Circuit Switching

Basic Categories of
Communication Networks
 Broadcast Networks
-a single node transmits the information
to all other nodes and hence, all stations
will receive the data
 Switched Networks
data are transferred from source to
destination through a series of intermediate
nodes (switching nodes).

Types of Switched Networks
 Circuit-Switched
 Packet-Switched
 Message-Switched
 Burst-Switched

Circuit-Switched Network
 “Line-Switched Network”.
 Originally developed for the analog-based
telephone system.
 a physical path is obtained for and dedicated
to a single connection between two end-
points in the network for the duration of the
connection.

3 Basic Elements
 End-Stations (or Terminals)
 Transmission Media
 Switching Nodes

3 Phases of a Circuit-Switched
Communication System
 Circuit Establishment
 DataTransfer
 Circuit Disconnect

 The connection path is established before the
transmission begins.
 Channel capacity must be reserved between
the source and destination throughout the
network.
 Each node must have available internal
switching capacity to handle the requested
connection.
 The Switching nodes must have the
intelligence to make proper allocations and to
establish a route through the network.

Example:
 PublicTelephone Network (PTN)
 Plain OldTelephone System (POTS)
 Long Distance Calls

Switching Node Architecture
Basic Functionalities:
 Signaling
 Control
 Switching
 Interfacing

 Signaling
 Monitors the activity of the incoming lines
and to forward appropriate status or control
information to the control element of the
switch.
 Also used to place control signals onto
outgoing lines under the direction of the
control element.

 Control
 process incoming signaling information and
sets up connections accordingly

 Switching
SWITCHING MATRIX
(FABRIC)
-an array of selectable
cross-points used to
complete connections
between input lines
and output lines.

 Interfacing
 Provides the hardware required to connect
different devices, such as:
 analog
 digitalTDM lines
 optical fibers
 etc.
to the switch matrix.

Characteristic of a Circuit-
Based Switch
 Blocking Switching Node
- occurs when the switching matrix does
not allow some input lines to be
connected to output lines.
- used on voice systems
 Non-Blocking Switching Node
- allows all inputs to be connected to all
outputs.
-used for data connections

Switching Technologies
 Space-Division
 Time-Division
 Frequency Division
 Wavelength Division

 Space-Division Switching
 Each input takes a different physical path
in the switch matrix depending on the
output.

2nd Generation
Space-Division System
 Step-by-Step Switch
 Crossbar Switch (Cross-Point Switch)

 A basic step-by-step switch has a single input
terminal and multiple output terminals.
 Connection from the input terminal to the
outputs is controlled by an internal rotary
contact, or wiper.
 As the wiper rotates, it establishes a contact
between the input and output terminals.
 Each time the user dials a rotary-dial digit,
the rotary contact is advanced one position,
and connects the input terminal to the next
output terminal.
( Chapuis, 1982 ; Clark, 1997 )

 Crossbar Switch
As digits are dialed, the control element of the
switch receives the entire address before
processing it.

 As digits are dialed, the control element of
the switch receives the entire address before
processing it.
 The cross-points of the crossbar switch are
mechanical contacts with magnets to setup
and hold a connection.
 Once the circuit is established, the switching
contacts are held by electromagnets
energized with direct current passing through
the established circuit.
 When the circuit is opened, the loss of current
causes the cross-points to be released.

NOTE:
 CROSSBAR arises from the use if crossing
horizontal and vertical bars to select contacts
on the cross-point.
 Step-by-Step and Crossbar Switching
Systems use electro-mechanical components
for both switching matrix and control
elements.

 Time Division Switching
The need for time division switching arises from the fact that
digital signals are often carrying multiple individual circuits, or
channels, in appropriate timeslots (TS).

 Time-division multiplexing (TDM) involves
dividing the carrier into two (or more)
channels based on time slices, i.e., the
common channel is allotted to several
different signals, one at a time, in alternating
time slots. Each individual data stream is
reassembled at the receiving end based on
the timing.

Timeslot Interchanging
In such systems, when two different multiplexed
channels are interconnected together through the
switch matrix a virtual circuit is established.
This is done by interchanging timeslots, each of
which maintain partial contents of a particular
channel.
(TSI) (Stallings, 1999) .

