Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
2000-01-18
2004-09-07
Patel, Ajit (Department: 2664)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S493000, C375S222000
Reexamination Certificate
active
06788705
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the transmission of information over a digital subscriber line (DSL).
2. Description of the Related Art
The use of digital subscriber line (DSL) technology has increased in recent years, allowing high-speed (i.e., broadband) access from a central location, typically a telephone central office, to multiple subscribers in homes or offices. The particular advantage of DSL over various other forms of broadband communications is that DSL allows the use of existing copper telephone lines from the central office to the end user, often referred to as the “local loop”, or simply “loop”. These existing lines are relatively small gauge (e.g., 24 or 26 gauge) twisted pair copper conductors, and were originally intended for use with a single analog voice channel occupying a relatively low bandwidth of about 3 to 4 kHz. However, the use of quadrature amplitude modulation (QAM) has allowed higher-speed digital communications over these lines, typically allowing data rates on the order of several hundred kilobits per second (Kbps). In addition, most recent DSL work has been concentrated in the area of discrete multi-tone (DMT) technology, which allow multiple channels to be sent over modulated carriers that are spaced apart in frequency; see, for example, U.S. Pat. No. 3,511,936. That technique can increase the data rate to over 1 megabits per second (Mbps). A typical DSL application is for connecting the subscriber to the Internet via the telephone central office, but other uses are possible.
There are numerous implementations of the above techniques that allow for sending data at a variety of rates over various distances, with shorter line distances generally allowing for higher data rates. For example, ADSL (Asymmetric Digital Subscriber Line) allows for a higher transmission rate from the central office to the subscriber than vice-versa. ADSL also typically provides for retaining the lower frequency portion of the phone line for analog voice communication, often referred to as POTS (Plain Old Telephone Service), while the higher frequencies are reserved for data. The data rates are typically about 1.5 Mbps downstream, (i.e. from the network to the end user's PC) and 384 Kbps upstream (i.e. from the end-user's PC to the network) in the standard “G.Lite” version of ADSL.
Referring to
FIG. 1
, a typical ADSL network topology is illustrated. A central office
100
includes a POTS splitter
101
that interfaces with the twisted pair subscriber line
108
, and provides an analog signal to the POTS line care
102
and a digital signal to the ADSL line card
103
. The POTS line card in turn interfaces with the voice switch
104
, for example a 5ESS® electronic switch from Lucent Technologies Inc., which provides switched circuit access to the telephone network
106
. The ADSL line card interfaces with the IP (Internet Protocol) router
105
, which provides switched packet access to the Internet
107
. The telephone subscriber line
108
connects to a subscriber
113
, typically a home or office, and terminates in a POTS splitter
109
, which provides voice frequencies to the telephones
111
and
112
, and provides the data over higher frequencies to a personal computer (PC)
110
. Note however that the G.Lite standard allows for eliminating the POTS splitter, by spacing the data frequency that carry the digital data sufficiently far in frequency from the lower analog frequencies that interference to the analog voice service is minimized. Then, the PC
110
and phones
111
and
112
may be directly connected to the subscriber line
108
, although the use of low-pass filters for the phones
111
and
112
is recommended in some installations.
Referring to
FIG. 2
, an illustrative G.Lite subscriber line frequency spectrum is illustrated, wherein the analog voice frequencies
200
lie below the downstream (slow) data channel
201
and the upstream (fast) data channel
202
. Both of the data channels comprise individually modulated tones that produce
256
bands approximately 4.3125 kHz wide (e.g.,
203
and
204
). The information in each band is allocated during modem training, so that more information is sent in a given band when the signal-to-noise ratio (S/N) is high, and less information is sent when the S/N is low. In this manner, the rate of information transmission may be maximized for a given subscriber line in the presence of phase and frequency response variations, cross talk, reflections, and noise from a variety of sources.
It is also known in the art to make use of the “analog” portion of the voice spectrum
200
for low-speed modems of the V. series (e.g., V. 32, V. 34 and V.90). The information transmitted or received by a modem may originate in digital form, for example computer files or Internet data, or in analog form, for example speech, that is converted to digital form. The modulation type depends on the particular V. series modem in use, but these modems are collectively referred to as “analog” modems because they utilize the POTS frequency portion of the telephone line; that is, below approximately 4 kHz. This limits the data rate to about 56 Kbps or less, which is referred to as “low speed” herein.
In order to establish high-speed DSL communications over the phone line, the modems at both ends must engage in a start-up sequence, often referred to as “handshaking”. The various handshaking protocols depend on the variety of DSL in use, but usually determine at least the line loss, frequency and phase response of the line, so that proper line equalization can be achieved. The information determined during handshaking can also be used for echo cancellation purposes and reduction of certain types of cross talk. However, over certain loop conditions, the DSL modems may encounter difficult loop impairments. In some cases, the modems on both sides of the loop may not be able to establish a connection. To solve this problem, two engineers, one on either side of the local loop cable, communicate with each other via a separate phone line in order to pass diagnostic data. However, that technique is slow and does not allow the “automatic” handshaking mode to work. Furthermore, the intervention of trained personnel increases the time and expense required to establish the DSL service.
SUMMARY OF THE INVENTION
I have invented a technique and apparatus for passing startup information between DSL modems connected by telephone lines. In my invention, DSL modem startup parameters are passed in digital form over the voice band channel. For this purpose, analog V. series modems may be used to communicate over the voice band channel.
REFERENCES:
patent: 5889856 (1999-03-01), O'Toole et al.
patent: 6137839 (2000-10-01), Mannering et al.
patent: 6263016 (2001-07-01), Bellenger et al.
patent: 6374288 (2002-04-01), Bhagavath et al.
patent: 6400759 (2002-06-01), Liu et al.
patent: 6442195 (2002-08-01), Liu et al.
patent: 6477595 (2002-11-01), Cohen et al.
patent: 6594306 (2003-07-01), Mehrabanzad et al.
Lucent Technologies - Inc.
Patel Ajit
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