Lecture 16

DATA COMMUNICATIONS & NETWORKING

LECTURE-14

Course Instructor : Sehrish Rafiq

Department Of Computer Science

University Of Peshawar

LECTURE OVERVIEW Communication Between Devices Switching Circuit switching Space Division Switching Cross-Bar switch Multistage Switch

COMMUNICATION BETWEEN DEVICES

Whenever we have multiple devices we have the problem of how to connect them to make one to one communication possible.

One solution is to install a point-to-point connection between each pair of devices(a mesh topology) .

OR A point-to-point connection between a

central device and every other device in a star topology.

LIMITATIONS OF THE PREVIOUS SOLUTION

These methods however, are impractical and waste ful when applied to very large networks.

The number and length of the links require too much infrastructure to be cost efficient and the majority of those links would be idle most of the time.

Other topologies employing multipoint connections such as bus are ruled out because the distances between devices and total number of devices increase beyond the capacities of the media and equipment.

WHAT SHOULD BE THE SOLUTION???

A better solution is switching. A switched network consists of a series of

interlinked nodes , called switches. Switches are hardware and/or software

devices capable of creating temporary connections between two or more devices linked to the switch but not to each other.

In a switched network, some of those nodes are connected to the communicating devices.

Others are used only for routing.

TYPES OF SWITCHING

Circuit switching Packet switching Message switching

CIRCUIT SWITCHING Circuit switching creates a direct physical

connection between two devices such as phones or computers.

CIRCUIT SWITCH A circuit switch is a device with n inputs and

m outputs that creates a temporary connection between an input link and output link.

FOLDED SWITCH An n-by-n folded switch can connect n lines

in full duplex mode. It can connect n telephones in such a way

that each phone can be connected to every other phone.

CIRCUIT SWITCHING TECHNOLOGIES

Circuit switching can use either of two technologies.

Space division switch Time division switch

SPACE DIVISION SWITCHING

In space division switching, the paths in the circuit are separated from each other spatially.

This technology was originally designed for use in analog networks but is used currently in both analog and digital networks.

CROSS-BAR SWITCH A crossbar switch connects n-inputs to m-

outputs in a special grid, using electronic micro switches(transistors) at each cross point.

The major limitations of this design is the number of cross points required.

Connecting n inputs to m outputs using a cross-bar switch requires n x m cross points.

CROSS BAR SWITCH

LIMITATIONS OF CROSS BAR SWITCH

A cross bar switch with a very large number of cross points is impractical.

Such a switch is also inefficient because statistics show that in practice fewer than 25% of the cross points are in use at any given time.

MULTISTAGE SWITCH

The solutions to the limitations of the cross bar switch is the multistage switch which combines cross bar switches in several stages.

In multistage switching, devices are linked to switches that in turn are linked to other switches.

The design of a multistage switch depends on the number of stages and the number of switches required or (desired) at each stage.

Normally the middle stages have fewer switches than do the first and last stages.

MULTISTAGE SWITCH

OBSERVATIONS Each of the first stage switches must have an

output to each of the intermediate switches. Each of the third stage switch must have

input from each of the intermediate switches. The intermediate switch must connect to all

three-first stage switches and all three last stage switches and so must have 3 inputs and 3 outputs each.

MULTIPLE PATHS Multi-stage switches provide several options

for connecting each pair of linked devices.

HAVE NICE TIME AT HOME!!!