Cable Data Network

Traditionally Cable TV systems were designed to provide one directional broadcast television signals the subscribers' homes. The Cable TV industry has become a major player in providing services by allowing data transport (bi-directional ? up and down link) over the evolved Cable TV networks. This was possible by re-creating a portion of the over the air RF spectrum within coaxial cable line.

CABLE DATA NETWORK
A Conceptual Insight

INTRODUCTION
Traditionally Cable TV systems were designed to provide one directional broadcast television signals the subscribers' homes. The Cable TV industry has become a major player in providing services by allowing data transport (bi-directional ? up and down link) over the evolved Cable TV networks. This was possible by re-creating a portion of the over the air RF spectrum within coaxial cable line.
The old coaxial cable systems operated with 330MHz or 450MHz of capacity ? however the modern HFC (Hybrid Fiber/Coax) systems have capacities of the order of 750MHz or more.
A standard analogue TV channel requires 6 MHz of RF spectrum. Therefore the traditional CATV system with 400 MHz of downstream bandwidth can deliver around 60 analogue TV channels. A HFC system with 700 MHz of downstream bandwidth has the capacity for over 110 analogue TV channels.

CABLE NETWORK ARCHITECTURE
At the core of the traditional CATV system is a shared coaxial cable network that transmits analogue television signals downstream to the subscribers. The cable network is similar to the over-the-air RF broadcast signals that is received via a TV antennae but in case of Cable TV the signals are carried on cable. All the subscribers within a neighborhood share the cable and use a tree-and-branch topology ? multiple households within a neighborhood share the same cable
Earlier CATV systems were all based on coaxial cable. At the advent of fiber optic cable more sophisticated HFC (hybrid Fiber/Coax) systems have come into play. Fiber cable helps reduce attenuation of signals over long distance and overcome problems of ageing. Now-a-days the old coax cable is commonly replaced with fiber due to the fact that it can provide more bandwidth for next gen services. In some cases the cable service providers have been installing fiber all the way to the curb ? which is commonly known as FTTx (x -> C or H)
Most of the cable network from past which were design for downstream transmission of TV signals are being evolved to support bi-directional data transmissions. Amplifiers are used at various points along the cable to boost signals. These amplifiers only work in one direction and upgrading these to support bi-directional boost is very expensive task.

CABLE ACCESS NETWORK
Subscribers of Cable data network are connected to the network via the cable modems. When they are connected to the data network, they obtain a continuous connection to the Internet via the cable network. Modems are the internal devices or connect to PC and home entertainment equipment via USB (Universal Serial Bus) and other interfaces.
The cable modems communicate with the CMTS (Cable Model Termination System) in the head office. Each CMTS provides connections for thousands of cable modems over a network that stretches over 100 km and with a potential bandwidth of 50Mbps. The CMTS helps connecting to the Internet and other content sources (other media sources) and is capable of sending and receiving user/subscriber data packets.
A cable modem is responsible for upstream and downstream conversions. Data packets from internet arrive at the providers head end as shown in Figure 2 and a dedicated processor in the CMTS converts them into MPEG packets, checks for errors and modulates the packets onto a carrier wave using QAM/FEC (Quadrature Amplitude Modulation/Forward Error Correction). The output from the CMTS is then transmitted downstream on a coaxial cable as RF signals to the subscriber. The cable modem converts the radio frequency information back to IP packets and sends them to the end device such as PC or a data terminal.
The head-end portion of the cable network (CMTS) can receive signals from a variety of sources, including Internet, terrestrial and satellite wireless transmissions.
To deliver data services over a cable network, one television channel (in the 50 to 750 MHz range) is typically allocated for downstream traffic to homes, and another channel (in the 5 to 42 MHz band) is used to carry upstream signals. A head-end CMTS communicates through these channels with cable modems at subscriber homes to create a virtual local area network (LAN) connection. The cable modem access cable network operates at Layer 1 (physical) and Layer 2 (media access control/logical link control) of the OSI Model. Therefore, IP traffic is seamlessly delivered to the end users.
A single downstream 6 MHz television channel may support up to 27 Mbps data throughput from the cable head-end (CMTS) by employing 64 QAM transmission technologies. The speed can be improved to 36Mbps by 256 QAM.
Upstream channels may deliver 500 Kbps to 10 Mbps from homes by using 16QAM or QPSK (Quadrature Phase Shift Key) modulation techniques. Using 64 QAM, upstream speeds can be improved to 30 Mbps. On a cable network the upstream and downstream bandwidth is shared by the active data subscribers, usually 500 to 2,000 homes on a modern HFC network.

