The physical layer moves data in the form of electromagnetic signals across a transmission medium. Generally, the data are not in a form that can be transmitted over a network. So the data must first be changed to a form that transmission media can accept. For transmission, data needs to be changed to signals. When this signal is transmitted over the transmission medium it is known as signal transmission.
Two approaches/types of signal transmission:
1. Baseband Transmission
2. Broadband Transmission
Transmission of digital signal: We can transmit a digital signal by using one of two approaches: baseband ‘transmission or broadband transmission.
A. Baseband Transmission
Baseband transmission means sending a digital signal over a channel without changing it to an analog signal. The Baseband transmission needs a low-pass channel i.e., a channel having a bandwidth that starts from zero. This is possible if we have a dedicated medium with a bandwidth constituting only one channel. Following are two cases of baseband communication:
a. Low-pass channel with wide bandwidth
- If we want to preserve the exact form of a non-periodic digital signal we need to send the entire spectrum between zero and infinity.
- This is possible if we have a dedicated medium with an infinite bandwidth.
- But we cannot have such a channel in real life.
- However, the amplitudes of the frequencies at the border of the bandwidth are so small that they can be ignored.
- This means that if we have a medium with a very wide bandwidth (such as fiber optic cable) two stations can communicate with good accuracy.
b. Low-pass channel with limited bandwidth:
- In a low-pass channel with limited bandwidth, we approximate the digital signal with an analog signal.
- The level of approximation depends on the bandwidth available.
B. Broadband Transmission:
Broadband transmission or modulation means changing the digital signal to an analog signal for transmission. Modulation allows us to use a bandpass channel i.e., a channel whose bandwidth does not start from zero. This type of channel is more available than a low-pass channel. In a bandpass channel, we need to convert the digital signal to an analog signal before transmission.
Read more about Congestion