Note: Descriptions are shown in the official language in which they were submitted.
CA 02353739 2001-07-24
AN IMPROVED SCHEME FOR THE INITIALIZATION OF ADSL MODEMS
The present invention relates generally to transfer of data using Digital
Subscriber Loop
(DSL) technology, and specifically to an improved scheme for initializing the
transfer.
BACKGROUND OF THE INVENTION
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. T'he 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.
One proposal for high-speed communication is the introduction of Digital
Subscriber
Line (DSL) technology. One of the most attractive i:eatures of DSL is that it
is
implemented using an infrastructure that already exists. I)SL 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 fre~~uency spectrum from
4kHz to
approximately 1.lMHz for transmitting data.
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 bits/QAM symbol.
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CA 02353739 2001-07-24
According to the ADSL ITU 6.992.2 standard, several phases occur in order to
initialize
a communication link. These phases include handshakin~;, transceiver training,
channel
analysis and exchange.
Handshaking is used for determining the nature and capabilities of
communication
endpoints (such as an ADSL modem) and for indicating v~rhich protocol will be
used for
the remainder of the initialization. The ADSL modem, o:r 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.
The signalling method used for the handshake interchange is designed to be
robust.
Biphase shift keying (BPSK) modulation is 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.
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 :>tep that can
also be performed
during this phase.
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.
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
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CA 02353739 2001-07-24
calculated channel parameters measured during the channel analysis phase, and
are not
necessarily the same as the preliminary rates offered during that phase.
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. 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.
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.
The information transferred during the exchange is important for establishing
the
communication between the ATU-C and the ATU-R. Therefore, the same bits are
also
modulated into a set of back-up tones for improving robustness. The tone sets
used by
6.992.1 Annex A and 6.992.2 standards are provided below in Table 1.
Primary Set (Index Backup Set (Index
No.) No.)
Upstream 43, 44, 45, 46 91, 92, 93, 94
Downstream 10, 1 l, 12, 13 20, 21, 22, 23
Table 1
Optimally, the receiver combines the bits carried in the two sets of tone for
improving
reliability of the transmission. However, the signal-to-noise ratio (SNR) in
the frequency
band of the backup tone is much lower than that in the frequency band of the
primary
tone. Therefore, on long loops, especially for the downstream tones, the
backup set of
tones is essentially ineffective. In these cases, the bit error ratio (BER) is
determined by
the SNR on the primary set. Within a set, the highest BER within the four
carriers,
determines the overall bit error rate on the message.
As a result, increasing the number of sets of carriers has limited benefits,
since it does not
guarantee best performance and further complicatE;s the messaging protocol.
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CA 02353739 2001-07-24
Furthermore, as is often the case, the tone assigned by the designated indices
may have a
poor SNR, causing the initialization to fail.
Therefore, there is a need for a messaging protocol that improves the
reliability of the
messages transferred during the initialization. It is an object of the present
invention to
obviate or mitigate at least some of the above mentioned disadvantages.
SUMMARY OF THE INVENTION
In accordance with an aspect of the present invention, there is provided a
method for
initializing a communication link between a first transceiver and a second
transceiver for
transferring data there between. The method comprises the steps of analyzing
channel
properties of a plurality of sub-channels within the communication link,
identifying a
predefined number of sub-channels having an anticipated highest performance
for
communication, communicating the identified sub-channels between the first and
second
transceivers, and transmitting information for initializing the communication
link using
the identified sub-channels.
BRIEF DESCRIPTION OF THE DRAWINGS
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 block diagram illustrating a typical system for providing ADSL
service
(prior art);
Figure 2 is a block diagram illustrating the flow of data during the exchange
(prior
art);
Figure 3 is a block diagram illustrating the flow o~f data during the exchange
in
accordance with an embodiment of invention;
Figure 4a is a graph illustrating the performance of the initialization
process over a
varying loop length with 24 ADSL NEXT and FEXT;
Figure 4b is a graph illustrating the performance of the initialization
process overa
varying loop length with 24 DSL NEXT;
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CA 02353739 2001-07-24
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
For convenience, like numerals in the description refer to like structures in
the drawings.
