The phone exchanges a principle

If have many users when, to make sure random can communicate between two users, very natural we can think of every two users use a pair of circuitry to rise repeatedly.
　 of <P Align=left><IMG Height=178 Src="/uploadfile/telecom/uploadfile/200804/20080421122115937.gif" Width=200 Align=right Vspace=5 Border=0> circumstance of 5 users connective: When N=5 of <BR> 　 　 , what use =4 3 2 1=10</P> of 　 of circuit <BR> 　
N of <P Align=left> 　 user connective circumstance: <BR> place uses circuit =n(n-1)/2</P>
The line logarithm that <P Align=left> 　 counts the place when increasing N to need when the user accordingly more mushroom, want, if be opposite for every user, need to receive N-1 in the home to the line, before calling, still need to mix him word machine line be callinged rises repeatedly, that is too troublesome! <IMG Height=162 Hspace=10 Src="/uploadfile/telecom/uploadfile/200804/20080421122116508.gif" Width=220 Align=left Vspace=5 Border=0></P>
Then people thought up <P Align=left> 　 a tweak, be in the concentrated center that the user distributings, install an equipment, this is just like is contact of a switch, open at ordinary times, when needing to communicate between two users arbitrarily, equipment join the line of two users puts through. Can see from this, equipment but the requirement of the person that the basis sends a word, finish with the task that information exchanges between another user, so this kind of equipment calls a phone switching equipment. Actual switching equipment is quite complex, but had a phone to exchange equipment, n user, need N to be able to satisfy a requirement to the line only, make the charge of circuit is reduced greatly. Although raised the cost of switching equipment, but it will be N the user serves, utilization rate is very high. </P>
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