Digital Switching System

1. TSN Lectures
ETE 310
Prof. A.H.M. Asadul Huq, Ph.D.
http://asadul.drivehq.com/students.htm
asadul@univdhaka.edu
5 June 2015 A.H. 1
Introduction to Switching

2. Introduction to Switching
• A switching system connects a call from one
subscriber to the desired subscriber.
• One important problem is how to manage
traffic within the switching system.
• In early telephone network, operators in the
switching office used to connect subscribers
manually.
5 June 2015 A.H. 2

3. Introduction to Switching … continued
• Switching and other relevant techniques have been
evolved to automatic telephone exchanges where
digital controls have been used.
• If no circuit is available to connect to the desired
subscriber, the call will be blocked.
• When a call is finished the connection will be
disconnected to make the circuit available for other
users.
5 June 2015 A.H. 3

4. Switching technology types
In general, two different types of technologies of
switching -
• Circuit Switching
• Packet Switching
5 June 2015 A.H. 4
Circuit Switching Packet Switching

5. Circuit Switch Network
Circuit switch network assigns a dedicated
communication path (physical and real time) between 2
stations. It involves -
• Direct connection from a station to network
• Internal switching (and multiplexing) among
switching nodes.
• Communication
• Circuit physically disconnection.
5 June 2015 A.H. 5

6. Advantages and disadvantages of Circuit-
Switched Network
Advantages
• Once connection is established network is transparent.
• Nodes seem to be directly connected.
• Fixed communication rate with no delay
Disadvantages
• Can be inefficient
• Resources are dedicated to the connection even if no data is
sent
• Delay (for set up) prior to usage of connection
• Probability of blocking for voice connection.
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7. Comparison
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8. 5 June 2015 A.H. 8

9. Elements of Switching System
• Switching network subsystem
• Signaling subsystem
• Control subsystem
• Accessory circuit
5 June 2015 A.H. 9

10. Direct Control vs Common Control
• If the control subsystem is an integral part of the switching
network then the system is called direct control switching
systems.
• If the control subsystem is outside the switching network then
the system is called common control switching system.
Examples
• Strowger exchanges are usually direct control system.
• Crossbar and electronic exchanges are common control
system.
• All stored program control systems are common control
system.
5 June 2015 A.H. 10

11. Direct v Common Control Signaling
5 June 2015 A.H. 11

12. Signaling
Signaling help exchanges to establish
connections between subscribers correctly
Signal category.
• Subscriber loop signaling.
• Inter-exchange signaling.
• Intra-exchange or register signaling.
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13. Telephone subscriber loop signaling
5 June 2015 A.H. 13

14. The Terminology for exchange units
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15. Remember this very basic point to point network
5 June 2015 A.H. 15
• There are n stations (in this case n=
6).
• Each station needs lines to n-1
others.
• All stations can call every other
stations. No blocking.
• The system needs a total number of
interconnecting lines -
𝑁 =
1
2
𝑛 𝑛 − 1 ≈ 𝑛2

16. 5 June 2015 A.H. 16
The N2 Problem
For N users to be fully connected directly
• Requires N(N – 1)/2 connections
• Requires too much space for cables
• Inefficient & costly since connections are not always on
N = 1000
N(N – 1)/2 = 499500
1
2
34
N
Solution: Use a telephone switching exchange office where
efficient switching will be performed

17. Manual switching Exchange Office
• Patch cord panel switch invented in 1877
• Operators connect users on demand
– Establish circuit to allow electrical current to flow
from inlet to outlet
• Only N connections required to the exchange
1
2
3
N – 1
N
5 June 2015 A.H. 17

18. 5 June 2015 A.H. 18
Strowger Switch
• Human operators are intelligent & flexible
– But expensive
– Possibility of human error
– Less switching speed
• Strowger invented automated switch in 1888
– Dialer is included in the telephone set
– Telephone user at home now send dial pulses to
the exchange and controls connection setup – no
manual operator needed
– Each current pulse produced by the dialing phone
at home advances the wiper of the Strowger
switch by 1 position at telephone exchange

19. Almon Brown Strowger
Father of the automatic telephone switching system
• A B Strowger invented
automatic telephone exchange.
He was awarded a patent in
1891.
• Bell Telephone Company
acquired the technology in
1916 and it was called Strowger
Switch, became standard
equipment throughout the
20th century.
5 June 2015 A.H. 19
Almon Brown Strowger
1839 - 1902

20. Basic Strowger switching components
2 types –
• Uniselector
• two motion selector
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21. Strowger two motion step by step Switch
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The movable contacts in a step by step Strowger switch
can connect to any of a 100 different pairs of fixed
contacts, each leading to a different line.

22. Timeline of Telecommunication
Source: www.telephonetribute.com/timeline.html
5 June 2015 A.H. 22

23. Multiple stage switching
• Decimal telephone numbering system
• multiple switching stages
• Hierarchical network structure simplifies call
connection
– Area code, exchange, station number can be
assigned
5 June 2015 A.H. 23
.
.
.
0
9
0
9.
.
.
0
9
0
9
.
.
.
1st digit 2nd digit . . .

