Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – Of data transmission
C379S022020, C379S022070, C379S024000, C379S030000, C379S032020, C324S525000, C324S527000
BACKGROUND OF THE INVENTION
This invention relates generally to communication networks and more particularly to systems for qualifying telephone lines for data transmission.
As is known in the art, public switch telephone networks, i.e., so-called plain old telephone service (POTS) lines, were originally designed for voice communications which cover a limited frequency bandwidth (i.e., about 4 KHz). Today, it is desired to use the same POTS lines for data transmission. Data signals, however, generally have different frequency characteristics than voice signals. As a result, a POTS line that works well transmitting voice signals might not work well, or at all, for data signals. Telephone companies need to know which lines are suitable, i.e., qualify, and which lines are not suitable for data transmission. Telephone companies also need to know why particular lines are unable to support data transmissions and where such faults occur so they can determine whether the transmission line can be corrected.
The telephone network was originally designed for voice communication. Voice communication covers a limited frequency bandwidth. In some cases, telephone lines were optimized for signals in this frequency range. Even where the lines were not optimized for voice signals, there was no incentive to make the lines operate at other frequencies and often they did not.
Now, it is desired to use those same lines to carry data signals. The data signals generally have different frequency characteristics than the voice signals. As a result, a line that works very well transmitting voice signals might not work well or at all for data signals. Phone companies need to know which lines will work for data signals and use those lines for data.
Line Qualification is the overall ability to make statements about the quality of a subscriber loop as it relates to its ability to deliver voice communications (i.e. POTS), or data services. Disqualification is the ability to make a statement with a high degree of confidence that a subscriber loop will not support a data service without remedial actions. Pre-qualification is the ability to make a statement with a high degree of confidence that a subscriber loop will support a data service without remedial actions.
Telephone operating companies (TELCO's) have two problems to solve in qualifying subscriber loops for delivery of data. The first problem is strategic. Telco's are reluctant to deploy emerging technologies for the delivery of data (e.g., ISDN or ADSL) because there is uncertainty in their knowledge that sufficient of the subscriber loops are of high enough quality to make deployment economically successful. This discourages early adopters because there is significant risk in being first to deliver a technology that may not work in their access network. If Telco's could be given a technology to take much of this risk out of initial deployment, they can secure market share and lead in the face of competition
The second problem is tactical and comes after a Telco has made a decision to deploy a particular technology. There is a need to qualify, either pro-actively or reactively, specific lines for service as that service is requested by subscribers or targeted by the Telco for delivery. For example, if a Telco is to market and deliver the new service, they would like to target those subscriber loops most likely to support the service out of the box and/or with a minimum of work. As another example, a Telco receiving a new service request from a subscriber desires information to either accept or reject that request for new service based on the condition of their line.
4TEL, a product sold by Teradyne, Inc., of Deerfield, Ill., USA, has been used in the past in support of line qualification for delivery of POTS. Techniques in 4TEL lend themselves to the accurate detection and location of conditions which impair both voice and FSK modems. Modern data transmission techniques (such as those used in V.34, V.90, ISDN, and ADSL) encode data in part by shifting the phase of the carrier frequency(s). As such, these technologies rely upon there being fixed end-to-end and differential transmission characteristics (e.g., phase and echo).
A telephone line is made up of a two wire pair, called Tip and Ring. Ordinarily, the Tip and Ring wires should have the same electrical properties. It is desirable for the lines to be balanced. In a balanced line, the resistance, capacitance and inductance of each wire are equal. Imbalances exist if capacitance, inductance, or resistance of one of the wires differ from the other.
A particularly difficult type of condition to identify on a telephone line using single point measurements is called a series resistive imbalance. A series resistive imbalance introduces a differential phase shift between the two wires of the loop. The cause of series resistance is likely due to non-cold welded wire wraps, IDC, or dry solder joints. The oxidation created at the junction of the failing connection causes the series resistance to be unstable, thus modifying the phase shift with time due to changes in current flowing through the junction, further oxidation of the junction, mechanical movement of the junction, and the like. Higher speed modems encode many bits into phase shifts on these carrier frequencies. Thus even minor instabilities of the series resistance cause reduced data throughput, errors, and retraining. With ISDN, the shifts in phase cause energy from one pulse to overlap into the synchronization signal or into the time occupied by another pulse, thus causing inter symbol distortion and/or loss of synchronization. As can be seen, there is quite general degradation of both analog and digital transmission methods, both being susceptible to minor instabilities in series resistance. Stable series resistance, even when values get very high can often be successfully compensated for by internal circuitry in analogue modems or at the U interface for ISDN.
It is important to detect series resistive imbalance because large imbalance values affect POTS by reducing loop current levels. It is possible that the imbalance might be so large, (2 kilo-ohms or more) that seizing a dial tone may not be possible, or the ringing current might not be sufficient to activate the bell circuitry in the telephone or modem. It is equally important to detect imbalance at values below 2 kilo-ohm when data transmission is concerned. Any series resistance and the noise that it causes in terms of phase shift have a detrimental effect on the data throughput that may be achieved on that subscriber loop.
A telephone company would like to pre-qualify a line for high data rate operation, such as ISDN and ADSL. Lines that have been pre-qualified can be leased at a higher price. Lines with imbalances would not be made available for these high data rate services.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method is provided for qualifying a transmission line to propagate data signals. The method includes measuring phase imbalance in the transmission line from a terminating end of the line.
When the wires get out of balance, a human user of the telephone line might notice a degradation in performance in the form of audible noise or reduced voice quality. When the line is used for data transmission, imbalance can limit the data throughput at which the line can operate. However, we have recognized that it is the change of imbalance that has most significant effect on data transmission.
In accordance with another feature of the invention, a method is provided for qualifying a transmission line to propagate data signals. The method includes measuring imbalance in the transmission line from a terminating end of the line.
In accordance with another feature of the invention, a method is provided for qualifying a transmission line to propagate data signals. The method includes applying a voltage in common (i.e., a common mode voltage) to the transmission line; and, determining phase imbalance in the line in response to the applied common mod
Schmidt Kurt E.
Teradyne Legal Dept.
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