard initialization procedures which require determination of
the actual parameter values to complete full initialization prior
to the establishment of any communication link. By providing
predetermined parameter values 31 and using these values for the
initialization steps and quickly establishing the data
communication link in step S35, the delay which would result in
determining the actual parameter values can be avoided.
[0073] Moreover, the seamless rate adaptation techniques allow the
data rate of the communication link between the modems of the
transmission system to be seamlessly adapted without requiring
disruption in the communication link or requiring full
initialization. In this manner, the initialization period during
which the user is precluded from using the system is greatly
reduced from, for example, 10 seconds to approximately 1 second or
even less. Although the transceivers may transmit and receive data
at sub-optimum data rates, this disadvantage may be quickly
overcome by seamlessly modifying the data rate after data
transmission and reception are underway.
[0074] FIG. 4 shows a flowchart outlining a method of initializing
modems in a multicarrier transmission system in accordance with an
exemplary embodiment of the present invention which may be used to
establish a communication link therebetween.
[0075] Upon beginning the initialization in step S40, the method
includes the step S41 of providing at least one predetermined
parameter value that approximates a corresponding actual parameter
value of the communication link between the transceivers.
Predetermined parameter values can be generated in a number of
ways. For example the actual parameter values from a previously
completed standard initialization may be used as predetermined
parameter values. Alternatively, for example, predetermined
parameter values may be determined by using the lowest possible
actual value for a parameter, e.g., using a 1 bit constellation on
a subchannel.
[0076] Alternatively, the predetermined parameter value may
actually be an estimated parameter value based on partial training
or the inaccurate measurement of initialization functions. For
example the predetermined parameter value may be a bit allocation
table that is generated based on a partial inaccurate SNR
measurement. A partial SNR measurement is accomplished if, for
example, the SNR is measured over a short period of time, e.g.,
less than 1 second in ADSL systems. In this case the bit allocation
table based on the inaccurate SNR measurement would be sub-optimum
and would therefore need to be adapted by seamlessly adapting the
bit allocation table to achieve the optimum data rate.
[0077] Alternatively still, for example, the transceivers can
monitor one or more characteristics of the line and/or the
transmitted or received data, store this data, and either increase
or decrease the data rate through an initialization according to
the principles of this invention to maintain the communications
link. Furthermore, for example, profiles of various parameter value
sets can be stored such that in anticipation of certain conditions,
such as varying line conditions that may occur, the transceivers
can preemptively initialize and update based on the stored
parameter sets.
[0078] The provided at least one predetermined parameter value is
used to establish a data communication link between the modems of
the transceivers. As previously explained, the at least one
predetermined parameter value is used as an approximation of the
actual parameter value. The data communication link established in
step S42 using the predetermined parameter value allows the user to
transmit and receive data between the transceivers. Of course, the
data communication link established using the at least one
predetermined parameter value has a data rate that may be
different, e.g., greater or lesser, than a data rate obtained when
the actual parameter value corresponding to the at least one
predetermined parameter value is used. In step S43, the actual
parameter value corresponding to the at least one predetermined
parameter value is determined. Then, using the actual parameter
value determined in step S43, the data rate of the established data
communication link is seamlessly adapted in step S44 to provide an
updated communication link, for example, a steady state
communication link, e.g., showtime, with a different data rate. In
the illustrated embodiment, the initialization between the modems
is then completed in step S45.
[0079] Of course, in other exemplary embodiments, the at least one
predetermined parameter value may actually be a plurality of
predetermined parameter values that approximate a plurality of
actual parameter values, these actual parameter values being
determined and used to seamlessly adapt the communication link's
data rate. In this regard, FIG. 5 shows a flowchart outlining an
exemplary method of initializing modems in a multicarrier
transmission system in accordance with another embodiment of the
present invention. Again, the initialization starts in step S50 and
a plurality of predetermined parameter values are provided in step
S51, which are used to establish a data communication link in step
S52. Once the data communication link is established in step S52,
at least one actual parameter value corresponding to at least one
of the predetermined parameter values is determined in step S53.
Then, using the actual parameter value determined in step S53, the
data rate of the established data communication link is seamlessly
adapted in step S54 to provide a steady state communication link
with an updated, for example, optimized, data rate.
[0080] As can be seen, a determination is then made in step S55 as
to whether the optimization of the communication link is complete.
In this regard, optimization as used here refers to the change in
data rate of the communication link to the data rate that would be
attained by a communication link established using the determined
actual parameter values. In the illustrated embodiment, this step
is present because a plurality of predetermined parameter values
are provided and the plurality of actual parameter values will be
determined. This allows the step of seamlessly adapting the data
rate of the communication link to be performed numerous times in an
iterative manner as one or more of the actual parameter values are
determined. For instance, it may be desirable to iteratively adapt
the data rate of the communication link as the actual parameter
values are determined. Thus, rather than waiting for all of the
actual parameter values to be determined before modifying the data
rate to an optimized data rate, when one or more of the actual
parameter values are determined, the data rate of the communication
link may be updated using only those actual parameter values that
have been determined.
[0081] Of course, the adapted data rate of the communication link
partly using some of the predetermined parameter values and partly
using the determined actual parameter values could be less than
data rate attainable in a communication link established using all
actual parameter values. Nonetheless, this provides the user of the
transmission system better data rate performance than that attained
in the initial communication link established using only the
predetermined parameter values. Moreover, subsequent to this
adaptation, the data rate, or other actual parameter values may be
determined and the data rate of the communication link seamlessly
adapted again based on the newly determined actual parameter
values.
[0082] Thus, the steps of determining each of the plurality of
actual parameter values and seamlessly adapting the data rate of
the communication link using the plurality of actual parameter
values may be attained iteratively for each of the plurality of
actual parameter values. This iterative determination of whether
optimization of the communication link is complete is made until it
is determined that optimization is completed which leads to the
ending of the initialization between the modems in step S56.
[0083] In this manner, rapid initialization of the modems may be
attained so that a data communication link can quickly be
established therein between. Moreover, the data rate of the
established communication link is seamlessly adapted as the actual
parameter values are determined thereby improving the performance
of the multicarrier transmission system.