Distribution Network
The regional cable head-end feeds to the distribution hub on a cable of bandwidth of the order of ~ 600 Mbps and eventually residential or business subscribers are connected via the Fiber Node. A distribution hub generally consists of an IP Switch Router that switches individual subscribers' traffic, CMTS towards the access end and few caching servers connected to the IP Switch Router.

STANDARDS
In order to provide interoperability between cable modems and the head-end gear (CMTS), cable standards are designed and developed. This helps subscribers to buy off-the-shelf cable modems that are guaranteed to work with cable network and are compatible with the CMTS installed at cable provider.
The most important of all the standards are listed below:
? DOCSIS (Data over Cable Interface Specification) DOCSIS is the result of work done by MCNS (Multimedia Cable Network System Partners Ltd.). This standard has become the most interesting and important, and is covered under the "DOCSIS" heading. (Visit http://www.cablemodem.com/)
? DAVIC (Digital Audio Visual Council) DAVIC was a non-profit group that promoted digital audio-visual applications and services based on specifications which maximized interoperability across countries and applications/services. DAVIC developed a digital video broadcast reference model that is popular in Europe and preferred by the European Cable Communications Association (ECCA), a European cable industry organization. DAVIC is oriented toward delivering digital video to home users, while DOCSIS is better positioned for data delivery. DAVIC completed its work and closed in 2000. (Visit http://www.davic.org.)
? IEEE 802.14 Working Group This group is defining the physical layer and a MAC (Medium Access Control) layer protocol for HFC networks. The architecture specifies an HFC cable plant with a radius of 80 KM from the head end. The group's goal is to develop a specification for delivering Ethernet traffic over the network. ATM networking was also considered for the delivery of multimedia traffic. In fact, MCNS began work on DOCSIS because the IEEE was not working fast enough on its specification.
? IETF IP over Cable Data Network (IPCDN) Working Group The IPCDN is defining how IP can be delivered over the cable network. Most of its work is centered on DOCSIS and addresses higher levels than the IEEE 802.14 Working Group, which is concentrating on physical and data link layer protocols. IPCDN is defining a specification to map both IPv4 and IPv6 into the HFC access networks. The group is interested in multicast, broadcast, address mapping and resolution (for IPv4), and neighbor discovery (for IPv6). IPCDN is also working on bandwidth management and guarantees using RSVP, security using IPSec, and management using SNMP. More information is at http://www.ietf.org/html.charters/ipcdn-charter.html. The Web site lists a number of Internet drafts that describe various aspects of delivering IP over cable networks.

MERITS & DEMERITS
Merits
? High speed uninterrupted broadband connection.
? Constant connectivity - Because cable modems use connectionless technology, a subscriber's PC is always online with the network, which means there is no need for dial-in to begin the session.
? Additionally, going online does not tie up their telephone line.
Demerits
? Because cable network is a shared medium, it has introduced security problems. Hackers can snoop and watch traffic and attempt to capture valuable information. In order to secure the individual's traffic the cable industry has come up with encryption techniques and other security measures that support privacy. Firewalls are now common in homes.
? Because the media is shared among many subscribers, in the peak usage times users may experience the bandwidth dropping.

CABLE MODEM VENDORS
A range of vendors are offering cable modems, including Ambit Broadband , Arris Group Inc. (Nasdaq: ARRS), Cisco Systems Inc. (Nasdaq: CSCO), D-Link Systems Inc. , Linksys , Motorola Inc. (NYSE: MOT), Terayon Communication Systems Inc. , and Thomson Broadband .

Anil Satram