Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LOOP DIAGNOSTIC MODE FOR ADSL MODEMS
BACKGROUND OF THE INVENTION
0001 The present invention relates to a system and method for diagnosing
errors on
asymmetric digital subscriber line (ADSL) subscriber loops, wherein a
successful connection
cannot be achieved.
0002 Remote access and retrieval of data is becoming increasingly popular in
data
communication. The proliferation of the Internet has provided a vast network
of information that
is available to the general public. As the Internet grows and technology
advances, this
information is becoming increasingly voluminous and the details are become
increasingly
intricate. What used to comprise mainly text information has grown to include
still and moving
images as well as sound. The increase in the volume of information to be
transferred has
presented a need for a high-speed Internet connection, since traditional
telephone modems
communicate at speeds to slow for efficient communication.
0003 One proposal for high-speed communication. is the introduction of Digital
Subscriber
Line (DSL) technology. One of the most attractive features of DSL is that it
is implemented
using an infrastructure that already exists. DSL shares copper twisted pair
lines typically used
for telephone communication. However, only a small portion of the available
bandwidth of the
twisted pair line (0 to 4kHz) is used for Plain Old Telephone Service (POTS).
DSL takes
advantage of the available frequency spectrum from 4kHz to approximately
1.lMHz for
transmitting data.
0004 Asymmetric DSL (ADSL) is currently the most practical form of DSL
technology, and
therefore the most widely implemented. ADSL is asymmetric in that its
downstream (to a
subscriber) capacity is larger than its upstream (from the subscriber)
capacity. Typically, a
Discrete Multi-tone (DMT) scheme is used. The spectrum from 4kHz to 1.lMHz is
divided into
256 sub-channels, or tones, each having a bandwidth of 4.3125kHz. Each sub-
channel uses
Quadrature Amplitude Modulation (QAM) to carry 2 to 15 bitslQAM symbol.
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0005 In accordance with ADSL standard ITU 6.992.2, several phases occur in
order to
initialize a communication link. These phases include handshaking, transceiver
training, channel
analysis and exchange.
0006 Handshaking is used for determining the nature and capabilities of
communication
endpoints (such as an ADSL modem) and for indicating which protocol will be
used for the
remainder of the initialization. The ADSL modem, or termination unit, at a
central office is
referred to as an ATU-C. Similarly, the ADSL termination unit at the
subscriber, or remote
location, is referred to as the ATU-R.
0007 The signalling method used for the handshake interchange is designed to
be robust.
Biphase shift keying (BPSK) modulation is often used to modulate multiple
single-tone
sub-carriers, all carrying the same data. Typically, the ATU-C and ATU-R
exchange a message
containing information about the endpoint type, frequency range, and number of
DMT
sub-carriers supported.
0008 During transceiver training, the transceivers at each end of the line
acquire a DMT
symbol stream, adjust receiver gain, perform symbol-timing recovery, and train
any equalizers.
There is an optional echo cancellation training step that can also be
performed during this phase.
0009 During channel analysis, the transceivers exchange capability information
and perform
detailed channel characterization. Both the ATU-R and ATU-C attempt to measure
specific
channel characteristics such as unusable sub-carriers, loop attenuation on a
per sub-carrier basis,
SNRs, and any other channel impairments that would affect the potential
transmitted bit rates.
Based on the discovered channel characteristics, the ATU-C makes the first
offer of the overall
bit rates and coding overhead that will be used for the connection.
0010 The exchange phase sets the final overall transmission rates in both the
upstream and
downstream directions for the connection. These final rates are determined
based on calculated
channel parameters measured during the channel analysis phase, and are not
necessarily the same
as the preliminary rates offered during that phase.
0011 Furthermore, the exchange phase sets forward error correction (FEC) and
interleaver
configurations. Generally, the configurations are close to the optimum bit
rate for the channels.
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Four earners are used to modulate the bits of the messages, each carrier being
loaded with 2 bits
using quadrature phase shift key (QPSK) modulation.
0012 Since the ATU-C controls data rates, if the ATU-R cannot support any of
the offered
rates, both terminals will return to the beginning of the initialization
process. Otherwise the
ATU-R responds with the rate it can support.
0013 However, sometimes the transceivers may not be capable of successfully
reaching a data
transfer stage, referred to as SHOWTIME, due to poor channel conditions. In
this case there is a
need for a system to be able to diagnose the problems preventing the
transceivers from
initializing so that they can be corrected or avoided.
0014 It is an object of the present invention to obviate or mitigate at least
some of the above
mentioned disadvantages.