[0084] For example, the bit allocation table (BAT) can be the
predetermined parameter set. As an example, this predetermined BAT
can be based on the BAT that was generated during a previous full
initialization. In a standard initialization, the BAT is generated
after measuring the SNR of the channel using long training
sequences. These training sequences can take, for example, more
that 4 seconds. In a fast initialization the SNR can not be
measured using such a long training sequence since the fast
initialization sequence will typically last less than 4 seconds.
Therefore, the BAT of the previous full initialization is used as a
predetermined parameter set for the fast initialization and a data
connection is established using this predetermined BAT. Since the
channel may have changed since the last full initialization, the
predetermined BAT may not be optimum for the current connection.
This means that the data rate resulting from the use of this BAT
may either be too low, because the channel conditions (SNR) have
improved, or too high, because channel conditions (SNR) have
worsened. In either case, the SNR of the channel is measured over
the data communication link connection over the required
measurement period which is typically, for example, greater than 4
seconds. After the SNR is measured more accurately the actual BAT
can be generated and the system can be modified to use the actual
BAT. This is accomplished by seamlessly adapting the data rate and
using the new BAT for communication to establish a steady state
connection.
[0085] The systems and methods of this invention will also work
equally well when a partial training is performed. With a partial
training, a combination of actual parameter values and
predetermined parameter values are used. Again, the predetermined
parameter values can be retrieved from a storage location and based
on any one or more of a previously used parameter value, a fixed
parameter value, a estimated parameter value, a dynamically
determined parameter value, or the like. For example, the system
may know that a certain actual parameter will be difficult and time
consuming to determine, for example, the SNR. Thus, the system
could use a predetermined parameter value for the SNR, allow data
communication, and determine the actual parameter values for the
remaining parameters, thus completing initialization. This approach
would also lead to at least an initialization time savings over
that of a conventional full initialization.
[0086] The principles of the invention may be employed using
transceivers that include a transmitter, such as that discussed
above in relation to FIG. 1, and a receiver. In general, an ADSL
system includes both a transmitter and a receiver for communication
in a particular direction. In the discussion that follows, an ADSL
DMT transmitter accepts data and transmits data over a transmission
line, which can be, for example, a twisted wire pair, or the like.
A transmission can also occur over a medium that includes other
kinds of wires, fiber optic cable, wireless links, or the like. In
order to utilize the transmitted signal, a second transceiver at
the remote end of the transmission line includes a receiver that
converts the received analog signal into a digital data stream for
use by devices, such as computers, digital televisions, digital
radios, communications equipment, or the like. For bi-directional
communication using a pair of transceivers, each transceiver
includes a transmitter that sends information to the receiver of
the other member of the pair, and a receiver that accepts
information transmitted by the transmitter of the other member of
the pair.
[0087] As discussed herein, the exemplary DMT system has the
capability of adapting the system bit rate on-line, during
initialization, in a seamless manner. The DMT system also provides
a robust and fast protocol for completing this seamless rate
adaptation. The DMT system also provides a framing and encoding
method with reduced overhead compared to conventional DMT systems.
This framing and encoding method enables, for example, a system
with a seamless rate adaptation capability.
[0088] The specific details of methods for seamlessly adapting the
data rate of the established communication link as set forth in
steps S44 in FIG. 4 and S54 in FIG. 5 is discussed herein below. In
this regard, various methods for seamlessly adapting the data rate
during initialization of the modems are generically described and
various examples are also discussed in U.S. application Ser. No.
09/522,870 filed Mar. 10, 2000 entitled "A Method for Seamlessly
Changing Power Modes and ADSL Systems," U.S. patent application
Ser. No. 09/522,869, filed Mar. 10, 2000 entitled "Seamless Rate
Adapted Adaptive Multicarrier Modulation System and Protocols,"
U.S. patent application Ser. No. 09/523,086, filed Mar. 10, 2000
entitled "A Method for Synchronizing Seamless Rate Adaptation,",
and U.S. patent application Ser. No. 09/918,033 filed Aug. 1, 2001
entitled "Systems and Methods for Transporting a Network Timing
Reference in an ADSL System", all of which are incorporated herein
by reference in their entirety.
[0089] It is highly desirable that this adaptation of the data rate
between the modems occur in a "seamless" manner, i.e., without data
bit errors or an interruption in service. However, the DMT ADSL
modem specified standards are not capable of performing seamless
data rate adaptation. Thus, the following discussion provides the
details of how the data rate of the communication link may be
seamlessly increased using various SRA methods described to provide
a data communication link with an updated data rate.
[0090] Condition 4 described above does not allow the size of the
BAT to change without modifying the R-S coding, interleaving and
framing parameters. If the BAT and N.sub.BAT could be modified
during operation, i.e., if more or fewer bits were allocated to
carriers in a DMT symbol, the data rate could be changed. Condition
4 requires that when the number of bits N.sub.BAT in the BAT
changes, the size of the R-S codeword, and therefore the
interleaving parameters, must also be modified. Modifying the
interleaving and coding parameters on-line requires the
re-initialization of the interleaver. Re-initialization of the
interleaver always results in a "flushing" of the interleave
memory. This flushing of the memory results in data errors and the
transition not being seamless.
[0091] In order to allow a DMT ADSL transmission systems to change
the data rate seamlessly, such as during initialization of the
modems, the framing and encoding of the data must be efficient such
that there is less overhead data bits per DMT symbol which thereby
increases the data bit rate. Additionally, the ADSL system must be
able to dynamically adapt to the data rate in a seamless manner.
Furthermore, there must exist a robust and fast protocol for
completing such a seamless rate adaptation such that the data rate
change can occur successfully even in the presence of high noise
levels.
[0092] As discussed hereinafter, and in the co-pending related
applications, an exemplary framing method is disclosed that
decreases the overhead, i.e., non-payload data in a DMT ADSL
system.
[0093] FIG. 6 illustrates an ADSL frame and R-S codeword 200 that
comprises at least one framing overhead byte 210, one or more
payload bytes 220 and one or more checkbytes 230. This framing
method enables seamless rate adaptation. As discussed above,
current ADSL systems place restrictions and requirements on the
ADSL frames, R-S codewords, and DMT symbols. This configuration as
shown in FIG. 6 allows for the de-coupling of the ADSL frames and
the R-S codewords from the DMT symbols. This de-coupling results in
a system that has, for example, lower overhead data per DMT symbol
and can also complete data rate adaptations in a seamless manner.
