Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – Of data transmission
Reexamination Certificate
2002-06-20
2004-11-30
Tieu, Binh (Department: 2643)
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
Of data transmission
C379S027010, C379S029010
Reexamination Certificate
active
06826258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to data transmission networks and more particularly to determining the data rates of a telephone network used for high speed data services.
2. Description of Related Art
With the recent growth in the use of the internet and expansion of small and home offices, there has been a great demand for high speed data services at numerous premises. Many phone companies have responded to this demand by offering DSL services over their existing telephone lines. DSL stands for “Digital Subscriber Loop.”
DSL services allow information to be transmitted over the phone line in digital form. With the digital encoding used in a DSL system, the number of bits that can be transmitted through a phone line is much greater than with a traditional analog modem. An analog modem provides a carrier signal that is modulated with the bits that represent the digital information. Modulation can be in amplitude, frequency or phase.
Because the modulating information is digital, the carrier can be modulated into one of several states. To transmit one bit at a time, the carrier only needs to be modulated into one of two states, with one state signaling that the bit has a 0 value and the other state signaling that the bit has a 1 value.
If it is desired to transmit more than one bit at a time, the carrier can be modulated into more than two states. For example, if the carrier is modulated into one of four states, two bits can be transmitted at one time. For three bits, eight modulation states are required. Whatever modulation scheme is used, a modem at the receiving end of the line detects the modulation state of the carrier and outputs a group of bits that have a value assigned based on the modulation state.
Generally increasing the number of modulation states of the carrier increases the rate at which bits can be communicated through the channel. However, the number of bits that can be simultaneously transmitted can not be set arbitrarily large. As the number of modulation states increases, the difference between each modulation state gets smaller. The less difference there is between the modulation states, the more likely it is that noise in the communication channel will disrupt communication. Noise can make a signal in one modulation state look as it is was modulated into a different state and the receiving modem will associate the signal in that channel with the wrong modulation state. Thus, the number of bits that can be simultaneously communicated on one carrier is limited because of the noise in the communication channel.
The number of bits that can be transmitted over a communication channel is also dependent on the attenuation in that channel. If the signal is attenuated, there is less energy available for the receiving modem to distinguish between the signal and the noise. Thus, the bit rate of a channel is also limited by the amount of attenuation in the channel.
To determine what bit rate can be supported, a conventional analog modem often goes through a “training sequence.” A modem attempting to initiate a connection to another modem will often try to transmit with several different modulation schemes. The modulation scheme that provides the highest bandwidth is selected for communication between those two modems.
A DSL system operates on a principle that is similar to a traditional analog modem. However, a DSL system may use multiple carrier frequencies. As part of the training sequence, the modem determines the number of modulation states that each carrier signal can carry. Those carrier frequencies that have a high attenuation or that have a lot of noise will be assigned a small number of modulation states. Some frequencies might not be used for communication at all, if they are too noisy or have too much attenuation.
A second difference between DSL and a traditional analog modem is that many of the carrier frequencies used by the DSL modem are at higher frequencies than used in a traditional analog modem. Having higher frequency signals allows more carrier signals in one phone line and therefore more bits to be simultaneously transmitted. However, traditional telephone lines were designed for voice signals that have a frequency range below approximately 20 kHz.
While most telephone lines can carry signals at frequencies higher than 20 kHz, the attenuation of a line generally increases with frequency, which limits the bit rate that can be transmitted through the line. Not all telephone lines have the same attenuation characteristics. Some lines will be able to carry enough high frequency signals that they can support DSL service at bit rates of many hundreds of kilobits per second. Others will be able to support DSL service, but at lower bit rates. Still others will not be able to carry enough high frequency signals that they could meet the lowest level of what would be considered acceptable as a high speed data service.
To offer DSL services, it is important for the phone company to know before it connects a particular subscribed for DSL service, whether the lines running to that subscriber's premises will support high speed data services. And, if the telephone company offers different service levels for DSL service based on the data rate selected by the customer, it is important for the phone company to know whether the lines support the minimum bit rate associated with the selected service level.
Therefore, there is a need for the telephone company to “prequalify” a subscriber line for high speed data services. “Prequalification” means that the telephone company determines, with a reasonable degree of certainty, that the line will support DSL service at the selected data rate and that this qualification is made before the service in installed.
One simple way that prequalification has been done is through the use of a technician in the field. A technician might actually go to the subscriber premises and attach measurement equipment to the far end of the subscriber line. Through a series of interactions with test equipment at the near end of the line, the technician could determine the data rate that could be supported.
Traditionally, the attenuation of the line at 300 kHz is used as a metric to pre-qualify subscriber lines. Though DSL service operates with multiple carrier frequencies that might range from 10 kHz to over 1 MHz, a large portion of the data transmitted at any time is generally done so on carriers with frequencies near 300 kHz. Further, the performance of the line at higher frequency is usually correlated to its performance at 300 kHz, so knowing performance at that frequency often allows an acceptable prediction of performance to be made across the full frequency spectrum.
However, sending a technician to the far end of the line to measure attenuation can be costly. Most telephone companies prefer a pre-qualification approach that does not require two-ended measurements.
An alternative approach to doing pre-qualification is to make an estimate of the length of the subscriber's phone line because attenuation of a line is heavily influenced by the length of the line. Phone companies have developed a relationship between what is called Equivalent Working Length (EWL) and the data rate for DSL services the phone line will provide. The EWL is defined in ANSI standard T1.417-2001 Spectrum Management for Loop transmission Systems.
Techniques for making estimates of EWL without two-ended measurements have been used. One approach is to estimate the length of the line from installation information. Some simple techniques use geographic location of the subscriber in relation to the switch to which their subscriber line is connected. A simple way to estimate the subscriber's location is through the use of postal codes. The post office assigns codes based on geography, making it possible to estimate distance between the subscriber and the switch.
Use of postal codes is not, however, very accurate. The postal code only gives an approximation of distance. If the line travels a circuitous r
Teradyne Legal Department
Teradyne, Inc.
Tieu Binh
LandOfFree
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