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Using Coax to Distribute Advanced IPTV Services in the Home
By: Bud Bates
      

We had the opportunity to talk with Coaxsys about how existing coaxial lines in a home can be used distribute multiple types of media (including IPTV) in the home. Coaxsys has developed a smart coaxial based media distribution system called "TVnet" that enables users to distribute many kinds of media throughout the home on a coaxial distribution system. Existing Coaxial Distribution Systems

Coaxial distribution of television channels from over-the-air antenna systems or cable television system sends the television programming on their broadcasted radio channel frequencies. Over-the-air program

Figure 1, Coax Television Distribution System

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ming frequency range and cable television systems typically transmit television channels up to 870 MHz. 

The upper frequency television channels are typically converted to digital channels to allow for channel expansion (multiple TV channels per RF channel) and advanced programming (data channels and music channels). The spectrum below 54 MHz is commonly used as a return path for cable modem systems.

Coaxial distribution systems in homes typically connect multiple televisions and devices through the use of splitters. Splitters divide the signal energy and adapt the impedance of the line to avoid signal reflections or signal distortion.

In addition to the signal energy that is lost through the splitters, some signal energy is lost through attenuation on the coax cable itself. The type of coaxial cable, the signal frequency and the length of the cable determine the amount of energy that is lost. As the length of coaxial lines increases and the number of ports (splitters) increases, the amount of attenuation also increases. 

Figure 1 shows how an in-home coaxial cable distribution system is typically used to distribute television signals in a home. This diagram shows that a cable connection is made at a demarcation ("demarc") point on the outside of a home or building. This cable connects to a signal splitter (and optionally an amplifier) that divides the signal and sends it to several locations throughout the home. This example shows that the coaxial systems may use lower frequency bands for return connections (data modems), middle frequency bands for analog television channels, and upper frequency bands for digital television channels. 		Why is a Smart Coax IPTV Premises Distribution Network (PDN) Important?

Smart coaxial premises distribution for IPTV is important because television and other media devices are already connected to a coaxial line that is already installed in a home or business. These lines can be used to get multimedia information where you need it. 

Smart coaxial distribution systems are usually able to distribute or interconnect media between several devices simultaneously. Smart coaxial systems modulate different media (Internet data, voice-over-IP packets, or IP video streams) into different portions of the coaxial spectrum. 

Because the coax cable is shielded and RF channels are virtually free from the effects of interfering signals, coaxial cable provides a large information pipe that is capable of distributing multiple wide radio frequency channels. Coax is already installed in many homes, and television outlets are commonly located near media equipment such as televisions, VCRs and cable modems. Coaxial cable is easy to install and expand.

How does the TVnet Coax Multimedia Distribution System Operate?

The TVnet multimedia distribution system operates by transferring media on unused frequency bands on the coaxial cable system. For example, the TVnet/C network uses the coax spectrum above 1 GHz. This ensures that TVnet signals will not interfere with channels received from another television service provider.

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Figure 2, TVNet Distribution System
Because multiple devices may be networked on a TVnet system, the master control device in the TVnet system directs the flow of traffic on the network, thus enabling target devices to send and receive IP traffic.
 
The TVnet master sends control commands between target devices, which allows for the establishment and control of communication sessions. Essentially, the master serves as the arbiter on the coax, deciding which target TVnet device is permitted to communicate at which time. This prevents a problem on the network, ensuring that devices do not try to communicate over the top of one another.

The TVnet system uses an Ethernet type access structure that allows IP packets to be transferred between devices. The TVnet system uses RF modulation technology to allow IP packets (whether video, voice, or data) to be transmitted over coax. TVnet devices convert the digital signal into an analog signal, transmit the new analog signal over coax, and convert the signal back to digital once it reaches its destina 		tion.