Referring to figure 1, a system for implementing ADSL service is illustrated
generally by
numeral 100. The system 100 comprises a central office transceiver (ATU-C)
102, a
splitter 104, a twisted pair loop 106, and a remote transceiver (ATU-R) 108.
The splitter
104 includes a high pass filter 110 and a low pass filter 112. The ATU-C 102
is coupled
between a broadband network 114, such as the Internet, and the high pass
filter 110 of the
splitter 104. The low pass filter 112 of the splitter 104 is coupled to a
narrowband
network 116 such as a General Switched Telephone Network (GSTN) or Integrated
Services Digital Network (ISDN). Output from the high pass 110 and low pass
filters
112 are combined and coupled with the twisted pair loop 106.
The twisted pair loop is, in turn, coupled with a customer-premises wiring
network 118.
The customer-premises wiring network 118 is coupled via a low pass filter 112
with
narrowband network devices 120, such as telephones, voiceband modems, and ISDN
terminals. The customer-premises wiring network 118 is fizrther coupled to the
ATU-R
108 via a high pass filter 110. 'The ATU-R 108 is further coupled to a
plurality of service
modules 122 via a home network 124.
The system 100 illustrated in figure 1 operates by transferring data between
the ATU-C
102 and the ATU-R 108 on a frequency spectrum above that used for the
narrowband
devices 120. Therefore, the system 100 provides the service modules 122 access
to a
high-speed network connection across the twisted pair loop 106, which is an
existing
infrastructure.
Oftentimes, the twisted pair loop 106 is long, resulting in a~n increase in
the bit error ratio
(BER) for the transmission. This is particularly important during the
exchange, since the
transmission parameters are established at this point. As it is known, the BER
for QPSK
modulation is
BER; = Q( SNR ; ) (1)
s
..~ M ~ e. . .~~~ . n.
CA 02353739 2001-07-24
and the overall BER over the 4 carriers (i.e. the average BER for the decoded
message) is
4
BER = 4 ~ BER; (2)
,_,
The Message Error Rate (MER) for a given message of L bits is then
MER =1- (1- BER)L (3)
The initialization message includes cyclic redundancy check (CRC) bytes and,
therefore,
L is the number of bits of the message the CRC bytes are computed from.
Because the
MER increases with L, one should consider the max value of L, Lmax. for the
initialization
messages when evaluating the reliability of the messaging scheme.
The following messages and corresponding message sizes are transferred during
the
exchange.
Downstream
The first group of messages includes C-RATES-RA, C-CIE~C-RAl, C-MSG-RA, and C-
CRC-RA2. The messages comprise 960 bits for C-RATES-RA, 16 bits for C-CRC-
RA1, 48 bits for C-MSG-RA, and 16 bits for C-CRC-RA2, yielding a total of
1,040 bits
or 130 Discrete Multi-tone (DMT) symbols.
The second group of messages includes C-MSG2, C-CRC3, C-RATES2, and C-CRC4.
The messages comprise 32 bits for C-MSG2, 16 bits for C-CRC3, 8 bits for C-
RATES2,
and 16 bits for C-CRC4, yielding a total of 72 bits, or 9 DnTT symbols.
T'he third group of messages includes C-B&G and C-CRC:>. The messages comprise
496
bits for C-B&G and 16 bits for C-CRCS, yielding a total of 512 bits, or 64 DMT
symbols.
Upstream
The first group of messages includes R-RATES-RA, R-CRC RA2, R-MSG -RA, and R-
CRC-RA1. The messages comprise 8 bits for R-RATES-RA, 16 bits for R-CRC-RA2,
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CA 02353739 2001-07-24
80 bits for R-MSG-RA, and 16 bits for R-CRC-RA1, yielding a total of 120 bits;
or 15
DMT symbols.
