Note: Descriptions are shown in the official language in which they were submitted.
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MEDIA CONVERTER FOR DATA COMMUNICATION ON AN EXTENDED
TWISTED-PAIR NETWORK
Back-r~ ound
This invention relates to data communication over an extended twisted-pair
telephone
wiring network.
Systems have been designed for data communication over active telephone lines
within a
building, for example as described in International Patent Application
US98/11197, "Techniques
for high-speech distribution of data over ordinary twisted pair telephone
wires." That patent
describes a system in which devices communicate over active telephone lines
using a collision
to detection/collision avoidance random access approach based on the Ethernet
signaling standard.
In another approach to data signaling on active telephone lines, Digital
Subscriber Loop
(DSL) standards address approaches to data communication between a telephone
exchange and a
residence over an active telephone local loop. In DSL, point-to-point
communication between
two DSL modems uses one of a number of signaling approaches, including
Quadrature
15 Amplitude Modulation (QAM), and carrierless amplitude/phase modulation
(CAP).
S ummary
In one aspect, in general, the invention is a media adapter. The media adapter
has a first
interface for receiving a first data signal and transmitting a second data
signal. An encoder
coupled to the first interface generates a first reduced bandwidth signal in a
data band from the
2o first data signal. The bandwidth of the reduced bandwidth signal is less
than the bandwidth of
the first data signal. A decoder coupled to the interface receives a second
reduced bandwidth
signal in the data band and generates the second data signal from the second
reduced bandwidth
signal. A second interface, which is coupled to the encoder and the decoder,
transmits the first
reduced bandwidth signal to and receiving the second reduced bandwidth signal
from a twisted
25 pair telephone network at frequencies within the data band. The data band
is at higher
frequencies than a voice band used for voice transmission on the twisted pair
network. A
collision signal generator generates a collision signal when the media adapter
is transmitting the
first reduced bandwidth signal to the twisted pair network. The collision
signal is transmitted to
the twisted pair network at frequencies within a collision band that is at
higher frequencies than
3o the voice band. The media adapter also includes a collision signal detector
that detects signals
within the collision band on the twisted pair network indicating that a device
connected to the
twisted pair network is transmitting a signal within the data band.
The media adapter can include one or more of the following features:
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A signal generator coupled to the collision signal detector and to the first
interface for
generating a signal indicating that another device is transmitting and
transmitting the generated
signal through the first interface.
The first data signal and the second data signal are Ethernet signals.
The data rate of the first data signal and the second data is 10 Mb/s or
greater and the
bandwidth of the data band is 2.5 MHz or smaller.
The data rate of the first data signal and the second data signal is 100 Mb/s
or greater and
the bandwidth of the data band is 12 MHz or smaller.
The encoder includes a quadrature amplitude modulator for modulating the first
data
1 o signal and the decoder includes a quadrature amplitude demodulator for
demodulating the second
reduced bandwidth signal.
The decoder further includes a signal pre-processor coupled to the quadrature
amplitude
demodulator for performing spectral equalization on the second reduced
bandwidth signal prior
to demodulation.
~ 5 In another aspect, in general, the invention is a method for passing data
over an active
telephone wiring network. The method includes receiving a first data signal
and transmitting a
second data signal over a first interface, generating a first reduced
bandwidth signal in a data
band from the first data signal, receiving a second reduced bandwidth signal
in the data band,
generating the second signal from the second reduced bandwidth signal, and
transmitting the first
20 reduced bandwidth signal to and receiving the second reduced bandwidth
signal from a twisted
pair network at frequencies within the data band . The method also includes
generating a
collision signal when the media adapter is transmitting the first reduced
bandwidth signal to the
twisted pair network, the collision signal being transmitted to the twisted
pair network at
frequencies within a collision band that is at higher frequencies than the
voice band, and
25 detecting signals within the collision band on the twisted pair network
indicating that a device
connected to the twisted pair network is transmitting a signal within the data
band.
The invention has an advantage of allowing high rate data communication on an
active
telephone network over an extended range. By compressing the bandwidth of a
data signal, such
as a 10 Mb/s or a 100 Mb/s Ethernet signal, a lower range of frequencies can
be used for the data
3o band. These lower frequencies suffer less attenuation than do higher
frequencies, thereby
increasing the signal-to-noise ratio at a receiver. The same frequency range
is used for all
communication in a random access approach, thereby using a reduced bandwidth
as compared to
approaches in which different directions of communication use non-overlapping
frequency
ranges. Use of different frequencies for collision signals improves detection
of transmissions of
35 other media converters. Use of collision detection band that is above the
voice band but at
relatively low frequencies fiirther improves detection of collisions on the
network.