Thus, the ADSL frames and the R-S codewords are constructed to have
the same length and to be aligned. The R-S codeword is made
sufficiently large to maximize the coding gain. The size of the R-S
codeword, and therefore the ADSL frame, can be negotiated at, for
example, the beginning of initialization or predetermined in
advance. A fixed number of R-S checkbytes and overhead framing
bytes are included in an ADSL frame. These parameters can also be
negotiated at the beginning of initialization or predetermined in
advance.
[0094] Unlike conventional DMT symbols, the DMT symbols produced in
accordance with the exemplary embodiment of this invention are not
aligned with the ADSL frames and the R-S codewords. Additionally,
the number of bits in a DMT symbol depends solely on the data rate
requirements and configurations, and is de-coupled from the R-S
codeword size, the interleaver depth, and the ADSL frame size.
[0095] The number of bits in a DMT symbol dictates the data rate of
the modem independently of the other framing, coding or
interleaving restrictions. Since overhead bytes are added at the
ADSL frame layer, a DMT symbol does not necessarily contain a fixed
number of overhead bytes. As the data rate gets lower, for example,
128 kbps, the overhead data remains low. In particular, this
framing method assigns a fixed percentage of overhead data to the
data stream, rather than a fixed number of overhead bytes. This
percentage does not change when the data rate of the modem changes,
as in the case with current ADSL modems. Consider the following
examples of conventional standard compliant framing methods:
CONVENTIONAL EXAMPLE #1
[0096] The line capacity is 192 bytes per DMT symbol (6.144 Mbps).
The codeword size is 192, which includes 16 checkbytes and one
overhead framing byte, assuming ANSI T1.413 Framing Mode No. 3. The
total framing overhead, i.e., checkbytes plus overhead framing
bytes, per DMT symbol is 16+1=17. Therefore, the framing overhead
is {fraction (17/192)}=8.8% of the available throughput. In this
case, the framing overhead is reasonable.
CONVENTIONAL EXAMPLE #2
[0097] The line capacity is 4 bytes (128 kbps). The codeword is
constructed from 16 DMT symbols and is 16.times.4=64 bytes. There
are 16 R-S checkbytes, one checkbyte per DMT symbol, and there is
one overhead framing byte, assuming ANSI T1.413 Framing Mode No. 3.
The total framing overhead, i.e., checkbytes plus overhead framing
bytes, per DMT symbol is 1+1=2 bytes. Therefore the framing
overhead is {fraction (2/4)}=50% of the available throughput. This
is highly inefficient.
[0098] Examples of embodiments of the framing method that may be
used to implement this invention provide the following results,
called the constant percentage overhead method:
[0099] EXAMPLE #
[0100] This is exactly the same as the standard compliant training
example, i.e., conventional example #1 above. The codeword sizes,
DMT symbol sizes and overhead are the same. Therefore, the framing
overhead is {fraction (17/192)}=8.8% of the available throughput.
EXAMPLE #
[0101] The line capacity is 4 bytes (128 kbps). The codeword is
constructed independently of the DMT symbol and therefore could be,
for example, set to 192 bytes. This is also the size of the ADSL
frame. Sixteen R-S bytes and one overhead framing byte per codeword
or ADSL frame are used. There are 192/4=48 DMT symbols in one
codeword. The total overhead, i.e., checkbytes plus overhead
framing bytes, per 48 DMT symbols is 1+16=17 bytes or 17/48=0.35
bytes per one DMT symbol. The framing overhead is thus {fraction
(0.35/4)}=8.8% of the available throughout.
[0102] Accordingly, from Examples 1 and 2 above, it is apparent
that a method of achieving a framing overhead that is a constant
percentage of the available throughput may be used, regardless of
the data rate or the line capacity. In these exemplary scenarios,
the framing overhead was 8.8% for both 6 Mbps and 128 kbps.
[0103] Another exemplary benefit of the framing method described
herein is that it enables seamless data rate adaptation during
initialization. Seamless Rate Adaptation (SRA) is accomplished by
changing the DMT symbol BAT, i.e., the number of bits allocated to
each subchannel in the multicarrier system. As shown above,
modifying the BAT changes the number of bits per DMT symbol and
results in a change in the data bit rate of the system. In an
exemplary embodiment, the DMT symbol size is changed without
modifying any of the R-S coding, interleaving and/or framing
parameters. This is possible because the constant percentage
overhead framing method described above removes the restrictions
imposed by the prior art on the relation between the DMT symbols
and the R-S codewords or ADSL frames. Since the R-S coding and
interleaving parameters do not change, interleaver flushing and
other problems associated with changing the parameters associated
with these functions do not occur. Thus, the transceiver can adapt
the data rate without errors or service interruption through an
updating of the BAT.
[0104] A BAT should be updated at the transmitter and the receiver
at exactly the same time, i.e., on exactly the same DMT symbol. If
the transmitter starts using a new BAT for transmission before the
receiver does, the data is not demodulated correctly and bit errors
can occur. Also, if the receiver changes to a new BAT before the
transmitter does, the same errors can occur. For this reason, the
transition to the use of the updated BAT for transmission and
reception needs to be synchronized at the transmitter and the
receiver. In an exemplary embodiment, a protocol is provided that
enables the synchronized transition to the use of an updated
BAT.
[0105] It is also important that, for example, this protocol be
robust in the presence of channel noise. For example, if the
protocol fails and the receiver does not switch to the updated BAT
at the same time as the transmitter, then bit errors occur and the
transition is not seamless. Furthermore, if the transmitter and
receiver are using different BATs, it is difficult to re-establish
an error-free link without performing a re-initialization of the
connection, which results in an interruption of service of up to,
for example, ten or more seconds as previously described.
[0106] It is also important that the transition between the BATs
occur very quickly, since the need to operate at a new data rate
during initialization is almost instantaneous.
[0107] Accordingly, the SRA protocol should at least provide a
method for synchronizing the transceivers to the updated BAT, a
robust transition to the new data rate and a fast transition to the
new data rate.
[0108] Two exemplary protocols are provided that satisfy these
requirements for seamless rate adaptation during initialization, in
particular, to seamlessly increase the data rate of the established
communication link as set forth in step S44 in FIG. 4 and step S54
in FIG. 5. The first protocol is the normal SRA (NSRA) protocol and
the second protocol is the fast SRA (FSRA) protocol.
