Cable television is an important part of the way in which citizens of the world are informed and entertained. The original purpose of Cable Television was to deliver broadcast signals in areas where they were not received in an acceptable manner with an antenna. These systems were called community antenna television, or CATV. The first CATV system was built in 1948, by Ed Parson of Astoria, Oregon, using twin-lead transmission wire strung from housetop to housetop. In  1950, Bob Tarlton built a system in Landford, Pennsylvania, using coaxial cable on utility poles.

Cable television, the technology involved and the services provided both, have a come a long way in these years. Cable television companies are in the midst of a transition from traditional core business of entertainment video programming to a position as a full-service provider of video, data and telecommunications services. Among the elements that have made this possible are technologies such as the "Cable Data Modem". A "Cable Modem" is a device that allows high-speed access to the Internet via a cable TV network. The cable technology was originally designed to optimize the one-way, analog transmission of programs to homes. However, the underlying coaxial cable has enough bandwidth to support a two-way transport of signals. Recent growth in the internet access and other two-way services caused the trend in the industry to enhance the cable systems with fiber optic technology. Upgrades of the cable plant are transforming cable systems into hybrid fiber-coaxial, or HFC networks. Replacement of a part of the distribution plant with fiber optics unleashed the inherent two-way capability of the coaxial plant, without the need to replace the individual subscriber connections.

Why Cable ?
 

Anyone who has ever surfed the net using a phone line can tell that they can be painfully slow, especially when downloading files, graphics or video. Besides, the dial-up process itself can be pretty tedious, time-consuming and unreliable. The cable modem technology, on the contrary provides you with an "always on" feature, meaning that you don't have to dial every time to get connected. And needless to say, you don't need to have an additional telephone line for internet access, while its broad band network means that users can move around quickly, owing to its high speed access. Cable modem speeds can vary widely. In the downstream (DS) direction (from the network to the home), speeds can be anywhere upto 36 Mbps, though a more realistic number is 3-10 Mbps. In the upstream (US) direction (from Computer to Network), the speeds can be upto 10 Mbps. However, most of modem producers select an optimum speed of between 200 Kbps to 2 Mbps.

 
Comparative Data Transmission Speeds
        Time to Transmit a Single 500 kBytes Image

Telephone Modem    (28.8 kbps)	6-8 minutes
ISDN   (64 kbps)	1-1.5 minutes
Cable Modem   10 Mbps	Approx. 1 second

In simple terms, this means that you can access internet at a speed hundreds of times faster than the usual telephone links, through your Cable connection which earlier just provided you with TV channels. All this, and you don't need to keep your phone free while you are on net, plus you can still watch TV. Cable modem has a ethernet interface which connects to the PC or hub, in parallel to the Coax which connects to the TV.

What's behind it?
 

Digital data signals are carried over radio frequency (RF) carrier signals on a cable system. Digital data utilizes cable modems, devices which convert digital information into a modulated RF signal and convert RF signals back to digital information. The conversion is performed by a modem at the subscriber premises, and by the headend equipment (cable router) handling multiple subscribers.

Even though, a cable modem modulates and demodulates, it is an understatement to call it a Modem, since these are far more complicated as compared to their Telephone counterparts. They can act as modems, tuners, encryption/decryption devices, part bridge, part router, SNMP agents, and part ethernet hubs. Cable industry typically uses 42-750 MHz RF range for VHF and UHF video broadcast as 6 MHz wide channels. A single 6 MHz channel can support multiple data stream or multiple users through the use of shared LAN protocols. Different modulation techniques are used to maximize data speed that can be transmitted through a 6 MHz channel. Modulation techniques include Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM) and Vestigial Side band (VSB) amplitude modulation. The frequency spectrum has been divided such that 5-42 MHz is used for upstream data, and 54-860 MHz is used for downstream transmission. In the DS direction, QAM-256 and QAM-64 are used, while QPSK is preferred in the US direction, since the frequency range (5-42 MHz) is a noisy environment, with interference's from HAM and CB radios etc.

A typical traditional Cable network consists of following major parts :

1) A Headend - This may be a Regional headend (RHE) or a Local headend (LHE). RHE's receive the national channels from satellites and transmit them to various LHE's. LHE's besides, collecting signals from RHE, also receive local channels and selective process and transmit them to subscribers.

2) Hybrid Fiber Coax (HFC) trunk which connects the subscribers to headends, and video flows as RF or optical signals. Headend consists of specialized hardware to achieve its functionality. A 'Receiver' receives a channel (a modulated signal for which it is tuned) and converts it to a base band signal. A 'modulator' converts a base band signal to a modulated signal of a desired channel's carrier frequency, while a 'combiner' combines various channels on one cable. 'Optical transmitters' convert electrical signal on the 75 Ohm Coax to an optical signal on the Fiber to be sent across HFC to subscribers. The HFC in turn, is a complex network, where optical fiber brings signals to the Optical Nodes that serve 500-2000 homes. Fiber Nodes convert optical signals from Fiber to electrical signals on 75 Ohm Coax. RF amplifiers increase the signal power for covering longer distances using a Coax. Trunks and Distribution Links form a tree and branch structure, from which Taps provide service to individual homes.



Now, as expected Data transmission over this network requires special considerations:-
Typically, one channel per Fiber Node is used for DS, while one or multiple channels per Fiber Node may be used in US direction. While the Fiber is separated for US and DS flow, the same coax with bi-directional amplifiers is used for bi-directional signaling. A HFC interface Unit (part of Cable Router) transmits an RF Modulated digital data signal downstream, which is received by one or all subscriber Cable modems, and vice-versa for upstream.

Besides having separate frequency bands for US and DS transmission, Cable network is used as a broadcast media in the DS direction, since all modems are listening to all channels. The security issues raised here are resolved by other means. In the US direction, all modems also share the bandwidth on the Cable (frequency band 5-42 MHz), but it is non-meshed Point to Multipoint media as only the headend's HFC interface is listening. So, for a modem to talk to another modem within a Fiber Node, it has to go through Headend's HFC interface unit (Cable Router). Another implementation, which avoids the IP ARP and ICMP Redirect issues raised by the first one, involves re-transmitting US signals on the DS channel by the Cable Router.

The Headend also consists of DHCP servers, Time of Day Servers, TFTP Servers which play their role in the bring-up and running of the subscriber modems, as a plug-n-play device. When a modem is powered on, it dynamically interacts with the headend devices to procure its IP address, software, configuration files etc., besides synchronizing and establishing a working link for user-data to flow. When Cable modems are powered on, they scan a defined spectrum range for downstream channel using Synch Pulses, and perform special hand shaking with Head End's HFC interface unit for timing, signal power level, authentication, addressing and other issues.

Standards
 

Modems are available from a variety of vendors, each with a unique technical approach. These modems have made it possible for cable companies to enter the data communications market now. However, inter operability of these devices across Cable Systems, geographic locations and other cable devices like Cable Routers needs to be ensured for greater market success. For this purpose, Cable companies came together in 1995 to begin working towards an open standards. CableLabs, a organization over-seeing these efforts, along with inputs from other participants, came up with Data Over Cable Service Interface Specifications (DOCSIS), which are the evolving standards. The project was started by Multimedia Cable Network System (MCNS) Partners, an organization formed by major cable companies of USA and Canada. DOCSIS documents describe the internal and external network interfaces for a system that allows bi-directional transfer of IP traffic, between cable system headend and customer premises equipment, over CATV.