1 Switching Systems pulse code modulation stored program common control time/digital electronic 1976 ESS—third generation reed switch stored program control space/analog Semi-electronic 1972 ESS—second generation reed switch common control space/analog Semi-electronic 1960 ESS—first generation X-bar switch common control space/analog Electro-mechanical 1918 cross-bar stepping switch train distributed stage-by-stage space/analog Electro-mechanical 1892 step-by-step plug/cord/jac k human space/analog manual 1878 manual operator Type of Network Type of Control Method of Switching Operation Switching System
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• In a folded network with N subscribers, there can be a maximum of N/2simultaneous calls.
• The switching network may be designed to provide N/2 switching paths, the network then is said to be non-blocking.
• Blocking Network (Switching Paths<N/2)
• The probability that a user may get blocked is known as blocking probability.
• Switching exchanges are designed to meet an estimated maximum average simultaneous traffic, usually known as busy hour traffic. (20-30% activity)
K. Chandrasekaran
Highlight
K. Chandrasekaran
Highlight
K. Chandrasekaran
Highlight
4
Elements of a switching system
• Trunk Interface (Incoming and Outgoing)• Subscriber Line Interface (I and O)• Service Circuit Interface• Junctors• Subscriber and Service line scanning and
distributor units.• Control: Direct or Common Control• Operator console• Trunk circuit scanning and distributor units.
SwitchingNetwork
Trunk Interface
SubscriberLine
Interface
ServiceCircuit
Interface
Trunk Interface
Subscriber Line
Interface
ServiceCircuit
Interface
SubscriberAnd
Service Line Scanning
and Distributor
Units
Trunk Circuit Scanning
and Distributor
Units
Control
Operator Console
JUNCTORS
Incoming Outgoing
ToIncomingTrunks
K. Chandrasekaran
Highlight
5
Switching Systems (Classification)
Switching Systems
Manual
Automatic
Electromechanical Electronic (SPC)
Strowger (Step-by-Step)
Crossbar
Space Division Switching
Time Division Switching
Manual Switch-Boards
• The first switching Systems utilized operators at manual switchboards.
• The operators asked the caller for the number they wanted to call and then established the connection by plugging in a cord between terminal jacks
6
RST
• R – Ring
• S – Sleeve
• T – Tip
• One wire of a wire pair is commonly referred to as the tip and other is referred to as ring, even on digital wire pairs
7
Manual
LB exchange CB exchange
Magneto exchange
If 200 subscribers terminate on a switch board, what will be
the maximum number of simultaneous calls
Ans. 100
K. Chandrasekaran
Highlight
K. Chandrasekaran
Highlight
8
Switch boards
Single Termination Multi Termination
Cyclic Assignment
Single termination board
TransferLines
LocalConnection
Non-LocalConnection
K. Chandrasekaran
Highlight
9
SubscriberLines
Multi-Termination Boards
Cyclic Assignment
300-399
700-799
0-99
400-499
100-199
500-599
200-299
600-699
300-399
700-799
0-99
400-499
10
• Almon Strowger was an undertaker who created the first automatic telephone switching machine.
• He wasn't a natural scientist nor was he immediately interested by telephones. What really spurred him on was when he imagined that his undertaking business was missing out because the lady at the phone exchange was connecting callers to a rival funeral business.
• Customers didn't immediately take to the system, and Strowger soon tired of it himself, selling his stake for a modest sum to buy a hotel. His system was later sold for $2.5 million and installed in exchanges worldwide until the 1960s.
Almon Brown Strowger
(1839-1902)
operator - get off the line!
Advantages: Automatic Switching Systems
• Language independent. • Privacy • Establishment and release of calls are
• It uses ‘direct progressive control’• The control elements of the switch are
integrated into the switching matrix• A call may be blocked even if an alternate
path exists through the switch.• Alternate routing for outgoing trunks is not
possible.• Signaling schemes other than dial pulses are
not directly usable• Number translation is impossible.
Limitations of strowger systems
Crossbar switching• a crossbar switch (also known as cross-point switch,
or matrix switch) is a switch connecting multiple inputs to multiple outputs in a matrix manner.
• The cross-points of a cross bar switch are mechanical contacts with magnets to set up and hold a connection.
• The term crossbar arises form the use of horizontal and vertical bars to initially select the contacts.
K. Chandrasekaran
Highlight
16
Crossbar switching• Uses ‘common control’
• Control function implementation is separate from the switch implementation
• Logical addresses instead of physical line numbers
K. Chandrasekaran
Highlight
17
• Early crossbar systems were slow in call processing as they used electro-mechanical components for common control subsystems.
