Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1999-06-04
2004-10-12
Phan, Man (Department: 2665)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S465000, C370S478000, C370S286000, C375S219000, C375S222000, C375S225000
Reexamination Certificate
active
06804267
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to Digital Subscriber Line (DSL) modems for communicating over telephone subscriber loops, and more particularly, to optimizing transceiver training for Time Compression Multiplexing DSL (TCM-DSL) modems.
BACKGROUND OF THE INVENTION
The demand for modem transmission speed continues to soar as more telephone customers in more countries send more data traffic over phone lines. While it is feasible to run high-speed fiber-optic cable to some new customers, existing customers are connected to the phone system by slower copper wires such as untwisted or twisted-pair lines. The cost of replacing all existing copper wires with higher-speed fiber-optic cable is prohibitive. Thus, higher-bandwidth technologies that use the existing copper-cable phone lines are desirable.
Basic rate Integrated Services Digital Network (ISDN) boosted data rates over existing copper phone lines to 128 kbps. Special termination and conditioning of the existing copper phone lines is required for ISDN.
DSL modems are now becoming available. Several variations of DSL technology (referred to generically as xDSL) are being explored, such as High Bit Rate DSL (HDSL), Rate Adaptive DSL (RADSL), Very High Bit Rate DSL (VDSL), and Asymmetric DSL (ADSL). ADSL is particularly attractive for consumer Internet applications where most of the data traffic is downloaded to the customer. Upstream bandwidth for uploading data can be reduced to increase downstream bandwidth since most Internet traffic is downstream traffic. ADSL provides a bandwidth of up to 8 Mbps in the downstream direction, or up to 2 Mbps if symmetric DSL is used. See, for example, U.S. Pat. Nos. 5,461,616, 5,534,912, and 5,410,343 for descriptions of ADSL technology.
Cross-talk Using Pulp Cables Limits xDSL
The wider bandwidth required for xDSL transmission creates higher cross-talk interference among copper pairs in the same cable-binder group. The level of the cross-talk varies for different cable structures and materials. In particular, some countries such as Japan and Korea use telephone cables with a paper-based “pulp” insulator rather than the plastic-insulated cables (PIC) used in the United States. These pulp cables produce much more cross-talk interference than the PIC cables. Thus, it is more difficult to deploy wide-band xDSL services in those countries since their existing telephone cables are prone to cross-talk interference.
FIG. 1
shows the problem of interference from existing ISDN lines. Central Office
8
(CO) contains several ISDN line cards
14
that connect the telephone network backbone to local lines
20
that are strung to the customer premises equipment (CPE). Remote ISDN terminal adapters or modems
12
are located at different remote customer sites within a few kilometers of central office
8
.
Local lines from ISDN line cards
14
to remote ISDN modems
12
are usually routed through one or more cable bundles
18
. These telephone-cable bundles
18
may contain dozens or more separate telephone lines or copper pairs. Standard voice services, ISDN services, and newer xDSL services often must share the same cable bundle. Since lines run close to other lines in cable bundles
18
for long distances, mutual inductances can create cross-talk interference or noise on lines
20
.
For voice services such as Plain-Old-Telephone Service (POTS), the frequencies are low, so interference is negligible. ISDN digital services use a higher bandwidth of around 80 to 320 kHz. Interference begins to cause problems at ISDN frequencies. New xDSL services usually use even higher bandwidths. For example, ADSL bandwidths are usually above 1 MHz and have significant cross-talk problems. Cross-talk from other digital services such as older ISDN and T1 in a cable bundle can severely restrict xDSL speeds.
Due to different cross-talk interference characteristics, different line codes are used for basic rate ISDN. In countries such as the U.S., where better-insulated PIC cables are used, full-duplex data transmission with echo cancellation is deployed. Echo cancellation by the receivers removes the echo by the locally-transmitted signal so that the remotely transmitted signal can be received. Thus, both ends of the line can transmit simultaneously. Full-duplex data transmission with echo cancellation is described in International Telecommunication Union-Telecommunication Standardization sector (ITU-T) G.961, Appendix II, or T1.601, which is incorporated by reference herein in its entirety.
Japan Uses Half-Duplex ISDN
In countries such as Japan, where the noisy pulp cables are installed, a different ISDN system is often deployed. To eliminate the near-end cross-talk (NEXT) interference, TCM-ISDN is used rather than echo cancellation full-duplex. In such a system, the ISDN line cards at the CO are synchronized so that they all transmit at the same time. The ISDN line cards all receive during a different time period. Thus, NEXT interference during reception is eliminated since none of the other ISDN modems at the same side are transmitting during the reception time period. Although far-end-cross-talk (FEXT) interference exists, it is usually much weaker than NEXT. TCM-ISDN service is described in ITU-T G.961, Appendix III, which is incorporated by reference herein in its entirety.
FIG. 2
is a timing diagram for a TCM-ISDN line. During time period or window
22
, data is output from the CO to the remote ISDN modem at the customer premises. This data arrives at the remote modem after a delay, during reception window
24
. The customer premises ISDN equipment uses a burst clock detector to determine the timing of the receive downstream burst and to generate the timing for its transmit upstream burst. A pause occurs when no data is transmitted. This pause is sometimes called the turnaround period. During period
26
, upstream data is transmitted from the remote modem to the CO, which arrives at the CO after a delay, during window
28
.
At any particular time, only one end of the TCM-ISDN line is transmitting, while the other end is receiving. Echo cancellation is not needed since the echo of the transmitted signal does not have to be removed. Since each side transmits in slightly less than half of the time, the data rate during transmission has to be approximately doubled to obtain the same average data rate. This translates to a higher frequency bandwidth, which in turn creates more cross-talk. While such a TCM-ISDN system is effective for reducing cross-talk in the TCM-ISDN system itself, it is difficult to add newer xDSL systems in the same cable bundle because of the cross-talk from the ISDN lines.
Synchronized ISDN Lines Create Interference for xDSL
Newer xDSL services, such as HDSL and ADSL, use full-duplex transmission based on frequency-division-multiplexing (FDM) or echo cancellation. Therefore, the receiver at either side receives all the time. If such an xDSL modem is installed in the same cable bundle as the TCM-ISDN, the strong NEXT during the transmission time for the same-side TCM-ISDN modems will severely affect the reception of the xDSL signal.
FIG. 3
is a diagram of interference at a CO from several ISDN lines transmitting in synchronization. During transmit window
22
, a burst of data is sent from the CO to the remote sites. NEXT is particularly strong during transmit window
22
, since the ISDN devices at the CO are all transmitting. During receive time window
28
, these ISDN devices at the CO are not transmitting. Interference is primarily FEXT, which is weaker than NEXT since it is attenuated by the length of the telephone line.
TCM-ISDN Transmitters Often Poorly Filtered—
FIG. 4
FIG. 4
is a transmitting-signal spectrum of a TCM-ISDN modem. For background information on a TCM-ISDN telephone system, see U.S. Pat. No. 5,265,088 by Takigawa et al., and assigned to Fujitsu Ltd. and Nippon Telegraph and Telephone Corp. (NTT). This coding scheme uses Pulse Amplitude Modulation (PAM) with alternate-mark inversion (AMI). In this scheme, a binary zero is represented by no pulse, and a binary one b
Bar-Ness Yaron
Long Guozhu
Centillium Communications Inc.
Fenwick & West LLP
Phan Man
LandOfFree
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