taxonomy of switched wide area networks - eecs.yorku.ca · telecommunication. systems....

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Circuit vs. Packet Switching Taxonomy of Switched Wide Area Networks intelligent core dumb nodes Datagram Networks (e.g. the Internet) WAN Telecommunication Systems Circuit-Switched Networks (e.g. telephone networks) Packet-Switched Networks connection-oriented service (TCP) connectionless service (UDP) dumb core intelligent nodes 1

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Page 1: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Circuit vs. Packet Switching

Taxonomy of Switched Wide Area Networks

intelligent coredumb nodes

Virtual Circuit(e.g. ATM)

Datagram Networks(e.g. the Internet)

connectionless connection-oriented

WANTelecommunication

Systems

Circuit-SwitchedNetworks

(e.g. telephone networks)

Packet-SwitchedNetworks

connection-orientedservice (TCP)

connectionlessservice (UDP)

dumb coreintelligent nodes

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Page 2: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Circuit vs. Packet Switching (cont.)Network Core – mesh of routers/switches that interconnect end systems

• two fundamental approaches to building a WAN core:

(1) circuit switching (example: telephone networks)• a sequence of links (communication path) between

two communicating nodes is determined ahead ofthe actual communication

• on each physical link, a channel is dedicated to the connection

• data is sent as a stream of bits through the network

(2) packet switching (example: the Internet)• data is sent through network in short blocks – packets• network links are dynamically shared by many packets;

each packet uses full link bandwidth

LAN 1

LAN 2WAN Core Network

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Page 3: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Circuit vs. Packet Switching (cont.)

Circuit-SwitchedNetworks

(e.g. telephone networks)

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Page 4: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Circuit vs. Packet Switching (cont.)

Packet Switching:Datagram Networks(e.g. the Internet)

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Page 5: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Communication via – involves three phases:Circuit Switching

Circuit Switching

(1) circuit establishment• before any data is transmitted, an end-to-end circuit

must be established, i.e. network resources on path/links between end-devices must be reserved

(2) data transfer• data transmission and signaling may each be digital

or analog

(3) circuit disconnect• after some period of data transfer, the connection

is terminated, by action of one of two stations, anddedicated resources are released

Vancouver

Toronto

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Page 6: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Circuit Switching (cont.)

Multiplexing inCircuit-SwitchedNetworks

– each link can be shared among (up to) n “circuits” ⇒each circuit gets a fraction 1/n of the link’s bandwidth• multiplexing = set of techniques that allows simultaneous

transmission of multiple signals across a single data link

• frequency division multiplexing (FDM) = each circuitcontinuously gets a fraction of the link’s bandwidth

• time division multiplexing (TDM) = each circuit gets allof the bandwidth periodically during brief intervals of time

frequency

frequency

FDM

time

time

TDM

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Page 7: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Circuit Switching (cont.)

Advantages of Circuit Switching

• inefficient use of capacity – channel capacity isdedicated for the duration of a connection, even ifno data is being transferred(example: silent periods in a phone call)

• circuit establishment delay – circuit establishmentintroduces ‘initial delay’

• network complexity – end-to-end circuit establishmentand end-to-end bandwidth allocation is complicatedand requires complex signaling software to coordinateoperation of switches

Disadvantages of Circuit Switching

• guaranteed Quality of Service – data are transmittedat fixed (guaranteed) rate; delay at nodes is negligible

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Page 8: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Packet Switching

Communication viaPacket Switching

(1) message segmentation• longer message is broken up into series of packets• packets contain user’s data + control data• control data (header) contains information that network

requires to route the packet

(2) data transfer• intermediate nodes perform following operations:

(a) receive entire packet(b) determine next node and link on route(c) queue packet to go out on that link

• when link is available, packet is transmitted to nextnode

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Page 9: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Further Details ofPacket Switching

– each packet is treated independently withno reference to packets that have gonebefore!

• each packet contains the full (IP) addressits destination as well as its source

• each packet switch has a forwarding (routing)table that maps destination addresses to an output link

• when packet arrives at a packet switch, theswitch examines packet’s destination addressand chooses the next node on packet’s pathbased on current traffic, line failure, etc.

• packets with the same destination addressdo not necessarily follow the same route⇒ packets may arrive out of sequence atthe destination !

• if packets arrive out of order, resequencing must be performed at the destination

Packet Switching (cont.) 9

Page 10: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Packet Switching (cont.)Main Principle ofPacket Switching

• statistical multiplexing () on-demand rather thanpre-allocated sharing of resources – link capacity is sharedon packet-to-packet basis only among those users whohave packets that need to be transmitted over the link(1) router buffers packets and arranges them in a queue(2) as the transmission line becomes available, packets

are transmitted one by one …

• store-and-forward () switch must receive entire packetbefore it can begin to transmit the first bit of the packet ontothe outbound link

A

B statistically multiplexed packets: packets are interleaved

based on the statistics of the senders

queue of packets waiting for output

Bandwidth division into “pieces”Dedicated allocationResource reservation

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Page 11: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Packet Switching (cont.)

Example [ circuit switching vs. packet switching ]

• N=35 users share a 1 Mbps link• each user generates 100kbps when “active”• each user is active 10% of time

How many users can be supported with circuitand how many with packet switching?

Circuit SwitchingWith circuit switching, 100kbps must be reserved for each user at all times. Hence, theoutput link can support 1Mbps/100kbps = 10 simultaneous users.

Packet Switching• 10 or fewer simultaneously active users ⇒ aggregate rate ≤ 1 Mbps ⇒ users’

packets flow through output link without delay, as in case of circuit switching

• more than 10 simultaneously active users ⇒ aggregate rate exceeds output capacity

With 35 users, probability of 10 or less simultaneously active users = 0.9996.Thus, packet switching can support all 35 users with virtually no delay!

N users 1 Mbps link

100 kbps

100 kbps

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Page 12: Taxonomy of Switched Wide Area Networks - eecs.yorku.ca · Telecommunication. Systems. Circuit-Switched. Networks ... Packet Switching: Datagram Networks ... Packet Switching (1)

Advantages of Packet Switching

• transmission delay – each time a packet passes througha packet-switching node, it incurs a delay not present incircuit switching = the time it takes to absorb the packetinto an internal buffer

• variable delay – each node introduces additional variabledelay due to processing and queueing

• overhead – to route packets through a packet-switchingnetwork, overhead information including the address ofdestination and/or sequence information must be added toeach packet

Disadvantages of Packet Switching

• greater line efficiency – node-to-node link dynamicallyshared by many packets / connections

• data rate conversion – two stations of different datarates can exchange packets, because each connects toits node at its proper data rate ⇒ nodes act as buffers

• no blocked calls – packets are accepted even underheavy traffic, but delivery delay increases

Packet Switching (cont.) 12