Data and Computer CommunicationsNinth Editionby William StallingsChapter 10 Circuit Switching and Packet Switching

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Switched Communications Networksswitching nodes provide a switching facility that move data between nodesstations devices attached to the networknodes switching devices that provide communicationconnected by transmission linksdedicated point-to-pointusually multiplexed using either FDM or TDM

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Switched Network

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Communication Networks

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Circuit Switchinguses a dedicated path between two stationscan be inefficientchannel capacity dedicated for duration of connectionif no data, capacity wastedset up (connection) takes timeonce connected, transfer is transparent

has three phases

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Public Telecommunications Networkexamples of circuit switching network:

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Public Circuit Switched Network

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Circuit Establishment

*Subscribers connect directly to an end office, which switches traffic between subscribers and between a subscriber and other exchanges. The other exchanges are responsible for routing and switching traffic between end offices. This distinction is shown here in Stallings DCC9e Figure 10.3. To connect two subscribers attached to the same end office, a circuit is set up between them. If two subscribers connect to different end offices, a circuit between them consists of a chain of circuits through one or more intermediate offices. In the figure, a connection is established between lines a and b by simply setting up the connection through the end office. The connection between c and d is more complex. In c's end office, a connection is established between line c and one channel on a TDM trunk to the intermediate switch. In the intermediate switch, that channel is connected to a channel on a TDM trunk to d's end office. In that end office, the channel is connected to line d.

Circuit-Switching TechnologyDriven by applications that handle voice trafficKey requirement is no transmission delay and no variation in delayEfficient for analog transmission of voice signalsInefficient for digital transmissionTransparentonce a circuit is established it appears as a direct connection; no special logic is needed

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Circuit-Switching Concepts

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Circuit Switch Elements

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Blocking or Non-blockingblocking networkmay be unable to connect stations because all paths are in useused on voice systems because it is expected for phone calls to be of short duration and that only a fraction of the phones will be engaged at any one time

non-blocking networkpermits all stations to connect at oncegrants all possible connection requests as long as the called party is freewhen using data connections terminals can be continuously connected for long periods of time so nonblocking configurations are required

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Space Division Switchingoriginally developed for analog, space division switching has been carried over into the digital realmsignal paths are physically separate from one anotherpath is dedicated solely to transfer signalsbasic building block of switch is a metallic crosspoint or semiconductor gate

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Space Division Switch

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3 Stage Space Division Switch

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Time Division Switchingmodern digital systems use intelligent control of space & time division elementsuse digital time division techniques to set up and maintain virtual circuitspartition low speed bit stream into pieces that share higher speed streamindividual pieces manipulated by control logic to flow from input to output

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Softswitch Architecturelatest trend in circuit-switching technologycomputer running specialized software that turns it into a smart phone switchcosts less and provides more functionalityMedia gateway (MG) physical switchingMedia gateway controller (MGC) call processing logic

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Traditional Circuit Switching

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Softswitch

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Packet Switchingcircuit switching was designed for voicepacket switching was designed for datatransmitted in small packetspackets contains user data and control infouser data may be part of a larger messagecontrol information includes routing (addressing)packets are received, stored briefly (buffered) and passed on to the next node

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Packet Switching

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Advantagesline efficiencysingle link shared by many packets over timepackets queued and transmitted as fast as possibledata rate conversionstations connects to local node at own speednodes buffer data if required to equalize ratespackets accepted even when network is busypriorities can be used

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Switching Techniquesstation breaks long message into packetspackets sent one at a time to the networkpackets can be handled in two ways:datagram

each packet is treated independently with no reference to previous packetsvirtual circuit

a preplanned route is established before any packets are sent

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DatagramDiagram

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VirtualCircuitDiagram

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Virtual Circuits vs. Datagramvirtual circuitsnetwork can provide sequencing and error controlpackets are forwarded more quicklyless reliabledatagramno call setup phasemore flexiblemore reliable

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There is a significant relationship between

Packet Size

and transmission time.

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Event Timing

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Comparison of Communication Switching Techniques

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External Network InterfaceITU-T standard for interface between host and packet switched networkalmost universal on packet switched networks and packet switching in ISDNdefines three layersPhysicalLinkPacket

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X.25 Use of Virtual Circuits

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Circuit vs. Packet Switchingperformance depends on various delayspropagation delay

time it takes a signal to propagate between nodestransmission time

time it takes for a transmitter to send a block of datanode delay

time it takes for a node to perform processing as it switches datarange of other characteristics, including:transparencyamount of overhead

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Summaryswitched communications networksstations / nodescircuit switching networkscircuit switching conceptsdigital switch, network interfacing, control unitsoftswitch architecturepacket switching principles

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Data and Computer CommunicationsNinth Editionby William StallingsChapter 12 Routing in Switched Data Networks

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Routing in Packet Switching Networkskey design issue for (packet) switched networksselect route across network between end nodescharacteristics required:correctnesssimplicityrobustnessstabilityfairnessoptimalityefficiency

