CIRCUIT SWITCHING

Basic Categories of Communication Networks

Broadcast Networks

-a single node transmits the information

to all other nodes and hence, all stations will receive the data

Switched Networksdata are transferred from source

to destination through a series of intermediate nodes (switching nodes).

Types of Switched Networks

Circuit-Switched Packet-Switched Message-Switched Burst-Switched

Circuit-Switched Network

“Line-Switched Network”. Originally developed for the analog-

based telephone system. a physical path is obtained for and

dedicated to a single connection between two end-points in the network for the duration of the connection.

3 Basic Elements

End-Stations (or Terminals) Transmission Media Switching Nodes

3 Phases of a Circuit-Switched Communication

System Circuit Establishment Data Transfer Circuit Disconnect

The connection path is established before the transmission begins.

Channel capacity must be reserved between the source and destination throughout the network.

Each node must have available internal switching capacity to handle the requested connection.

The Switching nodes must have the intelligence to make proper allocations and to establish a route through the network.

Example:

Public Telephone Network (PTN) Plain Old Telephone System (POTS) Long Distance Calls

Switching Node Architecture

Basic Functionalities:

Signaling Control Switching Interfacing

Switching Node Elements

Signaling

Monitors the activity of the incoming lines and to forward appropriate status or control information to the control element of the switch.

Also used to place control signals onto outgoing lines under the direction of the control element.

Control

process incoming signaling information and sets up connections accordingly

Switching

SWITCHING MATRIX(FABRIC)-an array of selectable cross-points used to complete connections between input lines and output lines.

Interfacing

Provides the hardware required to connect different devices, such as: analog digital TDM lines optical fibers etc.

to the switch matrix.

Characteristic of a Circuit-Based Switch

Blocking Switching Node- occurs when the switching

matrix does not allow some input lines to be connected to output lines.

- used on voice systems Non-Blocking Switching Node

- allows all inputs to be connected to all outputs.

-used for data connections

Switching Technologies

Space-Division Time-Division Frequency Division Wavelength Division

Space-Division Switching

Each input takes a different physical path in the switch matrix depending on the output.

2nd Generation Space-Division System

Step-by-Step Switch Crossbar Switch (Cross-Point Switch)

Step-by-Step Switch

A basic step-by-step switch has a single input terminal and multiple output terminals.

Connection from the input terminal to the outputs is controlled by an internal rotary contact, or wiper.

As the wiper rotates, it establishes a contact between the input and output terminals.

Each time the user dials a rotary-dial digit, the rotary contact is advanced one position, and connects the input terminal to the next output terminal.

( Chapuis, 1982 ; Clark, 1997 )

Crossbar Switch

As digits are dialed, the control element of the switch receives the entire address

before processing it.

As digits are dialed, the control element of the switch receives the entire address before processing it.

The cross-points of the crossbar switch are mechanical contacts with magnets to setup and hold a connection.

Once the circuit is established, the switching contacts are held by electromagnets energized with direct current passing through the established circuit.

When the circuit is opened, the loss of current causes the cross-points to be released.

NOTE:

CROSSBAR arises from the use if crossing horizontal and vertical bars to select contacts on the cross-point.

Step-by-Step and Crossbar Switching Systems use electro-mechanical components for both switching matrix and control elements.

Time Division Switching

The need for time division switching arises from the fact that digital signals are often carrying multiple

individual circuits, or channels, in appropriate timeslots (TS).

Time-division multiplexing (TDM) involves dividing the carrier into two (or more) channels based on time slices, i.e., the common channel is allotted to several different signals, one at a time, in alternating time slots. Each individual data stream is reassembled at the receiving end based on the timing.

Timeslot Interchanging

In such systems, when two different multiplexed channels are interconnected together through the switch matrix a virtual circuit is established.

This is done by interchanging timeslots, each of which maintain partial contents of a particular channel.

(TSI) (Stallings, 1999) .

Time-Space-Time Architecture

Note that the second time switch stage is necessary to ensure that multiple timeslots in one incoming stream are not superimposed or blocked. Having more stages

can further improve the switch performance.

Utilizes both time-division switch capability:

to shift channels between timeslots, and space-division switching capability:

to enable a different physical outgoing line system to be selected.

Frequency and Wavelength-Division Switching

Frequency Division Multiplexing

 is an analog multiplexing technique that combines analog signals.

The transmission facility is divided into channels by splitting the total frequency band (of the carrier) into narrow bands, each allotted to an individual signal (sub-channeling).

Analog signals are commonly multiplexed using FDM.

Wavelength Division Multiplexing

Wavelength Division Multiplexing

 is an analog multiplexing technique to combine optical signals

WDM is an optical transmission technique in which multiple streams of data are transmitted over a single optical fiber as light rays of different wavelengths. It exploits the fact that light of different wavelengths does not interfere.

WDM allows to simultaneous transmission of different data formats (e.g., IP, SONET, ATM) at different rates as each channel is demultiplexed at the end of the transmission back into the original source.

Multiplexing

Multiplexing means sending multiple signals (each with a given transmission capacity requirement) on a carrier (with large transmission capacity) at the same time as a single, complex signal and then recovering the separate signals at the receiving end.

Advantages of Circuit Switching

Guaranteed Bandwidth The communication performance in

Circuit Switching is predictable and there will be no "best-effort" delivery with no real guarantees.

Simple Abstraction Circuit Switching is a reliable

communication channel between hosts and one would not have to worry about lost or out-of-order packets.

Simple Forwarding The forwarding in Circuit Switching is

based on time slot or frequency and one would not need to inspect a packet header.

Low per-packet overhead There will be no IP (and TCP/UDP) header

on each packet in Circuit Switching.

Pitfalls in Circuit Switching

Wasted bandwidth Since most traffic occurs in bursts, in Circuit

Switching this may leads to idle connection during silent period. Because it is unable to achieve gains from statistical multiplexing that relies in identifying, predicting and allocating more time for the generally more active paths.

Blocked Connections When resources are not sufficient, the

connection will refuse to be connected and thus, Circuit Switching is unable to offer "okay" service to everybody.

Connection Set-up Delay There will be no communication until the

connection is set up. Plus, in Circuit Switching, it is unable to avoid extra latency for small data transfers.

Network State The network nodes in Circuit Switching

must store per-connection information and it is unable to avoid per-connection storage and state.

Sources:

http://searchnetworking.techtarget.com/definition/circuit-switched

http://www.tcpipguide.com/free/t_WhatIsNetworking.htm

http://computer.howstuffworks.com/ip-telephony2.htm

http://en.wikibooks.org/wiki/A_Bit_History_of_Internet/Chapter_2_:_Circuit_switching_vs_packet_switching

http://www.erg.abdn.ac.uk/~gorry/eg3567/intro-pages/cs.html

http://www.aafrin.com/2011/05/12/example-circuit-switching-vs-packet-switching/

http://www.computerworld.com/s/article/41904/Packet_Switched_vs._Circuit_Switched_Networks

http://en.wikipedia.org/wiki/Circuit_switching

Stallings, William “Data and Computer Communications”, Chapter 10: Circuit Switching and Packet Switching, Eight Edition.

http://voip.about.com/od/voipbasics/u/UsingVoIPUP.htm

http://www.highteck.net/EN/Basic/Internetworking.html

http://www.pcmag.com/encyclopedia/term/39698/circuit-switching

“Circuit Switching”, Dr. Farid Farahmand and Dr. Qiong (Jo) Zhang, Central Connecticut State University and Arizona State University at West Campus