24. Model of Switching Network
Switching network is a component of the switching
system
Main function: Establish an electrical path between
given inlet/outlet pair (2 telephone users or
subscribers)
5 June 2015 A.H. 24

25. Inlets/outlets Connections
• Types of connections
• Local call (Incoming and outgoing)
• Trunk (transit) call (call between 2 subscribers of
2 separate exchanges)
5 June 2015 A.H. 25

26. Folded network
• All inlet/outlets are connected to the subscriber lines.
• This is only for local exchange.
5 June 2015 A.H. 26
E
X
C
H
A
N
G
E
1
N

27. Blocking and Non-blocking System
• Non-blocking: A calling subscriber will always be able
to establish a connection to the called subscriber
• Blocking: Not all subscriber will be able to call to a
desired subscriber all time. The reason is that the
number of simultaneous switching paths are less
than the total number of subscriber in a system.
5 June 2015 A.H. 27
Note: All switching exchanges are designed to meet an
estimated maximum average simultaneous traffic, usually known
as busy hour traffic. Past records of the telephone traffic indicate
that even in a busy exchange, not more than (20-30)% of the
subscribers are active at the same time.

28. Elements of a telephone exchange
5 June 2015 A.H. 28

29. Switching Techniques (Review)
• Circuit Switching
– A path is established between the caller and destination.
– Real-time connection formed.
– Example: PSTN
• Message Switching
– Also called store and forward.
– A message is first stored in a buffer and then sent on in its entirety
step by step as resources become available.
– No real-time connection (i.e. connectionless).
– Example: E-mail
• Packet Switching
– A message is broken down into parts and each part is sent separately
(possibly via different routes).
– Example: Internet UDP protocol
5 June 2015 A.H. 29

30. Switching Circuits
• Four technologies for separating (Switching) circuits:
– Space, time, Radio frequency, Optical
• We want to connect circuits coming into a switch with circuits
at the output.
• Example of Space division switch: Strowger, crossbar Switch.
• Example “space division” equivalent interconnection pattern:
Input 1
Input 2
Input 3
Output 1
Output 2
Output 3
5 June 2015 A.H. 30

31. Space Division Switching
• Connecting two channels that are separated in space.
• Can be mechanical and/or electronic.
• Several problems:
– Slow
– Bulky with lots of interconnect wiring
– Subject to cross-talk
Input 1
Input 2
Input 3
Output 1
Output 2
Output 3
Input 1
Input 2
Input 3
Output 1
Output 2
Output 3
Equivalent
5 June 2015 A.H. 31

32. Strowger Switching
• Patented 12/March/1889 and in some places still in use
today.
• First widely-used automatic exchange system.
• A wiper assembly (contact arm) moves across a fixed set
of switch contacts (contact bank).
– Each contact is connected to an outgoing channel.
Uni-selector:
Strowger
uni-selector
5 June 2015 A.H. 32

33. Strowger Switching (2)
• Several uni-selectors can be “graded” together so multiple
incoming circuits can connect to multiple outgoing circuits.
• Or two uni-selectors can be wired back-to-back (line-finders).
– 1st uni-selector chooses the incoming circuit, the 2nd chooses the
outgoing circuit.
Graded uni-selectors:
Unless there is heavy
traffic, it is inefficient
and uneconomical to
provide each incoming
circuit with a uni-
selector.
Line-finder (hunter):
Incoming
Circuits
Outgoing
Circuits
or other
uni-selectors
Line-finders can be graded
together as well to form
large switches.
5 June 2015 A.H. 33

34. Strowger Switching (3) (Review)
• In general, multiple uni-selectors, line-finders, and two-
motion selectors (movable in two planes) can be connected in
series.
• These switches respond to dialled digits, automatically
switching an incoming circuit to the correct outgoing trunk.
– Step-by-step switching will respond to each digit
individually.
Strowger two-motion selector
5 June 2015 A.H. 34

35. Two motion Selector (Trowger)
• A set of wipers (in the figure there is one wiper shown for
simplicity) is moved in two different planes (horizontal and
vertical, you have seen that in a video clip)
• A typical switch can access 100 switching contacts (10 vertical
levels and each level contains 10 horizontal contact points).
Note: Actually there are 11 vertical positions and 11 horizontal contacts. The
lowest vertical position and first horizontal position in each vertical level are
home positions.
5 June 2015 A.H. 35

36. Block diagram of a multi-stage Strowger
switching system
• Subscriber Line circuit(SLC) and Line finder: When a
subscriber lifts his handset, current starts to flow on the line,
this is detected by SLC.
• Line finder and allotter finds a free selector. (see next figure)
5 June 2015 A.H. 36