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3
Description of Drawings
FIG. 1 illustrates a system in which multiple computers are coupled to a
twisted-pair
telephone network through media converters;
FIG. 2 is a diagram that illustrates a collision band and a data band at
higher frequencies
than the telephone voice band;
FIG. 3 is a block diagram of a media converter;
FIG. 4 is a block diagram of a decoder of a media converter; and
FIG. 5 is a block diagram of an encoder of a media converter.
1 o Description
Referring to FIG. 1, a number of computers 120 communicate with one another
over an
active twisted-pair telephone network 150. As illustrated in FIG. 1, twisted-
pair network 150
carries voice signals between telephones 140 and a telephone exchange 170.
Each computer 120
is coupled to twisted-pair network 150 through a media converter 110. Twisted
pair network 150
has two conductors over which both voice and data signals are transmitted. In
this embodiment,
computers 120 communicate with their corresponding media converters 110 using
a 10 Mb/s
Ethernet standard, 10 Base-T, in which data link 122 includes two pairs of
conductors, one pair
for each direction of communication. In a second embodiment, the computers and
media
converters communicate using a 100 Mb/s Ethernet standard, 100 Base-T.
Telephones 140 are
2o coupled to twisted pair network 150 through low-pass filters 130. Low-pass
filters 130 pass
signals in the telephone voice band and presents a high impedance at higher
frequencies to the
twisted pair network. A terminator 160 is coupled at the end of twisted pair
network 150 to
reduce reflections at high frequencies.
Referring to FIG. 2, media converters 110 communicate with one another using a
relatively small bandwidth as compared to the bandwidth of the signals passed
between the
media converters and computers 120 coupled to the media converters. Media
converters 110
pass data between one another within a data band 230. Each media converter
transmits data in
the same range of frequencies. When transmitting data in data band 230, a
media converter 110
also transmits a collision tone 220 within a collision band 220. Each media
converter is assigned
a different frequency for transmitting its collision tone. A receiving media
converter 110 detects
collision tones within collision band 220 at frequencies other than its own in
order to detect
collisions. In this way, a media converter does not have to simultaneously
transmit in data band
230 and detect transmissions of other media converters in the data band.
Collision band 220 and
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data band 230 are in different frequency ranges, both of which are at
frequencies above the
telephone voice band 210, which does not extend beyond 5 kHz.
Referring to FIG. 3, media converter 110 has a transmit section and a receive
section that
are each coupled to twisted pair network 150 through a junction 310. In the
receive section,
Ethernet signals from an inbound pair of wires 122b pass signal from computer
120 to the media
converter. These signals pass to an encoder 360, which converts the Ethernet
signals into a
narrower bandwidth format for transmission to other media converters in data
band 230 (FIG. 2).
In addition to encoding the Ethernet signal into the data band, media
converter 110 also
generates a collision tone in tone generator 370 whenever it receives an
Ethernet signal on wires
l0 122b. The generated collision tone is at a different frequency than
collision tones generated by
tone generators 370 in the other media converters 110 coupled to twisted pair
network 150. Both
the collision tone generated by tone generator 370 and the data signal encoded
by encoder 360
are coupled to twisted pair network 150 through junction 310.
In the receive section, signals from the twisted pair network are passed from
twisted pair
network 150 to a high-pass filter 320 and to a low-pass filter 322. High-pass
filter 320 passes
signals in data band 230 (FIG. 2), while low-pass filter 322 passes signals in
collision band 220
(FIG. 2).
When media converter 110 is not transmitting a signal onto twisted pair
network 150 and
it receives a data signal in data band 230, that signal passes through
junction 310 and high-pass
2o filter 320 to decoder 330. Decoder 330 converts the signaling format used
to communicate
between the media converters into a standard Ethernet signal. The Ethernet
signal is passed
through a coupler 350 and over an outbound pair of wires 122a of data link
122, which couples
media converter 110 and computer 120.
When media converter 110 is transmitting onto twisted pair network 150 and it
receives a
collision tone from the network, a tone detector 340 at a frequency other than
the frequency at
which tone generator 370 generates collision tones, tone detector 340 sends a
signal to signal
generator 342, which in turn generates a signal that is passed through coupler
350 to computer
120 over wire pair 122a. This generated signal is an Ethernet format signal
that computer 120
interprets as another computer transmitting, and the computer will thereby
detect that a collision
3o has occurred and stop transmitting.
Note that both the collision tones and data signals in this system are
expressed at
frequencies above the voice band. Separation of the voice signals from the
signals related to data
is achieved by passive low-pass and passive high-pass filters. Various
arrangements for such
separation, and for coupling signals above the voice band on branches of a
twisted pair wiring
network are described in patent application US98/11197 referenced above.
