Computer Networks
Chapter 8 – Circuit Switching versus Packet Switching
Switches
 What is a switch?
 A central device usually used with a star topology
 Can be built in hardware and/or software
 Used to provide temporary connections between any
two devices connected to the switch
 A network of switches can be made if a very large
number of devices spread in a large geographic space
need to be connected
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Telephone network – Local Switch
 The telephone network uses switches
 Every subscriber ( telephone jack in a house) has a twisted-pair
wire connected to the closest telephone exchange. They are called
local switches or local exchanges.
 This cannot provide
connection to subscribers
connected to another local
switch.
subscriber
 










switch
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A Circuit Switch
 Device with a number of inputs and outputs
 Creates temporary physical connection between an
input and output link
 The local switch can
connect each telephone
with each other
Subscribers connected to
the same swich


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Telephone network – Connecting Local Switches
 It is expensive to connect each local switch with every
other in a fully meshed topology

 Higher order

 
switches are
2nd order
introduced to save
switch

on cables
The trafic
 between the
 Redundancy is

introduced to
switches is
multiplexed
provide reliability




 Local switch
(1st order switch)
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Subscribers Connected to Different
Local Switches
 The connection goes through several switches.
 If any part of the connection cannot be reserved, the
connection is not established


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Circuit Switching
 Three phases of the connection:
 Circuit establishment
 Data transfer
 Circuit disconnect
 The bandwidth is guaranteed during the
connection
 The bandwidth cannot be used by anyone else, even if
it is not needed at certain moment (no flexibility)
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Circuit Switching –cont.
 The resources (for example, the bandwith) are
divided into pieces
 Pieces are allocated to “calls”.
 No sharing: no two calls can share the same piece
of bandwidth.
 A piece is regarded “idle” if not assigned to any
calls.
 A call is rejected if no ”idle” piece exists on any
part of the connection.
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Telephone calls – Typical Example for Circuit
Switching
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Types of Switches
 Space switches
 Crossbar
 Multistage – A way to save on the cross points
 Blocking – input point cannot be connected to the
output due to all cross points being used by others
 Time-division switches
 Time-slot interchange
 TDM bus
 Combined switches
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Characteristics of the Switches
 Space switches
 The advantage is that if a cross point is available, the
connection is almost instantaneous
 The disadvantage is the need for many cross points
which is expensive
 Time switches
 Advantage is that it does not need cross points
 Disadvantage is the time necessary for processing each
time slot.
 Combined switches combine the advantages of both
types
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Hierarchy of the Telephone Network
International
network
International gateway exchange
National tandem exchanges
regional tandem
exchanges
trunk network
Tandem offices
local tandem
exchanges
local
network
local
exchanges (toll
offices)
subscriber
lines (local
loops)
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Computer Networks
 Telephone network has been developing since the end of the
19th century
 Computer networks started their development late in the
20th century (ARPANet has emerged in the late 60s and
early 70s)
 Circuit switching was not convinient for computer traffic
because
 Data traffic is bursty – a lot of traffic might need to be transmitted
at one moment and nothing in another (dedicated bandwidth will
then be wasted)
 Data traffic is delicate – if binary file is transmiited, every bite
must be correct for the program to work (for voice this is not too
important)
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Packets – A New Paradigm
 The message is splitted in smaller chunks called
packets.
 Introducing packets is important for the following
reasons:
 Errors can appear only in a few packets. Then these
packets need to be retransmitted again. However,
retransmitting several packets instead of the whole
message saves bandwidth.
 A very long message means monopolizing bandwidth
by a single user. Using smaller packets provides better
sharing.
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Message versus Packets
Header
Data
Whole
message
Padding if
necessary
Each packet carries the destination address in the header and is
independent from the other packets in the message - datagram
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Store-and-forward Switching
 Instead of establishing a circuit and making reservation on
every hop of the path, packet switching is actually storeand-forward switching.
 In store-and-forward switching every packet is switched
(forwarded) as it comes to the switch.
 If the switch is busy, the packet waits in the input queue to be
processed
 Once the switch is idle it checks the header of the packet and based
upon that information forwards it to the output line which is closer
to the packet’s destination.
 If the output line is busy, the packet waits in the output queue
until it is transmitted to the next store-and-forward switch.
 The switch that is directly connected to the desination will deliver
the packet.
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Router – A Store-and-Forward Switch
incoming links
router
outgoing
links
Link 1
Link 2
Link 3
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Statistical multiplexing
 The physical link is shared over time (like in STDM)
 Data are transmitted on demand rather than during the
predetermined time slot (unlike in STDM)
Buffering (queues)
 Defines a cost-effective way for multiple users to share network
resources in fine-grained manner
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Packet Switching
 Two types of packet switching
 Datagram switching
 The store-and-forward switch forwards the packet to the output
link by looking only at its destination address.
 Packets belonging to the same message can travel different
paths and their ordering is done at the receiver
 Example technologies: IP
 Virtual circuit switching
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Datagram Switching
Each router forwards the datagram solely based upon the
destination address. Packets can arrive out of order.
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Delays with packet Switching
 Packet experiences delay on each hop:
 transmission delay (length of message/through.)
 propagation delay (distance/speed of prop.)
 nodal processing
 quieuing
transmission
A
propagation
B
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nodal
processing
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queueing
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