37. Routing of a local call in Strowger switching
system
• Group selector: Depends on the subscriber number. For a 4
digit number two selectors are required.
• Final selector: The final selector takes care of last two digits.
• If called subscriber is free the path setup is competed.
Otherwise a busy signal is retuned to the caller. (Gna P. 66-68)
5 June 2015 A.H. 37

38. A Typical
Public Circuit Switched Network (PSTN)
5 June 2015 A.H. 38

39. Circuit Establishment
5 June 2015 A.H. 39

40. Multi-stage Space Division Switch
5 June 2015 A.H. 40

41. Blocking or Non-blocking (Review)
• blocking network
– may be unable to connect stations because all
paths are in use
– used on voice systems
• non-blocking network
– permits all stations to connect at once
– used for some data connections .
5 June 2015 A.H. 41

42. An example of Blocking
Even though there are
free links throughout
the switch, there is a
conflict for specific
links required for the
brown connection.
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
5 x 5
• Analyze how blocking in a network occurs:
– There are generally free links in each stage.
– Problem is that they are mismatched from stage to stage.
So blocking occurs for this design.
5 June 2015 A.H. 42

43. Design 1: 100 line switching system using uniselectors
• 100 subscriber. 2 stage Strowger
exchange.
• This is a blocking design
• Each subscriber should given one
uniselector at the exchange (100
uniselector at the inlet)
• Only 10 uniselector at the outlet
• The corresponding outlets of all
the 1st stage uniselectors are
commond or connected.
• Probability of blocking PB=0.009.
• Equipment utilization factor EUF
= 0.18
• CCI = 9.09 (Thia P. 48-50)
5 June 2015 A.H. 43

44. Design 2: 100 line exchange with two-motion selectors
• 100 subscriber. Single stage. Non blocking.
• A subscriber is assigned a number in the range 00-99.
• The corresponding outlets in all the 100 two-motion selectors are
commoned and folded back to the corresponding inlets.
• Blocking probability PB = 0
• Equipment utilization factor EUF = 0.5
• CCI = 25 (Thia P. 51-52)
5 June 2015 A.H. 44

45. Switching Network Design Parameters
1. Number of subscriber lines = N
2. Total number of switching elements = S
3. Cost of switching system C=SxCs +Cc+Cch
where,
Cs=cost per switching system
Cc=cost of the common control system
Cch=cost of the common hardware
Since the control circuits are associated with switching elements in a
Strowger system, Cc = 0 for the system.
The common hardware is usually a small proportion of the total hardware
except for the power supplies and its cost is of the same order in different
comparable designs. Hence we ignore it, hence Cch =0 in most of the
calculations.
5 June 2015 A.H. 45

46. Switching Network Design Parameters … continued
4. Switching capacity = SC (Max no. of connections at a time)
5. Traffic handling capacity-
𝑇𝐶 =
𝑆𝑤𝑖𝑡𝑐ℎ𝑖𝑛𝑔 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑙𝑜𝑎𝑑
=
𝑆𝐶
𝑁
2
=
2(𝑆𝐶)
𝑁
6. Equipment utilization factor –
𝐸𝑈𝐹 =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑤𝑖𝑡𝑐ℎ𝑖𝑛𝑔 𝑒𝑙𝑒𝑚𝑒𝑛𝑡𝑠 𝑖𝑛 𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑤ℎ𝑒𝑛 𝑡ℎ𝑒 𝑆𝐶 𝑖𝑠 𝑓𝑢𝑙𝑙𝑦 𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑤𝑖𝑡𝑐ℎ𝑖𝑛𝑔 𝑒𝑙𝑒𝑚𝑒𝑛𝑡𝑠
7. Number of switching stages = K
8. Average switching time per stage Tst
9. Call setup time = Ts = (Tst x K) + To
To is time required for functions other than switching. To is significant quantity
in Common Control Systems where control functions are separated from
switching functions. In Strowger (direct control) systems, To may be
considered 0.
5 June 2015 A.H. 46

47. Switching Network Design Parameters …
continued
10.Cost capacity index –
𝐶𝐶𝐼 =
𝑁 × 𝑠𝑤𝑖𝑡𝑐ℎ𝑖𝑛𝑔 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦
𝑐𝑜𝑠𝑡 𝑝𝑒𝑟 𝑠𝑢𝑏𝑠𝑐𝑟𝑖𝑏𝑒𝑟 𝑙𝑖𝑛𝑒
=
𝑁(𝑆𝐶)
𝐶
Higher the value of CCI, better the design.
5 June 2015 A.H. 47

48. Design Parameters of the Design 1
• S=110, SC = 10, K=2, N=100
• TC=2(SC/N)=2(10/100)=0.2
• EUF = 20/110 = 0.18
Cost =C
=Total no. of switches x unit cost
=110x1=110
CCI=
𝑁(𝑆𝐶)
𝐶
=(100x10)/110 = 9.09.
5 June 2015 A.H. 48

49. TSN Lecture
Volume-2
THE END
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5 June 2015 A.H. 49