U.S. patent application number 09/259681 was filed with the patent office on 2001-08-09 for reducing crosstalk between communications systems.
Invention is credited to TERRY, JOHN BRIAN.
Application Number | 20010012321 09/259681 |
Document ID | / |
Family ID | 24776727 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012321 |
Kind Code |
A1 |
TERRY, JOHN BRIAN |
August 9, 2001 |
REDUCING CROSSTALK BETWEEN COMMUNICATIONS SYSTEMS
Abstract
Modems using a telephone line for high speed communications
between them are arranged to monitor crosstalk from other existing
communications systems with which they may mutually interfere, and
adjust the power spectral densities (PSDs) of their transmitted
signals to reduce overlap between the PSDs of the different
systems, thereby reducing near end crosstalk. Communications can
thereby be optimized for whatever crosstalk conditions may exist.
The modems can have a master-slave relationship for communicating
buffered frames in a half-duplex manner using a collision avoidance
protocol for computer network access. Analysis of monitored
crosstalk PSD information can be performed by each modem, by the
master modem, or by a separate computer on the network. A digital
signal processor used in each modem for receiving signals can be
configured to be used at other times for the monitoring of
crosstalk.
Inventors: |
TERRY, JOHN BRIAN; (CUMMING,
GA) |
Correspondence
Address: |
ARTHUR SCHWARTZ
FOLEY & LARDNER
3000 K STREET N W
SUITE 500 P O BOX 25696
WASHINGTON
DC
200078696
|
Family ID: |
24776727 |
Appl. No.: |
09/259681 |
Filed: |
March 1, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09259681 |
Mar 1, 1999 |
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08691486 |
Aug 2, 1996 |
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6055297 |
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Current U.S.
Class: |
375/227 ;
375/222 |
Current CPC
Class: |
H04B 3/32 20130101 |
Class at
Publication: |
375/227 ;
375/222 |
International
Class: |
H04Q 001/20; H04B
003/46; H04B 017/00; H04B 001/38 |
Claims
What is claimed is:
1. A method of determining a power spectral density (PSD) for
supplying signals from a signal transmitter to a communications
path, comprising the steps of: determining a PSD on the
communications path, due to other communications, in the absence of
signals supplied from the signal transmitter to the communications
path; supplying signals from the signal transmitter to the
communications path; and adjusting at least one parameter of the
signals supplied from the signal transmitter to the communications
path in dependence upon the determined PSD to reduce overlap
between the PSD of the signals supplied from the signal transmitter
to the communications path and the determined PSD.
2. A method as claimed in claim 1 wherein the step of determining a
PSD on the communications path comprises monitoring a PSD on the
communications path, while signals are not supplied from the signal
transmitter to the communications path, to produce the determined
PSD.
3. A method as claimed in claim 1 wherein the step of determining a
PSD on the communications path comprises the steps of: storing PSD
templates for communications systems; monitoring a PSD on the
communications path, due to other communications, in the absence of
signals supplied from the signal transmitter to the communications
path; comparing the monitored PSD on the communications path with
the templates to identify a communications system corresponding to
the monitored PSD; and producing the determined PSD in dependence
upon the identified communications system.
4. A method as claimed in claim 3 wherein the step of monitoring a
PSD on the communications path is performed signals are not
supplied from the signal transmitter to the communications
path.
5. A method as claimed in claim 1 wherein said at least one
parameter comprises a power of the signals supplied from the signal
transmitter to the communications path.
6. A method as claimed in claim 1 wherein said at least one
parameter comprises a frequency band of the signals supplied from
the signal transmitter to the communications path.
7. A method as claimed in claim 1 wherein said at least one
parameter comprises a modulation scheme of the signals supplied
from the signal transmitter to the communications path.
8. A method as claimed in claim 1 wherein said at least one
parameter comprises a power, a frequency band, and a modulation
scheme of the signals supplied from the signal transmitter to the
communications path.
9. A method as claimed in claim 2 wherein said at least one
parameter comprises a power of the signals supplied from the signal
transmitter to the communications path.
10. A method as claimed in claim 2 wherein said at least one
parameter comprises a frequency band of the signals supplied from
the signal transmitter to the communications path.
11. A method as claimed in claim 2 wherein said at least one
parameter comprises a modulation scheme of the signals supplied
from the signal transmitter to the communications path.
12. A method as claimed in claim 2 wherein said at least one
parameter comprises a power, a frequency band, and a modulation
scheme of the signals supplied from the signal transmitter to the
communications path.
13. A method as claimed in claim 3 wherein said at least one
parameter comprises a power of the signals supplied from the signal
transmitter to the communications path.
14. A method as claimed in claim 3 wherein said at least one
parameter comprises a frequency band of the signals supplied from
the signal transmitter to the communications path.
15. A method as claimed in claim 3 wherein said at least one
parameter comprises a modulation scheme of the signals supplied
from the signal transmitter to the communications path.
16. A method as claimed in claim 3 wherein said at least one
parameter comprises a power, a frequency band, and a modulation
scheme of the signals supplied from the signal transmitter to the
communications path.
17. A method as claimed in claim 4 wherein said at least one
parameter comprises a power of the signals supplied from the signal
transmitter to the communications path.
