# Switches-Circuit Switching

Switches are used in Circuit-Switched and Packet-Switched Networks.  The switches are used are different depending up on the structure and usage.

Circuit Switches (or) Structure of Circuit Switch:-

The Switches used in Circuit Switching are called Circuit-Switches

Space-Division Switch:-

• The paths are separated spatially from one switch to other.
• These were originally designed for analog circuits but currently used for both analog and digital Networks.

Cross-bar Switch:-

In this type of Switch we connect n inputs and m outputs using micro switches (Transistors) at each cross point to form a cross-bar switch of size n X m.

The number of cross points required = n X m.

As n and m increases, cross points required also increases, for example n=1000 and m=1000  requires n X m= 1000 X 1000 cross points. A cross-bar with these many number of cross points is impractical and statics show that 25% of the cross points are in use at any given time.

Multi stage Switch:-

The solution to Cross-bar Switch is Multi stage switching. Multi stage switching is preferred over cross-bar switches to reduce the number of cross points.  Here number of cross-bar switches are combined in several stages.

Suppose an N X N cross-bar Switch can be made into 3 stage Multi bar switch as follows.

1.  N is divided into groups , that is N/n Cross-bars with n-input lines and k-output lines forms n X k cross points.
2. The second stage consists of k Cross-bar switches with each cross-bar switch size as (N/n) X (N/n).
3. The third stage consists of N/n cross-bar switches with each switch size as k X n.

The total number of cross points = $2kN&space;+&space;k&space;(\frac{N}{n})^{2}$, so the number of cross points required are less than single-stage cross-bar Switch = $N^{2}$.

for example k=2 and n=3 and N=9 then a Multi-stage switch looks like as follows.

The problem in Multi-stage switching is Blocking during periods of heavy traffic, the idea behind Multi stage switch is to share intermediate cross-bars. Blocking means times when one input line can not be connected to an output because there is no path available (all possible switches are occupied). Blocking generally occurs in tele phone systems and this blocking is due to intermediate switches.

Clos criteria gives a condition for a non-blocking Multi stage switch

$n&space;=&space;\sqrt{\frac{N}{2}}$$k\geq&space;(2n-1)$  and  Total no.of Cross points $\geq&space;4N(\sqrt{2N}-1)$.

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