.jpg)
The most widespread of the advanced systems of cellular systems is the GSM. In a GSM system, the first step within the phone is to digitize the user’s voice via an analog/digital converter called a coder/decoder (codec), at an 8 kbps rate and with 13-bit resolution for a total of 104 kbps. The resulting stream of bits is then compressed using an advanced algorithm that eliminates redundancies (since there are many predictable patterns in spoken word patterns). This reduces the required bit rate to be transmitted to just 13kbps.
Next, error detection and correction bits are added to
eliminate or reduce errors due to channel noise, fading, and other problems,
bringing the final voice data rate to 22.8 kbps. The compression and EDC
functions are implemented by a specialized microprocessor in the phone called a
digital signal processor.
At the receiving GSM phone, the EDC bits are stripped off
and used for their intended function. The corrected bit stream is then
processed by an algorithm that undoes the compression of the transmitting
phone, recreating the original bit stream.
Finally, the bits go to the digital/analog converter part of
the codec, which creates the original analog voice signal in the phone
earpiece. The remainder of the phone operation such as dialling, call set-up,
handing off from cell to cell, and call termination is done much the same way
as analog cellular phone.
•The greatest is that three times as many user channels can
be fit into the available bandwidth as compared to analog cell phone.
•Since the signal is now in digital form, many of the
advanced techniques for handling and modulation can be used within the phones
and their systems.
•The bits can be encrypted at the phone to defeat
eavesdroppers who mow use a simple receiver to intercept conversations.
•The entire phone is now compatible with other
digital formats, such as computer files.
Be in touch...
0 comments:
Post a Comment