Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
1998-06-30
2003-04-08
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S545000, C375S222000
Reexamination Certificate
active
06546024
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 line 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 T
1
, 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” 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 T
1
.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 T
1
data rate over two twisted pairs as described in ANSI Committee T
1
TR-28 and ETR
152
, the entireties of which are incorporated herein by reference for all purposes. A single line digital subscriber line (SDSL) is a single pair version of HDSL, i.e., transmitting data at one-half the T
1
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
along with bits
112
represent an additional 16 kbps for an actual transmission rate of 784 kbps.
FIG. 2
is a simplified block diagram of a standard SDSL
202
connecting a central office or public branch exchange (represented by modem
204
) and a subscriber premises (represented by modem
206
). The data to be transmitted enters framing circuitry
208
of modem
204
at the raw data rate of 768 kbps. Framing circuitry
208
organizes the incoming data stream into the 6 ms frames described above with reference to
FIG. 1
using a 768 kHz oscillator
210
and 16 kbps of frame overhead data generated by framing circuitry
208
. 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 represented by modem
206
.
At the subscriber premises, the modulated framed data are received at the rate of 784 kbps and demodulated by bit pump
218
which is clocked by a 784 kHz oscillator
220
. The demodulated data are then received by framing circuitry
222
which strips off the 16 kbps frame overhead data and decomposes the 6 ms HDSL frames into a 768 kbps data stream. Framing circuitry
222
is clocked by a phase-locked loop (PLL) recovered clock (PLL circuitry
224
) derived from the incoming data stream.
As mentioned above, standard HDSL transmission is only capable of servicing subscribers on loops of 12,000 feet or less at the T
1
data rate. Unfortunately, a significant number of subscribers connected to the current network infrastructure are on loops greater than 12,000 feet. In fact, nearly one-fifth of all subscribers in the U.S. are on loops longer than 18,000 feet. Other subscriber loops have impedance mismatches caused by taps which dramatically reduce the high data rate utility of the loops. Thus, many subscribers are not able to take advantage of the increasing number of on-line services being offered with standard HDSL or SDSL data transmission. Obviously, neither will such subscribers be able to receive data transmitted according to higher rate standards such as DS2 or STS-1.
While plans to significantly decrease the average subscriber loop length in most telephone networks and replace copper with fiber optics are currently being implemented, it is likely that many subscribers will not benefit from such improvements for a number of years. Because it is desirable to provide high speed data transmission to most or all telephone network subscribers as soon as possible, efforts are currently under way to devise technologies which are capable of doing so over longer loop lengths than standard HDSL and SDSL. One technique for achieving this goal is illustrated with the block diagram of FIG.
3
.
FIG. 3
is a simplified block diagram of a transmission loop
302
connecting a central office or public branch exchange (represented by modem
304
) and a subscriber premises (represented by modem
306
). In this example, the data to be transmitted enter bit pump
308
at a sub-standard SDSL data rate, i.e., 512 kbps. The bit pump modulates the data using a clock (oscillator
310
) having an appropriately low frequency for the length of subscriber loop to be serviced. In this example, the frequency of oscillator
310
is 512 kHz. The modulated data are then transmitted via twisted pair
316
at an equivalent data rate of 512 kbps. At the subscriber end, the modulated data are received by bit pump
314
which is clocked by 512 kHz oscillator
316
, i.e., the same frequency as oscillator
310
. Bit pump
314
demodulates the data which results in the original 512 kbps data stream.
As is immediately obvious, this technique completely eliminates the framing circuitry to avoid being constrained by the frame format of HDSL technology. Unfortunately, this departure from the standard frame format presents its own difficulties. For example, there are a significant number of commercially available chip sets and other hardware which are designed to operate according to the HDSL standard using the HDSL frame format. However, for the lower speed technique described with reference to
FIG. 3.
, this hardware may not be used. This forces manufacturers of such hardware and service providers to provide different technologies for different subscriber loop lengths.
Even more significantly, much of the phone network infrastructure is also designed to use the standard frame format. Thus, the solution of
FIG. 3
requires modification or replacement of much of the existing system. Even if this were feasible, it would result in a system in which data are transmitted in a variety of different formats using a variety of different hardware depending upon the location of the subscriber. To the extent such a system goes against the development of transmission standards, it is clearly undesirable.
Moreover, departing from the standard frame format results in a loss of some of the advantages associated with having a standard frame. For example, 1) the ability to place all or a portion of the payload data into loopback mode to remotely test the link including bit error rate tests; 2) the ability to query the remote node customer premises device(s) or intermediate repeaters for configuration, status and the device type, version, or model numb
Aiyagari Sanjay K.
Coffeng Gregory M.
Henniger Mick
Meggitt Warren
Sharper Craig
Beyer Weaver & Thomas LLP
Cisco Technology Inc.
Hyun Soon-Dong
Nguyen Chau
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