Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
2000-07-14
2002-07-30
Isen, Forester W. (Department: 2644)
Telephonic communications
Subscriber line or transmission line interface
C379S399020, C379S026010, C379S026020, C379S412000, C379S027010, C379S093050
Reexamination Certificate
active
06427011
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communication systems, methods and devices used to communicate data between computers and in particular embodiments to telephone line coupled modem systems, methods and devices.
BACKGROUND OF THE INVENTION
The need to communicate between distant computers has led to the use of telephone lines for data communication. The telephone lines are a natural choice for communications because of their ubiquitous nature and ability for dedicated instantaneous transmission between points. Modems are often used to communicate data between computers across a telephone line. A modem is a device that accepts digital data (for example, from a computer) and uses the data to modulate an analog signal for transmission across a telephone line. At the receiving end of the transmission another modem converts the analog information sent by the first computer and modem to digital data by demodulating the analog signal. The process of MODulating a signal on the sending end and DEModulating the signal on the receiving end is how the term “MODEM” was derived.
FIG. 1A
illustrates a block diagram of a telephone modem coupled to a telephone line. In
FIG. 1A
the modem system
101
functionally comprises two blocks. The first block
103
is the actual telephone modem, which includes a telephone line interface circuit or Data Access Arrangement (DAA)
105
. The second block
111
is the plain old telephone system (POTS)
111
which both accepts information from and provides information to the DAA
105
portion of the telephone modem
103
. The telephone modem
103
commonly couples to the telephone system
111
via two telephone line terminals commonly denominated Tip or “T”
107
and Ring or “R”
109
. The data access arrangement
105
provides the interface between the telephone modem
103
and the analog telephone system
111
. The DAA
105
is typically an isolated Analog Front End (AFE), which the telephone modem uses to interface to the analog telephone system
111
.
From the early days of telephone modems and telephone line equipment in general isolation is required between the telephone modem system
103
and the telephone system
111
. The purpose of this requirement is to decouple any difference voltage potential between the telephone modem
103
and the telephone system
111
. Furthermore, the isolation protects the user of the telephone modem from such things as lightning strikes within the telephone system
111
, which could be destructive to the system and fatal to the user without adequate isolation. A transformer, such as
125
illustrated in
FIG. 1B
, were commonly used to address this isolation requirement. Typically, at least one driver, such as illustrated in
FIG. 1B
as
113
, drives the transformer. Additionally, each driver circuit typically includes a resistor such as
121
, which are used to set the impedance of the DAA seen by the phone line. The same transformer
125
may also be used for reception of signals. Signals are commonly coupled from the transformer
125
into a circuit known as a hybrid
119
and then further coupled into a receive amplifier
117
. Generally, the function of the hybrid circuit
119
is to couple signals received from the telephone system to the receive amplifier
117
often after cancelling as much as possible of any transmit signal injected to the telephone line transmit buffer
113
.
In the early days of modem development, the transformer
125
was used to carry DC “loop” current
129
from the telephone line as well as AC communication signals to and from the telephone line. Transformers with windings that carry DC current as well as AC signals are sometimes called “wet” transformers. The DC loop current
129
conducted by a wet transformer functions to inform the telephone system
111
that the modem is ready to communicate AC signals to and from a central office (CO) communications are impending. The process of causing a DC current in the telephone line is commonly referred to as going off-hook or seizing the telephone line. The magnitude of the DC current
129
used to inform the telephone system
111
that lines
109
and
107
are being seized is generally between 20 to 100 milliamps, depending on the distance of the modem system to the (CO).
Generally wet transformers used in modems had a limiting resistor
127
, to set the DC resistance of the modem seen from the telephone line within specified limits. A typical current limiting resistor (e.g.
127
) has a value of, for example, about 150 ohms and is typically placed in series with a primary winding of a transformer
125
, which also commonly has a resistance of about 150 ohms. The addition of the transformer winding resistance and resistor resistance results in an additive DC resistance of approximately 300 ohms. The telephone line system
111
is, thus, presented with this 300 ohms resistance when a user goes off-hook.
An example arrangement for a wet transformer to provide off-hook current is shown in FIG.
2
. The arrangement includes a relay
201
, in series with the current limiting resistor
127
. The closing of relay contact
201
couples the serial combination of the primary transformer
125
and resistor
127
to the telephone system. An advantage of a wet transformer system is that its primary winding is not polarized. Therefore, while
FIG. 2
shows one example in which the resistor
127
side of the transformer
125
is coupled to the tip-line, coupling the resistor
127
to the ring side of the transformer would work equally well. Many early low-speed modems were configured with wet transformers.
However, wet transformers tend to exhibit nonlinear operation when DC current flows through the primary winding which can be problematic for higher speed modems. Because a transformer is a mechanical device, it is subject to such variations as magnetization, temperature variations and varying permeability. In addition, as the amount of current passing through its primary winding changes, so does the permeability of the transformer's core. Modem systems generally function by detecting phase differences in incoming signals. As the speed of modem transmission increases above 2400 baud, modem systems became less tolerant of the distortion introduced by wet transformers, and wet transformers became less practical and more expensive to build than “dry” transformers for a specified linearity characteristic.
FIG. 3A
illustrates a dry transformer arrangement.
In
FIG. 3
, a direct current (DC) blocking capacitor
301
prevents DC from passing through the telephone line side primary winding of transformer
305
. Because no DC passes through the primary of transformer
305
, the linearity of the transformer
305
can be substantially improved over the wet transformer system, for the same physical size and cost. The dry transformer system does not inherently provide a path for the off-hook DC current however, and, so, another method is needed to provide the DC current for signaling the telephone system of an off-hook condition.
To draw off-hook current, a system referred to as an electronic inductor (EI)
303
was included with the dry transformer arrangement. An electronic inductor
303
has the ability to conduct off-hook DC current but appear as high AC impedance. It is beneficial for the electronic inductor
303
to appear as a high AC impedance so that the electronic inductor
303
does not contribute AC loading to either the transformer
305
, or to the telephone system
311
. The AC impedance value of the electronic inductor
303
is important because, in modern high-speed modems, an AC bandwidth from 10 hertz to 3.4 kilohertz is commonly desired and any impedance in parallel with the telephone line may affect the bandwidth of the signal and ultimately the performance of the system. Therefore, the electronic inductor must exhibit high impedance at frequencies from 10 hertz to 3.4 kilohertz and such as a coil, a passive inductor is not practical to use because its impedance is 2&pgr; times the frequency of a signal, times the in
Conexant Systems Inc.
Foley & Lardner
Isen Forester W.
Singh Ramnandan
LandOfFree
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