18. A method as claimed in claim 4 wherein said at least one
parameter comprises a frequency band of the signals supplied from
the signal transmitter to the communications path.
19. A method as claimed in claim 4 wherein said at least one
parameter comprises a modulation scheme of the signals supplied
from the signal transmitter to the communications path.
20. A method as claimed in claim 4 wherein said at least one
parameter comprises a power, a frequency band, and a modulation
scheme of the signals supplied from the signal transmitter to the
communications path.
Description
[0001] This is a continuation of pending U.S. patent application
Ser. No. 08/691,486 filed Aug. 2, 1996 in the name of John B.
Terry, entitled "Reducing Crosstalk Between Communications
Systems", the entire disclosure of which is hereby incorporated
herein by reference.
[0002] This invention relates to reducing crosstalk between
communications systems. The invention is particularly, but not
exclusively, applicable to reducing NEXT (near end crosstalk)
between twisted pairs of wires in telephone cables used
historically for providing telephone service to subscribers and now
being used increasingly to provide additional communications
services, for example for data communications and computer network
connections.
REFERENCE TO RELATED APPLICATION
[0003] Reference is directed to U.S. patent application Ser. No.
08/640,705 filed May 1, 1996 in the names of J. B. Terry et al.,
entitled "Information Network Access Apparatus And Methods For
Communicating Information Packets Via Telephone Lines", the entire
disclosure of which is hereby incorporated herein by reference.
This application, referred to below as the related application,
describes and claims methods and apparatus which can be used in
particular to facilitate remote access via conventional twisted
pair telephone lines to computer networks such as the global
computer information network which is generally known as the
Internet and is referred to herein as the Network. The present
invention is not limited in any way to the arrangements of this
related application, but can be applied in a particularly
convenient manner to such arrangements as is described later
below.
BACKGROUND OF THE INVENTION
[0004] Twisted pair public telephone lines are increasingly being
used to carry relatively high-speed signals instead of, or in
addition to, telephone signals. Examples of such signals are ADSL
(asymmetric digital subscriber line), HDSL (High Density Subscriber
Line, T1 (1.544 Mb/s), and ISDN signals. There is a growing demand
for increasing use of telephone lines for high speed remote access
to computer networks, and there have been various proposals to
address this demand, including using DOV (data over voice) systems
to communicate signals via telephone lines at frequencies above the
voice-band.
[0005] The provision in the public telephone network of varied
services using such diverse communications systems imposes a
requirement that different and similar systems not interfere with
one another. A predominant limiting effect in this respect is NEXT
(near end crosstalk) between wire pairs within multiple-pair cable
binder groups or between wire pairs within adjacent binder
groups.
[0006] Allocations of wire pairs within telephone cables in
accordance with service requests have typically resulted in a
random distribution of pair utilization with few precise records of
actual configurations. In addition, due to the nature of pair
twisting in cables, and where cable branching and splicing occurs,
a wire pair can be in close proximity to different other pairs over
different parts of its length. At a telephone C.O. (central
office), pairs in close proximity may be carrying diverse types of
service using various modulation schemes, with considerable
differences in signal levels (and receiver sensitivities)
especially for pairs of considerably different lengths.
[0007] Statistical data has been developed that can be used to
estimate crosstalk between services using different pairs of
multi-pair telephone cables, for example in terms of BER (bit error
rate) based on power spectral density (PSD, for example measured in
milliwatts per Hertz expressed in decibels, or dBm/Hz) overlap
between the services. However, this statistical data is of limited
use in practice in the provision of a new service using equipment
connected to a specific wire pair, in view of factors such as those
discussed in the preceding paragraph.
[0008] It is therefore a significant concern of telephone companies
that the signals and operation of existing systems may be adversely
affected, especially as a result of NEXT, by the deployment of new
equipment, particularly digital signal transmission equipment. This
concern is increased in accordance with the extent to which such
equipment is likely to be deployed, and hence particularly applies
to equipment that may be used in very large numbers for remote
access to computer networks. New equipment can be designed in a
manner largely to avoid interference with other systems in
accordance with the statistical data, but this imposes undesirable
constraints on signal spectra and signal levels, limiting its
usefulness in an unacceptable manner to accommodate a relatively
small proportion of situations for which such constraints may be
necessary.
[0009] An object of this invention is to provide a method that can
permit new communications systems to be added to existing
communications paths in a manner that is generally compatible with
existing systems where these exist, and that can make optimum use
of communications capacity.
SUMMARY OF THE INVENTION
[0010] This invention provides a method of determining a power
spectral density (PSD) for supplying signals from a signal
transmitter to a communications path, comprising the steps of:
determining a PSD on the communications path, due to other
communications, in the absence of signals supplied from the signal
transmitter to the communications path; supplying signals from the
signal transmitter to the communications path; and adjusting at
least one parameter of the signals supplied from the signal
transmitter to the communications path in dependence upon the
determined PSD to reduce overlap between the PSD of the signals
supplied from the signal transmitter to the communications path and
the determined PSD.
