Advantages of automatic switching system over the manual exchanges:
In an automatic exchange, the subscriber does not need to communicate
with the operator. The operation of automatic exchange is language
independent.
A greater degree of privacy is obtained in automatic exchange as no
operator is generally involved in monitoring and setting up a call.
Establishment and Release of calls are faster in automatic exchanges. In
manual exchange, operator consumes a few minutes to observe the end of
conversation and release circuits.
In an automatic exchange, the time needed to establish a call keeps more
or less of similar order irrespective of the load on the system or the time
of the day. In manual systems such may not be true.
Strowger switching
In the Strowger switching system, there are two types of selectors which form the
building blocks for the switching systems.
1. Uni-selector
2. Two-motion selector
These devices are called selectors as they enable selection of suitable switching
paths for establishing connections amongst subscribers.
Uni-selectors have a mechanism to extent across a bank of contacts that
lie in a horizontal plane and to select a free contact that can be used to
establish a connection.
Two-motion selectors have the ability to move both in the vertical and
horizontal directions to select a free contact.
Both of these selectors are constructed using electro-mechanical rotary
switches which in turn are built using an electromagnet, an armature and
a rachet wheel.
Uni-selector Drive mechanism of a rotary switch
When the input voltage energizes the Electromagnet, the armature is
pulled down towards the magnet. Now as the armature gets attracted
towards the electromagnet, the pawl falls one position down the previous
one in the ratchet wheel. The detent prevents the movement of the ratchet
wheel.
Once the electromagnet gets de-energized, the armature is released and
this action moves the pawl upwards, which further moves the ratchet
wheel to one position above. Hence, the wiper moves one position below
or in clockwise direction, to make a contact.
If the electromagnet is energized and de-energized five times, by
applying five pulses, the wiper moves by five contacts. The type of
switching mechanism is known as the reverse drive type because, here
the ratchet wheel moves when the armature return to its rest position.
If it is arranged such that the wheel moves during the forward motion of
the armature it is known as the forward drive type.
The Reverse drive type mechanism is used in uniselectors and the
forward drive type mechanism in the two-motion selectors.
Uniselector Schematic representation
A uniselector is a one which has a single rotary switch with a bank of
contacts.
Depending upon the number of switching contacts, uniselectors are
identified as 10 outlet or 24 outlet uniselectors. The former has 11 contact
positions for the wiper and the latter 25.
The first contact position is known as the home contact, and the remaining
as switching contacts.
The wiper in home contact position means that the uniselector is free and is
available for making a switched connection.
When in use, the wiper is positioned at one of the switching contacts.
The 11 or 25 contacts are said to constitute a bank of contacts. Typically
there are four banks of contacts on four horizontal planes, in a uniselector.
One of the banks, called homing bank, has a special arrangement of contacts.
o It has only two contacts: one at the first position corresponding to the
home contacts of the other banks, and the other extending as an arc
from the second position to the last position. This arc contact is often
referred to as homing arc.
Of the remaining three banks, two are used for carrying voice signals in the
two directions between the calling and the called subscribers.
The third bank, sometimes called the guard bank, is used to indicate the
busy/free condition of the uniselector. In other words, this bank guards a
busy uniselector from being accessed for new connections.
Two-Motion Switches:
A two-motion selector is capable of horizontal as well as vertical
stepping movement.
It has two rotary switches, one providing vertical motion for the wiper
assembly, and the other giving horizontal movement for the wipers.
The horizontal movement rotary switch of a two-motion selector has an
interrupter contact as in the case of uniselector.
Normally, there are 11 vertical positions and 11 horizontal contacts in
each vertical position.
The lowest vertical position and the first horizontal contact in each
vertical level are home positions, and the remaining ones are the actual
switching positions.
Thus, the wiper in a two-motion selector has access to 100 switching
contacts.
Access to any particular contact is obtained by moving the wiper
assembly vertically to the required level and then rotating the wipers to
the desired contact at that level.
At each level, there are three or four banks of contacts. Depending upon
the number of banks, a two-motion selector is known as a 330-point or
440-point selector.
For homing the wiper assembly, it is driven beyond the 11th contact
position by using the interrupter contact mechanism. The wiper assembly
then falls vertically to the home level and returns to the horizontal home
position under the influence of a restoring spring.
