Pulse or digital communications – Pulse width modulation
Reexamination Certificate
1999-10-26
2003-03-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Pulse width modulation
C375S256000, C375S257000, C375S296000, C370S205000, C370S212000, C327S031000, C332S109000, C332S115000
Reexamination Certificate
active
06532260
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ISDN basic service which achieves digital data transfer via metallic lines, and particularly relates to a long-distance transfer-pulse transmission device which transfers data a long distance via a metallic line (subscriber line) to a subscriber at a remote location.
2. Description of the Related Art
An ISDN basic service is designed and developed to achieve high-speed data transfer by using existing metallic lines which are conventionally used in analog communication. With regard to the ISDN basic service, a configuration shown in
FIG. 8
is defined in JT-G961 of the TTC standard. In
FIG. 8
, a plurality of line terminals LT are connected to an end terminal (switch-end terminal) ET on the station side. On the household side, a network terminal NT is connected to various household communication devices such as a terminal adaptor TA and terminal elements TE. The network terminal NT has a one-to-one connection with one of the line terminals LT via a metallic line. In Japan, digital transfer is implemented on the metallic line by using time-division transfer technology.
The ISDN basic service as described above is designed with an upper limit of line loss equal to 50 dB by taking into consideration a line balance against the ground, cross-talks, a line quality, etc. With respect to a user at a remote location farther away than the line-loss upper limit of 50 dB, basically, no service is provided. An initial estimate at the time of starting the ISDN basic service was that 99% of the entire telephone network in Japan would be covered by the ISDN basic service. A recent spread of population distribution, however, has resulted in 2-3% of service use being accounted for by use of the service in the areas where the line loss exceeds 50 dB. In order to provide a proper service to customers in such areas, remote stations need to be established, which inevitably requires a large investment. Because of this, currently, the ISDN service is not provided to all the customers.
Accordingly, there is a need for a long-distance-transfer system which can render the ISDN basic service at a low cost without establishing remote stations to areas where line losses from the line terminals LT exceed 50 dB.
FIG. 9
is an illustrative drawing showing a configuration of a line terminal LT on the station side and a configuration of a network terminal NT on the household side. The line terminal LT has a U-point interface on the metallic-line side and a V-point interface on the device (e.g., switch) side, and includes a transmission driver
1
, a receiver
2
, an equalizer (line termination)
3
, and a U-point/V-point-conversion unit
4
. The network terminal NT has a U-point interface on the metallic-line side and a T-point interface on the household-communication-equipment side, and includes the transmission driver
1
, the receiver
2
, the equalizer
3
, and a T-point/V-point-conversion unit
5
.
In this related-art configuration, a flow of signals going downstream is as follows. The line terminals LT is connected to an upper-level device (e.g., switch) via the V-point interface, and receives commands from the upper-level device at the U-point/V-point-conversion unit
4
. The U-point/V-point-conversion unit
4
changes speed of control signals and data so as to fit them to the U-point interface, and the transmission driver
1
sends them to the metallic line. The transmission driver
1
used in Japan is a U-point driver which attends to conversion to AMI signals.
In the network terminal NT, the receiver
2
receives signals that are degraded while traveling through the metallic line. The received signals have waveforms thereof reshaped by the equalizer
3
. Then, the T-point/V-point-conversion unit
5
extracts a clock from the signals, and changes speed of the signals so as to fit them to the T-point interface. The transformed signals are supplied to the T points.
A flow of signals going upstream is as follows. The network terminal NT is connected to a lower-level device (e.g., a terminal adaptor, a terminal element, etc.) via the T-point interface, and receives data from the lower-level device at the T-point/V-point-conversion unit
5
. The T-point/V-point-conversion unit
5
changes speed of status signals and data so as to fit them to the U-point interface. Timings of signal transmission to the U-point are determined by extracting a clock signal from the signals traveling downstream at the T-point/V-point/-conversion unit
5
. The transmission driver
1
converts the signals into AMI signals, which are transmitted to the metallic line.
In the line terminal LT, the receiver
2
receives signals that are degraded while traveling through the metallic line. The received signals have waveforms thereof reshaped by the equalizer
3
. Then, the U-point/V-point-conversion unit
4
identifies the status signals and data, and changes speed of the signals so as to fit them to the V-point interface with the upper-level device.
The equalizers
3
provided in the network terminal NT and the line terminal LT serves to correct signal degradation that is caused by the metallic line. This function of signal correction will be described below in detail.
The metallic line connecting between the network terminal NT and the line terminal LT serves as a subscriber line, and has frequency-to-line-loss characteristics as shown in
FIG. 10
in accordance with distributed parameters thereof. In
FIG. 10
, a horizontal axis shows a frequency f (Hz), and a vertical axis shows a line loss LOSS (dB). The characteristics are shown with respect to different lengths of metallic lines. As can be seen from the frequency-to-loss characteristics of
FIG. 10
, the line loss LOSS is in proportion to the square root of the frequency (i.e., f
½
) in a higher frequency region when a logarithm of the loss is considered. Namely, the higher the frequency, the greater loss the signal suffers. The lower the frequency, the easier the signals pass through the metallic line.
Frequency-to-line-loss characteristics inevitably vary, depending on parameters such as a type of a line, a diameter of a line, etc. In Japan, a type of a metallic line includes a lead cable, a paper insulated cable, a CCP cable line, etc., and a diameter of a line varies from &PHgr;0.4 to &PHgr;0.9. If all the frequency characteristics are averaged, a paper-insulated cable having a diameter of &PHgr;0.5 may represent characteristics that are closest to the average characteristics. When the equalizer
3
is used for equalizing a signal degraded by a metallic-line cable, a paper-insulated cable having a diameter of &PHgr;0.5 is used as a reference, and correction is made so as to cover the loss of this reference cable. In this manner, signal waveforms are corrected to have as little deformation as possible. This process is referred to as a square-root-f equalization.
FIG. 11
is an illustrative drawing for explaining a method of correcting signal waveforms.
The square-root-f equalization is made by combining gain characteristics of a flat amplifier, a first-order-slope amplifier, and a second-order-slope amplifier. The gain combined in this manner approximates for the frequency-to-loss characteristics of the paper-insulated cable with &PHgr;0.5 that is used as a reference as described above. In this manner, losses generated along the line are corrected.
When the related-art transfer system between a station and a household is used, three schemes as follows can be regarded as a viable option that achieves a long-distance transfer of data.
1) Signal transmission levels are boosted in the line terminal LT and the network terminal NT. This insures a greater signal level of signals received by the receivers, so that proper signal exchanges are attainable without making any changes to the existing receiver circuits.
2) Signal receipt sensitivities of the receiver circuits are boosted in the line terminal LT and the network terminal NT. This insures that signals are received by the highly se
Chin Stephen
Fujitsu Limited
Ha Dac V.
Katten Muchin Zavis & Rosenman
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