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Chapter: Computer Networks : Physical Layer

Cable TV for Data Transfer

1. Bandwidth 2. CM and CMTS 3. Data Transmission Schemes: DOCSIS



Cable companies are now competing with telephone companies for the residential customer who wants high-speed data transfer. DSL technology provides high-data-rate connections for residential subscribers over the local loop.


1. Bandwidth


Even in an HFC system, the last part of the network, from the fiber node to the subscriber premises, is still a coaxial cable. This coaxial cable has a bandwidth that ranges from 5 to 750 MHz (approximately). To provide Internet access, the cable company has divided this bandwidth into three bands: video, downstream data, and upstream data.

Downstream Video Band


The downstream video band occupies frequencies from 54 to 550 MHz. Since each TV channel occupies 6 MHz, this can accommodate more than 80 channels.


Downstream Data Band


The downstream data (from the Internet to the subscriber premises) occupies the upper band, from 550 to 750 MHz. This band is also divided into 6-MHz channels. Modulation Downstream data band uses the 64-QAM (or possibly 256-QAM) modulation technique. Downstream data are modulated using the 64-QAM modulation technique.


Upstream Data Band


The upstream data (from the subscriber premises to the Internet) occupies the lower band, from 5 to 42 MHz. This band is also divided into 6-MHz channels. Modulation The upstream data band uses lower frequencies that are more susceptible to noise and interference. For this reason, the QAM technique is not suitable for this band.


2. CM and CMTS


To use a cable network for data transmission, we need two key devices: a cable modem (CM) and a cable modem transmission system (CMTS).



The cable modem (CM) is installed on the subscriber premises. It is similar to an ADSL.



The cable modem transmission system (CMTS) 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 subscriber. The CMTS also receives data from the subscriber and passes them to the Internet. Figure 1.77 shows the location of the CMTS.



3. Data Transmission Schemes: DOCSIS

Several schemes have been designed for data transmission over an HFC network.


Upstream Communication

The following describes the steps that must be followed by a CM:


·           The CM checks the downstream channels for a specific packet periodically sent by the CMTS. The packet asks any new CM to announce itself on a specific upstream channel.


·           The CMTS sends a packet to the CM, defining its allocated downstream and upstream

·           channels.


·           The CM then starts a process, called ranging, which determines the distance between the CM and CMTS. This process is required for synchronization between all CMs and CMTSs for the minislots used for timesharing of the upstream channels.

·           The CM sends a packet to the ISP, asking for the Internet address.


·           The CM and CMTS then exchange some packets to establish security parameters, which are needed for a public network such as cable TV.


·           The CM sends its unique identifier to the CMTS.


·           Upstream communication can start in the allocated upstream channel; the CM can contend for the minislots to send data.


Downstream Communication


In the downstream direction, the communication is much simpler. There is no contention because there is only one sender. The CMTS sends the packet with the address of the receiving CM, using the allocated downstream channel.


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