Evolution of switching
system
Unit 2
INTRODUCTION
Switching system & associated signalling
system are essential to the operation of
telecommunication networks
The functions performed by a switching
system or a subsystem of it, in order to
provide customer with facilities
Classification of switching
system
Message switching
Message switching eg: delay system or
queuing system
Early days of telegraphy a customer might
sent a msg from town A to town B
although there was no telegraph ckt
between A and B.
if there was a ckt between A and C and
between C and B, this achieved by the
process known as msg switching.
The operator at A send msg to C, this
operator recognized the address of msg as
being at B & retransmitted the msg over5
The message received at C is automatically recorded on
punch tape & subsequently turn off the receiver by the
operator, who read the address from the tape.
Message then automatically retransmitted same tape (b)
& called torn tape relay system
Later outgoing route was selected automatically (c)
Finally paper tape was eliminated by storing the
message electronically & analysing their address by
electronic logic (d)
Message switching was first to adopt stored program
control(SPC)
6
Circuit switching
Invention of telephone introduced a new requirement:
simultaneous both way communications in real time.
Message switching could not meet this requirement
because of the delay.
It becomes necessary to connect the circuit of a calling
telephone to called telephone & to maintain this
connection for the duration of the call. This is called
circuit switching.
In circuit switching, if the required outgoing circuit
from a switch is already engaged on another call, the
new call offered to it cannot be connected.
The call cannot be stored in message switching. In
circuit switching is thus example of lost call system
Functions of switching
system
Attending: The system must be continuously
monitoring all lines to detect call requests.
The calling signal is sometimes known as a seize
signal because it obtains a resource from the
exchange.
Information receiving: In addition to receiving
calls and clearing signals, the system must
receive information from the caller as to the
called line (or other service) required.
This is called the address signal.
8
Information processing: The system must
process the information received in order to
determine the actions to be performed and to
control these actions.
Busy testing: Having processed the received
information to determine the required outgoing
circuit, the system must make a busy test to
determine whether it is free or already engaged
on an other call.
In an automatic system, busy testing is also
required on trunks between switches in the
exchange
9
Interconnection:
For a call between two customers, three
connections are made in the following sequence;
A connection to the calling terminal
A connection to the called terminal
A connection between the two terminals
some modern systems release the initial
connections a and b and establish connection c
over a separate path through the switching
network.
This is known as call-back or crank-back.
The calling line is called back and the connection
to the called line is cranked back.
10
Alerting: Having made the connection, the
system sends a signal to alert the called
subscriber.
Supervision: After the called terminal has
answered, the system continues to monitor the
connection in order to be able to clear it down
when the call has ended.
When automatic ticketing is employed, the
system must send the number of the caller to the
supervisory circuit when the connection is setup.
11
information sending: if the called
customers line is located on another
exchange, the additional function of
information sending is required.
the originating exchange must signal the
required address to the terminating
exchange
12
DISTRIBUTION SYSTEM
13
Necessity of distribution
frames
Many changes occur during the life of telephone
New customer join & old leave, customer move
from one part to another
Customer with PBX may increase their no. Lines &
growth of traffic may require additional switches
in exchange & more junctions to other exchange
Great flexibility is require in trunking of exchange
& obtained by inserting distribution frames into
permanent exchange cabling
14
Functions of distribution
frames
Distribution frames contain an array of terminal
blocks & terminal linked by wires called jumpers
MDF: Place where the cables of the customer
distribution n/w terminate
line side of MDF correspond to steet cabling & so
reflects geography of area
The terminating on exchange side of MDF are
arranged in directory no.
No. Lines is changed by moving its jumper
It also provide convenient point of access for
testing lines
15
IDF (Intermediate Distribution Frames)
Used to distribute incoming traffic evenly
over the group of 1st selector
On multiple side of IDF lines are arranged
in directory number order(DN)
On local side, order can be arbitrary to
obtain desired result
Terminals on this side correspond to
equipment no.(EN)
Customers uni-selector are connected to
local side of IDF.
16
If exchange is equipped with meters, these are
required to be associated with directory numbers
& cabled to multiple side of IDF
Meter pulses on P wires arrive at local side , IDF
provides EN-DN translation
Incoming call for sector terminate at final sector
on an outlet corresponding to directory no.
