Cable TV for Data Transfer
In this era, cable companies are competing with telephone companies for residential customers who want high-speed data transfer. To provide high-speed data transfer, the use of a cable TV network is used. To understand this technology following points needs to be considered:
As we have also studied in the HFC system, that it uses the fiber mode, but in this, the coaxial cable is still used for the fiber node to the subscriber premises. The bandwidth range of coaxial cable is 5 to 750 MHz. To provide internet access, the cable company divided this bandwidth into three bands i.e. video band, downstream, and upstream which is as shown in the following figure:
Downstream Video Band: The frequency of the downstream video band is of the upper band, i.e. about 54 to 550 MHz. Each TV channel occupies 6MHz, So, it can accommodate more than 80 channels.
Downstream Data Band: The frequency range of this is 550 to 750 MHz. Like the downstream video band, it is also divided into 6 MHz channels. Downstream data are modulated using the 64-QAM modulation technique.
Data Rate: As there are 6 bits in 64-QAM. Out of these 6 bits, one is used for checking of forwarding error and leaves the other 5 bits of data per baud. There is a standard that specifies 1hz for each baud. So, by calculation, we can say that downstream data can be received at 30Mbps. However, the cable modem that is connected to the computer through a 10 Base-T cable limits the data rate to only 10Mbps.
Upstream Data Band: It is quite opposite to the downstream data band, as it has a frequency of lower band, i.e. about 5 to 42 MHz. But from the division point of view, it is the same as the downstream data band because it also divides the band into 6MHz. Due to the lower frequency range, it is more prone to noise and interference, Due to this reason, the QPSK method is used for the modulation technique.
Data Rate: As it uses the QPSK modulation technique, it uses 2 bits/baud. The standard specifies 1 Hz for each baud. So, the upstream data can be sent at 12 Mbps.
Upstream and Downstream bands, both are shared by the subscribers.
Upstream Sharing: The bandwidth of upstream data is only 37 MHz, so only 6-MHz channels are available in the upstream direction. The timesharing method is used to share the 6 channels over subscribers.
Downstream Sharing: The downstream band has 33 channels of 6MHz, The number of subscribers is about 33. Here the multicasting solution is adopted to share each channel between the group of subscribers.
CM and CMTS
For data transmission using cable networks, the two key devices are used i.e. Cable Modem and Cable Modem Transmission System.
CM (Cable Modem): It is installed onto the subscriber premises. It is similar to ADSL.
CMTS (Cable Modem Transmission System): It is installed inside the distribution hub by the cable company. It receives data from the internet and passes them to the combiner, which sends them to the subscribers and vice-versa.
Data Transmission Schemes: DOCSIS
The most prevalent method used by the data transmission is devised by Multimedia Cable network Systems, called Data Over Cable System Interface Specification (DOCSIS). It specifies all the protocols that are necessary to transport data from CMTS to CM.
Upstream Communication: The following is the very simplified version of the protocol that is defined by DOCSIS for upstream communication. The following are the steps that are followed by CM:
- CM checks for a specific packet periodically sent by the CMTS. The packet itself asks any new CM to announce on a specific upstream channel.
- The CMTS sends a packet to CM, by defining its upstream and downstream.
- The process called ranging is started by CM which determines the distance between CM and CMTS for the synchronization between CM and CMTS.
- The CM sends a packet to the ISP, for accessing the internet address.
- For the establishment of security parameters, CM and CMTS exchange packets between them.
- Then CM sends its unique identifier to CMTS.
- In the allocated upstream channel, the upstream communication starts.
Downstream Communication: The communication in the downstream channel is much simpler. There is only one sender and CMTS send the packet with the address of the receiving CM using the allocated downstream channel.
Advantages of Cable TV:
- Cable TV is stable in its service.
- It is fairly inexpensive.
Disadvantages of Cable TV:
- Due to the availability of a single provider, it creates a monopoly.
- It will lead to less privacy when our TV cable is connected to the internet.