Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2001-02-09
2003-05-06
Pham, Chi (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C370S523000
Reexamination Certificate
active
06560277
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data communications equipment. More particularly, the present invention relates to methods for distinguishing different types of signals received by a PCM modem.
2. State of the Art
With the ever-increasing importance of telecommunications for the transfer of data as well as voice, there has been a strong effort to increase data transfer rates over the telephone wires. In 1994, the ITU-T adopted the V.34 Recommendation (International Telecommunication Union, Telecommunication Standardization Sector Recommendation V.34, Geneva, Switzerland 1994). The V.34 standard and subsequent amendments define modem operating speeds of 28.8 kbps up to 33.6 kbps, and the vast majority of modems being sold today adhere to the V.34 Recommendation. However, with the explosion in the use of the Internet, even at the V.34 transfer rates, downloading of large files available on the Internet can take long periods of time. Thus, even as the V.34 standard was being adopted, there was a thrust to provide additional standards recommendations which would increase data transfer rates even further.
Recognizing that further increases in data rates is theoretically limited where the telecommunication network is an analog system (see C. E. Shannon, “A Mathematical Theory of Communication,”
Bell System Technical Journal,
27:379-423, 623-656 (1948)), there have been various proposals to take advantage of the fact that much of the telecommunication network is now digital. For example, U.S. Pat. No. 5,394,437 to Ayanoglu et al., U.S. Pat. No. 5,406,583 to Dagdeviren, and U.S. Pat. No. 5,528,625 to Ayanoglu et al. (all assigned to AT&T/Lucent and all of which are hereby incorporated by reference herein in their entireties) all discuss techniques which utilize the recognition that the network is mostly digital in order to increase data transmission rates to 56 kbps and higher. Similarly, Kalet et al., “The Capacity of PAM Voiceband Channels,”
IEEE International Conference on Communications '
93, pages 507-511 Geneva, Switzerland (1993) discusses such a system where the transmitting end selects precise analog levels and timing such that the analog to digital conversion which occurs in the central office may be achieved with no quantization error. PCT application number PCT/US95/15924 (Publication WO 96/18261) to Townshend which is hereby incorporated by reference herein in its entirety) discusses similar techniques. All of the disclosures assume the use of PAM (pulse amplitude modulation) digital encoding technology rather than the QAM (quadrature amplitude modulation) currently used in the V.34 Recommendation. The primary difference between the AT&T technology and the Townshend reference is that the AT&T technology suggests exploiting the digital aspect of the telephone network in both “upstream” and “downstream” directions, while Townshend appears to be concerned with the downstream-direction only.
Recently, a new Recommendation for standard was adopted by the ITU-T for the purposes of standardizing a PCM-type modem. The new standard, known as “V.90”, which is hereby incorporated by reference herein in its entirety, relates primarily to the transmitter of a PCM-type modem, and relates to a modem which exploits the digital aspect of the telephone network in the downstream direction only. The ITU-T has also recently approved an additional standard known as “V.92” which relates to a modem which exploits the digital aspect of the telephone network in both the upstream and downstream directions.
In Section 8.4.1, the V.90 Standard requires the provision of a probing signal; also known in the art as digital impairment learning or “DIL”. The purpose of the DIL is to give the receiver of the receiving (analog) modem the opportunity to measure network impairments. The measurements and determinations made by the receiving modem are used by the receiving modem in formulating an appropriate constellation for the transfer of data. The constellation formulated by the receiving modem is transmitted back to the transmitting modem according to the format set forth in Section 8.5.2 of the V.90 standard.
While much attention has been paid in the prior art to the transmitters in the V.90 and V.92 modems, it will be appreciated that ability to design an appropriate transmission constellation plays a critical role in producing a high quality modem. In particular, according to V.90, the transmitter transmits 8-bit binary numbers (octets) which correspond to 128 positive and 128 negative &mgr;-law or A-law levels. These octets go through the digital network and are finally transformed into analog levels in a digital-to-analog (D/A) converter in the central office. To maximize data rates in the presence of network impairments, an optimal signal constellation must be utilized. Thus, it is necessary to relate (correspond) the transmitted octets to the levels received at the D/A output. This relation or correspondence is accomplished by reference to a translation table. Determination of the translation table is not a trivial task because the digital channel has uncertain parameters and the PCM signal is subjected to both digital and analog distortions including digital attenuation (PAD), robbed bits, etc. However, preparation of an appropriate translation table is critical to the high-quality functioning of the data communications. In addition, the translation table is necessary for generating an appropriate constellation design.
As set forth in previously incorporated Ser. No. 09/238,319, an important step in generating a translation table and constellation design is a determination as to whether the signal being received is an A-law signal or a &mgr;-law signal. Some countries (particularly European) utilize A-law encoding in their phone networks, and others (e.g., Japan and the U.S.) use &mgr;-law encoding. A few countries (such as South Korea) implement both A-law and &mgr;-law encoding in their networks.
As a rule, phone networks have only either A-law encoding or &mgr;-law encoding for most domestic calls. However, it is possible that the network between a client modem and a server modem may link the A-law and &mgr;-law networks when some domestic or international calls are placed. As a result, the client modem can receive any of four types of signals: a pure &mgr;-law signal, a &mgr;-law signal which is the result of an A-&mgr;conversion, a pure A-law signal, or an A-law signal which is the result of a &mgr;-A conversion. In establishing a channel, it is critical that the receiving modem determine whether the signals it is ultimately receiving are A-law (either pure or the result of a &mgr;-A conversion) or &mgr;-law (either pure or the result of an A-&mgr; conversion).
The technology set forth in previously incorporated Ser. No. 09/238,319 is effective and capable of distinguishing between a pure &mgr;-law signal and a pure A-law signal. In particular, in previously incorporated Ser. No. 09/238,319 a separation function was introduced:
F
1
(
n
1
,
n
2
)
=
∑
i
=
n
1
i
=
n
2
{
L
(
i
)
-
2
y
*
[
L
(
i
-
16
y
]
}
,
where L(i) is the i-th positive received level corresponding to transmitted Ucode=i, and y is a positive integer preferably equal to one. For pure A-law levels without noise (and with respect any PAD attenuation), for any n
2
>n
1
≧33, F
1
(n
1
,n
2
) will be zero. On the other hand, for pure &mgr;-law levels without noise and with 0 dB PAD attenuation, F
1
(n
1
,n
2
)=33(n
2
−n
1
+1). According to the preferred embodiment of Ser. No. 09/238,319, the value for the separation function is calculated for any non-robbed-bit signal within the frame, or for the average of non-robbed-bit signals. Then, the value for the separation function is compared to a threshold (e.g., five hundred). If the value of the separation function exceeds the threshold, the signal is determined to
Drucker Vitaly
Goldstein Yuri
Okunev Yuri
Wang Qin
Bayard Emmanuel
Gallagher Thomas A.
Gordon David P.
Jacobson David S.
PC Tel Inc.
LandOfFree
Distinguishing between final coding of received signals in a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Distinguishing between final coding of received signals in a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Distinguishing between final coding of received signals in a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3033128