[0109] In the normal SRA protocol (NSRA), either the transmitter or
the receiver of the transceiver can initiate this method as
illustrated in FIGS. 8-9. In particular, for receiver initiated
SRA, control begins in step S100 and continues to step S120, in
which during the initialization, a receiver determines whether the
data rate should be modified, i.e., increased or decreased. If the
data rate is to be modified, control continues to step S130.
Otherwise, control jumps to step S190, where the control sequence
ends.
[0110] In step S130, the capabilities of the transmitter are
checked based on the determined modified data rate. The data rate
may be modified because, for example, the channel conditions on the
desired Bit Error Rate has changed. Then, in step S140, a
determination is made whether the updated data rate is within the
transmitter's rate capabilities. If the updated data rate is within
the transmitter's capabilities, control continues to step S150.
Otherwise, control jumps back to step S120.
[0111] In step S150, data rate and the updated BAT, which in this
case is the determined actual parameter value, are forwarded to the
transmitter using, for example, the AOC or EOC channel. This
corresponds to an "NSRA Request" by the receiver. Next, in step
S160, the transmitter receives the "NSRA Request" and uses an
inverted synchronization (SYNC) symbol as a flag to signal the
receiver that the updated BAT is going to be used. The updated BAT
is used for transmission on the first frame, for a finite number of
frames, following the inverted SYNC symbol. The inverted SYNC
symbol operates as a rate adaptation "SRA GO" message sent by the
transmitter. Then, in step S170, the receiver detects the inverted
SYNC symbol, "SRA GO," and the updated BAT is used for reception on
the first frame, or for a finite number of frames, following the
inverted SYNC symbol. Control then continues to step S190, where
the control sequence ends.
[0112] FIG. 9 illustrates the method of performing a
transmitter-initiated NSRA during initialization. In particular,
control begins in step S200 and continues to step S220, where the
transmitter determines whether the data rate should be modified,
i.e., increased or decreased. If the data rate is to be modified,
control continues to step S230. Otherwise, control jumps to step
S295 where the control sequence ends.
[0113] In step S230, having determined the modified data rate, the
capabilities of the receiver are checked to determine if the
desired data rate is within the receiver's rate capability. Next,
in step S240, a determination is made whether the data rate is
acceptable. If the data rate is acceptable, control continues to
step S250. Otherwise, control jumps back to step S220.
[0114] In step S250, the transmitter forwards to the receiver the
updated data rate using the EOC or AOC channel. This corresponds to
an "NSRA Request" message. Next, in step S260, a determination is
made, based on the NSRA request, whether the channel can support
the new data rate. If the channel can support the new data rate,
control continues to step S270. Otherwise, control jumps to step
S265, where an "SRA DENY" message is sent back to the transmitter
using, for example, the EOC or AOC channel.
[0115] In step S270, the receiver forwards the updated BAT which is
the determined actual parameter value in this example, to the
transmitter using, for example, the AOC or EOC channel based on the
updated data rate. This corresponds to an "NSRA GRANT" request by
the receiver. Next, in step S280, the transmitter receives the
"NSRA GRANT" message and uses an inverted SYNC symbol as a flag to
signal the receiver that the new BAT is going to be used. This new
BAT is used for transmission on the first frame, or a finite number
of frames, following the inverted SYNC symbol. The inverted SYNC
symbol operates as a rate adaptation "SRA GO" message sent by the
transmitter. Then, in step S290, the receiver detects the inverted
SYNC symbol "SRA GO" and the updated BAT is used for reception on
the first frame, or for a finite number of frames, following the
inverted SYNC symbol.
[0116] The rate adaptation involves changing the number of bits in
a DMT symbol by changing the BAT, and not the R-S codeword size,
interleaver depth, or the ADSL frame size. This can be done without
any interruption in data flow or an introduction of data
errors.
[0117] This protocol is robust in that it does not use the EOC or
AOC channel to send the "SRA GO" message for synchronizing the
transition to the new data rate, such channels easily corrupting
messages transmitted therein.
[0118] With the above methods, the "SRA GO" message is communicated
via an inverted SYNC symbol. The SYNC symbol is defined in the ANSI
and IT standards as a fixed non-data carrying DMT symbol that is
transmitted every 69 symbols. The SYNC symbol is constructed by
modulating all the DMT carriers with a predetermined PN sequence
using basic QPSK (2-bit QAM modulation). This signal, which may be
used throughout the modem initialization process, has a special
auto-correlation property that makes possible the detection of the
SYNC symbol and the inverted SYNC symbol even in highly noisy
environments. An inverted SYNC symbol is a SYNC symbol in which the
phase information in the QAM signal is shifted by 180 degrees.
However, phase shifts other than 180 degrees of the SYNC symbol can
be used equally well for the "SRA GO" message. Using the SYNC
symbol for the "SRA GO" message makes the rate adaptation protocol
very robust, even in noisy environments. However, in general, any
symbol that can be detected in the presence of noise can be used in
place of the SYNC symbol.
[0119] The Fast SRA (FSRA) protocol seamlessly changes the data
rate on the line faster than the NSRA protocol. In the FSRA
protocol, the predetermined parameter values are stored BATs which
may be used to speed up the SRA and enable quick changes in the
data rate. Unlike the profiles used in G.992.2, the stored BATs do
not contain the R-S coding and interleaving parameters since these
parameters are not affected when a data rate change occurs using
the constant percentage overhead framing.
[0120] The BATs are exchanged using the NSRA method described in
the previous section. After the one-time NSRA is complete, and a
BAT that is based on the particular channel condition or
application condition is stored by both transceivers, the FSRA
protocol can use the stored BAT to complete fast on-line rate
adaptation. Stored BATs are identified so that both the transmitter
and receiver simply need to notify or point to the other
transceiver which table is being used without actually having to
transmit the information redundantly. For example, the stored BATs
may be numbered. The transmitter or receiver simply needs to tell
the other transceiver which BAT table number is to be used for
subsequent transmission. As with the NSRA method, either the
receiver or the transmitter can initiate the FSRA protocol.
[0121] In particular, and with reference to FIG. 10, the
receiver-initiated FSRA protocol commences in step S300 and
continues to step S320 where a determination is made whether the
data rate should be modified. If the data rate is to be modified,
control continues to step S330. Otherwise, control jumps to step
S390, where the control sequence ends.
