Circuit Switching

Circuit Switching


Switched network

A switched network consists of a series of inter-linked 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.

Classification of Switched Networks

Circuit-Switching

Circuit switching creates a direct physical connection between two devices such as phones or computers.
Circuit switch connects input links to output links
We can use switches to reduce the number and length of links.

Circuit switch

A circuit switch is a device with n inputs and m outputs that creates a temporary connection between an input link and an output link.
The number of inputs does not have to match the number of outputs.

Circuit Switch Types

Evolution of Switching System:
Manual operators
Step-by-step switch (Strowger switch)
Crossbar switch
Space-Division switches
Multistage switches
Time-Division switches
Time-slot interchange technique
Time-space-time switches
Hybrids combine Time & Space switching


Manual Switching

Patch-cord panel switch invented in 1877
Operators connect users on demand
Establish circuit to allow electrical current to flow from inlet to outlet
Only N connections required to central office

Step-by-step (Strowger) Switch

Electromagnetic switch
Sequence of dialed numbers selects output line

Crossbar Space Switch

N x N array of crosspoints
Connect an input to an output by closing a crosspoint
Non blocking: Any input can connect to idle output
Complexity: N2 crosspoints


Crossbar Switches: Problem

The number of switches is huge.
connect n inputs by m output -- require n * m crosspoint.
Ex : 1000 input, 1000 output → 1,000,000 crosspoint

Inefficient
fewer than 25% of the crosspoints are in use at a given time.

Folded switch

An n-by-n folded switch can connect n lines in full-duplex mode. For example, it can connect n telephones in such a way that each phone can be connected to every other phone.
Circuit switching uses space-division switch [paths in the circuit are separated from each other spatially] or time-division switch.


Circuit Switching

Uses a dedicated path between two stations
Has three phases
Establish (Call Setup)
Transfer
Disconnect
Inefficient
Channel capacity dedicated for duration of connection
if no data, capacity wasted
Set up (connection) takes time
Once connected, transfer is transparent


Blocking or Non-blocking

blocking network
may be unable to connect stations because all paths are in use
used on voice systems
non-blocking network
permits all stations to connect at once
used for some data connections


Space-Division Switch

Paths in the circuit are separated from each other spatially.
Crossbar Switch
Crossbar switch connects n inputs to m outputs in a grid, using electronic micro-switches (transistors) at each cross-point.
Limitation is the number of cross-points required.

Multistage switch

Multistage switch combines crossbar switches in several stages.
Design of a multistage switch depends on the number of stages and the number of switches required (or desired) in each stage.
Normally, the middle stages have fewer switches than do the first and last stages.

MULTIPLE Switching paths

Multiple paths are available in multistage switches.
Blocking refers to times when two inputs are looking for the same output. The output port is blocked.

Design of a Three-Stage Switch

Three steps
Divide the N input lines into groups, each of n lines. For each group, use one crossbar of size n x k, where k is the number of crossbars in the middle stage
Use k crossbars, each of size (N/n) x (N/n) in the middle stage
Use N/n crossbars, each of size k x n at the third stage
N/n(n x k) + k(N/n x N/n) + N/n(k x n) = 2kN + k(N/n)2
In a three-stage switch, the total number of crosspoints is 2kN + k(N/n)2 which is much smaller than the number of crosspoints in a single-stage switch (N2).

Multistage Switch: Example

Design a three-stage, 200 × 200 switch (N = 200) with k = 4 and n = 20.

In the first stage we have N/n or 10 crossbars, each of size 20 × 4. In the second stage, we have 4 crossbars, each of size 10 × 10. In the third stage, we have 10 crossbars, each of size 4 × 20. The total number of crosspoints is 2kN + k(N/n)2, or 2000 crosspoints. This is 5 percent of the number of crosspoints in a single-stage switch (200 × 200 = 40,000).

Time Division Switching

Modern digital systems use intelligent control of space & time division elements
Use digital time division techniques to set up and maintain virtual circuits
Partition low speed bit stream into pieces that share higher speed stream
Individual pieces manipulated by control logic to flow from input to output

Time-slot interchange

TSI consists of random access memory (RAM) with several memory locations. The size of each location is the same as the size of a single time slot.
The number of locations is the same as the number of inputs.
The RAM fills up with incoming data from time slots in the order received. Slots are then sent out in an order based on the decisions of a control unit.


TDM bus

Input and output lines are connected to a high-speed bus through input and output gates (microswitches)
Each input gate is closed during one of the four slots.
During the same time slot, only one output gate is also closed. This pair of gates allows a burst of data to be transferred from one specific input line to one specific output line using the bus.
The control unit opens and closes the gates according to switching need.

Comparison of SDM and TDM

SDM
Advantage:
Instantaneous.
Disadvantage:
Number of cross points required.
TDM
Advantage:
No cross points.
Disadvantage:
Processing delay.

Time- and Space-Division Switch Combination

Space division: instantaneous, crosspoints
Time division: no crosspoint, processing delay
Space-and time-division switching combinations take advantage of the best of both
TST(time-space-time)
TSST(time-space-space-time)
STTS(space-time-time-space)

TST switch

Combine Space division and time division switching.
This results in switches that are optimized both physically (the number of crosspoints) and temporally (the amount of delay).