lec2-CircuitSwitching

8/6/2019 lec2-CircuitSwitching

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Circuit Switching

by

Dr. Nguyen Minh [email protected]

March 2007
mailto:[email protected]:[email protected]


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References Reference books:

Data Communications and Networking, 4/e,

by Behrouz A. Forouzan, DeAnza College(http://highered.mcgraw-hill.com/sites/0072967757/information_center_view0/)

Data & Computer Communicationsby William

Stallings (http://williamstallings.com/DCC/DCC7e.html)
http://williamstallings.com/DCC/DCC7e.htmlhttp://williamstallings.com/DCC/DCC7e.html


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Introduction How can we connect multiple devices?

Point-to-point vs.

Multipoint Better solution switching Switches: hardware and/or software

devices allowing temporary connectionsbetween two or more devices

Three methods: circuit, packet, andmessage


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Example of a Switched Network


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Circuit Switching Creates a direct physical connection

between two devices

Originally developed for voice traffic Does not use point-to-point connectionsbetween every device to every otherdevice

Instead, devices link to a switch which mayin turn link to other switches Allows for a dedicated connection for

duration of transmission


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Circuit Switching

Space-division switching Time-division switching TDM Bus Combinations

Circuit switching: create adirect physical connectionbetween 2 devices such as

phones or computers


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Circuit Switch vs. Folded Switch

n inputs, m

outputs

n lines in full duplex

Folded Switch: An n-by-nfoldedswitch allows every device toconnect to every other device

in full-duplex mode


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Space-Division Switch The paths in the

circuit are separated

from each otherspatially

Originally for analognetworks, but now for

both analog and digital Crossbar switch Multistage switch


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Crossbar SwitchInputs and outputsconnected in a grid usingelectronic microswitches

(transistors) at crosspoints

Disadvantage???

Too many crosspoints, only afew used at any given time

Inefficient; fewer than 25%of switches are in used at a

given time


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Multistage Switches Combine crossbar switches in several stages Devices are linked to switches that are linked

to a hierarchy of other switches Middle stages usually have fewer switches

than first and last stages Fewer crosspoints but still allowing multiple

paths through the network increasedreliability


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Multistage Switch (1)

A set of crossbar switches in several stages

Use a 15x15 crossbar => 225 crosspoints

Use the above multistage =>78 crosspoints


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Multistage SwitchMultiple paths available for each pair of devices => reliable

Blocking problem when heavy traffic,

experienced in public phone systems

Disadvantage???


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Blocking Reduced number of crosspoints may

mean that at times of heavy traffic, an

input may not be able to connect to anoutput if there is no path available (i.e.all switches are occupied)

Does not occur in single-stage switch; anon-blocking path is always available


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Time-Division Switches Uses time-division multiplexing to achieve switching Used by most modern switches

TD switching involves partitioning low speed bitstream into pieces that share higher speed stream Two methods:

Time-slot interchange (TSI) TDM bus

Based on synchronous time division multiplexing (TDM) Each station connects through controlled gates to high speed

bus Time slot allows small amount of data onto bus Another lines gate is enabled for output at the same time


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TDM with a Time Slot

InterchangeWithout a TSI, order is unchanged

1->3, 2->4, 3->1, 4->2


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Time Slot Interchange (TSI)

#memory locations = # inputssize of mem. loc. = size of a time slot

Cannot handlebursty traffic

Consists of RAM withseveral memorylocations

Fills up with incomingdata from time slotsin order received

Slots are sent out inorder based oncontrol unit logic


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TDM Bus Input and output

lines are connectedto a high-speed busthrough input andoutput gates

Control unit opens

and closes gates asneeded


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Time vs. Space Division Switch Time division

No crosspoints Delay

Space division Many crosspoints Zero delay


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Space- and Time-Division

Switching Combinations Advantage of space-division switching is it is

instantaneous; however, requires too manycrosspoints

Advantage of time-division switching is itneeds no crosspoints; however, processingconnections creates delays

Combining them optimizes the number of

crosspoints and reduces the amount of delay TSS: Time-Space-Space TSST: Time-Space-Space-Time STTS: Space-Time-Time-Space


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TST Switch


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Telephone Networks1876: Alexander Graham Bell patented the telephone

1878: Bell Telephone Company first operated

1984: AT&T breakup => AT&T Long Lines and 23 Bell OperatingCompanies, and a few other pieces

Since 1996: Any company can provide local or long-distance services


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Telephone Network Structure

~22,000 end offices in US, each having up to 10,000 local loopsLocal loop: analog 1-10km => how far is it if we stretch out all thelocal loops in the world?