[0122] In step S330, the receiver attempts to locate a stored BAT
that matches the channel and/or application condition. Next, in
step S340, a determination is made whether a stored BAT has been
found that matches the conditions. If there is no stored BAT that
matches the condition, control continues to step S345, where an
NSRA is performed. Control then continues to step S390.
[0123] In step S350, if a BAT is found that matches the condition,
the receiver sends a message to the transmitter specifying which
stored BAT is to be used for transmission based on the new channel
and/or application condition. This corresponds to an "FSRA Request"
by the receiver. Next, in step S360, the transmitter receives the
FSRA request and uses an inverted SYNC symbol as a flag to signal
the receiver that the requested stored BAT will be used for
transmission. The stored BAT is used for transmission on the first
frame, or a finite number of frames, following the inverted SYNC
symbol. The inverted SYNC symbol corresponds to a rate adaptation
"SRA GO" message sent by the transmitter. Next, in step S370, the
receiver detects the inverted SYNC symbol. Then, in step S380, the
updated BAT is used for reception on the first frame, or for a
finite number of frames, following the inverted SYNC symbol.
Control then continues to step S390, where the control sequence
ends.
[0124] FIG. 11 illustrates a method of performing the fast seamless
rate adaptive transmission bit rate changes which are transmitter
initiated. In particular, control begins in step S400 and continues
to step S420 where a determination is made whether the data rate
should be modified. If the data rate is to be modified which, for
example, matches a channel condition, control continues to step
S440. Otherwise, control jumps to step S490, where the control
sequence ends.
[0125] In step S430, the transmitter attempts to locate a stored
BAT that matches the channel condition. Next, in step S440, a
determination is made whether the stored BAT is available. If the
stored BAT is not available, control continues to step S445 where
the NSRA sequence is initiated. Control then continues to step
S490.
[0126] However, in step S450, if a stored BAT matches the channel
condition, the transmitter sends a message to the receiver
specifying which stored BAT is to be used for transmission based on
the channel and/or application condition. This corresponds to an
FSRA request by the transmitter. Next, in step S460, the receiver
receives the FSRA request and returns to the transmitter the FSRA
Grant message to grant the FSRA request. Then, in step S470, the
transmitter uses an inverted SYNC symbol as a flag to signal the
receiver that the requested stored BAT will be used for
transmission. Control then continues to step S480.
[0127] In step S480, the specified stored BAT is used for
transmission on the first frame, or for a finite number of frames
following the inverted SYNC symbol. The inverted SYNC symbol
corresponds to a rate adaptation "SRA GO" message sent by the
transmitter.
[0128] In step S480, the receiver detects the inverted SYNC symbol
"SRA GO" and the stored BAT is used for reception on the first
frame, or for a finite number of frames, following the inverted
SYNC symbol.
[0129] The FSRA protocol can be completed very quickly. It only
requires the exchange of two messages, i.e., the FSRA Grant and the
FSRA Request and an inverted SYNC symbol. FSRA is faster than NSRA
because, for example, the BAT is stored and need not be
re-transmitted. As in the NSRA protocol, the FSRA protocol is also
very robust in noisy environments since it uses an inverted SYNC
symbol for the "SRA GO" message.
[0130] The SRA protocols described above may also be used to manage
power during the initialization of modems of the transceivers. Full
power mode is used during normal operations of the transceiver. Low
power transmission modes are often used in transceivers in order to
conserve power in cases when data does not need to be transmitted
over the line. Many modems have low power modes or "sleep" modes
that enable a transceiver to operate at a significantly lower power
level when the transmission requirements are reduced. Many modems
also have protocols that enable them to enter and exit these low
power modes very quickly so that the user is not negatively
effected by the modem's transition into the low power mode state.
The SRA protocols provided of the invention are used to enter and
exit from low power modes in a very fast and seamless manner. For
instance, the modems of the transceivers may be first operated at a
low power level to establish the communication link and then, the
data rate of the communication links be increased by seamlessly in
changing to an updated power level.
[0131] There are two basic types of low power mode (LPM). The first
is Low Data Rate LPM is low power mode with a very low data rate
(e.g. 32 kbps). Only a few of the subchannels are active. The data
connection is maintained. The pilot tone may also be transmitted in
order to maintain loop timing.
[0132] Another is the Zero Data Rate LPM which is a low power mode
with an effectively 0 kbps data rate, i.e., no subchannels are
modulating data. A data connection is not maintained. The pilot
tone may also be transmitted in this case in order to maintain loop
timing.
[0133] In both the Low Data Rate LPM and the Zero Data Rate LPM,
the sync symbol, which is sent in normal full power mode every 69
symbols, may be on or off. If the sync symbol is still transmitted
during the low power mode, the receiver can use the sync symbol to
monitor for channel changes and other fluctuations on the line.
However transmission of the sync symbol every 69 symbols can cause
non-stationary crosstalk and could be detrimental to other signals
on the same telephone wire or in the same wire bundle. If the sync
symbol is not transmitted during low power mode, there is no
non-stationary crosstalk on the telephone wire or the wire bundle.
However, in this case the receiver is not able to monitor the
channel with the sync symbol.
[0134] FSRA may be used to enter the low power mode during
initialization of the modems in the transceivers. In one example,
the receiver initiates the transition to low power mode using the
receiver-initiated FSRA protocol. A receiver initiating the
transition to low power mode uses a predetermined stored BAT
corresponding to the low power mode. The stored BAT table for the
low power mode may enable either a Low Data Rate LPM or a Zero Data
Rate LPM. The low power mode BAT can be predetermined by the system
or can be exchanged and stored using the NSRA process. In either
case the receiver uses the receiver-initiated FSRA protocol to
designate the low power mode BAT and synchronously switch to using
that BAT for transmission.
[0135] The transmitter may also initiate transition into the low
power mode. There are two exemplary ways the transmitter can use
the transmitter-initiated FSRA protocol to enter into the low power
mode. In one embodiment, the transmitter can use the entire
transmitter-initiated FSRA process and request the transition. As
in the case of receiver-initiated transition into low power mode,
transmitter initiating the transition to low power mode uses a
predetermined stored BAT for the low power mode. The stored BAT
table for the low power mode can enable either a Low Data Rate LPM
or a Zero Data Rate LPM. The low power mode BAT can be
predetermined by the system or can be exchanged and stored using
the NSRA process. In either case the transmitter uses the
transmitter-initiated FSRA protocol to designate the low power mode
BAT and synchronously switches to the low power mode using that BAT
for transmission.
