data and computer communications chapter 10 – circuit switching and packet switching (wide area...

Data and Computer Data and Computer CommunicationsCommunications

Chapter 10 – Circuit SwitchingChapter 10 – Circuit Switching and and Packet SwitchingPacket Switching

(Wide Area Networks)(Wide Area Networks)

Switched NetworkSwitched Network

NodesNodes

a collection of a collection of switching nodes switching nodes and and connectionsconnections is a communications network is a communications network

nodes may connect to other nodes only, or to nodes may connect to other nodes only, or to stations and other nodesstations and other nodes

network is usually partially connectednetwork is usually partially connected some some redundantredundant connections are desirable 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

Establish - Establish - an end-to-end (station-to-station) circuitan end-to-end (station-to-station) circuit Transfer – Transfer – Data transmitted through the network Data transmitted through the network Disconnect - Disconnect - the connection is terminated, usually by of one the connection is terminated, usually by of one

of the two stationsof the two stations

inefficientinefficient channel capacity is dedicated for duration of connectionchannel capacity is dedicated for duration of connection if no data, capacity wasted (Client / server)if no data, capacity wasted (Client / server)

set up (connection) takes timeset up (connection) takes time once connected, transfer is transparent (no delay)once connected, transfer is transparent (no delay)

Public Circuit Switched Public Circuit Switched NetworkNetwork

Circuit EstablishmentCircuit Establishment

Circuit Circuit Switch Switch

ElementsElements

Blocking or Non-blockingBlocking or Non-blocking

blocking networkblocking network may be may be unableunable to connect stations because 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 permitspermits all stations to connect (in pair) at once all stations to connect (in pair) at once used for some data connectionsused for some data connections

Space Division SwitchSpace Division Switch

3 Stage 3 Stage Space Division Space Division SwitchSwitch

Time Division SwitchingTime Division Switching

modern digital systems use modern digital systems use intelligent intelligent controlcontrol of space & time division elements 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 Slot InterchangeTime Slot Interchange

TSI receives input in sequence, but sends TSI receives input in sequence, but sends them to the output out-of sequence (as them to the output out-of sequence (as needed by the devices)needed by the devices)

Stored in memory, and scanned outStored in memory, and scanned out Delivery of data is delayedDelivery of data is delayed ## How many cross points are needed for a How many cross points are needed for a

crossbar switch to connect 1000 crossbar switch to connect 1000 telephones?telephones?

Traditional Circuit SwitchingTraditional Circuit Switching

SoftswitchSoftswitch

Packet SwitchingPacket Switching

circuit switching circuit switching was designed for voicewas designed for voice packet switching packet switching was designed for datawas designed for data transmitted in small packets (1000 octets)transmitted in small packets (1000 octets) packets contain user data and control infopackets contain 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 (control info includes routing (addressingaddressing) info) info

packets are received, stored briefly (buffered) packets are received, stored briefly (buffered) and passed on to the next nodeand passed on to the next node

Packet SwitchingPacket Switching

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

Packet Packet 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

DatagramDatagramDiagramDiagram

VirtualVirtualCircuitCircuit

DiagramDiagram

Virtual Circuits v DatagramVirtual Circuits v Datagram

virtual circuitsvirtual circuits network can provide sequencing and error network can provide sequencing and error

controlcontrol packets are forwarded more packets are forwarded more quicklyquickly less reliableless reliable

datagramdatagram No No setupsetup phase phase more flexiblemore flexible more reliablemore reliable

Packet Packet SizeSize

Circuit v Packet SwitchingCircuit v Packet Switching

performance depends on various delaysperformance depends on various delays propagation delay - negligiblepropagation delay - negligible transmission time transmission time node delay – processing timenode delay – processing time

range of other characteristics, including:range of other characteristics, including: Network size & topologyNetwork size & topology Load pattern Load pattern

Event TimingEvent Timing

## N = hops = 4, L = message length = 3200 N = hops = 4, L = message length = 3200 bits, B = data rate = 9600, P = packet (data bits, B = data rate = 9600, P = packet (data + overhead) size = 1024 bits, H = + overhead) size = 1024 bits, H = overhead = 16 bits, S = call set up time = overhead = 16 bits, S = call set up time = 0.2 s, D = propagation delay / hop = 0.001 0.2 s, D = propagation delay / hop = 0.001 s. (ignore the time for ACK in circuit s. (ignore the time for ACK in circuit switching and virtual circuit packet switching and virtual circuit packet switching)switching)

Find end to end delay for circuit switching, Find end to end delay for circuit switching, datagram and virtual circuit packet datagram and virtual circuit packet switching.switching.

Circuit:Circuit: T = C1 + C2T = C1 + C2 C1 = S, C2 = Tr. + Prop. TimeC1 = S, C2 = Tr. + Prop. Time Datagram:Datagram: T = D1 + D2 + D3 + D4T = D1 + D2 + D3 + D4 D1 = (Tr. + Prop. Time) of all packets over D1 = (Tr. + Prop. Time) of all packets over

11stst hop, Packet data = 1024 -16 =1008 hop, Packet data = 1024 -16 =1008 D2 = D3 = D4 = (Tr. + Prop. Time) of the D2 = D3 = D4 = (Tr. + Prop. Time) of the

last frame over 2last frame over 2ndnd, 3, 3rdrd, and 4, and 4thth hops. hops.

Virtual:Virtual: T = V1 + V2T = V1 + V2 V1 = S, V2 = Datagram TV1 = S, V2 = Datagram T

SummarySummary

circuit verses packet switching network circuit verses packet switching network approachesapproaches