CABLE TV FOR DATA TRANSFER

Cable companies are now competing with telephone companies for the residential customer who wants high-speed data transfer. In this section, we briefly discuss this technology.

Topics discussed in this section:

• Bandwidth
• Sharing
• CM and CMTS
• Data Transmission Schemes: DOCSIS

Division of coaxial cable band by CATV

Each video TV channel uses 6MHz

Note

Downstream data are modulated using

the 64-QAM modulation technique.

Note

The theoretical downstream data rate

is 30 Mbps.

Note

Upstream data are modulated using the

QPSK modulation technique.

Note

The theoretical upstream data rate

is 12 Mbps.

Sharing: Upstream sharing

• The upstream bandwidth is 37 MHz.
• There are six 6-MHz channels available.
• How can the channels be shared in an area with 1000,2000 or even 200,000 subscribers?
• Using FDM/timesharing.
• Subscribers have to contend for the channels with others.

Sharing: Downstream sharing

• The downstream band has 33 channels of 6 MHz.
• We have a multicast situation.
• If there is data for any of subscribers in the group, the data are sent to that channel.

Cable modem (CM)

In cable company: Cable modem transmission system (CMTS)

Data Transmission Schemes: Data Over Cable System Interface Specification (DOCSIS)

• Defines all the protocols necessary to transport data from a CMTS to a Cable Modem.
• Upstream Communication
• CM checks for specific packets sent by CMTS.
• The CMTS sends a packet to CM, defining its allocated downstream and upstream channels.
• The CM starts ranging process (to determine the distance for synchronization).
• The CM sends a packet to the ISP, asking for the IP address.
• The CM and CMTS exchange some packets to establish security parameters.
• The CM sends its unique identifier to the CMTS.
• Upstream communication can start in the allocated upstream channel.
• Downstream Communication
• No contention because only one sender.
• The CMTS sends the packet with the address of the receiving CM, using the allocated downstream channel.