The second group of messages includes R-MSG 2, R-CRC3, R-RATES2, and R-CRC4.
The messages comprise 32 bits for R-MSG2, 16 bits for R-CRC3, 8 bits for R-
RATES2,
and 16 bits for R-CRC4 yielding a total of 72 bits, or 9 DM:T symbols.
The third group of messages includes R-B&G and R-CRCS. The messages comprise
4080 bits for R-B&G and 16 bits for R-CRCS, yielding a total of 4096 bits, or
512 DMT
symbols.
Therefore, it can be seen that the maximum bit length for a downstream message
is Lm~ _
960 for C-RATES-RA. For upstream, the maximum bit length is Lmax = 4080 for R-
B&G.
In order to have the MER<10-2, substituting the values of L,m~ from equation 3
results in:
Downstream (Lm~ 960) BER < 10-5
Upstream (Lm~ 4080) BER < 2.5 ~ 10~
In terms of the required signal-to-noise ratio (SNR) in the carriers, this
means the
upstream messages require only a fraction of a dB higher SNR to compensate for
the
longer message.
Referring to figure 2, a timing diagram for the exchange :in accordance with
the state of
the art is illustrated generally by numeral 200. Generally, the nomenclature
for message
transmission uses an "R-" prefix for indicating that the message originated
from the
ATU-R, and a "C " prefix for indicating that the message originated from the
ATU-C.
The sequence of messages on the left side represents messages sent from the
ATU-C to
the ATU-R and the sequence of message on the right side represents messages
sent from
the ATU-R. For both sides, the message sequence begins ;~t the top of the
page.
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CA 02353739 2001-07-24
After C-MEDLEY 202 the ATU-C enters C-REVERB4 204 where it waits for messages
206 from the ATU-R. The messages 206 include R-RATES-RA, R-CRC RA2, R-MSG-
RA, and R-CRC-RA1. If the expected messages 206 are not received within 6,000
symbols, the ATU-C times out and the initialization fails. If the ATU-C
receives the
expected messages in the allotted time, it remains in C-REVERB4 204 for at
least
another 80 symbols before it enters C-SEGUE2 208. After C-SEGUE2 208, the ATU-
C
transmits a series of messages 210 to the ATU-R. These messages 210 include C-
RATES-RA, C-CRC-RA1, C-MSG-RA, and C-CRC-RA2.
Once the ATU-R has sent its messages 206 it enters R-REVERB-RA 212, where it
waits
for the messages 210 from the ATU-C. If the ATU-R does receive the messages
210
within 4,000 symbols, it times out and the initialization fails. The ATU-C and
ATU-R
use predefined tone indices for transmitting the messages. R-RATES-RA, R-CRC
RA2,
R-MSG-RA, R-CRC-RA1, C-RATES-RA, C-CRC-RAI, C-MSG-RA, and C-CRC-
RA2. An additional set of tone indices is used to transmit t~.hese messages as
a backup.
Referring to figure 3, a timing diagram for improving the reliability of the
exchange is
illustrated generally by numeral 300. Additional ATU-C transmissions C-REVERBx
302, C-SEGUEx 304, and C-MSGx/C-CRCx 306 axe inserted between C-MEDLEY 202
and C-REVERB4 204. Similarly, additional ATU-R transmissions R-REVERBx 308, R-
SEGUEx 310, and R-MSGx/R-CRCx 312 are inserted between R-MEDLEY 314 and R-
REVERB4 316.
The content of the messages C-MSGx and R-MSGx inclludes the indices of four
tones
with the best SNR available. C-MSGx includes the indicE;s for upstream
communication
and R-MSGx includes the indices for downstream communication. Therefore,
rather
than use fixed indexes to transfer the messages, the indices of the four tones
are selected
adaptively, in accordance with an estimated line SNR.