[0011] Thus a new communications system, operating in accordance
with this method, determines PSD on the communications path,
primarily due to NEXT from other existing communications systems
using adjacent communications paths, and then adjusts its own PSD
to reduce, and desirably to minimize, overlap between the PSDs. On
the basis that crosstalk between different communications paths is
equal for opposite directions, the method consequently reduces, and
desirably minimizes, interference from the new communications
system with any existing communications systems that may be
affected by the new system. Thus each new communications system
that is deployed, for example in a public telephone network where
the communications paths comprise twisted pair telephone lines, can
be operated in a manner that is adaptively adjusted to minimize
interference with existing systems in its own particular
communications path environment. The adaptive adjustment can be
performed only once on deployment of the new system, or much more
desirably in an ongoing manner.
[0012] The step of determining a PSD on the communications path can
comprise monitoring a PSD on the communications path, while signals
are not supplied from the signal transmitter to the communications
path, to produce the determined PSD. As an alternative to not
supplying signals from the signal transmitter to the communications
path during the monitoring, the PSD of signals supplied from the
signal transmitter to the communications path could be subtracted
from the monitored PSD representing the PSD on the communications
path, due to other communications, in the absence of signals
supplied from the signal transmitter to the communications
path.
[0013] Thus the determined PSD can be constituted by the monitored
PSD. Such a determination can be valid where the existing
communications systems are symmetric systems, for which the PSD of
signals having opposite directions of transmission can be
substantially the same, but can be inaccurate for asymmetric
systems for which the PSD of signals having opposite directions of
transmission can be substantially different. For example, in an
ADSL system the spectral utilization, and hence the PSDs, of
signals in the two opposite directions of transmission are
substantially different.
[0014] In view of this, preferably the step of determining a PSD on
the communications path comprises the steps of: storing PSD
templates for communications systems; monitoring a PSD on the
communications path, due to other communications, in the absence of
signals supplied from the signal transmitter to the communications
path; comparing the monitored PSD on the communications path with
the templates to identify a communications system corresponding to
the monitored PSD; and producing the determined PSD in dependence
upon the identified communications system. This enables the PSD of
signals supplied from the signal transmitter to the communications
path to be adjusted to reduce overlap with the PSD of signals of an
existing system transmitted in the opposite direction of an
adjacent communications path, this being appropriate because of the
predominance of NEXT.
[0015] The at least one parameter that is adjusted to reduce PSD
overlap can comprise the power (i.e. level), frequency band, and/or
modulation scheme of signals supplied from the signal transmitter
to the communications path. Desirably, all of these parameters are
adjusted collectively to achieve minimal interference with existing
communications systems consistent with optimal performance of the
new communications system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The invention will be further understood from the following
description with reference to the accompanying drawings, in
which:
[0017] FIG. 1 illustrates a communications arrangement using
twisted pair telephone lines for communicating high-speed signals,
to which the invention is particularly applicable;
[0018] FIG. 2 illustrates a communications arrangement in
accordance with an embodiment of this invention;
[0019] FIG. 3 illustrates parts of a modem used in the arrangement
of FIG. 2; and
[0020] FIG. 4 illustrates a flow chart with reference to which
operation of an arrangement in accordance with an embodiment of the
invention is explained.
DETAILED DESCRIPTION
[0021] Referring to FIG. 1, a line 10 represents a telephone line
which it is desired to use to provide communications between a
modem 12, connected to the line 10 at a telephone C.O. end thereof,
and a complementary modem 14 connected to the line 10 at a
subscriber end thereof. As is well known, diplexing filters (not
shown) can be provided at the ends of the line 10 to permit the
line to carry telephone communications simultaneously with
communications at higher frequencies between the modems 12 and 14.
For simplicity, associated telephone equipment is not shown in FIG.
1.
[0022] The telephone line 10 physically comprises a twisted wire
pair which is typically in close proximity along at least part of
its length with other such twisted pairs in one or more multi-pair
cables. These other pairs may carry arbitrary communications
signals, including telephone and high-speed data signals, which as
a result of crosstalk may interfere with, and may be interfered
with by, signals at similar frequencies on the line 10. As
discussed in the background of the invention, the environment of
the telephone lines and associated records make it impractical to
determine what communications equipment may be connected to which
twisted pairs over what parts of their lengths, so that the nature
and extent of any such interference can not generally be
predetermined. Because the deployment of high-speed communications
systems is relatively small compared to the total deployment of
telephone lines, there frequently may be no significant
interference. In some cases, however, there is potential for
interference with an existing communications service.
[0023] For example, FIG. 1 illustrates that over part of its length
the line 10 may be in close proximity to another telephone line 16
which is used for high-speed communications between a modem 18 at
the C.O. end of the line 16 and a modem 20 at a subscriber end of
this line. In the description below it is assumed, for example and
in order to describe principles of the invention fully, that the
existing modems 18 and 20 provide for ADSL communications via the
line 16, but the invention is applicable regardless of the nature
of the communications signals on the lines. A line 22 represents
other telephone lines which may or may not carry potentially
interfering signals which would also be susceptible to
interference.
[0024] Because of the close proximity of the lines 10 and 12,
crosstalk undesirably can occur between these lines; i.e. signals
on one line are coupled to some extent to the other line, the
extent depending upon numerous factors such as the physical
characteristics of the cable including these lines, and the levels
and frequencies (i.e. the power spectral density) of the signals.