In some designs, a third magnet, known as release magnet is used for
homing.
The numbering of a standard 100-contact bank is shown in Table 2.1.
It may be noted that the lowest vertical level commences with 11 and
ends with 10, while the tenth level commences with 01 and ends with 00.
This is so because digit zero produces 10 pulses when dialled.
Step-by-step switching system
A step-by-step switching system may be constructed using uniselectors
or two-motion selectors or a combination of both.
The wiper contacts of these selectors move in direct response to dial
pulses.
The wiper steps forward by one contact at a time and moves by as many
contacts (takes as many steps) as the number of dial pulses received or as
required to satisfy certain signalling conditions.
A switched circuit is established in a step-by-step manner in response to
the dialled digits. Hence the name is step-by-step switching.
Most of the necessary control circuits are built in as an integral part of the
selectors, thus enabling them to directly receive and respond to user
signalling.
The relevant signalling tones are sent out to the subscriber by the
switching elements (selectors) at the appropriate stages of switching.
Thus, a step-by-step switching system is a direct control system.
Step-by-step switching system
A typical step-by-step switching system has three major parts or stages.
Pre-selector stage
The line equipment part --- also known as --- preselector stage consists of
selector hunters or line finders --- interfaces with the incoming
subscriber lines on the one side and the switching resources of the group
selector stage on the other.
The main purpose of the preselector stage is to --- establish a connection
between the subscribers and the switching resources in the group selector
stage.
The Selector Hunters hunt for selecting a switching matrix part.
Usually, 24-outlet Uniselectors are used as selector hunters and so this can be
called as Subscriber Uniselector scheme as there is a dedicated Uni-selector for
each subscriber in the system.
These can also be build using two-motion selectors.
One preselector is used per connection.
The group selector stage
The group selector stage which constitutes --- the switching matrix part of
the switching system consists of one or more sets of --- two-motion
selectors. These sets may be viewed as --- substages of the group selector
stage.
A selector in substage 1 of the group selector stage is called first group
selector, one in substage 2 as second group selector, and so on.
The number of substages depends upon the size of the exchange.
When a subscriber lifts his hand set --- a preselector associated with his incoming
line --- connects his line to a free first group selector.
Once this connection is established, --- the preselector simply provides an
electrical path --- between the subscriber and the first group selector --- until it is
released. At this time, the preselector and the first group selector are marked ---
‘busy’. Dial tone is then sent to --- the subscriber by --- the first group selector via
--- the preselector.
The first group selector is now ready to --- receive the -- dialing pulses from -- the
subscriber.
When the subscriber starts dialing, --- the first selector cuts off --- the dial tone and
receives --- the pulse train corresponding to the first digit --- dialed by the
subscriber. The wiper assembly --- of the selector --- steps vertically as many steps
as --- the number of pulses --- in the first pulse train. This action is called
numerical selection as the selector is responding to the digit dialed.
After the end of the first pulse train and before the start of the second pulse train,
--- the wipers move in the --- horizontal plane in --- the auto stepping mode until a
contact to which a free second group selector is connected is found. We term this
action as --- resource selection, as the selector picks a free switching resource from
the next stage using --- the interrupter mechanism.
It may be recalled that the inter digit gap is around 240 ms and the resource
selection is completed within this period. Thereafter, the first group selector just
provides an electrical path to the second group selector.
The final selector
The final selector performs --- two numerical selections by processing --- the
last two digits of the dialed number. It first --- steps vertically according to
the 4th digit dialed and --- steps horizontally according to the last digit. The
last digit dialed, --- establishes electrical connection to the called subscriber.
Since the final selector responds to both the digits in vertical and horizontal
directions this final selector is also called a Numerical Selector
If the called subscriber is free, as sensed from a signal at the corresponding
bank contact, the final selector sends out a ringing current to the called
subscriber, and a ringing tone to the calling subscriber.
When the called subscriber lifts his handset, the ringing current and tone are
cut off and a switched electrical path is established between the calling and
the called subscriber. At the same time, call metering circuits are enabled by
the control circuits associated with the final selectors.
If the called subscriber is busy, the final selector sends out a busy tone to the
calling subscriber. At any stage of switching, if there is no free selector at
the next stage, a busy tone is returned to the calling subscriber.