Final sector multiples are cabled to multiple side
of the IDF
17
TDF(Trunk Distribution
Frame)
Between the ranks of selectors TDF
If additional selectors are needed at any
switching stage to cater for growth in
traffic, these can be accommodated by
rearranging connections in a TDF
Incoming calls for a customer terminate at a
final selector on an outlet corresponding to
directory no.
18
BASICS OF CROSSBAR
SYSTEM
19
Strowger switch need regular maintenance
Banks need cleaning , mechanism need
lubrication& adjustment Wiper & cords
wear out
A switch with N Inlets & N outlet requires N2
relays for cross point which uneconomical
so crossbar switch came into existence
20
Principles of crossbar switch
It retains a set of contact at each cross
point but these are operated through
horizontal & vertical bars by magnet
The idea of crossbar switches to provide a
matrix of nxm set of contacts with only
n+m activators or less to select one of
nxm sets of contact
Magnets horizontal bar select magnet
21
Vertical bar hold magnet or bridge
Contact point form pairs & Contact point
pair act as a cross point switch & remain
separated or open when not in use
Contact points are mechanically mounted
on set of horizontal &vertical bars called
dotted lines
This bars in turn attached to set of
electromagnets
22
Marker control crossbar
switch
23
Strowger selector perform counting & searching
But crossbar switch has no intelligence
Something external to switch must decide which magnet to
operate & called marker
It takes less than a second to operate the switch
Marker can control many switches & serve many registers
Large exchange needs few markers & hence can achieve
common control stage
A crossbar switch can make more than 1 connection at a
time & can make as many connection as it has vertical bars
Can be used as uniselector instead of 2 single motion
selector
24
2 stage link network
25
In order to produce larger switches, a 2 stage link
system of primary & secondary switch is used as shown
Fig shows 20 switches 10x10 used to connect 100
incoming trunks to 100 outgoing trunks
There is one link from each primary switch to each
secondary switch & these links arranged systematically
No. Of outlet in primary switch corresponds to no.of
secondary switch to which it links goes
No. Of inlet in secondary switch corresponds to no.of
primary switch from which link comes
26
concentrator
27
A concentrator can be constructed by
multiplying together the horizontal of a no.
Primary switch as shown
Fig shows n/w with 500 incoming trunks &
100 outgoing trunks using switches of
10x10
Although this n/w provide access from every
incoming trunk to every outgoing trunk,
it is not possible to make connection even
when required outgoing trunk is free
28
It may not always possible to make
connection even when the required outgoing
trunk is free because the link may be busy as
it is already in use for a connection
Then the call attempts fails, although the
outgoing trunk is free. The situation is called
blocking & also sometimes called mismatch
Therefore the traffic capacity of link n/w is
affected by internal blocking in addition to
congestion of the external trunks
29
4 stage switching network
30
This is implemented as 2 stage n/w
in which each switch is itself a 2
stage n/w
Hybrid capacity can be easily
realised by cascading lower capacity
switches across matrix arrangement
31
ELECTRONIC
SWITCHING(SPC)
32
This enables a digital computer to be used as
central control and perform different functions
with the same hardware by executing different
programs.
SPC exchanges can offer a better range of
facilities than other systems
In order to develop fully electronic exchange it
was necessary to replace the eletro-mechanical
switches in addition to electronic common control
33
One approach is to replace the relay
contacts of the switch by electronic
devices
A diode can be used as a cross point
A+VED is R.B cross point is
open
A-VED is F.B cross point is close
But external circuit needed to keep
point A negative for the duration of
34
In order to implement cross point ,1 bit
memory is required in addition to switching
Another approach is to use a multiplex
system instead of multiplexed elements
FDM used as a switch
If the modems of the path operate at fixed
frequencies, but those at other end can
operate at frequency of any channel
35
Then any trunk at one side of switch can
be connected to any trunk at other side
It is very expensive to put into practise
TDM can be used for transmission system,
fixed timing can be used
If the pulse timing can be altered, then
any incoming trunk can be connected to
any outgoing trunk
36
Switching systems may be classified
as:
Space Division (SD) systems:
Each connection is made over a
different path in space which exists for
the duration of the connection.
Time Division (TD) system:
Each connection is made over the same
path in space, but in different instants in
time.
37
Class of services
Call banning (outgoing or incoming): The
customer can prevent unauthorized calls being
made and can prevent incoming calls when
wishing to be left in peace.