Crosstalk is a particular concern for high-speed transmission, i.e.
where both lines carry signals at relatively high frequencies using
similar or overlapping frequency bands. For the majority of
telephone lines represented by the line 22, the addition of the
modems 12 and 14 for high-speed communications via the line 10 is
not a significant problem as far as crosstalk is concerned, because
the majority of telephone lines do not also carry high frequency
signals for high-speed transmission. For a relatively small number
of telephone lines such as the line 16, the addition of the modems
12 and 14 for high-speed communications via the line 10 presents a
potential problem as far as crosstalk is concerned, because
crosstalk from the line 10 to the line 16 can impair the signal
transmission on the line 16, and vice versa.
[0025] The main concern in this respect is NEXT, represented
schematically in FIG. 1 by double-headed arrows 24 and 26 at the
C.O. and subscriber ends, respectively, of the adjacent parts of
the lines 10 and 16. This can be seen to be the case from the fact
that a signal from the modem 12 coupled by crosstalk from the
nearest end of the line 10 to the line 16 and back to the modem 18
is attenuated by the lines 10 and 16 to a relatively small extent,
whereas a signal transmitted from the modem 14 coupled by crosstalk
from the line 10 to the line 16 and received by the modem 18 (FEXT
or far end crosstalk) is subject to significant attenuation by the
length of the lines 10 and 16. Conversely, a signal from the modem
14 coupled by crosstalk from the nearest end of the line 10 to the
line 16 and back to the modem 20 is attenuated by the lines 10 and
16 to a relatively small extent, whereas a signal transmitted from
the modem 12 coupled by crosstalk from the line 10 to the line 16
and received by the modem 20 is subject to significant attenuation
by the length of the lines 10 and 16. Reciprocally, signals on the
line 16 are also coupled by crosstalk to the line 10, and again
NEXT is the most significant factor in view of the relative
attenuation of signals by the lengths of the lines over which they
are communicated.
[0026] For explaining principles in accordance with which
embodiments of this invention operate, it is assumed for example
that signals from the ADSL modem 18 to the ADSL modem 20 (referred
to as the downstream direction) occupy a broad frequency band in a
range of about 100 kHz to about 1 MHz, and that signals from the
ADSL modem 20 to the ADSL modem 18 (referred to as the upstream
direction) occupy a narrower frequency band in a range of about 50
to about 150 kHz. These figures are given only for illustrative
purposes and for convenience in explaining the invention.
[0027] A desirable result that the invention facilitates achieving
is to permit the addition and operation of the new modems 12 and 14
in a manner that reduces crosstalk between the line 10 and other
lines 16 and 22 to the extent that this is necessary to avoid
interfering undesirably with any communications, such as the ADSL
communications between the modems 18 and 20, that may (but
frequently will not) exist on such other lines, while permitting
the new modems 12 and 14 to communicate via the line 10 in an
optimum manner, e.g. with the highest allowable signal levels and
the greatest allowable frequency bandwidths.
[0028] To this end, a sequence of steps described below is carried
out initially and/or in an ongoing manner (e.g. periodically,
irregularly as desired, or dependent upon parameters such as
traffic conditions). These steps include measurements or monitoring
of the line 10 to which the modems 12 and 14 are connected. As
described initially below, it is convenient for the modems
themselves not to transmit signals to the line 10 during
measurement or monitoring periods, but there can be alternatives to
this, and to other aspects of the immediately following
description, that are discussed later below.
[0029] The new modems 12 and 14 first suspend transmission of
signals to the line 10 during a monitoring period that can be
relatively brief, e.g. 50 ms, or more protracted as desired or
necessary. During one or more such monitoring periods, in each of
the modems 12 and 14, separately or simultaneously, the modem
receiver performs a spectral analysis of any signals that may be
received via the line 10. This spectral analysis is conveniently
performed by controlling the bandwidth and centre frequency of a
receive bandpass digital filter in the modem to receive signals
within a narrow bandwidth that is progressively changed over a
desired spectrum, for example 20 kHz to 1 MHz, while monitoring the
power level of any signal that is received. It can be appreciated
that any such signal will be predominantly a result of NEXT from
signals on adjacent lines 16 and 22, and that the power spectrum
that is constituted by this monitoring will represent the extent of
the crosstalk and will be characteristic of the type of
communications signals contributing to this crosstalk.
[0030] Thus each of the modems 12 and 14 can determine not only the
extent of crosstalk between the line 10 and any other lines 16 and
22, but also, from a comparison of the monitored power spectrum
with stored templates of power spectra for various systems,
suitably modified to take into account the known NEXT frequency
characteristics of the twisted pair cable, the type of
communications system predominantly contributing to such crosstalk.
Each of the modems 12 and 14 then adjusts the power spectral
density (PSD) for signals that it will transmit to the line 10 to
minimize overlap with the PSD of signals for the opposite direction
of transmission for any determined communications system
contributing to the monitored NEXT. On the basis that crosstalk
between the line 10 and each other line is reciprocal, this also
minimizes NEXT from signals on the line 10 between the modems 12
and 14 to any such determined communications system, whereby the
different systems can both operate with minimal interference
between them. The modems 12 and 14 then resume (or start)
transmission of signals to the line 10 in accordance with the
adjusted PSD.