18
Stored Program control (SPC)
• Carrying out the exchange control functions through programs stored in the memory of computer led to the nomenclature stored program control.
• SPC lead to full scale automation of exchange functions and introduction of variety of new services to the users.
SPC
Centralized Distributed
K. Chandrasekaran
Highlight
19
Centralized
Processor
Signal distributor
Scanners
MaintenanceConsole
Memory Secondary Storage: Call Recording,
Program Storage etc.
• Redundancy may also be provided at the level of exchange resources and function programs.
• A dual processor architecture may be configured to operate in one of three modes:- Standby, Synchronous duplex, and Load Sharing.
K. Chandrasekaran
Highlight
20
Standby Dual Processor Config.
Exchange Environment
P1 P2
Secondary Storage
Synchronous Duplex
Exchange Environment
P2
M2M1
P1 C
21
Load Sharing
Exchange Environment
P2
M2M2
P1
ED
Distributed SPC
• In distributed control, control functions are shared by many processors within the exchange itself.
• Exchange functions may be decomposed either “horizontally” or “vertically” for distributed processing.
• Better reliability and availability• Owes its existence to low cost processors
22
Levels of Control Functions(Horizontal decomposition)
Event Monitoring and Distribution
O & M and Charging
Call Processing
Dual Chain Distributed Control
Exchange Environment
EM & DP
O & MP
CP
EM & DP
O & MP
CP
23
• 1965: Bell system installed the first electronic switching system (No. 1 ESS)
• Allowed introduction of new features such as abbreviated dialing, call forwarding, call waiting,…
• Simplified administration and maintenance tasks.
• 1976: AT&T’s No.4 ESS is a high capacity toll switch using SPC and digital electronics for its switching matrix.
1
Drive Mechanism of Rotary Switch
Uniselector
2
2-motion selector
2-motion selector
3
Step-by-step switching
Selector HuntersOr
Line Finders
Group SelectorStage
Final Selector
From Calling Subscriber
To CalledSubscriber
Line Equipment Part (Pre-selector
Stage)
Switching NetworkPart
1st Selector, 2nd Selector, &
so on….
Connector Part(Numerical Selector)
Selector Hunters
o Off hook --- interrupter mechanism activated ---wiper steps till find a free selector group --- first selector once located ---interrupter mechanism deactivated --- first selector sends dial tone ---accept pulses --- group selection continues till final selector.
4
Line Finders
o Off-hook signal sensed by all line finders ---interrupter mechanism of line finder whose associated first selector is free, gets activated ---line finder wiper steps till it reaches the subscriber contact --- first selector sends dial tone.
Design Parameters
• Number of Subscriber lines, N• No. of Switching Elements, S• Cost of Switching System, C = S×Cs + Cc + Cch
• Switching Capacity, SC – maximum number of simultaneous calls.
• Equipment Utilization Factor, EUF = Number of switching elements in operation when SC is fully utilized / total number of switching elements in the system.
K.Chandrasekaran
Highlight
5
• Number of Switching Stages, K• Average Switching Time per stage, Tst
• Call set up time, Ts = Tst × K + T0
• Cost capacity index, CCI = switching capacity / cost per subscriber line = N(SC) / C
• Blocking Probabilty, PB or B
Design Parameters
Design 1: 100-line exchange (uniselectors)
K.Chandrasekaran
Highlight
K.Chandrasekaran
Highlight
6
Design 2: 100-line exchange (uniselectors)
• 10 uniselectors in the second stage for every one uniselector in the first stage of Design 1.
Design 3: 100-line exchange (2-motion selectors)
7
Design 3: 100-line exchange with selector finders
Design 4: 100-line exchange with line finders
8
1000-line exchange
Common Control Switching System
9
3X3 Crossbar Switching
6X6 Crossbar Matrix
10
Connection in Crossbar
• Energize horizontal bar --- energize vertical bar --- de-energize horizontal bar.
• Energize vertical bar --- energize horizontal bar --- de-energize vertical bar.
• Suitable arrangement so that Latch is maintained, even after de-energizing the concerned bar.
Diagonal Crosspoint Matrix
11
Blocking Crossbar Switch
• Energize horizontal, A
• Energize vertical, P
• De-energize horizontal, A
• Energize horizontal, B
• Energize vertical P’
• De-energize, B
• Energize A & B, • Energize P, • De-energize A & B