Trunks between switching offices: coax, microwave, fiber optics

All trunks and switches are now digital


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Major Components Local loops twisted-pair cable connecting

subscriber phone to nearest end office or local

central office Bandwidth of 4000 Hz (4 KHz) Trunks transmission media that handles

communication between offices

Muxing through optical fibers or satellite links Switching Office connects local loops or

trunks allowing connections betweensubscribers


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Major Components


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LATAs Local Access and Transport Area defines

the boundary of a service area Intra-LATA services are provided by local

exchange carriers (LEC) Incumbent local exchange carriers

Competitive local exchange carriers Communication is handled by end switches

and tandem switches


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Services Inter-LATA services are handled by

interexchange carriers (IXCs) long-distance companies To provide these services, each IXC

must have a POP (Point of Presence)inside each LATA


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Local Access Transport Areas

(LATAs) 200+ LATAs in the US Intra-LATA services

Services offered by the common carriers(telephone companies) inside a LATA

Such a carrier = local exchange carrier (LEC) Before 1996: 1 LEC in a LATA => monopoly

After 1996: 1+ LEC inside a LATA The LEC that owns the local loops system is called

incumbent LEC (ILEC) => provide main services Extra carrier = competitive LEC (CLEC) => extra

services: mobile, toll calls, etc.


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Switching Offices in a LATA

Calls that go through a tandem office is charged

Calls that are completed by using only end offices are toll-free


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Inter-LATA Services IXCs: inter-exchange carriers, a. k. a. long-

distance companies, provide communication

between 2 customers in different LATAs AT&T, MCI, WorldCom , Sprint, Verizon

IXCs provide general data communicationsservices including telephone. Phone service

is normally digitized

How do IXCs interact with LECs? Use POP


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Point of Presence (POP)


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Phone Local loop

C.O.

P.O.P.

Phone

C.O.P.O.P.

Local loop

Inter-exchangecircuit

Belongs to IXC(Inter-eXchange Carrier)

Belongs to LEC(Local-Exchange Carrier)

LATA - A

LATA - B

trunk

line

trunk line

Example PSTN Network


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Representative Voice Network Hierarchy

Residentialcustomer

Businesscustomer

Residentialcustomer

Businesscustomer

Class1:regional centers

Class2:sectional centers

Class3:primarycenters

Class4:toll centers

Class5:

local central office

Localloops

Tandemoffice

Localloops

Local Carrier'sDomainof Influence, Intra-LATA

Class1:regional centers

Class2:sectional centers

Class3:primarycenters

Class4:toll centers

Class5:

local central office

Example PSTN Network


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Making Connections Subscriber telephones are connected

through local loops to end offices

Accessing the switching station at the endoffices is accomplished via dialing Dialing

Before: use Rotary or pulse dialing

for each number dialed, a digital signal is sent to endoffice Prone to errors due to inconsistency of humans during

dialing Today: touchtone dialing


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Rotary and Touchtone DialingTouchtone: Number 8 = 2bursts of signals withf1=1336Hz and

f2=852Hz


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Analog Switched Telephone

Services Familiar dial-up service, analog local loop Local calls Toll call

intra- (when LATA is large) or inter-LATA 800 (888, 877, 866) services:

organization pays for call Wide area telephone service (WATS):

Organization pays for outbound call at rate cheaper thanlong distance

900 services Caller pays for call, much more expensive than long-

distance


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Analog Leased Lines A line is dedicated permanently

between 2 customers This line goes through a switch, but

seen as a single line because theswitch for it is always closed.

No dialing is needed


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Digital Telephone Services Less sensitive to noise and other interference Switch/56

digital version of analog switched line, data up to 56Kbps Line is digital, no need for modem, but need a digital

service unit (DSU) Support bandwidth on demand, can use more lines for

more bandwidth => can use video conferencing, fast fax,fast data transfer, among other services

Digital Data Service (DDS) Digital version of analog leased line, up to 64Kbps

High-speed digital: T-lines and home networks


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