Question { 9044 }

what is the difference between hub and switch?

Answer

A hub is a unsophisticated broadcast device. Any packet
entering any port is broadcast out on every port and thus
hubs do not manage any of the traffic. Since every packet is
constantly being sent out through every port, chances of
data collision is more and thus it impedes the smooth flow
of traffic.
A switch isolates ports, meaning the every received packet
is sent out only to the port on which the target may be
found. If proper port is not found, then it will broadcast
the packet to all ports. So using switch network performance
can be greatly increased.

Question { ONGC, 29724 }

Why we use 4 to 20 mA, (This is International Standard)
instaed of this can we use 4-16, or 4-24 or 3- 15 or any
other range

Answer

Answers 5 & 6 are totally correct.. just adding another point.
20 mA as upper value is used to keep electrical components
intrinsically safe at hazardous area.