Question

A Hybrid Fiber-Coaxial (HFC) network has a bandwidth that ranges from 5 to 750 MHz. Describe the purposes of the three bands in this bandwidth.

Answer 1

The Internet-access using HFC is accomplished by dividing its bandwidth into three bands: video/TV, downstream data and upstream data. The division is made to avoid interference among the TV, downstream and upstream subscribers data signals while passing through medium. The TV band is placed in between upstream and downstream bands because upstream/downstream bands are used extensively and in random fashion while the channels TV band remain silent for most of the time. Hence TV band's interference is very low over the other two. As the up and down stream bands are spaced largely, they cannot interfere each other. On the other hand, interference over TV band by the other two is not an very important issue as TV applications can tolerate a slight loss of data to run it smoothly.

The video/TV band occupies frequencies from 54 to 550 MHz. Each TV channels occupies 6 MHz and so it can accommodate more than 80 channels.

The downstream data i.e. data from the Internet to the user/subscriber occupies the band from 550 to 750 MHz. This band is also divided into 6 MHz channels.

The upstream data i.e. data from user/subscriber to the Internet occupies the band from 5 to 42 MHz. This band is also divided into 6 MHz channels. The transmission of upstream data follows Quadrature Phase Shift Keying (QPSK) modulation where transmission capacity is 2 bits/Hertz. Hence the theoretically upstream data is sent at 12 Mbps (2 bits/Hz * 6 MHz) transmission rate. But, the actual rate is much lower as there is always bit errors and losses due to effects on the medium.