Circuit Switching

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Information about Circuit Switching

Published on March 13, 2014

Author: milesgalario



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 Networks data 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  DataTransfer  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:  PublicTelephone Network (PTN)  Plain OldTelephone 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  digitalTDM 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.  SimpleAbstraction  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 (andTCP/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:     _packet_switching    etworks   Stallings, William “Data and Computer Communications”,Chapter 10: Circuit Switching and Packet Switching, Eight Edition.     “Circuit Switching”, Dr. Farid Farahmand and Dr. Qiong (Jo) Zhang, Central Connecticut State University and Arizona State University atWest Campus

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