The choice of data transmission mainly depend on the type of the channel. The communication channel can be classified into two main type, based on the frequencies supported by it. These are:
1. Low-pass channel.
2. Band-pass channel.
If a channel is having a bandwidth between 0 and f, it is called as a low-pass channel. This means, the lowest supported frequency of a low-pass channel is zero. A digital signal needs a bandwidth between 0 and infinity. If we relax the upper limit, the bandwidth 0 to f can be suited for a digital transmission. Thus, we can say that digital transmission would be suitable, if the communication channel is a low pass channel.
A band pass channel is having a bandwidth between fl and f2. This means, the least frequency supported by a band-pass channel is not zero. If the network is having a band pass channel, analog transmission is suitable for that.
The bandwidth of an analog signal is generally narrower than digital signal with frequencies between f1 and f2. The main advantage of using a band-pass channel is that we can shift a signal with a bandwidth from f1 to f2 to a signal with bandwidth from f3 to f4 till the width of the bandwidth remains the same.
This means a signal having frequencies between 200 and 300 KHz be shifted into a signal with frequencies between 400 and 500 KHz.
The other advantage is that the bandwidth of a band-pass channel can be divided into n logical channels to carry n analog transmissions. The best example of this is cellular telephony in which a limited bandwidth is divided into n users and the signals are appropriately shifted.