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Unit 4: Physical Layer-2

reach at destination. The telephone set at destination converts the electrical signal on the Notes
line back into the voice of the calling party. The phone set has passed through various
developmental stages from simple magneto systems to electromechanical system to digital
system.
(b) Local Loop: Last mile connections or a local loop (a drop wire from home phone to a
connection box of local telegraph pole or cable at the street) is the physical wiring to
connect the telephone subscriber to the PSTN. This line may carry voice or data signal or
both. The physical wiring for the local loop consists of a pair of twisted copper wires from
the telecom service provider’s central office (CO) to the subscriber’s premise and another
pair of twisted copper wires from subscriber’s premise to the CO, thus making a loop. The
subscriber’s telephone connection goes to a connection box outside home that also collects
drop wires from other houses in the same area. This connection box is referred to Network
Interface Device (NID).

(c) NID: The NID provides a duplex connection between the home wiring to the local loop
wiring to the CO over which an analog electrical signal is passed. The voice signal produced
by the microphone in the subscriber’s telephone set is converted into a series of electrical
pulses to form an analog signal. The house wiring enables this analog signal to reach NID
so that it may be passed to the RCU (Remote Concentrator Unit) of CO through local loop
wiring.
(d) CO: At CO, thousand pairs of copper cables merge forming bundle sets of 26 pairs of wires
which are split into individual pairs and then punched down into punch blocks mounted
on subscriber or loop side in a distribution frame. The other side of the distribution frame
is wired to digital cross connect switches to connect the phone calls to the other part of
world. Time division multiplexing devices are provided within cross connect switches to
multiplex multiple channels into a single higher speed circuit. Some examples are that 24
DS0 circuits are converted to a T1 circuit.

The telephone exchange is considered as a set of one or more cross-connect switches in one or
more central offices to respond to a single three digit code. The first three digit code specifies the
exchange of the subscribers to which they belong. A central office may serve more than one
exchange. A foreign exchange is any exchange outside the subscriber’s calling area exchange or
local exchange and connected to a local exchange through large, high-speed trunk lines preferably
T3 or better. The foreign exchange is referred to extended local calling area.
A foreign exchange is considered as any exchange in a circuit-switched telephone system outside
the subscriber’s local exchange’s calling area. Another local exchange, national exchange and
international exchange are examples of a foreign exchange. When a subscriber calls a telephone
number outside the local exchange, the subscriber’s call is completed by opening a connection to
another exchange over a trunk line. The external exchange facilitating the completion of the call
is referred to as a foreign exchange.
The national exchange provides connections from the regional telephone providers to the long-
distance telephone providers. This exchange defines the area code. The international exchange is
the point at which the long distance providers connect to other long distance providers overseas.
The international exchange provides country codes. An example of dialing number for
Johannesburg, South Africa from U.S.A. will look like as follows:

Country Code Area Code Exchange Code Number

27 11 xx xxxxxxx

The International dialing codes are comprised of country codes (World Zones), area codes,
exchange codes and local numbers.

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