d means data .since what ever we give as input same is 
reproduced at output.

as vinay says d stands for data is not right as far as my
knowledge coz d can also be delay flip flop as d-flip flop
is also used to generate delay

i think chebsew is more efficient

The theory is that 3dB differences are "just perceptible,"
so a speaker whose response curve lies within that tolerance
window is a reasonably accurate speaker.

or

As a rough rule of thumb, a change of 3 dB at mid
frequencies is just noticeable by the average person.

or
mathematically to say 
It is the half power point. At this point, the
electronic/communication system has achieved the maximum
power transfer.

MOBILE IS ON ACTIVE MODE AND BCCH (BROADCAST CONTROL 
CHANNEL)WILL WORK.

CDMA (CODE DIVISION MULTIPLE ACCESS) IS A CHANNEL SYSTEM 
USED FOR VARIOUS RADIO TECHNOLOGIES.

what girish siad above is for sure right but if noise is
dominant over message signal then message wont be lost just
it would be distorted. to over come this we go for digital
communication where it is more immune to noise.
a communication is said to very good if its SNR is very high.

SNR or "Signal to Noise Ratio" is a parameter used in
measuring the effectiveness of a communication system or
network.  It is simply obtained as a ratio of the signal
power  to the noise power in the received signal.
In communication systems, the message signal should be
dominant than the noise in the received signal for the
message to be detected and decoded. Hence a threshold limit
is established beyond which if the noise increases, the
message would be lost. Thus for effective communication, the
SNR should be as large as possible and always be greater
than "1".