Circuit Switching pot

Circuit Switching
by
Dr. Nguyen Minh Hoang

March 2007

References

Reference books:

Data Communications and Networking, 4/e, by Behrouz A.
Forouzan, DeAnza College
(raw-
hill.com/sites/0072967757/information_center_view0/)

Data & Computer Communications
by William Stallings (
/>
Introduction

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

Circuit Switching

Space-division switching

Time-division switching

TDM Bus


Combinations
Circuit switching: create a
direct physical connection
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

Multistage switch

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
Blocking problem when heavy traffic,
experienced in public phone systems
Disadvantage???

Blocking

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
bursty traffic


Consists of RAM with
several memory
locations

Fills up with incoming
data from time slots
in order received

Slots are sent out in
order based on
control unit logic

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

Time division

No crosspoints


Delay

Space division

Many crosspoints

Zero delay

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

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