SU1VINIARY OF THE INVENTION
0015 In accordance with an aspect of the present invention, there is provided
a procedure for
exchanging diagnostic information between an ATU-R an the ATU-C when line
conditions are
too poor for the modems to initialize in a standard compliant manner.
0016 It is an advantage of the present invention that the transceivers are
able to exchange
diagnostic information during training.
0017 It is a further advantage of the present invention that the measured
diagnostic information
can be exchanged reliably, even in poor channel conditions.
0018 In accordance with another aspect of the invention there is provided a
method for
establishing communication in an ADSL subscriber loop, the method comprising
the steps of
determining that showtime cannot be entered during initialisation of
communication between the
modems; requesting entry into a diagnostic mode by one of the modems upon the
determining;
diagnosing line conditions as being unable to support communication at a
predetermined
standard; and establishing communication at a standard lower than the
predetermined standard.
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BRIEF DESCRIPTION OF THE DRAWINGS
0019 An embodiment of the invention will now be described by way of example
only with
reference to the following drawings in which:
Figure 1 is loop diagnostic timing diagram;
Figure 2 is a schematic diagram of a message format;
Figure 3 is a schematic diagram of a channel attenuation message;
Figure 4 is a schematic diagram of a quiet line noise power spectral density
message;
Figure 5 is a schematic diagram of a signal to noise ratio message;
Figure 6 is a schematic diagram of a message combining all the parameters of
Figures 3-5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIIVVIENT
0020 For convenience, like numerals in the description refer to like
structures in the drawings.
Referring to Figure 1, a loop diagnostic timing diagram is illustrated. The
timing diagram is
illustrated with two columns. A first column represents signals transmitted
from the ATU-C. A
second column represents signals transmitted from the ATU-R. Time is represent
vertically and
progresses from the top of the columns to the bottom.
0021 During Initialization, if it is determined that SHOWTIIVVIE cannot be
entered
satisfactorily, either the ATU-C or the ATU-R requests entry into a diagnostic
mode. Once a
request to enter diagnostic mode is made, the transceivers proceed to repeat a
normal
Initialization. However, after a signal-to-noise ratio SNR measurement is
performed, the
transceivers enter into a diagnostic link mode. In current standards, the SNR
is determined
during C-MEDLEY and R-MEDLEY, and the diagnostic mode is entered after
C-EXCHMARKER and R-EXCHMARKER, which immediately follow the MEDLEY states.
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During the diagnostic states, channel information that has been gathered
during previous
Initialization states are exchanged.
0022 During the loop diagnostic sequence of states, a counter that is
typically initialized at the
start of the-MEDLEY states is kept counting. The counter is used to fix the
size of the state
transitions. Preferably, any state transition occurs at multiples of 64 of the
counter value.
0023 Further, it is preferable that the messages conveying channel information
use a 1/8
bit/DMT modulation, where a "1" is encoded as eight consecutive REVERB symbols
while a
"0" is encoded as eight consecutive SEGUE symbols. REVERB and SEGUE symbols
are
defined in current standards. Alternately, it is possible that a "0" is
encoded as eight consecutive
REVERB symbols while a "1" is encoded as eight consecutive SEGUE symbols. 64
SEGUE
symbols, referred to as C -SEGUE-LD or R-SEGUE-LD, are defined as a time
marker and
precede a message.
0024 It is preferable that the diagnostic mode is very robust;. or at least as
robust as the G.hs
signaling technique. It is for this reason, that the REVERB and SEGUE
messaging is used
together with 8 symbols repeat technique. During both REVERB and SEGUE, a
periodic multi-
tone signal is transmitted across the loop. Since the signal is periodic, the
effects of inter-symbol
interference ISI and inter-channel interference ICI are greatly reduced and
the effect of timing
fitter on performance is improved. Also, time diversity can be exploited to
further improve
performance, either by averaging the repeated symbols prior to demodulation or
using majority
selection on the demodulated frames that nominally contain the same data.
0025 Messaging between the ATU-C and ATU-R is half-duplex in order to reduce
the effect of
echo on performance. This is preferable since echo-cancellers will not be able
to train properly
due to bad channel conditions.
0026 A corrupted received message does not trigger an Initialization reset
procedure, since
there is no benefit in going back to handshaking. On channels with poor signal
to noise ratio
(SNR) a second Initialization has a relatively high chance of failing with the
consequence that
loop diagnostic would be further delayed and there is a potential for the
process to degenerate
into an infinite loop condition. Therefore, the messaging protocol is designed
as a
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"repeat/request" technique, for which a certain number of attempts are made to
properly convey
a message. The specific number can be set by a vendor and is implementation
dependent.
