Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1998-03-03
2001-12-04
Pham, Chi (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C379S093240
Reexamination Certificate
active
06327301
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to modem technology, more particularly to PCM modem technology, and more particularly to a method for detecting and adapting to digital network impairments in a PCM modem.
BACKGROUND OF THE INVENTION
PCM modem technology as proposed requires no changes need to be made in the existing telecommunications system. Much attention has been paid to impairments affecting the local analog loop. However, there are several known digital impairments which can equally affect the performance of the PCM modems. The known impairments are the following:
a) Digital attenuation pads (3 dB and 6 dB)
b) Robbed bit signaling
c) PCM sample slips
d) PCM code conversions
It would be impractical to require the telephone carriers to change their transmission facilities. These digital impairments can and should be accommodated by PCM modems. Each of the above-identified impairments is further discussed below.
Digital Attenuation Pads
The network model diagrams for evaluation of PCM modems described in Sierra Semiconductor, “Liaison to TR30.1 Conceming PCM Modems,” Ft. Lauderdale, Jan. 23-24, 1997, Doc. FL-08, and Intel, “Proposed Provisional PCM Modem Network Model,” Ft.
Lauderdale, Jan. 23-24, 1997, Doc. FL-02, show configurations with 3 and 6 dB pads within the digital transport/switching network.
FIG. 1
presents the network model of the Intel paper, which illustrates the two-wire analog to four-wire digital PCM interface. Included in the interface are the A/D and DIA converters that process input and output of the PCM modern, analog attenuation elements including IMD and RTDIAD/EDD elements, noise generators summed with transmitted signals and a balance network interface. G.
121
further identifies additional attenuation pads used by various countries outside of North America. These pads are implemented via digital circuits or ROMs which provide a systematic remapping of PCM codes to other PCM codes. This mapping to a new space in the digital network is invariant for a telephone connection.
Conventional PCM modem technology relies on the network being digital with a single DIA converter in the path between the digital modem and the analog modem. During training, PCM modems will determine whether the channel can support the technology. If the network connection cannot support the PCM mode (i.e. offer a higher data rate than can be achieved with V.34), the modems will either fall back to another mode (e.g. V.34) or disconnect. However, there are a variety of different network configurations having differing digital attenuation elements.
FIGS. 2
a
-
2
f
illustrate various existing network configurations deployed between an analog PCM modem and a digital PCM modem. It is seen by comparison of these figures that a variety of configurations involving various combinations of digital switches, digital transport, and 3 dB and 6 dB attenuation pads are possible.
Accordingly, since significant variation exists in the implementation of attenuation pads, static predetermination of all possible remappings of PCM codes to other PCM codes is not viable.
Robbed Bit Signaling
Robbed bit signaling is used on North American telephone trunks for interoffice signaling. The least significant bit of every sixth PCM sample is used for signaling. Within the telephone network from end-to-end, typically two PCM samples within every six may be used for signaling, although in the worst case it may be all six. The effect of this impairment is the inability to convey information in this bit position. It further may affect PCM code transformations due to attenuation pads or law conversions.
PCM Sample Slips
PCM sample slips occur for purposes of synchronization within the telephone network or between telephone networks. Either a PCM sample is completely dropped or repeated. Detection of such impairments relies on redundancy or trellis coding or forward error correction of some form.
PCM Code Conversions
PCM code conversions between u-aw and A-law occur over international calls. This is a systematic mapping of codes from one law to the other. This mapping is not exactly one to-one according to the recommendations in G.711.
Regulatory Analog Transmission Power Limits
In addition to the digital impairments, telecommunications regulatory agencies have established power limits for circuits which terminate in an analog local loop, see Communications Certification Laboratory, “Liaison to T1A1.7 and TR30.1 on FCC Part 68 Compliance for PCM Modems,” Ft. Lauderdale, Jan. 23-24, 1997, Doc. FL-10. This limitation is a result of real implementation restrictions already within the deployed telephone equipment.
The highest data rates in the downstream direction (from the digital to the analog modem) can only be achieved if all of the &mgr;-law encoded levels can be used. However, the average energy of a sequence that uses all 255 &mgr;-law levels with equal probability is well above the maximum permitted transmit power level of −12 dBm0 specified in FCC part 68.
Thus it is apparent that the need to comply with this limitation requires use of a subset of the available PCM codes in the set of PCM symbols to be used for the high speed data transmission. Non-equally probably mappings (i.e. shell mapping) could be used to alter and control the probability distribution of each of the PCM codes in the set of symbols. Even with non-equally probably mapping techniques, the analysis in RSA Communications, “&mgr;-law signal power,” Orlando, Dec. 4, 1996, Doc. Orlando —10R#, suggests that both the highest and lowest segments of the &mgr;-law codes would not be used. The highest segments are eliminated because of signal power limits. The lowest are eliminated to increase the minimum distance between signal values.
SUMMARY OF THE INVENTION
The invention provides a means of detecting the presence of attenuation pads (3 dB or 6 dB) in the analog PCM modem, a determination of an appropriate set of PCM symbols for greatest data rates and communication of the attenuation and/or the selected set of PCM symbols to the digital modem. Without selection of alternate sets of PCM symbols, the data rate would be reduced and/or more problematic to receive without error when digital attenuation is present. The techniques of the invention eliminate the need for uniqueness in mapping for successful interoperation of L-law and A-law codes. Accordingly, the invention may be embodied in a method of compensating for systemic impairments in a telecommunications network. The method may involve transmitting a candidate set of digital symbols from a digital modem to an analog modem, processing an analog representation of said candidate set of digital symbols in the analog modem, deriving a set of data information symbols from the analog representation of the candidate set of digital symbols, transmitting a representation of the data information symbols from the said analog modem to the digital modem, and using the representation of the data information symbols in digital modem for subsequent transmission of data from the digital modem. The method may also include the preceding steps of sending digital symbols from the digital modem to the analog modem, converting the digital symbols into analog signals in the telephone network, processing the analog signals in the analog modem, and training a receiver of the analog modem to recognize changes in the analog signals that correspond to known changes in digital symbols transmitted by the digital modem.
REFERENCES:
patent: 5663982 (1997-09-01), Hodge et al.
patent: 5671251 (1997-09-01), Blackwell et al.
patent: 5754594 (1998-05-01), Betts et al.
Demjanenko Victor
Hirzel Frederic J.
Foley & Lardner
Pham Chi
Tran Khai
Vocal Technologies Ltd.
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