Each device that will be part of the TVnet network requires a TVnet "target" adapter; this target will be located next to the Ethernet device, such as an IP set-top box, PC, or IP telephone. If the customer desires, they can also use the device's pass-through port to allow cable television signals to pass directly through the TVnet target adapter. 

Figure 2 shows how TVnet systems can allow a coaxial distribution system to dynamically interconnect devices by connecting devices to coax and using unutilized coax spectrum for the purposes of IP networking. This example shows that a master coordinates the transmission of signals without interfering with the transmission between other devices. In this diagram, an IPTV head end streams a movie through a broadband modem to a television in another room. At the same time, a computer is connected through a coax cable to the broadband modem to the Internet and a television is connected to a media server through the broadband modem. This example also shows that the master device 

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in this coax system has the capability to setup connections and prioritize transmissions based on the type of media (such as real time video over web browsing data).

Who sets up IPTV Coax PDNs?

While it is possible for customers to self-install a TVnet system, in the initial launch phase of IPTV, carriers install the equipment to ensure that both the PDN and IPTV signals are operating correctly.

As carriers gain experience with the installation of coax distribution systems, they can develop standardized self-installation procedures to allow customers to self-install their system.

This will allow the carrier to verify that the modem or gateway configuration is correct and to set up each television's set-top box. Based on results from deploying operators, the time required for a

TVnet installation ranges from minutes (when existing coax is present) to a few hours (if rewiring is necessary). 

How Many Devices Can Simultaneously Send and Receive?

The TVnet/C is setup to allow eight target connections for each master unit, providing a total of 9 devices with the basic system. However, the TVnet system can be expanded to virtually an unlimited number of connections.

The user can also connect to multiple digital devices at any one connection point, so the number of devices that can be connected is far higher than nine. In practice, operators deploying in single-family 

 Figure 3, TVnet Devices

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homes typically deliver IPTV to 2-4 televisions. However, there are some applications such as IP video on demand or IPTV installations in multiple dwelling units (MDUs) that use far more Targets per Master than the typical 2-4.

What Media Types can be Sent on TVnet?

RF cable television may be transmitted. Any IP traffic (voice, video, or data) may travel over TVnet.

What Types of Equipment Are Needed for Coaxial TVnet PDNs?

There are two key types of equipment that are needed for a TVnet system; a TVnet Master and TVnet Targets. Alternatively, TVnet technology can be integrated into the modem or set-top box. No other software, switch settings or configuration setups are required. Figure 3 shows some TVnet devices.

What Transmission Speeds can be Achieved?

The TVnet system typically achieves actual throughput rates of 70 to 100 Mbps, and over 200 Mbps has been demonstrated. An important consideration about data transmission rates is the difference between physical data transmission rates (the "PHY" rate) and actual data throughput. The throughput is affected by the amount of overhead (control) data and the amount of information that is lost via interference and the access control process. Because coaxial cable systems can transfer signals with minimal signal leakage, they are relatively immune from interfering signals and can have higher throughputs that other types of premises distribution networks. 

Are There Key Compatibility Issues for Coax PDN Products?

Some of the potential compatibility issues include poor installation practices, excessive cable lengths, and the use of RF amplifiers.

Poor installation practices for coaxial distribution systems include non-terminated lines, poor connection installations and crushed cables. Poor installation can cause excessive signal loss and reflections that result in distorted signals. 

Long cable lengths can attenuate RF signals beyond a usable level and different types of coaxial cable have different types of signal 

option is to either add an amplifier or to use a cable that has a lower loss rating (lower insertion loss).

To overcome some of the loss in a cable distribution system, RF amplifiers are used. If these RF amplifiers are used, they can introduce distortion and may block signals from two-way operation. If amplifiers are needed to strengthen the RF signal, they should be installed before the TVnet master device so as to prevent disruption of the IP network.

Important User Features for Coax PDN?

Some of the key factors for selecting the right coaxial premises distribution network include simplicity of use, signal quality and reliability. If the coax PDN is a good system, the users should never know that it's there.