0027 The duration of the states during Channel Discovery, Transceiver Training
and Channel
Analysis of the loop diagnostics procedure is fixed. That is, a state
transition on one end does
not rely upon the detection of a state transition on the far end. This
makes,the procedure much
more robust that relying on state detection at another end of the loop,
especially when the loop
condition is poor.
0028 A message is allowed to be longer in the upstream direction than in the
downstream
direction, where the channel conditions are typically better. It is often
possible to provide an
upstream message that is between two and three times longer than a downstream
message.
0029 Further, for debugging purposes, during the loop diagnostic procedure a
message
communicating the reason why the last Initialization in normal mode failed is
exchanged. In one
embodiment, this message is exchanged during C-MSG1 and R-MSG1 of the
Initialization
procedure instead of the standard message. Alternately, this message is
exchanged with other
messages, as will be described is detail later in the description. This
message further includes an
index of the last state that was successfully reached during the last
Initialization procedure. This
helps discriminate between failures due to bad loop conditions and failures
due to other reasons,
such as interoperability issues for example. Some examples of possible
failures and
corresponding message codes include:
a) Failed - Cause: Insufficient Capacity "00010001";
b) Failed - Cause: CRC error in one of the received messages "00100010";
c) Failed - Cause: Time outs "01000100";
d) Failed - Cause: Unexpected received message content "10001000";
e) Failed - Cause: Unknown "00000000"; and
f) Successful "11111111".
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0030 In the example illustrated in Figure 1, three different messages are sent
by the ATU-R.
These messages are referred to as R-MSG-LD1, R-MSG-LD2 and R-MSG-LD3. Only one
message, C-MSG-LD, is sent by the ATU-C. The number of messages has been
selected for
ease of illustration only, as will be appreciated by a person skilled in the
art.
0031 Referring to Figure 2, a format for the messages listed above is
illustrated generally by
numeral 200. The message comprises a sequence number field 202 and a body 204.
For the
upstream direction, that is data sent from the ATU-R, the sequence number
field indicates which
of the three messages is being sent. The first message R-MSG-LD1 is identified
by "00010001"
shall indicate, the second message R-MSG-LD2 is identified by "00100010", and
the third
message R-MSG-LD3 is identified by "01000100". There is only one message sent
downstream
C-MSG-LDl, and its sequence number is set to "00010001". As illustrated by the
various
sequence field numbers, a portion identifies the direction of the data, that
is upstream or
downstream, and a portion identifies the message number.
0032 In the present embodiment, the information fields of the different
messages are defined as
follows. Referring to Figure 3, data fields for the first message R-MSG-LD1
are illustrated
generally by numeral 300. The data in the first message represents the channel
attenuation of the
loop. Channel attenuation (ATN) is an estimate of the channel transfer
function referred to tip
and ring. Any receive gain and filter is removed from the estimate. Any
spectral shaping
applied at the transmitter is also taken into account by subtracting it from
the estimate. The ATN
is provided for each of the number of downstream channels Nds. One octet is
used to represent
each ATN(i), with its decimal value ATN(i), i=O,l,...,Nds-1, representing the
attenuation as a
multiple of 0.5 dB. That is, the channel attenuation in dB for bin i is
ATN(i)*0.5. An ATN(i)
value of 255 is reserved to indicate that a channel attenuation estimate is
not available for that
bin.
0033 Refernng to Figure 4, data fields for the second message R-MSG-LD2 are
illustrated
generally by numeral 400. The data in the second message represents the quiet
line noise power
spectral density (PSD) of the loop. Quiet Line Noise PSD (NSD) is an estimate
of the quiet line
noise PSD referred to tip and ring. Any receive gain and filter shall be
removed from the
estimate. One octet is used to represent each NSD(i), with its decimal value
NSD(i),
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i=0,1,...,Nds-1, representing the NSD in multiples of -ldBm/Hz. That is, the
NSD for bin i is
-NSD(i) dBm/Hz. An NSD(i) value of 255 is reserved to indicate that the noise
PSD estimate is
not available for that bin.
0034 Referring to Figure 5, data fields for the third message R-MSG-LD3 are
illustrated
generally by numeral 500. The data in the third message represents the signal
to noise ratio
(SNR) of the loop. The SNR represents as estimate of the Signal-to-Noise ratio
as measured
during the MEDLEY state. One octet is used to represent each SNR(i), with its
decimal value
SNR(i), i=0,1,...,Nds-l, representing the SNR in multiples of 0.5 dB. That is,
the SNR for bin i
is 0.5*SNR(i) dB. An SNR(i) value of 255 is reserved to indicate that the
channel attenuation
estimate is not available for that bin.
