1Network Taxonomy,Network Taxonomy,Packet vs. Circuit SwitchingPacket vs. Circuit Switching

Required reading:Kurose 1.1, 1.2, 1.3

CSE 3214, Winter 2011Instructor: N. Vlajic

2Network Basics

Network Network set of devices (i.e. nodesnodes) connected by communication linkscommunication links network uses a combination of both hardware and softwarecombination of both hardware and software to send

data from one location to another

Nodes Nodes desktop PC, UNIX-workstation, printer, PDA, cell phone, sensoror any other device capable of sending and/or receiving data

Communication LinksCommunication Links direct communication pathway between two or more devices communication links made of different physical

media transmit data at different rates

3Communication Links

Transmission MediaTransmission Media can be divided into two categories: unguided (wireless) mediaunguided (wireless) media signal is broadcast

openly, transmission and reception are achieved bymeans of an antenna easy to set up, but low easy to set up, but low security and prone to interferencesecurity and prone to interference

guided mediaguided media characteristics and quality of transmitted signal are constrained by physical limits of the medium typically provides higher data rates than unguided media

from

4Link ConfigurationLink Configuration pointpoint--toto--pointpoint: dedicated connection between 2 devices entire link capacity is reserved for the 2 devices

multipointmultipoint: channel capacity is shared between 3 or more devices

Network Topology Network Topology geometric representation of the relationship of all linksand linking devices to one another

Network Topologies

5Mesh TopologyMesh Topology every device has a dedicated point-to-point connectionto every other device fully connected mesh with n nodes has n(n-1)/2 links advantagesadvantages: 1) dedicated links no need for load balancing

2) ensured privacy and security only intendedrecipient sees data

3) robustness to link failure many 2-hop routes

disadvantagesdisadvantages: 1) complex installation every device must be connected to every other device

2) expensive hardware each device must have multiple I/O ports

Mesh topology is implemented mostly in backbone/core networks.Mesh topology is implemented mostly in backbone/core networks.

Network Topologies (cont.)

6Network Topologies (cont.)

Star TopologyStar Topology each device has a dedicated point-to-point link onlyto a central controller, so-called hubhub

no direct traffic between devices advantagesadvantages: 1) simpler and less expensive installation than in

mash topology each device needs only onelink and one I/O port

disadvantagesdisadvantages: 1) hub = single point of network failure2) possibly slower performance

7Bus TopologyBus Topology one long cable, so-called backbonebackbone, links all devicesin the network multipoint connection / link advantagesadvantages: 1) simple installation

2) less cabling than in mesh or star topologies one cable stretches through entire facility

disadvantagesdisadvantages: 1) backbone = single point of network failure2) collisions diminishing capacity

if two or more devices transmit simultaneouslytheir signals will interferecollision controlcollision control: MAC control, scheduling or channelization

Network Topologies (cont.)

8Ring TopologyRing Topology each device has a dedicated point-to-point connection only with the two devices on either side of it

signal is passed along the ring in one direction, from deviceto device, until it reaches its destination

advantagesadvantages: 1) fairness in access token-passing provideseach station with a turn to transmit

2) relatively easy to install and reconfigure eachdevice is linked only to its immediate neighbors

disadvantagesdisadvantages: 1) entire network will fail if there is a failure inany transmission link or in the mechanismthat relays the token

Network Topologies (cont.)

9Categories of Networks

Categories of NetworksCategories of Networks (1) Local Area Network Local Area Network (LAN)(LAN)(2) Metropolitan Area Network Metropolitan Area Network (MAN)(MAN)(3) Wide Area Network Wide Area Network (WAN)(WAN)

LAN LAN computer network concentrated in a smaller geographic area (d < 1 km), such as an office, building, or campus

LAN is owned by the same organization that owns the attached devices!!! LANs typically employ only one type of transmission medium (wired or

wireless), and provides low-delay, relatively error-free communication

internal data rates of LANs are more uniform and much greater than those of WANs (100 Mbps LANs are common, 1/10 Gbps LANs are available)

most common LAN topologies: bus, ring, star

10Categories of Networks (cont.)

MAN MAN a number of LANs connected into a larger network so that resources can be shared MAN extends over a larger geographic area (5 to 50 km), e.g. entire city MAN can be wholly owned and operated by a private company, or it

may include point-to-point links provided by a public company (e.g. local telephone company) to connect its remote sites

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WAN WAN computer network that extends over large geographic area (>100 km), such as a country, continent, or even the whole world can be thought of as a collection of LANs interconnected via core

network (set of switching stations)

Categories of Networks (cont.)

Packet SwitchesPacket Switches connecting devices whose purpose is to route (i.eswitch) data to their final destination(Routers)(Routers)

Core Network

12Circuit vs. Packet Switching

Taxonomy of Switched Wide Area NetworksTaxonomy of Switched Wide Area Networks

intelligent coredumb nodes

Virtual Circuit(e.g. ATMATM)

Datagram Networks(e.g. the Internetthe Internet)

connectionless connection-oriented

WANTelecommunication

Systems

Circuit-SwitchedNetworks

(e.g. telephone networkstelephone networks)Packet-Switched

Networks

connection-orientedservice (TCP)

connectionlessservice (UDP)

dumb coreintelligent nodes

13Circuit vs. Packet Switching (cont.)Network CoreNetwork Core mesh of routers/switches that interconnect end systems

two fundamental approaches to building network core:(1) circuit switchingcircuit switching (exampleexample: telephone networks)

a sequence of links (communication path) betweentwo communicating nodes is determined in advance

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

data is sent as a stream of bits through the network

(2) packet switchingpacket switching (exampleexample: 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

14Circuit vs. Packet Switching (cont.)