Repeat last call: If called line is engaged the
caller can try again later without having to redial
the full number.
Reminder calls: the exchange can be instructed
to call customer at a pre-arranged time (wake up
call).
38
Call diversion: exchange can be instructed to
connect calls to a different number when the
customer goes away.
Three way calls: The customer can instruct
exchange to connect a third party to a call that is
already in progress
Charge device: after call has been ended the
exchange will send information back to indicate the
call duration & charge
39
DIGITAL SWITCHING
SYSTEMS
40
In digital switching ,TDM transmission was
introduced for trunk & junction circuits in
the form of PCM
If TD transmission is used with space
division tandem switching
It is necessary to provide demultiplexing
equipment to demodulate every channel
to audio before switching & multiplexing
equipment to retransmit it after switching
41
If TDM is used no multiplexing & demultiplexing is
needed
42
Evolution of digital
switching
A tandem exchange has a mixture of PCM junction
& analog audio junction
PCM terminal equipment is needed instead of
analog junctions
Large cost saving can be done by using TDM
43
Tandem exchanges has no customer lines, so there is
no problem of fully electronic switching for customer
line circuits which is uneconomical to be implemented
Local exchange can be formed by adding
concentrators to the periphery of tandem exchange
Therefore system were developed using reed relay
44
concentrator
This enables cheap line circuits to be retained &
large no. Subscriber to share a PCM equipment to
access TD routing switch
When
development
had
taken
place
in
semiconductor technology, large scale circuits were
appeared
This enables a PCM coder/decoder(codec ) to be
manufactured in single chip & made it practicable to
have one for each customer line
This
development
enabled
electromechanical
45
Next evolutionary step after the above is to move
PCM codec from the exchange end of the customers
line to customers end
This provide digital transmission over the customer
line which may have no. Advantages
If there is analog customer line modem must be
46
If line is digital ,data can be transmitted by
removing modem or codec & any digital signal
can be transmitted whose rate doesn't exceed
64kbps
This led to ISDN, in which the customers terminal
equipment & local exchange can be used to
provide many different services all using 64kbps
digit stream
Acess to ISDN provides 2 forms
Basic rate acess
Primary rate acess
47
Basic rate acess
The customer line carries 2 64kbps B channel
plus a 16kbps D channel for signalling in each
direction
Primary rate acess
2 lines provided to carry a complete PCM frame
at 1.5Mbps or 2Mbps in each direction
This gives the customer 23 or 30 circuit at
64kbps plus a common signalling channel at
64kbps
48
Subscribers line interface circuit for digital
exchange
49
Advance technology came into existence
Ie,
implemented
all
necessary
function
economically on subscribers line interface
circuit(SLIC)
Functions are
Battery feed
Over voltage protection
Ringing
Supervisory signalling
Coding
Hybrid
Testing
Acronym as BORSCHT
50
DIGITAL SWITCHING
SYSTEM
51
Purpose of Analysis
The reliability of digital switching systems is
becoming increasingly important for users of
telephone services
Internet access takes place through digital
switching systems.
Reliability of DSS is very serious matter as it
can impact on nations commerce security &
efficiency
DSS represent very complex system
It has multiple owners & is equipped by many
different supplier
52
Basic central office linkages
53
The following relate to the basic linkages
of a typical central office:
Main distributing frame (MDF):
Location where all lines and other related
links are cross-connected to a central
office switch also referred to as the line
side of a switch.
The MDF has two sides: a vertical and a
horizontal
The
subscriber cables terminate on the vertical
side. The wiring from the digital switching
system
54
Trunk distributing frame
(TDF)
Location where all trunks and other related
links are cross-connected to a central
office switch, also referred to as the trunk
side of a switch.
The TDF is usually smaller than the MDF.
All trunk cabling from different locations
terminates in the TDF.
The TDF has two sides: a vertical and a
horizontal. The trunk cables terminate on
the vertical side.
The wiring from the digital switching
55
Power plant.
A combination of power converters,
battery systems, and emergency power
sources which supply the basic -48- and
+24-V direct-current (dc) power and
protected alternating-current (ac) power
to a CO switch or a group of switches.
56
Carrier facilities.
Facilities which provide carrier or multiplex
transmission mode between central offices and
with other parts of the telephony network.
These facilities typically employ coaxial cables
(land or undersea) and radio and satellite
systems.
