HTU-C clocking from a single source

Details HTU-C clocking from a single source HTU-C clocking from a single source

2000-12-14

2002-04-16

Chin, Stephen (Department: 2634)

Pulse or digital communications

Transceivers

Modems

Reexamination Certificate

active

06373887

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to transmission of data on a subscriber loop in a public network such as, for example, a telephone network. More specifically, the present invention provides an improvement of standard single pair digital subscriber line (SDSL) technology.
A wide variety of technologies and transmission standards have been developed for transmission of data via currently existing public network resources. A substantial portion of these resources comprise copper twisted pair transmission lines. This is especially true for the final connections to individual subscribers, i.e., subscriber loops. Without other limitations such as core network filters, such copper lines can achieve practical data rates on the order of tens of megabits per second (Mbps). Of course, substantial attenuation occurs at the higher data rates thereby limiting the length of the subscriber loop which may be serviced at such rates. For example, 24 gauge copper supports reliable transmission of data at the DS1 standard, i.e., 1.544 kbps, also commonly referred to as T1, for up to 12,000 feet. By contrast, the same 24 gauge copper will only support the STS-1 standard, i.e., 51.840 Mbps, for lines of less than 1000 feet.
The term “digital subscriber line” (DSL) refers to a modem or modem pair connected by one or more twisted pairs having a specific data frame format and associated transmission rate. The first digital subscriber line technology, referred to as IDSL, corresponds to what is also known as basic rate ISDN. IDSL technology transmits duplex data at 144 kbps over copper lines using a 2B1Q modulation scheme. The modems multiplex and demultiplex the data stream into two B channels (64 kbps each) and a D channel (16 kbps) as described in ANSI T1.601, the entirety of which is incorporated herein by reference for all purposes.
High data rate digital subscriber lines (HDSL) are related to the earlier IDSL using the same modulation scheme to transmit data at the T1 data rate over two twisted pairs as described in ANSI Committee T1 TR-28, the entirety of which is incorporated herein by reference for all purposes. A single pair digital subscriber line (SDSL) is a single pair version of HDSL, i.e., transmitting data at one-half the T1 data rate, i.e., 768 kbps, over a single twisted pair. For both HDSL and SDSL and as shown in
FIG. 1
, data are organized into 6 ms frames
102
comprising alternating overhead and payload sections
104
and
106
. The four payload sections
106
each include twelve 97-bit payload blocks
108
,
96
bits (
110
) of which are data and one bit (
112
) of which represents block overhead. This works out to the well known data rate of 768 kbps. Overhead sections
104
and
112
represent an additional 16 kbps for an actual transmission rate of 784 kbps.
FIG. 2
is a simplified block diagram of one portion
200
of a standard SDSL, configured as a HDSL Terminal Unit—Central Office (HTU-C) or public branch exchange (represented by modem
204
). The central office typically transmits data via twisted pair line
216
to a subscriber premises (represented by modem
206
, FIG.
3
). The data to be transmitted enters framing circuitry
208
of modem
204
at the raw data rate of, for example, 768 kbps. Framing circuitry
208
organizes the incoming data stream into the 6 ms frames described above with reference to FIG.
1
. In order to perform this task, the framing circuitry
208
utilizes a signal generated from a 768 kHz oscillator
210
. The signal provided by oscillator
210
to framing circuitry
208
functions as a data clock, and is used in communicating and synchronizing with incoming raw data on line
201
. While the framing circuitry
208
organizes the incoming data stream into the 6 ms frames, it generates frame overhead data, which is multiplexed with the raw data, and inserted into the frame at a rate of 16 kbps. During the time that the frame overhead data is being inserted into the frame, the incoming raw data is queued in a FIFO buffer. When the insertion of frame overhead data is finished, the data from the FIFO buffer is then inserted into the frame, followed by incoming raw data from line
201
. The framed data are then sent to bit pump
212
where, using a 784 kHz oscillator
214
, they are encoded according to the 2B1Q modulation scheme and transmitted via twisted pair line
216
to the subscriber premises.
As shown in
FIG. 2
, a conventional HTU-C system, requires two different clock sources: one for the data rate and one for the signaling rate. The solution to this requirement is classically solved via two externally provided clock signals, each signal being generated from a separate oscillator. In
FIG. 2
, the 768 kHz oscillator
210
is utilized as a data clock by the framing circuitry for generating the data frames and for communicating and synchronizing with the incoming raw data. The 784 kHz oscillator
214
is utilized as a signaling clock by the bit pump for modulating the data frames according to the 2B1Q modulation scheme. However, use of two separate clock sources increases cost, complexity, power consumption, and space utilization of the DSL modem. What is desirable, therefore, is to provide a DSL modem having reduced, cost, complexity, power consumption, and space requirements compared to conventional DSL modems configured as HTU-C devices.
SUMMARY OF THE INVENTION
This and art additional objects are accomplished by the various aspects of the present invention wherein, briefly, according to a principal aspect, a digital subscriber line (DSL) modem is provided wherein the clocking signals utilized by the framing circuitry and bit pump are both derived from a single external clock source.
Accordingly, a first aspect of the present invention is directed to a DSL modem for receiving an incoming data stream and generating a sequence of data frames for a digital subscriber line. The modem comprises a clock source for providing a first clock signal; a modulation circuit; and framing circuit which provides a receiver overhead signal for indicating the incorporation of frame overhead bits into generated data frames; and a clocking circuit adapted to receive the first clock signal and the receiver overhead signal for providing a second clock signal to the framing circuit. The clocking circuit includes logic means for combining the first clock signal and the receiver overhead signal to generate the second clock signal, wherein the generated second clock signal has a specific timing relationship to the first clock signal and the receiver overhead signal such that, while the receiver overhead signal is inactive, a second clock signal is active at a frequency substantially equal to the frequency of the first clock signal, and while the receiver overhead signal is active, a second clock signal is inactive. This generated second clock signal is utilized by the framing circuit for communication with the incoming data stream.
A second aspect of the present invention is directed to an apparatus included within a DSL modem. The DSL modem includes a modulation circuit; a clock source for providing a first clock signal; and a framing circuit which provides a receiver overhead signal for indicating the incorporation of frame overhead bits into each data frame. The framing circuit further includes a pulse code multiplexed (PCM) interface circuit for communication with incoming PCM data. The apparatus comprises a clocking circuit adapted to receive the first clock signal and the receiver overhead signal for providing a second clock signal to the framing circuit The clocking circuit includes logic means for logically combining the first clock signal and the receiver overhead signal to generate the second clock signal, wherein the timing of the generated second clock signal is such that, while the receiver overhead signal is inactive, the second clock signal is active at a frequency substantially equal to the frequency of he first clock signal, and while the receiver overhead signal is active, the second clock frequency is inactive. This g

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