CircuitSwitching

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