[0031] More specifically, in this example the modem 12 will receive
and monitor a power spectrum having components in the broad
frequency range of 100 kHz to 1 MHz of the downstream ADSL signals
on the line 16, and will determine from this that it must adjust
the PSD of its transmitted signals to avoid NEXT with upstream
signals of an ADSL system which are received by the existing modem
18. It therefore adjusts the spectrum of its transmitted signals to
avoid the 50 to 150 kHz range of upstream ADSL signals, for example
controlling a digital transmit filter to provide a pass band from
150 kHz to 1 MHz, and transmits its signals at an appropriate level
within this frequency band, where even at a relatively high power
level they do not interfere with the ADSL communications on the
line 16. Conversely, the modem 14 will receive and monitor a power
spectrum having components in the narrower frequency range of 50 to
150 kHz of the upstream ADSL signals on the line 16, and will
determine from this that it must adjust the PSD of its transmitted
signals to avoid NEXT with downstream signals of an ADSL system
which are received by the existing modem 20. It therefore adjusts
the spectrum of its transmitted signals to avoid the 100 kHz to 1
MHz range of downstream ADSL signals, for example controlling a
digital transmit filter to provide a pass band from 50 to 100 kHz,
and transmits its signals at an appropriate level within this
frequency band, where they also do not interfere with the ADSL
communications on the line 16.
[0032] Where the monitored power spectrum relates to a symmetrical
communications system, for example ISDN signals, rather than an
asymmetrical system such as ADSL as described by way of example
above, the adjusted PSDs for signals to be transmitted by the
modems 12 and 14 can be the same rather than different as described
above.
[0033] Although as described above transmission of signals by the
modems 12 and 14 is suspended during each monitoring period, this
need not necessarily be the case. For example, for the monitoring
by the modem 12, it is not essential for transmission of signals on
the line 10 from the modem 14 to be suspended, because the PSD of
such signals at the modem 12 can be known and compensated for in
the monitoring by the modem 12. However, this would require the
modem 12 to have separate facilities for receiving the signals from
the modem 14 and for monitoring purposes, in contrast to using the
same receive filter at different times for receiving signals and
for spectral analysis as described above, which is much more
preferable. Also, the modem 12 can conceivably be arranged to
monitor for NEXT at the same time that it is itself supplying
signals to the line 10, these signals having a known PSD which can
be subtracted in the monitoring and spectral analysis process.
However, this may be relatively difficult to achieve in practice,
especially because the monitored NEXT has a much lower power than
the signals supplied to the line 10 by the modem 12. Accordingly,
it is more desirable in practice to suspend the transmission of
signals by the modem doing the monitoring of NEXT, and preferably
by both modems, during the monitoring.
[0034] In addition, although as described above the modems 12 and
14 operate relatively independently of one another and each
performs the necessary spectral analysis, this need not be the
case. More particularly, and for example as described in greater
detail below, the modem 14 may be subordinate to the modem 12 in a
master-slave relationship, the modem 14 performing monitoring and
adjusting the PSD of its transmitted signals in response to command
messages from the modem 12, the spectral analysis process being
performed by the modem 12. As a further alternative, as described
further below, the spectral analysis for a plurality of lines 10
and associated C.O. modems 12 (which may be multiplexed for
different lines 10) and customer modems 14 may be performed
centrally by a separate computer unit which communicates messages
with the modems in a time multiplexed manner.
[0035] These alternatives are particularly advantageous in a
network access arrangement in accordance with the related
application referred to above. In such an arrangement, access to a
CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
network, such as the Network using Ethernet frames, is provided via
a telephone line by providing a master modem at the head end and a
slave modem at the subscriber end of the line. The master modem
provides a CSMA/CD interface to the Network and controls half
duplex communications with the slave modem via the line to avoid
collisions of Ethernet frames on the line. The Ethernet frames are
enveloped in frames on the line with error checking information;
control information between the modems is contained in the same
and/or in separate frames. The modulation method and signal
bandwidth can be varied depending on errors to provide optimum
communications capacity via any particular line, and a ratio of
upstream to downstream frames can be varied depending on buffer
fills at the modems. The master modem can be multiplexed for
multiple lines. The modulated signal frequencies are above
telephone signal frequencies so that each line can be frequency
diplexed for simultaneous telephone communications.
[0036] FIG. 2 illustrates such an arrangement for one subscriber.
In FIG. 2, the modem 12 at the C.O. end of the line 10 in FIG. 1 is
constituted by a master modem 32, and the modem 14 at the customer
end of the line 10 in FIG. 1 is constituted by a slave modem 34.
The master and slave modems 32 and 34 are coupled to the telephone
line 10 via diplexing filters (DF) 30, which serve in known manner
to separate low frequency telephone signals, communicated between
the telephone C.O. (not shown) and a customer telephone (not shown)
at the respective ends of the line 10, from higher frequency
signals between the modems 32 and 34, these signals being
frequency-multiplexed on the line 10.
