References Telecommunication Switching and Networks, by P.
Gnanasivam, New Age International Publishers, 2 nd edition Network
Infrastructure and Architecture, Designing High-Availability
Networks, by Krzysztof Iniewski, Carl McCrosky and Daniel Minoli,
Wiley Data and Computer Communications, William Stallings, 5 th
edition Wireless Communications Principles and Practice, Theodore
S. Rappaport, 2 nd edition
Slide 4
Course Outline Introduction Call setup process Multiplexing Old
switching systems Digital switching systems Digital time switching
systems ST, TS, STS and TST switching architectures Blocking in
switching systems
Slide 5
Course Outline Packet switching Datagram switching Virtual
circuits Routing Signaling Traffic and QoS Cellular systems ATM PDH
SDH
Slide 6
Introduction General types of communication networks What is
switching Elements of switching systems Types of switching Example
of switch operation Other types of switching Manual switching
Slide 7
Types of Communication Networks Point-to-point (PTP) One
transmitter and one receiver Simple and small No need for
addressing Signaling may be required to indicate start and end of
transmission E.g. computer peripherals
TransmitterReceiverChannel
Slide 8
Point-to-multipoint Broadcasting (TV, radio) Addressing and
signaling are usually not required Network size can be very large
(worldwide) Non-broadcasting (control networks) An address is used
to identify the node that should receive the data Signaling is
typically used to indicate start and end of transmission.
Acknowledgements may be used The transmitter controls the channel
Typically the size of the network is small TransmitterChannel
Receiver 2 Receiver NReceiver 1
Slide 9
Multipoint-to-point Network size usually small An address is
required to identify the transmitter The receiver is in control of
the channel Signaling is required to decide which transmitter
should send. Acknowledgements may be used E.g. sensor networks
ReceiverChannel Transmitter 2 Transmitter N Transmitter 1
Slide 10
Multipoint-to-multipoint Can be small (LAN) Or large (WAN)
Addressing and signaling are required to organize access/usage of
channel and network TX/RX 2 TX/RX 3TX/RX 1 TX/RX 5TX/RX 4TX/RX 6
Channel Switching Network
Slide 11
What is switching? Switching is the process of calculating and
creating a route (path) between a source and a desired
destination(s) and relaying data (or call) via this route Switching
is required when there are no permanent channels between entities,
but there are limited common channel The main objective is to
minimize cost by replacing a large number of dedicated links by a
single one shared between users upon request Switching is divide
into path calculation route establishment data forwarding
Slide 12
Switch Trunk Switch Subscriber Loop/ Local Loop
Slide 13
Elements of Switching Systems End system or instrument
Transmission system Switching system Signaling
Slide 14
End Systems Transmitters and/or receivers responsible of
sending information and/or decoding the received signal or message
into intelligible message, e.g. telephone, cell phone, computer,
fax
Slide 15
Transmission System The physical links that transfer
information and control signals between terminals and switching
centers A transmission path between two distinct points can be
setup by connecting a number of transmission lines in tandem
Communication channels between switching systems are referred to as
trunks Can be bidirectional or unidirectional. Bidirectional links
can be constructed from 2 unidirectional links in opposite
directions Types of transmission links include twisted pair,
coaxial cables, optical fiber, terrestrial links and satellite
links
Slide 16
Switching System The switching centers receive the control
signals and messages or conversation from end systems and forward
them to the required destination after any necessary modifications
(e.g. amplification, A/D or D/A conversions) A switching system is
a collection of switching elements arranged and controlled in order
to setup a communication path between any two distant points For
telephone networks, a switching center consisting of a switching
network and its control and supporting equipments is called a
central office The switching method used in telephone networks is
known as circuit switching In computer communications a method
known as packet switching/message switching/store and forward
switching is usually applied
Slide 17
Signaling System The signaling system exchanges signaling
information between Subscribers and switching centers Switching
centers of the service provider Switching centers of other service
providers There are two types of signaling: In-band signaling,
signaling information and data/voice uses the same channels. Common
channel signaling, some channels are dedicated for sending
signaling information and not used for data transmission
Slide 18
Types of Switching Systems Distributed Centralized Hybrid
Slide 19
Distributed Switching System Each terminal is responsible of
establishing a path to the desired destination Diagram shows a
5-node network fully interconnected (fully connected mesh) T2 T1 T3
T5T4
Slide 20
Fully-connected Mesh Each terminal has 2 switches, one to make
a call and another to receive a call The number of bidirectional
links is N(N-1)/2, where N is the number of nodes For large
networks (N large) the number of links is too high ~O(N 2 )
Slide 21
Alternative Configuration Requires N links only Nodes are
permanently connected to the receive link Other terminals are
accessed via the switch Suitable for small networks T1T2T3T4
Slide 22
Centralized Switching Here, central switch(es) are responsible
of creating the route upon request Model 1: Single central switch
For large networks, the subscribers loops become too long Also the
Switch becomes too complicated Centralized Switching System T1 T2
T5 T4 T3
Slide 23
Centralized Switching Model 2: The complexity of the switching
center is less than that in model 1 The subscribers loops are
shorter High capacity links (trunks) are used between switching
centers Switching Center 1 Switching Center 2 T1 T2 T3 T4 T8 T7 T6
T5 Trunk
Slide 24
Hybrid Switching System Local Telephone Network T1 T2 T3 T4 T8
T7 T6 T5 Trunks Core Network
Slide 25
Hybrid Switching System Depending on the topology of the
network, several paths usually exist between any two nodes Under
heavy load/due to link failures some routes may become unavailable
Alternative routes are used in this case For telephones, a
hierarchical structure is used to organize route selection
(AT&T and ITU-T)
Slide 26
AT&T Hierarchy Final Class 1 (Regional Center) Class 2
(Sectional Center) Class 3 (Primary Center) Class 4 (Toll Center)
Class 5 (Central office/ Telephone Exchange) Subscriber High Usage
Trunks
Slide 27
AT&T Hierarchy Class 5 (Local Exchange): Also known as
branch exchange. It delivers dial tone to the customer and is the
closest connection to the end customer. Typically 10s of thousands
are available Class 4 (Toll Center): Calls between two end offices
not directly connected, or connected but the trunks are busy, are
routed via the Toll center. Typically these serve as intrastate
facilities Class 3 (Primary Center): Calls being made beyond the
limits of a small geographical where circuits are not connected
directly between class 4 toll offices would be passed from the toll
center to the primary center. These use high usage trunks to
complete the connection between toll centers. Number of primary
links in the US was between 150 to 230 Class 2 (Sectional Center):
Connects typically major toll centers within one or two states to
connect interstate long- distance calls. There were 50-75 active
centers in the US Class 1 (Regional Center): Used as a last resort
for call setup when no routes were available in the lower centers.