 Time-Space-Time Architecture
Note that the second time switch stage is necessary to
ensure that multiple timeslots in one incoming stream are
not superimposed or blocked. Having more stages can
further improve the switch performance.

Utilizes both
 time-division switch capability:
 to shift channels between timeslots, and
 space-division switching capability:
 to enable a different physical outgoing line system
to be selected.

Frequency and Wavelength-
Division Switching

Frequency Division
Multiplexing
 is an analog multiplexing technique that
combines analog signals.
 The transmission facility is divided into
channels by splitting the total frequency band
(of the carrier) into narrow bands, each
allotted to an individual signal (sub-
channeling).
 Analog signals are commonly multiplexed
using FDM.

Wavelength Division
Multiplexing

Wavelength Division
Multiplexing
 is an analog multiplexing technique to combine
optical signals
 WDM is an optical transmission technique in which
multiple streams of data are transmitted over a
single optical fiber as light rays of different
wavelengths. It exploits the fact that light of
different wavelengths does not interfere.
 WDM allows to simultaneous transmission of
different data formats (e.g., IP, SONET, ATM) at
different rates as each channel is demultiplexed at
the end of the transmission back into the original
source.

Multiplexing
 Multiplexing means sending multiple signals
(each with a given transmission capacity
requirement) on a carrier (with large
transmission capacity) at the same time as a
single, complex signal and then recovering
the separate signals at the receiving end.

Advantages of Circuit
Switching
 Guaranteed Bandwidth
 The communication performance in Circuit
Switching is predictable and there will be no "best-
effort" delivery with no real guarantees.
 SimpleAbstraction
 Circuit Switching is a reliable communication
channel between hosts and one would not have to
worry about lost or out-of-order packets.

 Simple Forwarding
 The forwarding in Circuit Switching is based on
time slot or frequency and one would not need to
inspect a packet header.
 Low per-packet overhead
 There will be no IP (andTCP/UDP) header on each
packet in Circuit Switching.

Pitfalls in Circuit
Switching
 Wasted bandwidth
 Since most traffic occurs in bursts, in Circuit Switching
this may leads to idle connection during silent period.
Because it is unable to achieve gains from statistical
multiplexing that relies in identifying, predicting and
allocating more time for the generally more active
paths.
 Blocked Connections
 When resources are not sufficient, the connection will
refuse to be connected and thus, Circuit Switching is
unable to offer "okay" service to everybody.

 Connection Set-up Delay
 There will be no communication until the
connection is set up. Plus, in Circuit Switching, it is
unable to avoid extra latency for small data
transfers.
 Network State
 The network nodes in Circuit Switching must store
per-connection information and it is unable to
avoid per-connection storage and state.

Sources:
 http://searchnetworking.techtarget.com/definition/circuit-switched
 http://www.tcpipguide.com/free/t_WhatIsNetworking.htm
 http://computer.howstuffworks.com/ip-telephony2.htm
 http://en.wikibooks.org/wiki/A_Bit_History_of_Internet/Chapter_2_:_Circuit_switching_vs
_packet_switching
 http://www.erg.abdn.ac.uk/~gorry/eg3567/intro-pages/cs.html
 http://www.aafrin.com/2011/05/12/example-circuit-switching-vs-packet-switching/
 http://www.computerworld.com/s/article/41904/Packet_Switched_vs._Circuit_Switched_N
etworks
 http://en.wikipedia.org/wiki/Circuit_switching
 Stallings, William “Data and Computer Communications”,Chapter 10: Circuit Switching and
Packet Switching, Eight Edition.
 http://voip.about.com/od/voipbasics/u/UsingVoIPUP.htm
 http://www.highteck.net/EN/Basic/Internetworking.html
 http://www.pcmag.com/encyclopedia/term/39698/circuit-switching
 “Circuit Switching”, Dr. Farid Farahmand and Dr. Qiong (Jo) Zhang, Central Connecticut
State University and Arizona State University atWest Campus

Circuit Switching

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