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METHOD FOR TESTING A
COMMUNICATION CHANNEL
TECHNICAL FIELD
The present invention relates generally to a method for testing a communication channel and, more particularly, concerns a communication system that uses modern line probing data in an evaluation method to determine the circuit characteristics of a customer subscriber loop.
BACKGROUND OF THE INVENTION
Numerous personal computers are connected to other computer systems or to computer networks such as the Internet through the public switched telephone network 15 (PSTN). A common way of connecting a computer to the PSTN for communication with other computers or computer networks is through the use of a modem. Most modems are capable of operating at different transmission rates at different times, and have a typical maximum communication 20 rate of 56 kbps.
Generally, both the transmitting modem and the receiving modem must evaluate the communication channel first in order to optimize the performance. One way of achieving this is, during "handshaking," to have the transmitting 25 modem transmit, as part of its set-up sequence to the receiving modem, a composite signal which will be recognized by the receiving modem. This allows the receiving modem to evaluate the communication channel and condition itself to have an optimized data rate for this connection. 30 Modems operating under the V.34 and V.90 standards perform such line probing to maximize the data communication rate.
The maximum communication rate that a modem can achieve is related to the hardware or circuit characteristics 35 associated with a particular customer subscriber loop. The subscriber loop is the twisted copper pairs of wires running to a home or a business from the telephone Central Office switching system. The physical characteristics of a particular subscriber loop may effectively limit the transmission 40 rate for a particular customer. There are numerous other problems, however, unrelated to the physical characteristics of the customer subscriber loop, which can degrade communication performance resulting in lower modem transmission rates. Therefore, for customers experiencing 45 degraded modem performance, there is a need for determining whether the characteristics of the customer subscriber loop is responsible for the degraded modem performance.
Presently, a telephone company technician must be dis- 5Q patched to the particular customer location with special testing equipment to determine the characteristics of the customer subscriber loop. Troubleshooting customer modem transmission problems by dispatching technicians is expensive and time consuming. Accordingly, what is needed 5J is an inexpensive and remotely located system and method for testing subscriber loop characteristics for a particular customer.
BRIEF DESCRIPTION OF THE DRAWINGS
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For a more complete understanding of this invention, reference should now be had to the embodiments illustrated in greater detail in the accompanying drawings described below by way of examples of the invention. In the drawings:
FIG. 1 is a schematic block diagram of one embodiment 65 of the present invention in the environment of the public switched telephone network.
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FIG. 2 is a logic flow diagram of one embodiment of the method for testing a communication channel.
FIG. 3 is a logic flow diagram showing one embodiment of implementing the method according to FIG. 2.
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENT(S)
FIG. 1 shows a schematic block diagram illustrating one example of a modem connection between a customer location and a network service provider in accordance with an aspect of the present invention. In this example, the customer location includes a computer system 10 which represents any type of computer system capable of receiving and transferring data. This includes computer systems such as a personal computer or a network server. The computer system 10 is connected to a data modem 12 which may be built into the computer system 10 or exist independently from the computer system 10. Modem 12 is a conventional data modem operating at frequencies below 4000 Hz such as a V.34 or V.90 type modem. The modem 12 is connected to the Central Office switching system 14 by a subscriber loop 16. Each subscriber loop 16 for a particular customer comprises pairs of twisted wires running from the customer location, such as a home or business, to the telephone Central Office switching system 14 by way of the Public Switched Telephone Network (PSTN) 18. The PSTN 18 routes and carries telephone signals between a plurality of different Central Office switching systems.
In this example, the customer modem 12 is connected to modem 22 of the network service provider 20 for subscriber line evaluation. The network service provider 20 represents, for example, the customer's local phone company. Alternatively, the network service provider 20 could be an ISP providing access to the Internet. The modem 22 associated with the network service provider 20 is also, preferably, a conventional modem operating at frequencies below 4000 Hz, such as a V.34 or V.90 type modem. Alternatively, the modem 22 can be a modem having an all-digital interface.
Because the subscriber loop represents the physical connection between a customer premises and another computer system or network, it will likely be different for each customer. Each subscriber loop may have any of several characteristics or circuit elements that can effectively limit the modem communication rates available for that customer. In particular, the physical distance the communication signals must travel along the twisted pair of wires directly affects signal strength. Due to capacitive reactance in the line, signal strength becomes attenuated as the distance increases. To counteract these losses, the lines must be "loaded" with an inductive reactance. The loaded line, however, has the characteristics of a low pass filter with significant signal attenuation above 3000 Hz.
In addition to the loaded or unloaded condition of a particular subscriber loop, the subscriber loop may include one or more analog-to-digital (A/D) conversions. Portions of the subscriber loop may be connected through a digital network such as a digital loop carrier network. Data transmissions across such a digital network require A/D conversions to convert the analog modem signals to digital data for transmission across the digital network and vice-versa. Most of the switching equipment associated with the PSTN digitizes analog signals on subscriber loops by sampling the analog signals at a frequency of 8 kHz. Because of the Nyquist limit, this sampling rate imposes a maximum frequency of 4 kHz that can be sampled and passed from a