They are used for control functions and billing of international
calls. In North America, only 12 regional centers were used
Slide 28
Example of Switch Operation Assume U1 is trying to call U8 U1
picks his phone handle Switch 1 should: Sense that U1 picked the
phone handle Provide U1 with a dial tone U1 dials the number for
user 8 U1 U2U8 S1 S2 S3 S4
Slide 29
Example of Switch Operation S1 should Receive the dialed number
from U1 Remove the dial tone Find an idle path to U8. This can be
via the trunk S1-S4 or via S2 or S3 if the S1-S4 trunk is busy. We
assume the trunk S1-S4 is used Inform the corresponding switch(es)
in the route to setup the route via signaling Switch S4 should
Establish the route as signaled by S1 Check if U8 is free Send a
ring signal to U8 Connect U8 to the route Signal S1 that the route
is established and U8 is ringing U1 U2U8 S1 S2 S3 S4
Slide 30
Example of Switch Operation Switch S1 should Connect U1 to the
route established Send the ringing tone to U1 The switches should
also Supervise the call (e.g. if U8 does not pick up) Make records
of the call for billing purposes Clear down, i.e. free the route
when the call terminates If U1 hangs first, S1 sends clear forward
signal If U8 hangs first, S4 sends clear backward signal Not all
these steps are used in all networks and some come before others in
some networks U1 U2U8 S1 S2 S3 S4
Slide 31
Types of Switching (again!!) Based on how the media is shared
between the users, switching (multiplexing) is divided into the
following types: Space division switching/multiplexing Time
division switching/multiplexing Frequency division
switching/multiplexing Wavelength division switching/multiplexing
Based on how the information is switched, there are two types of
switching: Circuit switching Packet switching Based on the devices
used in the switches, switches are divided into the following
types: Manual Switching Automatic Switching, which is divided into
Electromechanical Switching Electronic Switching
Slide 32
Media Based Switching Space Division Switching Several circuits
(wires/fibers) Switching requires physically connecting input and
output wires Used in analog and digital system
Slide 33
Media Based Switching Time Division Switching Time is divided
into intervals called time slots Each time slot is assigned to a
specific input Heavily used in digital systems Difficult to
implement and gives poor performance in analog system
Slide 34
Media Based Switching Frequency/Wavelength Division Switching
Media bandwidth is divided into smaller frequency bands (or
wavelengths) Each band is assigned to a certain input Each band
should have a dedicated modulator/demodulator The switch connects
the input with the modulator of the assigned band Hence, similar to
space switching It is possible to combine several switching systems
together (e.g. Space and frequency, time and frequency, , or even
space, time and frequency)
Slide 35
Based on how information is switched Circuit Switching Before
sending any data a path is setup Path is reserved and remains
dedicated for the connection All data travels in the same path Path
is released when the call ends Applies for digital and analog
transmissions
Slide 36
Based on how information is switched Packet/message switching
Relatively new, appeared after computers Applies only for digital
transmissions No path is setup or reserved Data chunks
(packets/messages) are sent from transmitter and contains addresses
Network elements use the address to determine where to forward the
message Consecutive packets may take different paths
Slide 37
Manual Switching First type of switching used An operator sits
in the central office When a user lifts the handle, a lamp
corresponding to the user illuminates The operator connects his
handset to the users circuit and asks him for the number If the
number is not within the office, the operator must call the
operator of the desired office The operator checks if the desired
user is free or not If the user is free, the operator activates the
ring switch of the user When the user picks up, the operator
connects him with the dialing party and tells them to start When
either of the users hangs the phone, his lamp goes off The operator
then disconnects both circuits.
Slide 38
Limitations of Manual Switching Language dependent Lack of
privacy Switching delay Limited system capacity