[0136] In a second exemplary embodiment, the transmitter can
transition directly to send the inverted sync symbol to indicate
transition into the low power mode during the transmitter initiated
FSRA protocol described above. The receiver detects the inverted
sync and transitions to the low power mode. In this case, since an
FSRA request has not been sent by the transmitter, the receiver
recognizes that an inverted sync symbol received without a FSRA
request transmitted indicates that the transmitter is switching to
low power mode. The low power mode BAT is predetermined by the
system or is identified and stored previously so that both the
transmitter and the receiver use the BAT. In an alternative second
embodiment, the transmitter sends a different signal that is
predetermined by the transmitter and the receiver to be the signal
used for transition into low power mode without an "FSRA request."
For example, the transmitter may send a sync symbol with 45 degree
phase rotation, rather than the inverted (180 degree) sync symbol.
A sync symbol with a 45 degree phase rotation indicates that the
transmitter is transitioning into low power mode using the stored
BAT associated with the low power mode on the first frame, or a
finite number of frames, following the sync symbol with a 45 degree
rotation. The transmitter-initiated entry into low power mode as
defined in the second embodiment has the advantage that it does not
require the reverse channel to make the transition. The reverse
channel is defined as the communications channel in the opposite
direction, i.e., here, the communications channel used to send the
FSRA messages from the receiver to the transmitter.
[0137] This is advantageous because the reverse channel may already
be in low power mode with no data connection. If there is no data
ready to be sent, the transmitter can simply transition to low
power mode. This is an important power savings technique since the
transmitter consumes a large portion of the power, as it is
required to send the signal down the line. Transmitter-initiated
transition into low power modes is also useful in "soft modem" (PC
host based) implementations. In a soft modem implementation, the
host processor is performing the modem transceiver functions and
many other PC applications at the same time. If the host processor
must perform another task that does not allow it to run the ADSL
transmitter, the processor can quickly transition the transmitter
to the low power mode by sending the inverted sync symbol, or the
sync symbol with 45 degree rotation. After this the host processor
resources can be consumed by the other task. The ADSL transmitter
sends no signal (0 kbps) onto the line. The transmitter-initiated
and receiver-initiated protocols described above enable the
communication system to enter a low power mode in each direction
(upstream and downstream) separately or in both directions
together. The cases described above each focus on one direction.
The protocols can be combined to accomplish transition in both
directions at the same time. As an example, assume that the
customer premise transceiver (CPT) is designed to enter into a low
power mode in response to a PC that is also entering a similar
state. The CPT first uses receiver-initiated low power mode
transition to put the downstream (CO to CPT direction) into low
power mode. Afterwards the CPT uses the transmitter-initiated low
power mode transition to put the upstream (CPE to CO direction)
into low power mode.
[0138] According to the SRA protocols, there are two embodiments
the receiver can use to exit the low power mode during
initialization of the modems of the transceivers. In the first
embodiment, receiver-initiated exit from low power mode can be
accomplished using the receiver initiated NSRA or FSRA protocol if
the low power mode still has at least a slow data connection in the
reverse direction (low data rate LPM). This is necessary because
the receiver must be capable of sending the SRA request back to the
transmitter along with the BAT to be used. If the transmitter has
not turned off the sync symbol in low power mode the NSRA or FSRA
protocols would be used as described above. If the transmitter sync
symbol is turned off while in low power mode, the "SRA Go" is sent
by the transmitter by turning the sync symbol back on. The receiver
detects the presence of the sync symbol (with or without inversion)
as a flag to synchronize the change in data rate.
[0139] In a second embodiment, there is no data connection in the
reverse direction (Zero Data Rate LPM). The receiver initiates an
exit by first completing a "transmitter-initiated exit from low
power mode (described below) in the reverse direction. This enables
the data connection in the reverse direction. The receiver uses the
receiver initiated NSRA or FSRA protocol to exit from low power
mode in it's own direction. As described above, if the transmitter
sync symbol is turned off while in low power mode, the "SRA Go" is
sent by the transmitter by turning the sync symbol back on. The
receiver detects the presence of the sync symbol (with or without
inversion) as a flag to synchronize the change in data rate.
[0140] According to the SRA protocols, there are two embodiments
the transmitter can use to exit from low power mode during
initialization of the modems of the transceivers. In the first
embodiment, the transmitter uses the entire transmitter initiated
FSRA or NSRA process and requests the transition. This requires
that there is a data connection in both directions (Low data rate
LPM) so the protocol messages can be exchanged. As in the
receiver-initiated exit from low power mode, if the transmitter has
not turned off the sync symbol in low power mode the NSRA or FSRA
protocols would be used as described above. If the transmitter had
turned the sync symbol off while in low power mode, then the "SRA
Go" is sent by the transmitter by turning the sync symbol back on.
The receiver detects the presence of the sync symbol (with or
without inversion) as a flag to synchronize the change in data
rate.
[0141] In the second embodiment, the transmitter can exit the low
power mode by sending the inverted sync symbol to indicate
transition out of the low power mode. This requires that a sync
symbol be sent during the low power mode. This protocol does not
require a low data rate LPM. The receiver detects the inverted sync
and exits the low power mode. The receiver is designed to recognize
that an inverted sync symbol received without a FSRA request
indicates the transmitter is exiting from low power mode. The full
power mode BAT is identified and stored previously so that both the
transmitter and the receiver have the BAT. For example, the BAT to
be used upon exiting a low power mode can be defined by the system
to default to the BAT of the last full power connection.
Alternatively, the transmitter can send a different signal that is
predetermined by the transmitter and the receiver to be the signal
used for transition out of low power mode without an "FSRA
request." For example, the transmitter can send a sync symbol with
45 degree phase rotation, rather than the inverted (180 degree)
sync symbol. When the receiver detects the sync symbol with a 45
degree phase rotation, the receiver recognizes that the transmitter
is transitioning out of low power mode using the stored BAT
associated with the full power mode on the first frame, or a finite
number of frames, following the sync symbol with a 45 degree
rotation. If the transmitter had turned the sync symbol off while
in low power mode, then the "SRA Go" is sent by the transmitter by
turning the sync symbol back on. The receiver detects the presence
of the sync symbol (with or without a phase shift) as a flag to
synchronize the change in data rate.
