Packet Switching versus Circuit Switching

Consider two restaurants:

One which requires reservations and another which neither requires

reservations nor accepts them. For the restaurant that requires reservations,

we have to call (or send an e-mail!) before we leave home.

But when we arrive at the restaurant we can, in principle, immediately

communicate with the waiter and order our meal. (Circuit Switching)

For the restaurant that does not require reservations, we don't need to bother

to reserve a table. But when we arrive at the restaurant, we may have to wait

for a table before we can communicate with the waiter. (Packet Switching)

Circuit switching has often argued that the packet switching is not suitable for real-time

services (e.g., telephone calls and video conference calls) due to its variable and

Unpredictable delays.

Proponents of packet switching argue that:

1- It offers better sharing of bandwidth than circuit switching

2- It is simpler, more efficient, and less costly to implement than circuit-switching.

Generally speaking, people who do not like restaurant reservations, prefer packet

switching to circuit switching.

Circuit Switching

Circuit switching

A simple circuit-switched network consisting of three circuit switches

interconnected with two links. Each link has n circuits; each end-to-end circuit over a link

gets the fraction 1/n of the link's bandwidth for the duration of the circuit. The n circuits in

a link can be either TDM or FDM circuits.

FDM and TDM

A circuit in a link is implemented with either:

- Frequency Division Multiplexing (FDM)

- Time-Division Multiplexing (TDM).

FDM

With FDM, the frequency spectrum of a link is shared among the

connections established across the link.

Specifically, the link dedicates a frequency band to each connection for the

duration of the connection.

In telephone networks, this frequency band typically has a width of 4kHz.

The width of the band is called “bandwidth”.

FM radio stations also use FDM to share microwave frequency spectrum.

TDM

For a TDM link, time is divided into frames of fixed duration and each frame

is divided into a fixed number of time slots.

When the network establish a connection across a link, the network

dedicates one time slot in every frame to the connection.

These slots are dedicated for the sole use of that connection, with a time slot

available for use (in every frame) to transmit the connection's data.

FDMvsTDM