The carrier facilities usually terminate on the
TDF for cross connection to the digital switching
system.
57
Digital X-connect.
Digital cross-connect provides automatic
assignments and cross-connection of trunks to
digital switching systems. It can be considered
a small switching system for trunks.
Special services.
Those services which require special interfaces
or procedures to connect central office
facilities to a customer, eg., data services and
wireless services.
58
Outside Plant versus
Inside Plant
Most of the telephone companies classify
their telephone equipment as Outside
plant or inside plant.
This classification becomes important
during the analysis of a switching system,
since indirectly it defines the extent of a
CO and consequently the scope of
analysis.
59
Any element of telephony equipment
outside the CO box, such as MDF and
carrier systems, is classified as
outside plant.
CO equipment, such as central
processors, switching fabric, and tone
generators, are considered inside
plant.
60
Switching System
Hierarchy
61
The search for a path through the network
for a long distance call follows a hierarchy
After a call leaves a class 5 switch , a path
is hunted through the class 4 office
followed by class 3 ,class 2 & class 1
Different classes of switching system are
62
Local exchange (class 5).
It is also referred to as the end office (EO).
It interfaces with subscribers directly and
connects to toll centres via trunks.
It records subscriber billing information
Tandem and toll office (class 4).
Most class 5 COs interface with the
tandem offices.
The tandem offices primarily switch trunk
traffic between class 5 offices; they also
interface with higher-level toll offices. Toll
operator services can be provided by
63
these offices.
Primary toll center (class 3).
The class 3 toll centre can be directly served
by class 4 or class 5 offices, depending upon
the trunk deployment.
In other words, if the normal number of
trunks in these offices are exhausted, then.
traffic from lower-hierarchy offices can home
into a class 3 office.
Class 3 offices have the capability of storing,
modifying, prefixing, translating, or codeconverting received digits as well as finding
64
Sectional toll centre (class 2).
It functions as a toll centre and can home
into class 1 offices.
- Regional toll centre (class 1).
It functions as a toll centre and can home
into international gateway offices.
International gateway.
These offices have direct access to
international gateway offices in other
countries. They also
provide international operator assistance.
65
Stored Program Control Switching
Systems
66
With the advent of software-controlled central
processors, the control of switching functions was
programmed into memory and actions were executed
by the controlling processor.
The basic function of an SPC system is to control line
originations and terminations and to provide trunk
routing to other central or tandem offices.
The SPC system also provided control of special
features and functions of a central office, identified
here as ancillary control.
.
67
The intelligence of an SPC system resided in one
processor, and all peripherals were controlled by
this single processor. These processors were
duplicated for reliability.
A modern digital switching system employs a
number of processors and uses distributed software
and hardware architectures
Control of the maintenance functions of the modern
digital switching system also evolved from earlier
SPC systems.
68
Digital Switching System
Fundamentals
A switching system is called digital when the
input to and output from the switching system
network can directly support digital signals.
A digital signal can be defined as coded pulses
that can be used for signalling and control.
Analog signals can still be processed through the
digital switching system via analog-to-digital
(A/D) or digital-to- analog (D/A) converters.
69
This shows line side & trunk side of
switch
The basic function of a switching
system is to switch lines and trunks.
main objective of digital switching
systems is to switch subscribers and
trunk facilities.
70
shows the next step in the evolution of
digital switching
This phase uses analog lines and analog
trunks but employs A/D and D/A
converters for digital processing of calls.
Digitized signals are sent through the
switch.
71
The next step in this process, in which
digital switches can "talk" to other digital
switches
via
digital
trunks
while
simultaneously supporting analog lines
and trunks.
72
shows the ultimate, an all-digital linkage. In
this arrangement, there are no analog lines
or trunks involved; all communication
between digital switches is via digital
signaling.
This plan assumes that all incoming lines
coming to a CO are digital and that all
outgoing trunks are digital as well.
73
Basic building of DSS
The development of this digital switching
system model is described in four stages
The switching fabric switches lines and the
trunks under the control of a central processor
and network controller.
The second stage of this development
introduces the concepts of line and trunk
modules.
The third stage introduces the notion of
interface controllers and distributed processing.
The fourth stage presents a high-level design of
a digital switching system equipped with service
circuits.
74
Stage 1.