[0142] Although throughout this description, the BAT is defined to
be a table that specifies the number of bits allocated to each
subchannel, the BAT can also contain other parameters associated
with allocating bits to subchannels in a multicarrier system. An
example of an additional parameter is the Fine Gain per subchannel
as defined in the ANSI and ITU standards. In this case, when the
BAT is exchanged during the NSRA protocol or the BAT is stored
during the FSRA protocol, the BAT also contains the Fine Gain value
for each subchannel.
[0143] The seamless rate adaptive system and associated protocols
described above which may be used for seamlessly increasing the
data rate of the established communication link may also be applied
to DMT systems that implement dual (or multiple) latency paths. A
dual latency system is defined in the ITU and ANSI standards as a
DMT system that supports two data streams with different latency
specifications in the Framer/FEC block.
[0144] FIG. 7 illustrates a standard ADSL DMT system 300 that
implements dual latency, as an example of a system having a
plurality of latencies. The system 300 includes three layers: the
Modulation layer 310, the Framer/FEC layer 320, and the ATM TC
layer 340, which are similar but not identical to the three layers
described above in relation to FIG. 1.
[0145] The Modulation layer 310 provides the functionality
associated with the DMT modulation. The DMT modulation is
implemented using a Inverse Discrete Fourier Transform (IDFT) 112.
The IDFT 112 modulates the bits from the dual input Quadrature
Amplitude Modulation (QAM) 314 encoder into the multicarrier
subchannels. The operation of the Modulation layer 310 is analogous
to that of Modulation layer 110 of FIG. 1, with the difference that
the Modulation layer 310 has multiple inputs, rather than only one
input.
[0146] The Framer/FEC layer 320 shown in FIG. 7 has two paths. This
layer contains a first path that includes the same portions as in
the Frame/FEC layer 120 of FIG. 1, namely the Interleaving (INT)
portion 122, the Forward Error Correction (FEC) portion 124, the
scrambler (SCR) portion 126, the Cyclic Redundancy Check (CRC)
portion 128 and the ADSL Framer portion 130. The layer further
contains a second path that includes a second one of each of the
Forward Error Correction (FEC) portion 124', the scrambler (SCR)
portion 126', the Cyclic Redundancy Check (CRC) portion 128' and
the ADSL Framer portion 130'. The Frame/FEC layer 320 provides
functionality associated with preparing a stream of bits for
modulation.
[0147] The lower path through the Framer/FEC layer 320 has a
different amount of latency than the original upper path
corresponding to FIG. 1, because the lower path does not perform
interleaving on the data stream. Dual latency is used to send
different bit streams with different latency requirements through
the ADSL DMT modem. The ATM TC layer 340 includes an ATM TC portion
342 having multiple inputs and multiple outputs that transforms
bits and bytes in cells into frames for each path.
[0148] The exemplary seamless rate adaptation system and method of
the present invention also applies to a system with dual latency,
or even multiple latencies. In the case of dual latency, the FEC
and interleaving parameters for both paths are decoupled from the
DMT symbol size. The BAT contains, in addition to the number of
bits allocated to each subchannel, the data rate for each latency
path in the form of bits per DMT symbol. When seamless rate
adaptations are performed using the FSRA and NSRA protocols, the
BAT also indicates the data rate for each latency path. For
example, if the dual latency system runs with 1.536 Mbps on the
interleaved path, e.g., a high latency upper path, and 256 kbps in
the non-interleaved path, e.g., a low latency lower path, and an
SRA is initiated, then the SRA protocol specifies the updated
parameter value such as an updated BAT containing the number of
bits per subchannel and also the new data rate for each latency
path. At a 4 kHz DMT symbol rate, a system running at 1.536
Mbps+256 kbps=1.792 Mbps. 1792000/4000=448 total bits per symbol.
The BAT specifies that 1536000/4000=384 bits per symbol are
allocated to the interleaved path and 256000/4000=64 bits per
symbol are allocated to the non-interleaved path. In the example,
when an SRA is performed, the updated data rate for the interleaved
path can be 1.048 Mbps, i.e., 1048000/4000=262 bits per symbol, and
the new data rate for the non-interleaved path can be 128 kbps,
i.e., 128000/4000=32 bits per DMT symbol, resulting in a total
throughput rate of 1.176 kbps, or 294 total bits per DMT symbol.
The NSRA and FSRA protocols combined with the framing method
specified herein complete this data rate change in both latency
paths in a seamless manner. It is also possible to not change the
data rate on both latency paths.
[0149] These basic concepts can be expanded to encompass the
transportation of a network timing reference (NTR) in an single or
multiple latency ADSL DMT system. Specifically, the transportation
of the NTR involves sending a timing reference signal from a CO
modem to a CPE modem. This enables the CPE modem to reconstruct the
network clock in order to send and receive signals or applications
that are synchronous to the network clock, such as voice over
DSL.
[0150] As discussed above, the framing layer is decoupled from the
modulation layer. As a result, the NTR signal cannot be inserted at
the framing layer as is done in the current ADSL standards
specified in the ITU and ANSI. Furthermore, the SRA enables the
system to change the data rate in a seamless manner by updating the
total number of bits per DMT symbol. This is exactly what is
necessary in order to transport the NTR since by using a subset of
the subchannels to transport the NTR on a specific DMT symbol, the
number of bits per DMT symbol is changing from one DMT symbol to
another. The SRA methods discussed above allow this to happen
seamlessly. However, it is to appreciated that the SRA enables the
transport of the NTR regardless of whether the BAT is actually
modified on the DMT symbol transporting the NTR, since the total
number of bits per DMT symbol for the regular information data is
changing from one DMT symbol to another.
[0151] Therefore, the NTR signal is inserted and transported at the
modulation layer by sending the NTR bits, for example, as specified
in the ADSL standard, on a set of carriers of a specified DMT
symbol in a superframe. For example, the NTR bits can be sent on
the first DMT symbol of the superframe. Thus, for the other DMT
symbols in the superframe, the set of carriers used for
transporting the NTR can be used to transport other data, such as
information data.
