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Circuit Switching
Dr. Indranil Sen Gupta
Circuit Switching
Slide 1
Introduction
• For transmission of data beyond a LAN, communication is
typically achieved by transmitting data from source to
destination through a number of intermediate switching
nodes.
– The switching nodes are not concerned with the contents of data.
• Their purpose is to provide a switching facility that will move the data
from node to node until they reach the destination.
– The end devices that wish to communicate are called stations.
– The switching devices are called nodes.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 2
Simple Switching Network
Dr. Indranil Sen Gupta
Circuit Switching
Slide 3
Some Observations
• Some nodes connect only to other nodes.
– Their sole task is the internal switching of data.
• Inter-node links are usually multiplexed, either using FDM
or TDM.
• Multiple paths between a source-destination pair enhances
the network reliability.
• Two different techniques used in wide-area switching
networks.
– Circuit switching and packet switching.
– Differ in the way the nodes switch information from one link to
another.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 4
Circuit Switching Networks
• Communication via circuit switching implies that there is a
dedicated communication path between two stations.
– The path is a connected sequence of links between network nodes.
– On each physical link, a logical channel is dedicated to the
connection.
• Communication via circuit switching involves three
phases:
– Circuit establishment
– Data transfer
– Circuit disconnect
Dr. Indranil Sen Gupta
Circuit Switching
Slide 5
Contd.
• Circuit establishment
– An end-to-end circuit must be established before any signal can be
transmitted.
– Some segments of the circuit may be a dedicated link, while some other
segments may be shared.
• Data transfer
– Information is transmitted from the source to the destination.
– The data may be analog or digital, depending on the nature of the network.
– The connection is generally full-duplex.
• Circuit disconnect
– The connection is terminated at the end of the data transfer.
– Signals must be propagated to deallocate the dedicated resources.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 6
Contd.
• Circuit switching can be inefficient.
– Channel capacity is dedicated for the duration of the connection,
even if no data is being transferred.
• For voice connection, utilization is typically high.
• For terminal-computer connection, the capacity may be idle during
most of the time of the connection.
– Performance-wise, there is a delay prior to signal transfer.
• Call establishment phase is there.
• Once the circuit is established, the network is transparent to the users.
• Information is transmitted at a fixed rate with no delay other than that
required for propagation through the communication medium.
• Originally developed for handling voice traffic, but is now
also used for data traffic.
– Best known example is the Public Telephone Network.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 7
Public Telephone Network
Dr. Indranil Sen Gupta
Circuit Switching
Slide 8
Switching Concepts
• We first examine the operation of a single circuit switched
node.
– Collection of stations attached to a central switching unit.
– The central switch establishes a dedicated path between any two
devices that wish to communicate.
• Major elements of a single-node network.
– Digital switch: provides a transparent (full-duplex) signal path
between any pair of attached devices.
– Network interface: represents the functions and hardware needed
to connect digital devices to the network (like telephones).
– Control unit: establishes, maintains, and tears down a connection.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 9
Elements of a circuit-switch node
Dr. Indranil Sen Gupta
Circuit Switching
Slide 10
Blocking and Non-blocking Networks
• An important characteristic of a circuit-switch node is
whether it is blocking or non-blocking.
– A blocking network is one which may be unable to connect two
stations because all possible paths between them are already in use.
– A non-blocking network permits all stations to be connected (in
pairs) at once and grants all possible connection requests as long as
the called party is free.
• For a network which supports only voice traffic, a blocking
configuration may be acceptable, since most phone calls
are of short duration.
• For data applications, where a connection may remain
active for hours, non-blocking configuration is desirable.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 11
Space Division Switching
• Originally developed for the analog environment, and has
been carried over to the digital domain.
• In a space division switch, the signal paths are physically
separate from one another (divided in space).
• The basic building block of the switch is a metallic
crosspoint or semiconductor gate that can be enabled or
disabled by a control unit.
– Essentially a crossbar matrix.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 12
Space-division Switch: block diagram
Dr. Indranil Sen Gupta
Circuit Switching
Slide 13
Limitations of crossbar switch
• The number of crosspoints grows with the square of the
number of attached stations.
– Costly for a large switch.
• The failure of a crosspoint prevents connection between
the two devices whose lines intersect at that crosspoint.
• The crosspoints are inefficiently utilized.
– Only a small fraction are engaged even if all of the attached
devices are active.
• Solution is to build multistage space division switches.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 14
3-stage Space Division Switch
Dr. Indranil Sen Gupta
Circuit Switching
Slide 15
Multistage Switches
• By splitting the crossbar switch into small chunks and
interconnecting them, it is possible to build multistage
switches with many fewer crosspoints.
• In the previous example:
– The number of crosspoints needed goes down from 100 to 48.
– There is more than one path through the network to connect two
endpoints, thereby increasing reliability
• Multistage switches may lead to blocking.
– The problem may be tackled by increasing the number or size of
the intermediate switches, which also increases the cost.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 16
Time Division Switching
• Both voice and data can be transmitted via digital signals.
• All modern circuit switches use digital time-division techniques for
establishing and maintaining circuits.
– Synchronous TDM allows multiple low-speed bit streams to share a highspeed line.
– A set of inputs is sampled in turn. The samples are organized serially into
slots (channels) to form a recurring frame of slots.
– During successive time slots, different I/O pairings are enabled, allowing a
number of connections to be carried over the shared bus.
• To keep up with the input lines, the data rate on the bus must be high enough
so that the slots recur sufficiently frequently.
• For 100 full-duplex lines at 19200 bps, the data rate on the bus must be greater
than 1.92 Mbps.
– The source-destination pairs corresponding to all active connections are
stored in the control memory.
• Thus the slots need not specify the source & destination addresses.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 17
Routing in Circuit Switched Networks
• In large circuit-switched networks, connections often require a path
through more than one switch.
• Routing function in public telecommunication networks has been
traditionally quite simple.
– Switches are organized as a tree structure.
– To add some resilience to the network, additional high-usage trunks are
added that cut across the tree structure to connect exchanges with high
volumes of traffic between them.
– Usually a static approach.
• To cope with growing demands, all providers today use a dynamic
approach.
– Routing decisions are influenced by current traffic conditions.
– Switching nodes have a peer relationship with each other rather than a
hierarchical one.
– Routing is more complex and more flexible.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 18
Method 1: Alternate Routing
• The possible routes to be used between two end offices are
predetermined.
– It is the responsibility of the originating switch to select the
appropriate route for each call.
• In practice, usually a different set of pre-planned routes is
used for different time periods.
– Takes advantage of different traffic patterns in different time zones
and different times of day.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 19
Alternate Routing Example
Dr. Indranil Sen Gupta
Circuit Switching
Slide 20
Method 2: Adaptive Routing
• This is designed to enable switches to react to changing
traffic patterns on the network.
– Greater management overhead (switches must exchange
information).
– Has the potential for more effectively optimizing the use of
network resources.
Dr. Indranil Sen Gupta
Circuit Switching
Slide 21