CircuitSwitching

Too many crosspoints, only a few used at any given time Inefficient; fewer than 25% of switches are in used at a given time... Multistage Switches Combine crossbar switches in several

 Reference books:

by Behrouz A Forouzan, DeAnza College

 How can we connect multiple devices?

 Point-to-point vs.

 Multipoint

 Better solution  switching

 Switches: hardware and/or software devices allowing temporary connections between two or more devices

 Three methods: circuit, packet, and

message

Example of a Switched Network

Circuit Switching

 Creates a direct physical connection

between two devices

 Originally developed for voice traffic

 Does not use point-to-point connections

between every device to every other device

 Instead, devices link to a switch which may

in turn link to other switches

 Allows for a dedicated connection for

duration of transmission

between 2 devices such as

phones or computers

Circuit Switch vs Folded Switch

n inputs, m outputs

n lines in full duplex

Folded Switch: An n -by -n folded

switch allows every device to

connect to every other device

in full-duplex mode

Space-Division Switch

 The paths in the

circuit are separated

from each other

spatially

 Originally for analog

networks, but now for

both analog and digital

Crossbar Switch

Inputs and outputs connected in a grid using electronic microswitches ( transistors ) at crosspoints

Disadvantage???

Too many crosspoints, only a few used at any given time Inefficient; fewer than 25%

of switches are in used at a given time

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  increased

reliability

Multistage Switch (1)

A set of crossbar switches in several stages

Use a 15x15 crossbar => 225 crosspoints Use the above multistage =>78 crosspoints

Multistage Switch

Multiple paths available for each pair of devices => reliable

experienced in public phone systems

Disadvantage???

 Reduced number of crosspoints may

mean that at times of heavy traffic, an input may not be able to connect to an output if there is no path available (i.e all switches are occupied)

 Does not occur in single-stage switch; a non-blocking path is always available

Time-Division Switches

 Uses time-division multiplexing to achieve switching

 Used by most modern switches

 TD switching involves partitioning low speed bit

stream 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 line’s gate is enabled for output at the same time

TDM with a Time Slot Interchange

Without a TSI, order is unchanged

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

Time Slot Interchange (TSI)

#memory locations = # inputs

size of mem loc = size of a time slot Cannot handle

TDM Bus

 Input and output lines are connected

to a high-speed bus through input and output gates

 Control unit opens and closes gates as needed

Time vs Space Division Switch

Space- and Time-Division

Switching Combinations

 Advantage of space-division switching is it is instantaneous; however, requires too many crosspoints

 Advantage of time-division switching is it

needs no crosspoints; however, processing

connections 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

TST Switch

Telephone Networks

1876: Alexander Graham Bell patented the telephone

1878: Bell Telephone Company first operated

1984: AT&T breakup => AT&T Long Lines and 23 Bell Operating

Companies, and a few other pieces

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

Telephone Network Structure

~22,000 end offices in US, each having up to 10,000 local loops

Local loop: analog 1-10km => how far is it if we stretch out all the local loops in the world?

Trunks between switching offices: coax, microwave, fiber optics

All trunks and switches are now digital

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 between

subscribers

Major Components

 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

 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

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.

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

Inter-LATA Services

 IXCs: inter-exchange carriers, a k a distance companies, provide communication between 2 customers in different LATAs

long- AT&T, MCI, WorldCom , Sprint, Verizon

 IXCs provide general data communications

services including telephone Phone service

is normally digitized

Point of Presence (POP)

Phone Local loop

C.O.

P.O.P.

Phone

C.O P.O.P.

Local loop

Inter-exchange circuit

Belongs to IXC (Inter-eXchange Carrier)

Belongs to LEC (Local-Exchange Carrier)

Representative Voice Network Hierarchy

Residential

customer

Business customer

Residential customer

Business customer

Tandem office

Local loops

Local Carrier's Dom ain of Influence, Intra-LATA

Class 1: regional centers

Class 2: sectional centers

Class 3: primary centers

Class 4: toll centers

Class 5: local central office

Example PSTN Network

Making Connections

 Subscriber telephones are connected

through local loops to end offices

 Accessing the switching station at the end offices is accomplished via dialing

 Dialing

 Before: use Rotary or pulse dialing

 for each number dialed, a digital signal is sent to end office

 Prone to errors due to inconsistency of humans during dialing

 Today: touchtone dialing

Rotary and Touchtone Dialing

Touchtone : Number 8 = 2

bursts of signals with

f1=1336Hz and f2=852Hz

Analog Switched Telephone

 organization pays for call

 Wide area telephone service (WATS):

 Organization pays for outbound call at rate cheaper than long distance

 900 services

 Caller pays for call, much more expensive than

long-distance

Analog Leased Lines

 A line is dedicated permanently

Digital Telephone Services

 Less sensitive to noise and other interference

 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 version of analog leased line, up to 64Kbps

Question?