The indices of the four tones are selected by the ATU-C; and ATU-R to
correspond to
tones with the best SNRs. The SNR estimate is available at the exchange
because it takes
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CA 02353739 2001-07-24
place after both C-MEDLEY and R-MEDLEY (during channel analysis). During C-
MEDLEY an estimate of the downstream SNR is determined at the ATU-R. The ATU-R
determines the indices of the tones having the four highest SNRs for
downstream
communication and compiles them into R-MSGx. Similarly, during R-MEDLEY an
estimate of the upstream SNR is determined at ATU-C. The ATU-C determines the
indices of the tones having the four highest SNRs for upstream communication
and
compiles them into C-MSGx. The sets of four indices, that is C-MSGx and R-
MSGx, are
exchanged between the ATU-R and the ATU-C using a more reliable 1-bit per
symbol
modulation.
The format of R-MSGx and C-MSGx is describes as follows. The message comprises
a
prefix, a first carrier index, a second Garner index, a third carrier index,
and a fourth
carrier index. The prefix is four bytes and each of the carrier indices is one
byte as
illustrated in Table 2 below.
Cer Carrier Carrier Carrier
Prefix index index #2 index #3 index #4
#1
Number 4 1 1 1 1
of
bytes
Table 2
The prefix is f 01010101 01010101 01010101 010101012}. The carrier index
fields
contain the four carrier indexes with the best SNR in decreasing order.
Therefore, the
SNR of carrier index #1 is greater than or equal to the SNR of carrier index
#2, which is
greater than or equal to the SNR of carrier index #3, which is greater than or
equal to the
SNR of carrier index #4. The byte for each Garner index is the binary
representation of
the selected index.
The message is followed by a 16-bit CRC that is transmitted using the same 1-
bit/symbol
modulation format. Thus, 80 DMT symbols are required for transmitting each of
the 80-
bit C-MSGx/C-CRCx message and 80-bit R-MSGx/R-CRCx message.
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CA 02353739 2001-07-24
Referring to figure 4a and figure 4b the performance of the messaging scheme
described
herein is compared to that currently in use, with respect t:o the MER of C-
RATES-RA.
Figures 4a and 4b refer to two different cross talk scenarios. Figure 4a has
24 ADSL near
end cross talk (NEXT) and far end cross talk (FEXT). Figure 4b has 24 DSL
NEXT. The
vertical axis represents an increase in the MER. The horizontal axis
represents an
increase in loop length. 'The loop lengths are selected in order to allow for
a non-zero net
throughput in presence of a coding scheme. In particular, when Reed Solomon
(RS) FEC
only is used, a non-zero throughput is guaranteed for the l7kft and l8kft
loops in both
figures 4a and 4b. When Trellis and RS are used, reach can be extended to
l9kft with 24
ADSL NEXT and FEXT (figure 4a) and to 20kft with 24 A.DSL NEXT (figure 4b).
As illustrated in both figures 4a and 4b, for these conditions the current
standard
messaging scheme is inadequate, since the MER approachf;s 1 for these loops.
Therefore,
even though the channel allows a non-zero net data rate, the unreliability of
the messages
does not allow the link to activate. However, the mess<~ging scheme described
in the
preferred embodiment is sufficiently reliable for all of these cases.
Furthermore, as a
result of the improved reliability of the selected set of caxriers, only one
carrier set is
required.
In yet an alternate embodiment, each transceiver sends a stream bits as
numerous as the
number of the tones capable of being received. Each bit corresponds to a tone.
If a bit is
set to 1 then its associated tone is to be used during for transmitting the
messages that
help establish the communications link. For example, the ATU-C transmits
messages
that include C-MSG-RA and C-RATES-RA. The ATU-R transmits messages that
include R-MSG-RA and R-RATES-RA. If the bit is set to~ zero, its associated
tone is not
used for modulating the messages.
In all of the embodiments described above, it is possible to use greater or
fewer than four
tones for communicating the message as will be apparent to a person skilled in
the art.
Although the invention has been described with reference to certain specific
embodiments, various modifications thereof will be apparent to those skilled
in the art
CA 02353739 2001-07-24
without departing from the spirit and scope of the invention as outlined in
the claims
appended hereto.
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