Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2002-12-06
2004-06-01
Tran, Khai (Department: 2631)
Pulse or digital communications
Transceivers
Modems
Reexamination Certificate
active
06744812
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
FIELD OF THE INVENTION
The present invention relates generally to computer systems and particularly to networking computer systems. More particularly, the present invention relates to implementing a computer network using conventional telephone wiring.
BACKGROUND OF THE INVENTION
Computer networks permit multiple computer systems to communicate and share data over one or more transmission lines. Although a network may connect computers located anywhere in the world, a smaller local area network (LAN) is commonly used to connect computers located in fairly close proximity, such as within the same room or building. A LAN typically comprises one or more physical wires or cables that connect the computers, although wireless networks are available. In a multidrop LAN, all of the networked computers connect to the same physical cable (or wireless link), either directly or through an intermediate device such as a terminator or adapter. Local area networks have enjoyed widespread popularity in the workplace, where they permit users to share documents and send and receive electronic messages. Local area networks also permit each personal computer to connect with one or more “fileserver” computers, which generally provide a central storage location for documents and program files and which often form gateways to other networks, such as the Internet.
Until recently, computer networks in the home have not been widely implemented due to the high cost of purchasing the computers and because of the expense, trouble, and difficulty faced by the average consumer of installing network cables and running the equipment. As computers have become cheaper and more powerful, however, many consumers have purchased multiple computers for the home; and recent developments in computer networking have made it easier to form multidrop home networks by connecting computers through existing home telephone lines. The networked computers then send data to each other over the phone wires. Similar networks rely on standard telephone wiring in businesses, schools, apartments, dormitories, hotels, and other structures which have the wiring in place.
Even though these network devices communicate over telephone lines, they are not required to wait for a dial tone before establishing communication or to follow other standard protocols required for outgoing calls. As a result, the home networking devices can communicate over a phone line that is in use by another service, such as “plain old phone service” (POTS) or Digital Subscriber Line (DSL) service. In order to coexist with other services, the phone line network signals are transmitted at high frequencies which a telephone, POTS modem, or DSL modem does not use. The high frequencies used for phone line networking typically range from approximately 2 million hertz (MHz) to 10 MHz, in contrast with POTS, which occupies the frequency spectrum from 0 hertz to about 4 thousand hertz (kHz). The POTS frequency band is within the audio frequency band, which includes frequencies that the human ear can detect. Digital Subscriber Line (DSL) modems generally communicate in the range of around 100 kHz to approximately 1.5 MHz.
Computers on a phone line network system generally send digital data by (1) transmitting a high frequency “carrier” signal and (2) altering, or modulating, the carrier signal based on the data. The number of possible alterations typically is fixed (hence the “digital” nature of the data), and each type of alteration represents a particular data value. The computer that is receiving the data monitors the carrier signal and determines the data that was transmitted by detecting, or “demodulating,” the alterations. Current phone line networking equipment uses pulse position modulation, in which the carrier is a short pulse that is transmitted periodically at fixed time intervals. To transmit a particular data value, the transmitting computer alters the timing between two successive pulses by delaying the second pulse relative to the first pulse. Accordingly, the pulses represent the carrier signal, and the delays represent the modulation. By determining the time delay between pulses, the receiving computer can detect which data value was transmitted.
Unfortunately, pulse position modulation tends to be particularly susceptible to electronic interference (called “noise”) picked up by the phone lines. The delay values can be adaptively lengthened to combat the noise, but doing so generally lowers the data throughput, or the speed at which data is transmitted. Although the voltage threshold level can be raised above the noise level, doing so requires the transmitting computer to transmit at higher voltage levels, requiring more expensive signal filtering circuitry and increasing power consumption.
In addition, the analog circuitry currently used to measure the time delays in pulse position modulation signals is not highly sensitive to small time increments. As a result, with conventional circuitry the different delay values available to a single pulse must be spaced somewhat far apart in time, limiting the achievable data rate to no higher than about one million bits (or “megabits”) per second, a relatively slow throughput compared to that of conventional LAN's. The analog circuitry also is susceptible to normal manufacturing tolerances in the crystals which control timing in the transmitter and receiver. The crystal in the transmitter controls the intervals between carrier pulses, while the crystal in the receiver controls the circuitry that measures the intervals to decode the data. If the transmitter crystal frequency deviates significantly from the receiver crystal frequency, however, then the receiver may not decode the data properly.
Another problem with pulse position modulation is that, unlike other types of digital modulation techniques, standardized software code is not widely available for implementing pulse position modulation algorithms. As a result, “time-to-market” can be slow for products that incorporate pulse position modulation. For these and other reasons, pulse position modulation is rarely used as a standard modulation technique in industry and thus suffers from lack of compatibility with many existing systems, dramatically increasing the time and effort required to design and implement a network.
An added difficulty with pulse position modulation is the “bursty” nature of the modulated carrier, which means that the maximum voltage of the signal is significantly higher than the average signal voltage. The signal burstiness is related to the fact that the signal reaches its highest level whenever a pulse is transmitted but remains at a zero level during the delays between pulses. Bursty signals place high performance demands on filtering circuitry, requiring greater expense and complexity in the receiver. Unless costly precautions are taken, bursty signals can interfere with normal telephone conversations.
Existing phone line networking systems also suffer from relying on analog comparator circuitry to detect when the transmitted signals exceed the voltage threshold. Although analog circuitry can be cheaper and simpler to implement that digital circuitry in some cases, analog comparators are sensitive to noise, a condition that ultimately can limit the data throughput. The analog circuitry also tends to be relatively difficult and expensive to upgrade as technology progresses. For example, existing analog hardware typically cannot be modified to support standard modulation techniques or higher data rates. The analog circuitry also prevents full duplex communication, which allows computers to transmit and receive at the same time over the network. Instead, current systems operate in half duplex mode, which means that a single computer cannot transmit and receive data at the same time.
Further, the analog nature of existing phone line network equipment makes compensating for unpredictable conditions in phone line quality difficult. Certain telephone cables,
Anne Ramakrishna
Watts Robert F.
Hewlett--Packard Development Company, L.P.
Tran Khai
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