[0037] Each of the modems 32 and 34 includes an Ethernet interface
of known form. At the customer end of the line 10, Ethernet (ENET)
frames communicated via the slave modem 34 are coupled to an
Ethernet interface (E I/F) of known form in a terminal device (TD)
36 which may for example be constituted by a personal computer.
Thus Ethernet frames are communicated between the slave modem 34
and the terminal device 36 in known manner, for example using
twisted pair wiring and the 10BASE-T CSMA/CD standard; this
communication can be expanded in known manner into a more extensive
LAN (local area network). At the C.O. end of the line, Ethernet
frames communicated via the master modem 32 are coupled via an
Ethernet hub or switch 38 and a router 40 to the Network. The
Ethernet hub or switch 38 and router 40 are both of known form. In
addition, a spectral compatibility manager (SCM) 42, for example
constituted by a computer, is also connected to the Ethernet switch
38 as shown or elsewhere in the Network. The function of the SCM 42
is described later below.
[0038] As shown at the top of FIG. 2, Ethernet frames are thus
communicated on the customer side of the slave modem 34 and on the
Network side of the master modem 32. Between the modems 32 and 34,
Ethernet frames are communicated using a point-to-point protocol
which uses collision avoidance and for convenience is referred to
as ECAP (Ethernet Collision Avoidance Protocol). Reference is
directed to the related application for a detailed description of
this, but it is outlined below.
[0039] The master and slave modems buffer Ethernet frames to be
communicated downstream (from the master modem 32 to the slave
modem 34) and upstream (from the slave modem 34 to the master modem
32). The ECAP communications of the buffered Ethernet frames
involves half-duplex transmission in which the master modem 32 has
priority and control over the slave modem 34. Thus the master modem
32 determines when to send information downstream via the line 10,
and informs the slave modem 34 when it is permitted to send
information upstream via the line 10. To facilitate these
communications, the information sent via the line 10 comprises not
only the data packets of Ethernet frames but also control packets
downstream and response packets upstream between the master and
slave modems. The data and control packets are incorporated into
ECAP frames which can take various forms. Control units in the
master and slave modems perform the necessary conversions between
the Ethernet frames and ECAP data frames, and generate and respond
to the ECAP control and response frames. Each of the master and
slave modems 32 and 34 includes an Ethernet interface as described
above and hence has a unique network address provided by this
interface; these addresses are used to address messages (control
and response packets) between the modems and can also be used for
addressing the modems from other devices such as the SCM 42 as
described below.
[0040] Each of the modems 32 and 34 includes a modulator,
demodulator, and related functions that are conveniently
implemented in known manner using one or more DSPs (digital signal
processors) with analog-digital conversion in known manner. The
DSPs are conveniently controlled to provide arbitrary different
signal bandwidths, low frequency limits (or, equivalently, filter
centre frequencies), modulation methods (for example the DSPs are
programmed to select any of a plurality of modulation methods such
as QAM (quadrature amplitude modulation), QPSK (quadrature phase
shift keying), and BPSK (binary phase shift keying)), and (e.g. for
QAM) different numbers of bits per symbol. The programming and
control of DSPs in this manner is known in the art and need not be
further described here. However, it is observed that this
programming and control, and control of the signal levels
transmitted from the modems to the line 10, provides extensive
control over the power spectral density (PSD) of signals supplied
to the line 10.
[0041] It can be appreciated from the above outline that the
collision avoidance protocol ensures that the modems 32 and 34
operate in a half-duplex manner for communications between them via
the line 10, with the total transmission capacity of the line being
shared between the downstream and upstream directions of
transmission. The protocol provides for control of the signal
bandwidth, modulation method, etc. to provide a maximum throughput
of Ethernet frames via the line 10 as described in the related
application. However, the same control principles can be used in
accordance with the present invention to adjust the PSD of signals
supplied by the modems 32 and 34 to the line 10 to reduce NEXT as
described above with reference to FIG. 1.
[0042] Furthermore, it can be appreciated that the half-duplex
communications between the modems 32 and 34 also provides, or can
very easily provide, periods during which signals are not supplied
to the line 10 and accordingly that can be used for monitoring the
line 10 as described above. For example, the control packets
communicated from the master modem 32 to the slave modem
conveniently provide a facility for the master modem 32 to instruct
the slave modem 34 not to supply signals to the line 10 for a given
period, and to monitor the line 10 as described above. During the
same period, the master modem 32 similarly can suspend supply of
any signals to the line 10 and can monitor the line 10, whereby
each modem monitors signals on the line 10 primarily due to NEXT.
Monitoring data from the slave modem 34 is then communicated in
response packets to the master modem 32, so that only the master
modem 32 performs a spectral analysis and the slave modem 34 can be
simplified accordingly (it must still be capable of monitoring NEXT
PSD, but does not need to analyse the resulting data). As described
in the related application and indicated above, the master modem 32
is advantageously used in a multiplexed manner for a plurality of
lines 10 and associated slave modems, and accordingly a single
master modem can perform the spectral analysis, in an ongoing
manner, for all of the lines 10 which it serves. In each case the
master modem 32 then sets its own PSD parameters, and via control
packets commands the respective slave modem 34 to set its PSD
parameters, in accordance with the determined PSD on the respective
line 10 to minimize PSD overlap, and hence NEXT, as described above
and to achieve an optimal (for the prevailing conditions applicable
to that particular line 10) throughput of data frames as discussed
above.