Circuit-SwitchedNetworks

(e.g. telephone networkstelephone networks)

15Circuit vs. Packet Switching (cont.)

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

Packet Switching:Virtual Circuit

(e.g. ATMATM)

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Communication viaCommunication via involves three phases:Circuit SwitchingCircuit Switching

Circuit Switching

(1) circuit establishmentcircuit 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 transferdata transfer data transmission and signaling may each be digital

or analog

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

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

Vancouver

Toronto

17Circuit Switching (cont.)

Multiplexing inMultiplexing inCircuitCircuit--SwitchedSwitchedNetworksNetworks

each link can be shared among (up to) n circuits each circuit gets a fraction 1/n of the links bandwidth

multiplexingmultiplexing = set of techniques that allows simultaneoustransmission of multiple signals across a single data link

frequency division multiplexing (FDM)frequency division multiplexing (FDM) = each circuitcontinuously gets a fraction of the links bandwidth

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

frequency

frequency

FDM

time

time

TDM

18Circuit Switching (cont.)

Advantages of Advantages of Circuit SwitchingCircuit Switching

inefficient use of capacityinefficient 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 delaycircuit establishment delay circuit establishmentintroduces initial delay

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

Disadvantages of Disadvantages of Circuit SwitchingCircuit Switching

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

19Packet Switching

Communication viaCommunication viaPacket SwitchingPacket Switching

(1) message segmentationmessage segmentation longer message is broken up into series of packets packets contain users data + control data control data, at a minimum, contains information that

network requires to route the packet

(2) data transferdata 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

20Packet Switching (cont.)

Main Principle ofMain Principle ofPacket SwitchingPacket Switching

statistical multiplexingstatistical 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

storestore--andand--forwardforward (/) 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 piecesDedicated allocationResource reservation

21Packet 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 SwitchingCircuit Switching

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

Packet SwitchingPacket Switching

10 or fewer simultaneously active users aggregate rate 1 Mbps userspackets flow through output link without delay, as in case of circuit switching

more than 10 simultaneously active users aggregate rate exceeds output capacityWith 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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Advantages of Advantages of Packet SwitchingPacket Switching

transmission delaytransmission 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 delayvariable delay each node introduces additional variabledelay due to processing and queueing

overhead 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 Disadvantages of Packet SwitchingPacket Switching

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

data rate conversiondata 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 callsno blocked calls packets are accepted even underheavy traffic, but delivery delay increases

Packet Switching (cont.)

23Packet Switching: Datagram Networks

Datagram Approach toDatagram Approach toPacket SwitchingPacket Switching(Internet)(Internet)

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

each packet (i.e. datagramdatagram) contains the fulladdress of its destination

each packet switch has a forwarding table thatmaps destination addresses to an output link

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

packets with the same destination addresspackets with the same destination addressdo not necessarily follow the same routedo not necessarily follow the same route packets may arrive out of sequence atpackets may arrive out of sequence atthe destinationthe destination !

if packets arrive out of order, resequencingmust be performed at the destination

24Packet Switching: Datagram Networks (cont.)

Example [ packet switching: datagram approach ]

data destination address

25Packet Switching: Datagram Networks (cont.)

Datagram Approach:Datagram Approach:Advantages Advantages

Datagram Approach:Datagram Approach:Disadvantages Disadvantages

(1) call setup avoidedcall setup avoided if a station wishes to send only one or a few

packets, datagram delivery is quicker no callsetup delay !

(2) better flexibility in case of congestionbetter flexibility in case of congestion if congestion develops in one part of the network

packets can be routed away from the congestion

(3) network survivabilitynetwork survivability if a node fails, subsequent packets may find an

alternate route that bypass the failed node

(1) slower packet switchingslower packet switching

(2) outout--ofof--order datagram deliveryorder datagram delivery

(3) best effortbest effort quality of servicequality of service

26Packet Switching: Virtual Circuit Networks

Virtual Circuit Approach toVirtual Circuit Approach toPacket SwitchingPacket Switching(ATM)(ATM)

relationship between all packets belongingto a message or session is preserved

a single route, so-called virtual circuitvirtual circuit, ischosen between sender and receiver at the beginning of the session

virtual circuit virtual circuit dedicated path dedicated path packets are still buffered at each node and queuedfor output

all packets of a given session travel one after another along virtual circuit

each packet contains virtual circuit IDvirtual circuit ID as well as data

node on a preestablished route has a tablethat maps virtual circuit IDs to output links

virtual circuit packet switching can be donevirtual circuit packet switching can be donequicklyquickly by looking up virtual circuit ID ofincoming packet in small translation table

27Packet Switching: Virtual Circuit Networks (cont.)

Example [ packet switching: virtual-circuit approach ]

data path identifier

28Packet Switching: Virtual Circuit Networks (cont.)

Virtual Circuit Approach:Virtual Circuit Approach:Advantages Advantages

Virtual Circuit Approach:Virtual Circuit Approach:Disadvantages Disadvantages

(1) small looksmall look--up tables up tables fast packet switchingfast packet switching number of virtual circuit IDs is reduced to the

maximum number of simultaneous virtualcircuits over an input port

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Circuit Switching Virtual Circuit Switching Datagram Switching

Route data along pre-established paths

all data sent between user1 and user2flows along the same (logical) circuits

advantage: faster switching & in-order delivery

It appears that Virtual Circuit Switching inherits the advantages of both Circuit and Packet switching. Why is, then, the Internet built on the Packet Switching technology?!

Statistical Multiplexingcut data into packets send packets through the network

without any preallocation (reservation) of resources

advantage: more efficient use of links/resources

FDMA or TDMA

No fixed paths.

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Warriors of the Nethttp://www.youtube.com/watch?v=LGnJw9rtjas