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Performance Criteriaused for selection of routesimplest is to choose minimum hopcan be generalized as least cost routingbecause least cost is more flexible it is more common than minimum hop

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Elements of Routing Techniques for Packet-Switching Networks

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Example of Packet Switched Network

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Decision Time and Place

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Network Information Source and Update Timingrouting decisions usually based on knowledge of network, traffic load, and link costdistributed routing

using local knowledge, information from adjacent nodes, information from all nodes on a potential routecentral routing

collect information from all nodes

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Routing Strategies - Fixed Routinguse a single permanent route for each source to destination pair of nodesdetermined using a least cost algorithmroute is fixeduntil a change in network topologybased on expected traffic or capacityadvantage is simplicitydisadvantage is lack of flexibilitydoes not react to network failure or congestion

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Fixed RoutingTables

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Routing Strategies - Floodingpacket sent by node to every neighboreventually multiple copies arrive at destinationno network information requiredeach packet is uniquely numbered so duplicates can be discardedneed to limit incessant retransmission of packetsnodes can remember identity of packets retransmittedcan include a hop count in packets

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Flooding Example

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Properties of Flooding

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Routing Strategies - Random Routingsimplicity of flooding with much less traffic loadnode selects one outgoing path for retransmission of incoming packetselection can be random or round robina refinement is to select outgoing path based on probability calculationno network information neededrandom route is typically neither least cost nor minimum hop

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Routing Strategies - Adaptive Routingused by almost all packet switching networksrouting decisions change as conditions on the network change due to failure or congestionrequires information about networkDisadvantages:decisions more complextradeoff between quality of network information and overheadreacting too quickly can cause oscillationreacting too slowly means information may be irrelevant

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Adaptive Routing Advantages

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Classification of Adaptive Routing Strategieson the basis of information source

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Isolated Adaptive Routing

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ARPANET Routing Strategies1st Generation

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ARPANET Routing Strategies2nd Generation

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Oscillation

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ARPANET Routing Strategies3rd Generation

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ARPANET Delay Metrics

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Least Cost Algorithms

alternatives: Dijkstra or Bellman-Ford algorithms

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Example of Least-Cost Routing Algorithms (using Figure 12.1)

Stallings DCC9e Table 12.2*

Dijkstras Algorithmfinds shortest paths from given source nodes to all other nodesdevelop paths in order of increasing path lengthalgorithm runs in stageseach time adding node with next shortest pathalgorithm terminates when all nodes have been added to T

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Dijkstras Algorithm Method

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Dijkstras Algorithm Example

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Dijkstras Algorithm Example

Iter TL(2)PathL(3)PathL(4)PathL(5)PathL(6)Path1{1}21251-3114 --2{1,4}21241-4-311421-45-3{1, 2, 4}21241-4-311421-45-4{1, 2, 4, 5}21231-4-5311421-4541-4-565{1, 2, 3, 4, 5}21231-4-5311421-4541-4-566{1, 2, 3, 4, 5, 6}21-231-4-5-311-421-4541-4-5-6

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Bellman-Ford Algorithmfind shortest paths from given node subject to constraint that paths contain at most one linkfind the shortest paths with a constraint of paths of at most two linksand so on

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Bellman-Ford Algorithm

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Example of Bellman-Ford Algorithm

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Results of Bellman-Ford Example

hLh(2)PathLh(3)PathLh(4)PathLh(5)PathLh(6)Path

0 -----121-251-311-4--221-241-4-311-421-4-5101-3-6321-231-4-5-311-421-4-541-4-5-6421-231-4-5-311-421-4-541-4-5-6

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Comparison

Bellman-Fordcalculation for node n needs link cost to neighboring nodes plus total cost to each neighbor from seach node can maintain set of costs and paths for every other nodecan exchange information with direct neighborscan update costs and paths based on information from neighbors and knowledge of link costs

Dijkstraeach node needs complete topologymust know link costs of all links in networkmust exchange information with all other nodes

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Evaluation

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Summaryrouting in packet-switched networksrouting strategiesfixed, flooding, random, adaptiveARPANET examplesleast-cost algorithmsDijkstra, Bellman-Ford

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*Subscribers connect directly to an end office, which switches traffic between subscribers and between a subscriber and other exchanges. The other exchanges are responsible for routing and switching traffic between end offices. This distinction is shown here in Stallings DCC9e Figure 10.3. To connect two subscribers attached to the same end office, a circuit is set up between them. If two subscribers connect to different end offices, a circuit between them consists of a chain of circuits through one or more intermediate offices. In the figure, a connection is established between lines a and b by simply setting up the connection through the end office. The connection between c and d is more complex. In c's end office, a connection is established between line c and one channel on a TDM trunk to the intermediate switch. In the intermediate switch, that channel is connected to a channel on a TDM trunk to d's end office. In that end office, the channel is connected to line d.*

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Stallings DCC9e Table 12.2**

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