[0152] This versatility allows the same BAT to be used for the DMT
symbol with the NTR bits and the DMT symbol without the NTR bits.
However, a different BAT can be used for the DMT symbol that sends
the NTR bits, than the DMT symbol(s) that do not send the NTR
bits.
[0153] In the first case, for the DMT symbol with the NTR bits, a
number of subchannels are used to transport the NTR bits, while for
DMT symbols without NTR bits, these subchannels are used to
transport other data, such as information data. For the second
case, where the different BATs are used, the use of different BATs
can take advantage of sending the NTR bits with, for example, a
higher margin than the regular information bits. This can be
especially useful since, the NTR signal may or may not be coded
with the FEC coding scheme as the regular information bits.
[0154] As an example, during the DMT symbol that transports the NTR
bits, the BAT in Table 2 can be used. During the DMT symbols
without NTR bits, the BAT in Table 3 can be used. For example,
during the DMT symbol that transports the NTR bits, the NTR signal
is transmitted in a 4 bit message, as is done in the current ADSL
standard, on subchannels 1, 3 and 6 with a high margin.
2TABLE 2 Subchannel Number Bits Allocated to Subchannel 1 1 (NTR) 2
6 3 1 (NTR) 4 5 5 4 6 2 (NTR) 7 5 8 5 9 6 10 4 11 5 Total bits per
symbol allocated to NTR = 4 Total bits per symbol allocated to
regular information data = 40
[0155] When the NTR is not being sent, Table 3 illustrates that the
BAT changed and that subchannels 1, 3 and 6 are used to transport
information data.
3TABLE 3 Subchannel Number Bits Allocated to Subchannel 1 5 2 6 3 6
4 5 5 4 6 4 7 5 8 5 9 6 10 4 11 5 Total bits per symbol allocated
to NTR = 0 Total bits per symbol allocated to regular information
data = 55
[0156] While the above examples illustrate the use of subchannels
1, 3 and 6, it is to be appreciated that any subchannels, or
combination thereof, can be used with equal success in accordance
with this invention.
[0157] FIG. 12 illustrates an exemplary method of transporting an
NTR from a CO modem, to a modem according to this invention. In
particular, control begins in step S500 and continues to step S510.
In step S510, a determination is made wether to update the network
clock. This update is typically done on a periodic basis, for
example, every 69 DMT symbols, in order to allow the receiver to
track the network clock using a timing recovery method, such as a
phase lock loop. If the network clock is to be updated, control
continues to step S520. Otherwise, control jumps to step S595 where
the control sequence ends.
[0158] In step S520, the NTR information is assembled. Next, in
step S530, a determination is made whether the same BAT is to be
used for both the normal DMT symbols, i.e., those that do not
contain the NTR bits, and the DMT symbols that are used for
transmission of the NTR bits. If the same BAT is to be used,
control jumps to step S550. Otherwise, control continues to step
S540.
[0159] In step S540, a BAT for use in transporting the NTR bits is
selected. Control then continues to step S550. In step S550, the
NTR is inserted at the modulation layer. This is done, for example,
on the first DMT symbol of a superframe. Next, in step S560, a
determination is made whether additional information bits are also
to be added to the BAT. If additional information bits are to be
added, control continues to step S570. Otherwise, control jumps to
step S580. In most cases, additional information bits are added to
the BAT. However, if the data rate is very low, then the NTR bits
may be the only bits transmitted on that DMT symbol.
[0160] In step S570, the information bits are added to the BAT.
Control then continues to step S580. In step S580, the NTR is
transported to the CPE modem. Then, in step S590, the CPE modem
receives the NTR and synchronizes the CPE clock. Control then
continues to step S595 where the control sequence ends.
[0161] The present invention for initializing modems of
transceivers in a multicarrier transmission system and
related-components can be implemented either on a DSL modem, such
as an ADSL modem, or separate programmed general purpose computer
having a communication device. However, the present method can also
be implemented in a special purpose computer, a programmed
microprocessor or a microcontroller and peripheral integrated
circuit element, an ASIC or other integrated circuit, a digital
signal processor, a hardwired or electronic logic circuit such as a
discrete element circuit, a programmable logic device, such as a
PLD, PLA, FPGA, PAL, or the like, and associated communications
equipment.
[0162] Furthermore, the disclosed method may be readily implemented
in software using object or object-oriented software development
environments that provide portable source code that can be used on
a variety of computers, workstations, or modem hardware and/or
software platforms. Alternatively, the method may be implemented
partially or fully in hardware using standard logic circuits or a
VLSI design. Other software or hardware can be used to implement
the methods in accordance with this invention depending on the
speed and/or efficiency requirements, the particular function, and
the particular software and/or hardware or microprocessor or
microcomputer being utilized. Of course, the present method can
also be readily implemented in a hardware and/or software using any
known later developed systems or structures, devices and/or
software by those of ordinary skill in the applicable art from the
functional description provided herein and with a general basic
knowledge of the computer and telecommunications arts.
[0163] Moreover, the disclosed methods can be readily implemented
as software executed on a programmed general purpose computer, a
special purpose computer, a microprocessor and associated
communications equipment, a modem, such as a DSL modem, or the
like. In these instances, the methods and systems of this invention
can be implemented as a program embedded in a modem, such as a DSL
modem, or the like. The method can also be implemented by
physically incorporating the method into a software and/or
hardware, such as a hardware and software system of a multicarrier
information transceiver, such as an ADSL modem, VDSL modem, network
interface card, or the like.
[0164] Thus, it should be evident from the discussion above how the
present invention provides an improved method for initializing
modems of transceivers in a multicarrier transmission system to
establish a communication link between the transmitter and the
receiver. By providing and using a predetermined parameter value
that approximates a corresponding actual parameter value of the
communication link, a data communication link may be attained very
quickly to allow the transmission of data. Then, the actual
parameter value may be determined and the data rate of the
communication link may be seamlessly updated using the determined
actual parameter value and the SRA methods described to provide an
steady state communication link.
[0165] While this invention has been described in conjunction with
a number of embodiments, it is evident that many alternatives,
modifications and variations would be or are apparent to those of
ordinary skill in the applicable art. Accordingly, applicants
intend to embrace all such alternatives, modifications, equivalents
and variations that are within the spirit and the scope of this
invention.
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