data and computer communications circuit switching and packet switching
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Data and Computer Data and Computer CommunicationsCommunications
Circuit SwitchingCircuit Switching and Packet and Packet SwitchingSwitching
NodesNodes
a collection of nodes and connections is a a collection of nodes and connections is a communications network communications network
network is usually partially connectednetwork is usually partially connected some redundant connections are desirable some redundant connections are desirable
have two different switching technologieshave two different switching technologies circuit switchingcircuit switching packet switchingpacket switching
Circuit SwitchingCircuit Switching uses a uses a dedicated path dedicated path between two stationsbetween two stations has three phaseshas three phases
establishestablish transfertransfer disconnectdisconnect
inefficientinefficient channel capacity dedicated for duration of connectionchannel capacity dedicated for duration of connection if no data, capacity wastedif no data, capacity wasted
set up (connection) takes timeset up (connection) takes time once connected, once connected, transfer is transparenttransfer is transparent
Blocking or Non-blockingBlocking or Non-blocking
blocking networkblocking network may be unable to connect stations because may be unable to connect stations because
all paths are in useall paths are in use used on voice systemsused on voice systems
non-blocking networknon-blocking network permits all stations to connect at oncepermits all stations to connect at once used for some data connectionsused for some data connections
Time Division SwitchingTime Division Switching
modern digital systems use intelligent modern digital systems use intelligent control of space & time division elementscontrol of space & time division elements
use digital time division techniques to set use digital time division techniques to set up and maintain virtual circuitsup and maintain virtual circuits
partition low speed bit stream into pieces partition low speed bit stream into pieces that share higher speed streamthat share higher speed stream
individual pieces manipulated by control individual pieces manipulated by control logic to flow from input to outputlogic to flow from input to output
Time-division Switching (cont)Time-division Switching (cont)
Control of a TDM bus switchControl of a TDM bus switch
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X
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X
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X1 32 54 63 15 26 4
Controllogic
Controlmemory
The latest trend in the development of circuit-switching The latest trend in the development of circuit-switching technology is generally referred to as the softswitch. In technology is generally referred to as the softswitch. In essence, a softswitch is a general-purpose computer running essence, a softswitch is a general-purpose computer running specialized software that turns it into a smart phone switch. specialized software that turns it into a smart phone switch. Softswitches cost significantly less than traditional circuit Softswitches cost significantly less than traditional circuit switches and can provide more functionality. In particular, in switches and can provide more functionality. In particular, in addition to handling the traditional circuit-switching functions, addition to handling the traditional circuit-switching functions, a softswitch can convert a stream of digitized voice bits into a softswitch can convert a stream of digitized voice bits into packets. This opens up a number of options for transmission, packets. This opens up a number of options for transmission, including the increasingly popular voice over IP (Internet including the increasingly popular voice over IP (Internet Protocol) approach.Protocol) approach.
In any telephone network switch, the most complex element is In any telephone network switch, the most complex element is the software that controls call processing. This software the software that controls call processing. This software performs call routing and implements call-processing logic for performs call routing and implements call-processing logic for hundreds of custom calling features. Typically, this software hundreds of custom calling features. Typically, this software runs on a proprietary processor that is integrated with the runs on a proprietary processor that is integrated with the physical circuit-switching hardware. A more flexible approach physical circuit-switching hardware. A more flexible approach is to physically separate the call processing function from the is to physically separate the call processing function from the hardware switching function. In softswitch terminology, the hardware switching function. In softswitch terminology, the physical switching function is performed by a physical switching function is performed by a media gatewaymedia gateway (MG) and the call processing logic resides in a (MG) and the call processing logic resides in a media media gateway controllergateway controller (MGC). (MGC).
Packet SwitchingPacket Switching
circuit switching circuit switching was designed forwas designed for voice voice packet switching packet switching was designed forwas designed for data data transmitted in small packetstransmitted in small packets packets contains user data and control infopackets contains user data and control info
user data may be part of a larger messageuser data may be part of a larger message control info includes routing (addressing) infocontrol info includes routing (addressing) info
packets are received, stored briefly (buffered) packets are received, stored briefly (buffered) and past on to the next nodeand past on to the next node
AdvantagesAdvantages
line efficiencyline efficiency single link shared by many packets over timesingle link shared by many packets over time packets queued and transmitted as fast as possiblepackets queued and transmitted as fast as possible
data rate conversiondata rate conversion stations connects to local node at own speedstations connects to local node at own speed nodes buffer data if required to equalize ratesnodes buffer data if required to equalize rates
packets accepted even when network is busypackets accepted even when network is busy prioritiespriorities can be used can be used
Switching TechniquesSwitching Techniques
station breaks long message into packetsstation breaks long message into packets packets sent one at a time to the networkpackets sent one at a time to the network packets can be handled in two wayspackets can be handled in two ways
datagramdatagram virtual circuitvirtual circuit
Virtual Circuits vs. DatagramVirtual Circuits vs. Datagram
virtual circuitsvirtual circuits network can provide sequencing and error network can provide sequencing and error
controlcontrol packets are forwarded more quicklypackets are forwarded more quickly less reliableless reliable
datagramdatagram no call setup phaseno call setup phase more flexiblemore flexible more reliablemore reliable
X.25X.25
ITU-T standard for interface between host ITU-T standard for interface between host and packet switched networkand packet switched network
almost universal on packet switched almost universal on packet switched networks and packet switching in ISDNnetworks and packet switching in ISDN
defines three layersdefines three layers PhysicalPhysical LinkLink PacketPacket
X.25 - PhysicalX.25 - Physical
interface between station node linkinterface between station node link two ends are distinct two ends are distinct
Data Terminal Equipment DTEData Terminal Equipment DTE (user (user equipment)equipment)
Data Circuit-terminating Equipment DCEData Circuit-terminating Equipment DCE (node)(node)
physical layer specification is X.21physical layer specification is X.21 can substitute alternative such as EIA-232can substitute alternative such as EIA-232
X.25 - LinkX.25 - Link
Link Access Protocol Balanced (LAPB)Link Access Protocol Balanced (LAPB) Subset of HDLCSubset of HDLC see chapter 7see chapter 7
provides reliable transfer of data over linkprovides reliable transfer of data over link sending as a sequence of framessending as a sequence of frames
X.25 - PacketX.25 - Packet
provides a logical connections (provides a logical connections (virtual virtual circuitcircuit) between subscribers) between subscribers
all data in this connection form a single all data in this connection form a single stream between the end stationsstream between the end stations
established on demandestablished on demand termed termed external virtual circuitsexternal virtual circuits
Frame RelayFrame Relay designed to eliminate most X.25 overheaddesigned to eliminate most X.25 overhead has large installed basehas large installed base key differenceskey differences::
call control carried in separate logical connectioncall control carried in separate logical connection multiplexing and switching at layer 2multiplexing and switching at layer 2 no hop by hop error or flow controlno hop by hop error or flow control hence end to end flow and error control (if used) are hence end to end flow and error control (if used) are
done by higher layerdone by higher layer
a single user data frame is sent from source to a single user data frame is sent from source to destination and destination and higher layer ACKhigher layer ACK sent back sent back