Write short notes on the following – (i) 56 K modems (ii) Cable modem. Or Write short note on high speed modems.

Write short notes on the following – (i) 56 K modems (ii) Cable modem. Or Write short note on high speed modems.

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


 56k modems

 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 modem

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

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