[0043] It can be appreciated that the ECAP communications
established between the master and slave modems 32 and 34 provide a
very simple and convenient facility for both establishing silent
periods for monitoring NEXT on the line 10 and adjusting the PSD of
signals supplied by the modems to the line 10, not least because
the Ethernet frames to be communicated are already buffered in
buffers in the modems. This provides a distinct advantage over
other arrangements using conventional modem communications, for
which the establishment of silent periods for monitoring, and the
control of the PSD of signals supplied to the telephone line, may
be considerably more complex.
[0044] As described above, the analysis of data provided by the
monitoring of NEXT by both modems 12 and 14 can be carried out by
the modem 32, and this can be multiplexed for a plurality of lines
10. This multiplexing or concentration of the analysis of data to
determine appropriate PSD parameters for the modems can be further
extended to be carried out by the SCM 42 instead of by the modems,
with messages being communicated between the SCM 42 and the modems
accordingly. In this respect, the modems 32 and 34 can operate
independently and can be addressed individually, using their
respective Ethernet addresses, for communications between the SCM
42 and the respective modem. Alternatively, as described below in
greater detail, the SCM 42 can communicate with the master modem 32
at the C.O. end of the line 10, the master modem 32 communicating
with the slave modem 34 using ECAP communications as described
above.
[0045] This is described further below with reference to FIG. 3,
illustrating a block diagram form of the modems 32 and 34, and FIG.
4 showing a flow chart.
[0046] Referring to FIG. 3, each of the modems 32 and 34 comprises
a hybrid unit 50 connected (optionally via a diplexing filter 30,
not shown in FIG. 3) to the telephone line 10 and an Ethernet
interface (ENET I/F) 52 for connection to the terminal device 36 or
Ethernet switch 38. Analog signals received via the line 10 are
supplied via the hybrid unit 50 to an analog-digital (A-D)
converter 54 to be converted into digital form, the digital signals
being passed via a configurable digital signal processor (DSP) 56
to a buffer 58, which exchanges control (or response) information
with a control unit 60 and data to be passed on in Ethernet frames
with the interface 52. In the opposite direction, a buffer 62
exchanges control or response information with the control unit 60
and Ethernet frame data with the interface 52, and information from
the buffer 62 is supplied via a configurable transmitter (Tx.) 64
and a digital-analog (D-A) converter 66 to the hybrid unit 50 and
thence to the line 10. Digital components of the master modem 32
can be multiplexed for a plurality of lines 10.
[0047] The control unit 60 controls the operation of the modem as
either a master modem 32 or a slave modem 34. For a master modem
32, Ethernet frames are exchanged with the Network from the buffers
58 and 62 via the interface 52. The control unit 60 controls
encapsulation into ECAP frames of Ethernet data frames from the
buffer 62 and control information which it generates for the slave
modem 34, and controls the downstream transmission of these via the
transmitter 64, converter 66, hybrid unit 50, and the line 10. The
control information includes polls which permit the slave modem 34
to transmit in the upstream direction, whereby the master modem
ensures half duplex transmission on the line 10 without collisions
between the downstream and upstream transmission directions.
Upstream ECAP frames are received via the hybrid unit 50, converter
54, and DSP 56, with response information being supplied to the
control unit 60 and Ethernet data frames being supplied via the
buffer 58 to the Ethernet interface 52.
[0048] Conversely, for a slave modem 34 ECAP frames on the line 10
are received via the hybrid unit 50, converter 54, and DSP 56, with
control information supplied to the slave modem's control unit 60
and Ethernet data frames being supplied via the buffer 58 and
Ethernet interface 52 to the terminal device 36. In response to a
poll in the control information received from the master modem, the
control unit 60 in the slave modem controls transmission upstream
of one or more frames containing response information and/or
Ethernet data frames from the buffer 62 in the slave modem, as
instructed by the master modem 32.
[0049] The control unit 60 in each modem also controls the
configuration of the DSP 56 and transmitter 64 of the modem. In
particular, for example, it controls parameters of the transmitter
64 such as the on/off state, signal level, amplitude slope
(variation in signal amplitude with frequency over the pass band),
centre frequency, and modulation scheme (e.g. QPSK or QAM and
number of bits per symbol), which affect not only the transmission
rate but also the PSD of the transmitted signal. It controls
similar parameters for the DSP 56 in a receive mode of the modem
used for normal operation, and in a monitoring mode used for
monitoring NEXT as described above it controls the DSP centre
frequency and bandwidth to provide for level measurement of any
received crosstalk.
[0050] The flow chart in FIG. 4 illustrates steps associated with
this monitoring, this in this case being controlled by the SCM 42
as indicated above. Each step is identified by a reference number
that is given in parentheses in the following description. Steps 70
to 77 are performed by the SCM 42 and are shown at the left of FIG.
4, steps 78 to 92 are performed by the master modem 32 and are
shown in the middle of FIG. 4, and steps 93 to 98 are performed by
the slave modem 34 and are shown at the right of FIG. 4.
