TWO WIRE CIRCUITS VS FOUR WIRE CIRCUITS Flashcards
is the physical path that runs between two or more points.
circuit
It terminates on a port (that is, a point of electrical or optical interface), and that port can be in a host computer (that is, a switching device used to establish connections), on a multiplexer, or in another device.
circuit
does not define the number of simultaneous conversations that can be carried; that is a function of the type of circuit it is.
circuit
Two Types of Circuits
Two-Wire Circuits and Four-Wire Circuits
has two insulated electrical conductors. One wire is used for transmission of the
information. The other wire acts as the return path to complete the electrical circuit.
Two-Wire Circuit
are generally deployed in the analog local loop, which is the last mile between the
subscriber and the subscriber’s first point of access into the network.
Two-Wire Circuits
When both directions are carried on the same pair of wires, it is called
two-wire transmission
The telephones in our homes and offices are connected to a local switching center (exchange) by means of
Two-Wire Circuits
A more proper definition for transmitting and switching purposes is that when oppositely directed portions
of a single telephone conversation occur over the same electrical transmission channel or path, we call
this
two-wire operation
is characterized by supporting transmission in two directions simultaneously, as opposed to four-wire circuits, which have separate pairs for transmit and receive
two-wire circuit
The subscriber local loop from the telco central office are almost all two wire for analog baseband voice
calls (and some digital services like ISDN), and converted to four-wire at the line card back when
telephone switching was performed on baseband audio.
Today the audio is digitized and processed completely in the digital domain upstream from the local loop.
The reason for using two wires rather than four is simple economics–half the materials cost half as much
to purchase and install.
Note the use of the past-tense “cost,” as installation of two-wire copper local loops for telephony was done
primarily during the mid 20th century. In the first world there is no new infrastructure planning for new
copper-based technology, and as customers are migrating to cellular telephony and high-speed Internet,
wireline carriers are abandoning their copper local loops, tearing out the copper and replacing it with fiberoptic cable and/or selling the rights-of-way to third parties for private use.
In developing nations, wireless communications are considered to be the most cost-effective from an
infrastructure perspective. Two-wire circuits in new installations are limited to intercom and military field
telephone applications, though these too are being supplanted by modern digital communication modes.
To communicate in both directions in the same wire pair, conversion between four-wire and two-wire was
necessary, both at the telephone and at the central office.
A ________ accomplishes the conversion for both. At the central office, it is part of a four-wire terminating
set, more often as part of a line card.
hybrid coil
has no two-to-four wire conversion whatsoever; it is strictly an analog/digital interface to a system that has a completely digital and integrated signal path internally
modern line card
Using actual wires to circuit switch a telephone call became obsolete when the crossbar switch (a mechanical system) was replaced by ________ in the 1970s by the Bell System in the US
4ESS electronic switches
The __________ have been replaced by inexpensive IC chip-based components that perform the same functions at greatly reduced cost
old telephone hybrids of yore
refer to the number of wires in the internal cabling plan
Two-pair and four-pair
have to do with the number of electrical conductors associated with a transmission
line
Two-wire and four-wire
has two pairs of conductors.
That is, it has two sets of one-way transmission paths: one path for each direction and a complementary
path to complete the electrical circuit.
Four-Wire Circuits
are used where there is distance between the termination points which requires that the signal be strengthened periodically
Four-Wire Circuits
PSTN
public switched telephone
network
are also used with leased lines, where a customer may be connecting locations of its own that are separated by distance. Also, all digital circuits are provisioned on a four-wire basis.
Four-Wire Circuits
is a two-way circuit using two paths so arranged that the respective signals are transmitted
in one direction only by one path and in the other direction by the other path
Four-Wire Circuit
gets its name from the fact that is uses four conductors to create two complete electrical
circuits, one for each direction. The two separate circuits (channels) allow full-duplex operation with low
crosstalk
Four-Wire Circuits
was historically used to transport and switch baseband audio signals in the phone company telephone exchange before the advent of digital modulation and the electronic switching system eliminated
baseband audio from the telco plant except for the local loop
four-wire circuit
The local loop is a two-wire circuit for one reason only: to save copper. Using half the number of copper wire
conductors per circuit means that the infrastructure cost for wiring each circuit is halved.
Although a lower quality circuit, the local loop allows full duplex operation by using a telephone hybrid to keep
near and far voice levels equivalent.
As the public switched telephone network expanded in size and scope, using many individual wires inside the
telco plant became so impractical and labor-intensive that in-office and inter-office signal wiring progressed to
high bandwidth coaxial cable, microwave radio relay and ultimately fiber-optic
communication for high speed trunk circuits
two types of four-wire circuit
a. physical four-wire
b. logical four-wire
In physical four-wire you can actually count four wires.
In logical four-wire, physically there are only two wires, but you derive the four individual paths by splitting the
frequency. Half of the frequency band carries the transmit signal, and the other half carries the receive signal.
So you can’t always tell just by looking what kind of circuit you’re dealing with; the application dictates the type
of circuit it is. (with operator)
Whenever you release energy into space, it loses power as it’s traveling over a distance. So, because networks were designed to carry communications over a distance, we need tools to augment signals that have
been losing power as they have traveled across the network, which are called
attenuated signals
boosts an attenuated signal back up to its original power level so it can continue to make its way across the network
amplifier
The distance requirement between amplifiers is relatively short on copper wires—generally about
6,000 feet (1,800 meters)
were unidirectional. They could only amplify a signal moving in one direction, so
any time you needed to provision a circuit that was going to be crossing a distance, you had to literally provision two circuits—one to amplify the information in the transmit direction and a second to amplify the information in the receive direction
First-generation amplifiers
Therefore, whenever a network was crossing a distance, it needed to use a four-wire circuit. But in building out
the millions of local loops for subscribers, it was seen as being cost-effective to have to pull only two wires into
every home rather than four.
Therefore, the local loops were intentionally engineered to be very short; some 70% to 80% of the local loops
worldwide are less than 2 miles (3.2 kilometers) long. Because the local loops are short, they don’t need
amplifiers, and therefore the subscriber access service can be provisioned over a two-wire circuit. However,
the local loop is increasingly being digitalized, so as we migrate to an end-to-end digital environment,
everything becomes four-wire.