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Refernng to FIGS. 4 and 5, encoder 360 and decoder 330 convert between
Ethernet
signals and narrower band signals used for transmission over twisted pair
network 150. In this
embodiment, encoder 360 and decoder 330 include a modulator and a demodulator,
respectively,
which uses quadrature amplitude modulation (QAM) for communication over
twisted pair
network 150. In this embodiment in which 10 Base-T Ethernet is used, a QAM-16
approach is
used in which each transmitted symbol is chosen from a 16-point constellation,
yielding a
transmission of 4 bits per transmitted symbol. This results in data band 230
being approximately
2.5 MHz or less to carry the 10 Mb/s bit stream of 10 Base-T Ethernet. In the
second
embodiment in which 100 Base-T Ethernet is used, QAM-128 is used, and data
band 230 is
1 o approximately 12 Mhz wide or less. In alternative embodiments, other
encoding approaches
which reduce the bandwidth of Ethernet signals are used.
Referring to FIG. 4, decoder 330 includes a series of functional elements. An
analog-to-
digital converter (A/D) 410 converts the received signals in the data band
into a digitized
sampled waveform representation. This digitized representation is digitally
filtered in equalizer
420. Equalizer 420 compensates for the frequency dependency of attenuation on
twisted-pair
network 150. In alternative embodiments, equalizer 420 adapts to the
characteristics of the
received signal. The equalized signal is passed to QAM decoder 430, which
outputs a serial bit
stream that was encoded in the received signal. The serial bit stream is
converted by an Ethernet
encoder 440 for transmission to the destination computer. In alternative
embodiments, other
implementations of this processing, for example performing some functions in
an analog domain
rather than a digitized domain, are used. Also, additional elements, for
instance, providing error
correction or echo cancellation mechanisms may be included in the encoder and
decoder.
Referring to FIG. 5, encoder 360 essentially performs the inverse operations
of decoder
330. An Ethernet decoder 510 receives an Ethernet signal from the computer and
converts it into
a serial bit stream. QAM encoder 520 converts the bit stream into a digitized
sampled waveform
representation of a QAM encoding of the bit stream. This digitized waveform is
processed in
signal pre-processor 530. In this embodiment, signal pre-processor pre-
emphasizes the higher
frequency component of the encoded signal in order to mitigate the greater
attenuation of those
higher frequency components. Finally, the processed digital waveform is passed
through digital-
to-analog converter 540 before transmission over the twisted pair network.
By reducing the bandwidth of the encoded data signals as compared to their
original
Ethernet representations and thereby allowing transmission at lower
frequencies, the data signal
suffers less overall attenuation on the twisted pair network than would the
original Ethernet
representation. This has the effect of increasing the signal-to-noise ratio
(SNR) at the receiving
media converter, or alternatively, increasing the transmission range of a
media converter.
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In order to allow collision detection, which as described above makes use of
collision
tones 222 in collision band 220, the transmission path length is limited to
allow detection of a
collision at a sending media converter before the sending media converter has
completed the
transmission. In this system, the transmitted packets are at least 512 bits in
length, as is
prescribed by the Ethernet standard. At 10 Mb/s, 512 bits corresponds to 51.2
~s. If a
transmission speed of 2x10 m/s is assumed, such a packet will be "stretched
out" over to over
km. In the second embodiment in which 100 Mb/s signaling is used, the packet
will be
stretched out to over 1 km. In the case of 10 Mb/s signaling, if the maximum
distance between
media adapters is one half the length of a packet, or approximately 5 km, then
if a second media
to adapter starts transmitting just before a packet transmitted from a first
media adapter reaches it,
then the first media adapter will detect the second media adapter's collision
tone before it has
finished sending the packet, thereby detecting the collision. Similarly, in
the second embodiment
with 100 Mb/s signaling, the maximum separation between media adapters is
approximately
500 m.
In alternative embodiments, other arrangements of signals in the frequency
spectrum are
used. In order to limit the upper frequency of the data band, the same band is
used for
transmission in both directions on the twisted pair network in a half duplex
arrangement. In
order to support robust collision detection, collision signals at different
frequencies for each
media adapter are used. In the embodiment described above, collision band 220
is at different
2o frequencies than data band 230. In alternative embodiments, collision
signals may be sent within
the data band. For example, if a multi-tone signaling approach is used for
transmission within
data band 230 rather than QAM, collision tones could be interspersed with the
signaling tones.
Also, alternative collision signals rather than tones can alternatively be
used. For example,
orthogonal narrow-band signals or spread spectrum signals that are unique to
each media adapter
can be used in place of the collision tones.
The signaling approach described above can be applied within a single
building, in a
campus of buildings, or between a telephone switching office and multiple
residences. A
centralized hub can couple computers on different branches twisted pair
networks, and subject to
the limitations of maximum total distance between computers, all the computers
can function
within a single collision domain.
It is to be understood that the foregoing description is intended to
illustrate and not to
limit the scope of the invention, which is defined by the scope of the
appended claims. Other
embodiments are within the scope of the following claims.