[0051] Referring to FIG. 4, the SCM 42 initially selects (70) a
telephone line 10 and direction to test, i.e. whether to monitor
NEXT at the master modem 32 or the slave modem 34 on the selected
line, and then selects (71) a centre frequency and bandwidth for
this monitoring, sending (72) via the Network a message containing
this information in an Ethernet frame addressed to the master modem
32 using its address (determined by the Ethernet interface 52 of
this master modem). The master modem 32 receives (78) this Ethernet
frame and its control unit 60 determines (79) whether the
monitoring is to be carried out by the slave modem 34. If not, then
the control unit 60 of the master modem 32 suspends (80)
transmission of frames downstream (thereby also suspending polling
for the slave modem 34 so that frames are also not transmitted
upstream on the line 10), configures the DSP 56 in accordance with
the provided message from the SCM 42 to perform (81) the desired
measurement or monitoring of NEXT on the line 10, sends a resulting
message in a conventional Ethernet frame addressed to the SCM 42
via the Network, and resumes (83) its transmission of frames
downstream (and polling of the slave modem to permit upstream
transmission).
[0052] The SCM 42 receives (73) the Ethernet frame containing the
monitoring information and determines (74) whether a desired test
has been completed. If not, it returns to the step 71 and the above
sequence is repeated for another selected centre frequency and/or
bandwidth. If the test is complete, then the SCM 42 analyses (75)
the monitoring data provided and determines (75) PSD parameters for
signals sent to the respective line 10 by the respective modem to
minimize interference with any other communications signals that it
determines, in the manner described above, may be affected by
crosstalk with signals from this respective modem. It then sends
(76) an Ethernet frame addressed to the master modem 32 containing
a message with the determined parameters. The master modem 32
receives (84) this Ethernet frame and its control unit 60
determines (85) whether the message is for the slave modem 34. If
not, then the control unit 60 of the master modem 32 adjusts (86)
the configuration of its transmitter 64 in accordance with the PSD
parameters provided, and sends (87) an Ethernet frame to the SCM 42
with a message confirming this adjustment. This is received (77) by
the SCM 42, which returns to the step 70. Obviously, these steps of
the SCM 42 can be carried out contemporaneously for many lines
10.
[0053] In the event that the master modem determines (79) that a
monitoring message from the SCM 42 is intended for the slave modem,
then it sends (88) the message in an ECAP frame to the slave modem
and then suspends (80) its transmission of frames downstream. The
slave modem receives (93) this message, performs (94) the desired
monitoring (without supplying signals to the line because it is not
being polled to do so), and sends (95) the resulting monitoring
information in an ECAP response frame to the master modem 32. The
master modem receives (90) this information and sends (82) it to
the SCM 42, continuing as described above.
[0054] Similarly, in the event that the master modem determines
(85) that a PSD adjustment message from the SCM 42 is intended for
the slave modem, then it sends (91) the message in an ECAP frame to
the slave modem, which receives (96) this message, adjusts (97) the
configuration of its transmitter 64 in accordance with the PSD
parameters provided, and sends (98) a message confirming this
adjustment in an ECAP frame to the master modem 32. This is
received (92) by the master modem 32 and forwarded (87) to the SCM
42, continuing as described above.
[0055] It can be seen that in the manner described above analysis
of monitoring data is performed centrally by the SCM 42 and can be
performed efficiently for many lines 10. In each case monitoring is
performed while signals are not supplied to the relevant line 10,
the existing DSP being configured for this purpose. The relatively
brief monitoring periods do not significantly interrupt the
transmission of information in either direction on the line 10,
because this information is already buffered in the buffer 62 in
each modem. During the monitoring, Ethernet frames from the buffer
58 in each modem can still be supplied via the respective Ethernet
interface 52. Furthermore, it can be appreciated that brief
monitoring periods may be established during otherwise unused or
idle periods of the half duplex communications on the line 10,
without any extra interruption of the information transmission on
the line 10, and/or that the same monitoring periods can be used
for monitoring at both ends of the line 10.
[0056] Within the constraints imposed by the PSD parameters
provided by the SCM 42 to reduce crosstalk, the master modem 34 can
still optimize communications on the line 10 in the manner fully
described in the related application, for example controlling a
ratio of upstream and downstream frame transmission in dependence
upon buffer fills.
[0057] Although as described above the slave modem 34 communicates
with the SCM 42 via the master modem 32, communications could
instead be carried out using Ethernet frames addressed directly
between the SCM 42 and the slave modem 34. Such frames would, of
course, still be communicated via the master modem 32.
[0058] The invention has been described above in terms of a new
communications system being provided, and adjusting the PSDs of its
signals, to be compatible with any existing system with which there
might otherwise be excessive interference. It can be appreciated
that the same advantages can apply in respect of two or more new
systems each of which can adjust its PSDs so that they do not
interfere with one another or with any other existing systems, so
that multiple systems can co-exist in a compatible manner.
[0059] Thus although particular embodiments of the invention have
been described in detail, it should be appreciated that these and
numerous other modifications, variations, and adaptations may be
made without departing from the scope of the invention as defined
in the claims.
* * * * *