MURALI PPT
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Transcript of MURALI PPT
8/7/2019 MURALI PPT
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SWITCHING CONCEPTS
BY
N.V.D.MURALIDHAR
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SWITCHES
Problem
-Each user can potentially call any other user
-Cant have direct line
-Switches establish temporary circuits
-Switching systems come in two partsSwitch and switch controller
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Routing and Switching
Many connections will need path through more
Than one switch
Static routing uses same approach all the time
Dynamic routing allow for changes in routing
Depending on traffic
Switching is local connectivity between input
And output line
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BasicConcepts and Terminology
-Transfer medium- buses that deliver cells
-Queues-Memory locetions that temporarilyStore cells
-Switch element -building block for switch
-Switch fabric -the shape of the switches
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BasicConcepts and Terminology
Blocking:-A network is unable to connect stations
because all paths are in use
-A blocking network allows this-Used on voice systems
- short duration calls
Non Blocking-Permits all stations to connect (in pairs)At once
-Used for some data connections
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SWITCHING
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-When there are many devices, it is necessary to develop suitablemechanism for communication between any two devices.
- One alternative is to establish point-to-point communicationbetween each pair of devices using mesh topology.
-However, mesh topology is impractical f or large number of device
because the number of links increases exponentially (n(n-1)/2 ) with the number of devices.
-A better alternative is to use switching techniques leading to
switched communication network.
-In the switched network methodology, the network consists
of a set of interconnected nodes, among which inf ormation is
transmitted from source to destination via different routes,
which is controlled by the switching mechanism.
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Switching Networks
Long distance transmission is typically doneover a network of switched nodes
Nodes not concerned with content of data
End devices are stations Computer, terminal, phone, etc.
A collection of nodes and connections is a
communications network Data routed by being switched from node to
node
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Nodes
Nodes may connect to other nodes only, or to
stations and other nodes
Node to node links usually multiplexed Network is usually partially connected
Some redundant connections are desirable forreliability
Two different switching technologies
Circuit switching
Packet switching
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Simple Switched Network
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The switching perf ormed by different
nodes can be categorized into theFollowing three types:
*CircuitSwitching
*PacketSwitching
*MessageSwitching
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Circuit-Switching
Circuit switching creates a direct physicalconnection between two devices such as
phones or computers.
We can use switches to reduce the number
and length of links.
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A circuit switch
A circuit switch is a device with n inputs and m outputsthat creates a temporary connection between an inputlink and an output link.
The number of inputs does not have to match the
number of outputs.
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A folded switch
An n-by-n folded switch can connect n lines in full-duplex
mode. For example, it can connect n telephones in such away that each phone can be connected to every otherphone.
Circuit switching uses space-division switch [paths in the
circuit are separated from each other spatially] or time-division switch.
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Circuit switching uses threetechnologies:
Space-division switching,
Time-division switching
or
combination of both
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Space division switching
-Developed for analog environment
-Separate physical paths
*Cross bar switch- Number of cross points grows as square of number of stations-Loss of cross point prevents connection
Inefficient use of -All stations connected only a few cross pointsin use
-Non Blocking
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Crossbar matrix
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Crossbar switches
-In the simplest form n input lines and n output lines feed to form n*nelectronic switches
-If all the lines are full duplex , half the number of switches are required
-A part from the number of number of I/O pins are also a problem
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Multistage switch
Multistage switch combines crossbar switches in several stages. Design of a multistage switch depends on the number of stages
and the number of switches required (or desired) in each stage.
Normally, the middle stages have fewer switches than do the
first and last stages.
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MULTIPLE Switching paths
Multiple paths are available in multistage switches.
Blocking refers to times when two inputs are looking forthe same output. The output port is blocked.
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Figure 8.7 Time-division multiplexing, without and with a time-slot interchange
Time-division switching uses time-division multiplexing to
achieve switching. Two methods used are:
Time-slot interchange (TSI) changes the order of the slots based onthe desired connection.
TDMbus
Time-Division Switch
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Time-slot interchange
TSI consists of random access memory (RAM) with several
memory locations. The size of each location is the same asthe size of a single time slot.
The number of locations is the same as the number of inputs.
The RAM fills up with incoming data from time slots in the
order received. Slots are then sent out in an order based onthe decisions of a control unit.
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Packet Switching Principles
Circuit switching designed for voice
Resources dedicated to a particular call
Much of the time a data connection is idle Data rate is fixed
Both ends must operate at the same rate
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Basic Operation
Data transmitted in small packets Typically 1000 octets
Longer messages split into series of packets
Each packet contains a portion of user data plussome control info
Control info
Routing (addressing) info Packets are received, stored briefly (buffered)
and past on to the next node Store and forward
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Use of Packets
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Advantages
Line efficiency
Single node to node link can be shared by many packets overtime
Packets queued and transmitted as fast as possible Data rate conversion
Each station connects to the local node at its own speed
Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy Delivery may slow down
Priorities can be used
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Switching Technique
Station breaks long message into packets
Packets sent one at a time to the network
Packets handled in two ways Datagram
Virtual circuit
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Datagram
Each packet treated independently
Packets can take any practical route
Packets may arrive out of order Packets may go missing
Up to receiver to re-order packets and
recover from missing packets
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Datagram
Diagram
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Virtual Circuit
Preplanned route established before anypackets sent
Call request and call accept packets establishconnection (handshake)
Each packet contains a virtual circuitidentifier instead of destination address
No routing decisions required for each packet Clear request to drop circuit
Not a dedicated path
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Virtual
Circuit
Diagram
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Virtual Circuits v Datagram
Virtual circuits Network can provide sequencing and error control Packets are forwarded more quickly
No routing decisions to make Less reliable
Loss of a node looses all circuits through that node
Datagram No call setup phase
Better if few packets More flexible
Routing can be used to avoid congested parts of thenetwork