75
In this particular case, which is a simple one, only
lines and trunks are defined
all inputs and outputs to a digital switching
system there can be many types of lines and
trunks.
this is only a convention and does not mean that
trunks appear on one side of the network and lines
on the other.
The central processor controls the network
controller, which in turn controls the switching
fabric.
For the time being, regard the switching fabric as
a "switched" path through the CO.
76
Stage 2
77
The concept of line modules (LMs) and trunk
modules (TMs) is introduced here.
The line and trunk modules are the building blocks
of a modern digital switching system, and
conceptually they represent
some lines or trunks grouped together on circuit
packs, termed line or trunk equipment, and
connected to the switching fabric through a
controlling interface.
Modern digital switching systems use various
schemes to terminate lines on the line module.
Some digital switching systems allow termination of
only one line on one line module, while others allow
termination of multiple lines on a single line module
78
However, if a piece of line equipment
becomes defective; the line can easily be
assigned new line equipment if the LM
carried multiple pieces of line
In a modern digital switching that a number
of these units can be added on an asneeded basis without reengineering the
system.
This allows for easy growth and offers
flexibility in offering new services.
The impact of these design ideas on system
reliability and on digital switching system 79
Stage 3
80
The concept of distributed processing in a digital
switching system environment is developed here.
The task of controlling the switching fabric is
usually assigned to a series of network control
processors that control a part of the switching fabric
and a group of LMs and TMs.
The central processor controls the actions of the
network control processors.
This type of architecture is very flexible and
allows the construction of different sizes of central
office by increasing the number of network control
processors.
81
Stage 4
82
digital switching system design may appear to be
the final stage of a digital switching system
model, but it is not. In reality, it is only an initial
model of a digital switching system which is
needed to develop a more detailed model.
This basic model introduces the duplicated
scheme now commonly used in modern digital
switching systems.
Since telephony processing is a nonstop process
requiring high reliability, a duplicated scheme for
processing units and associated memory units is
almost mandatory.
83
This basic conceptual model also shows the
attachment of interface controllers and service
circuits to the line and trunk modules.
The interface controllers allow interfacing and
control of LMs and TMs through the network
control processors.
The purpose of the service circuits is to provide
dial tone, ringing, and other associated functions.
84
Basic call processing
This section describes some basic
types of calls that are usually
processed through a digital switching
system:
-
Intra-LM calls
Inter-LM calls
Incoming calls
Outgoing calls
85
Intra-LM calls
86
When a customer dials from a telephone that is
connected to a specific line module and
calls another customer who is also connected to the
same line module, this type of call is classified as an
intra-LM call.
The off-hook (line origination request) condition is
detected by the line module, and service circuits are
attached to supply a dial tone to the calling
customer.
Many other functions are performed before a dial
tone is given to a calling customer.
87
The line module's request for a path through the
switching fabric is processed by the interface
controller, which in turn works with the network
control processor to make a path assignment.
Consequently, a path is established through the
switching fabric for the called line, and a service
circuit is attached to ring the line.
Again, many other functions are performed before
ringing is applied to the called customer
Since this is an intra LM call, the same line module
will be involved in controlling the origination and
88
termination of a call.
Inter-LM Calls.
89
Inter-LM Calls.
The workings of an inter-LM call are similar
to those of an intra-LM call,
except that the terminating line equipment
is located in another line module. Figure 1.6
& show interconnections for such a call.
There are some subtle differences in how an
inter-LM call is handled versus an intra-LM
call.
90
Incoming/outgoing
91
Outgoing Calls.
When a LM processes a call which has terminating
equipment outside the CO, the LM requests a path'
through the switching fabric to a trunk module via the
interface controller.
The interface controller works with the network
control processor to establish a path to an outgoing
trunk.
Once a path is established through the switching
fabric, the TM connects a service circuit for
controlling the call to the called CO or a tandem
office.
92
Functions such as out pulsing and multi
frequency (MF) signalling are provided by
the trunk service circuits.
An outgoing call from an originating office
is an incoming call to a terminating office
. Figure 1.6c shows the paths of incoming
and outgoing calls.
93
Incoming Calls.
When a TM detects an incoming call, it attaches
service circuits to control the call and requests a
path through the switching fabric from the
interface
controller
and
network
control
processor.
Once a path is found through the switching fabric
to a LM that has the terminating line/ service
circuits are attached to ring the called telephone.
This also provides functions such as audible
ringing to the calling line.
94
95