0035 Referring to Figure 6, data fields for the downstream message C-MSG-LD1
are
illustrated generally by numeral 600. The data in the upstream messages
represent the ATN,
NSD, and SNR of the loop. There are 3 upstream and one downstream messages, as
discussed
above regarding Figure 1.
0036 Generally, a 16-bit cyclic redundancy check (CRC) is appended to the
message and is
computed the same way as the CRC for a C/R-MSG1 signal used in the current
standards. The
same modulation technique as the one used for the message shall be used to
transmit the 16-bit
CRC.
0037 The parameters described above, those are the ATN, NSD, and SNR, are the
preferable
minimum parameters that should be transmitted for the diagnostics to be
considered useful.
Further it is preferred that an additional parameter, that is the attainable
data rate (ATTNDR), is
also transmitted. If the ATTNDR is not transmitted, it can typically be
estimated from the other
parameters. However, such an estimate is not always accurate and thus it is
preferred to transmit
the ATTNDR as well. While the transmission of the ATTNDR is not described it
detail herein,
it and other parameter's transmission will be apparent to a person skilled in
the art.
0038 Referring once again to Figure 1, the operation of the current embodiment
is described as
follows. Once the transceivers enter diagnostic mode, the ATU-C transmits a
filler signal
C-TREF-LD, which is the same as C-TREF1 in the current standard, which is a
symbol that
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contains a single tone. The filler signal C-TREF-LD lasts for a predefined
period of time, which
is preferably a multiple of 64. During this time, the ATU-R transmits the R-
SEGUE-LD signal,
which prefaces a message signal, and then the message signal itself, R-MSG-
LD1. The ATU-R
then transmits a filler signal R-QUIET-LD.
0039 Once the message R-MSG-LDl is received, the ATU-C acknowledges the
message using
C-ACK. In the present embodiment, the acknowledgement message C-ACK is
represented by as
"01010101" and is transmitted using the same 1/8 bit/symbol modulation
technique as that used
for the messages. If the ATU-C does not recognize or receive the message, it
continues to
transmit its filler signal C-TREF-LD. If the ATU-R does not receive the
acknowledgement
signal C-ACK within a predefined time period, it retransmits the first message
R-MSG-LD1 by
retransmitting R-SEGUE-LD and then first message R-MSG-LD 1.
0040 Further it is possible that the ATU-C transmits the acknowledgement
signal C-ACK, but
the ATU-R does not receive it. The ATU-R retransmits the first message R-MSG-
LDl as
described above. The ATU-C receives the message and, parsing the identifier,
determines that
the message has already been received. The ATU-C notes the message was
repeated and
retransmits the acknowledgement signal C-ACK. If a predefined number of
attempts to transmit
the message R-MSG-LDl all fail, then the process is aborted. The predefined
number is defined
by the vendor.
0041 If the ATU-R receives the acknowledgement signal C-ACK, it transmits the
second
message R-MSG-LD2, by transmitting R-SEGUE-LD and then the second message
R-MSG-LD2. A similar procedure is followed for the second and third messages
as it is for the
first message.
0042 After sending the last acknowledgement message C-ACK in response to the
third
message R-MSG-LD3 message, the ATU-C sends at least 256 symbols of C-TREF-LD.
The
ATU-R is programmed to send the three messages ATN, NSD and SNR as outlined
above. It
only repeats if it does not receive the C-ACK If an R-SEGUE-LD state is
detected, this is an
indication that the ACK message was corrupted and the ATU-R has transmitted R-
MSG-LD3
again.
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0043 However, the ATU-R begins to send a filler signal, which in the present
embodiment is
R-QUIET-LD, or no signal, once the last acknowledgement signal C-ACK is
received. Thus, if
The ATU-C does not detect an R-SEGUE-LD state within the timeframe, the ATU-C
sends a C-
SEGUE-LD state and its message C-MSG-LDl. A similar sequence of states to that
when the
ATU-R was transmitting the message follows.
0044 Similar conditions apply to the last acknowledgement message R-ACK
received from the
ATU-R than to the last acknowledgement message C-ACK received from the ATU-C.
After
sending the acknowledgement message R-ACK in response to the last C-MSG-LD1
message, the
ATU-R sends at least 256 symbols of R-QUIET-LD. If no C-SEGUE-LD state is
detected
within this timeframe the ATU-R assumes that the loop diagnostic procedure is
terminated and
enter the R-QUIET state.
0044 The procedure can also be used in the reverse direction from the ATU-C to
the ATU-R to
communicate the ATN, NSD and SNR values at the ATU-C, with all the messages
reversed.
0045 Although the invention has been described with reference to certain
specific
embodiments, various modifications thereof will be apparent to those skilled
in the art without
departing from the spirit and scope of the invention as outlined in the claims
appended hereto.
