Write short notes on the following – (i) 56 K modems (ii) Cable modem. Or Write short note on high speed modems.Ripunjay Tiwari
Ans. (i) 56 K Modems – The data rate of traditional modems is limited
to maximum of 33.6 kbps, as determined by the Shannon formula. But, now,
new modems with a rate of 56,000 bps, known as 56 K modems, are on the
market. These modems can be used only when one party is using digital
signalling. They are asymmetrical in that the down-loading (flow of data from
Internet provider to the PC) is a maximum of 56 kbps, while the uploading
(flow of data from the PC to the Internet provider) can be maximum of 33.6
kbps. When one side is an Internet provider and the signal does not have to
pass through a PCM converter, quantization is eliminated in one direction and
the data rate can be increased to 56 kbps, as shown in fig. 2.35.
Transmission of data from the subscriber to the Internet provider
(uploading) occurs in the following steps –
(a) Modem modulates the digital data at site A.
(b) Modem transmits the analog data to the switching station
at site A on the local loop.
(c) Data are converted to digital using PCM at the switching station.
(d) digital data travel through the digital network of the telephone company and are received by the internet provider computer.
Step (c) is again the limiting factor in these steps, which means there is
no improvement here. The user does not require such a high data rate because
in this direction, only small blocks of data (like an e-mail or a small file) are
Downloading or transmission of data from the Internet provider to the
modem at site A occurs in the following steps –
(a) Computer of the Internet provider transmits the digital data
through the digital telephone network.
(b) Digital data are converted to analog using inverse PCM at
the switching station.
(c) At site A, the switching station transmits the analog data to
the modem on the local loop.
(d) At site A, analog data are demodulated by the modem.
There is no quantization of data using PCM in this direction. The limitation
when uploading is not an issue here; data can be transmitted at 56 kbps. This
is what the user is looking for, because large files are typically downloaded
from the Internet.
Thus, it is concluded that the maximum data rate in the uploading direction
is still 33.6 kbps, but the data rate in the downloading direction is now 56 kbps.
(ii) Cable Modem- The traditional modems have the limitation of
data rate, which is mostly due to the narrow bandwidth of the local loop
telephone line (up to 4 kHz). A modem can be designed to handle much higher
data rates by using higher bandwidths.
The cable TV provides residential premises with a coaxial
cable which has a bandwidth upto 750 MHz and sometimes Even more. Generally,
this bandwidth is divided into 6 MHz bands using frequency
division multiplexing, and each band provides a TV channel. In order to permit
a user to download and upload information from the Internet, two bands can
be set aside.
The cable modem concept is shown in fig. 2.36, in which a splitter is
used instead of the traditional cable box. The splitter directs the TV bands to
the TV set and the Internet access bands to the PC.
Downloading, in general, needs a 6 MHz bandwidth in a range above 40
MHz. The 64-QAM technique is used for demodulation. It means that a user
can download information at a rate of-
6 MHz x 6 = 36 Mbps
But, PCs are not yet capable of receiving data at this rate. Therefore, the