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November 1999 Issue
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Focus on Telephony
Home Networking: It's Closer Than You Think
By Justin J. Junkus

• Networking 101
• You can get there from here
• How it works
• What installers must know
• Phone wire alternatives
• For more information

Justin J. Junkus

Where there are two computers, there is potential for a network to hook them together so that both machines can share files. The network also can be used to share other resources, such as printers, scanners, disk drives and even cable modems. This column won't get into the software and configuration of the computers, however. We're going to concentrate on the things you can see as soon as you open the door to the family "computer room."

Networking 101

There are lots of ways to build a network of computers suitable for home use. In the distant past (about a year ago), the only way to do it was to build a small version of a business local area network (LAN).

The most apparent sign of a networked house used to be the maze of wires running between the devices on the network. Because the home computer user was mimicking a corporate network, he or she had to string network-grade media between the devices on the network. If everything is in the same room, this is not a big problem. However, many two-computer folks want to keep the machines in separate rooms.

Here's where telephony comes into the picture. Your home already comes equipped with wires between most of the rooms, courtesy of the contractor who installed phone and electrical service. Obviously, you can't just plug an Ethernet into the phone or electric lines. The signals that are already on those lines would wreak havoc on a computer system. (Picture 110 VAC on the transmit and receive leads of a modem!)

You can get there from here

With a network interface that provides filtering and operates at frequencies outside the range of the signals usually found on the media, however, those media can be shared. Here you have two choices: the electric lines or the phone lines.

To date, most commercial development is focused on using phone lines as the networking media. A growing group of vendors including Intel, Lucent Technologies, Compaq, and Tut Systems has formed the Home Phoneline Networking Alliance (HomePNA). Together, they have established a de facto standard that builds upon the Institute of Electrical and Electronics Engineers' 802.3 (Ethernet) protocol to specify physical and logical interfaces between networked devices and phone wire used as a network medium.

Today's potential in-home networkers can buy a kit for less than $200 that typically consists of two or more network interfaces that plug into a phone outlet, connecting cords and software for the PCs being networked. The network interfaces also must be connected to the computer's parallel port and to a nearby electrical outlet for power.

PC cards also are available as network interfaces if the parallel port is being used for another application, such as a Zip Drive. Software installation is relatively simple with the Install Wizard that is part of the kit.

How it works

Home phoneline uses frequency division multiplexing (FDM) to assign each communications service on the media to a frequency spectrum that is different from all others. The in-home network operates between 5.5 MHz and 9.5 MHz. Passband filters in the network interfaces block both standard voice communications signals in the 20-Hz to 3.4-kHz range and universal asynchronous digital subscriber line (UADSL) signals in the 25-kHz to 1.1-MHz range.

The HomePNA literature states that network throughput at 1 Mbps is possible today. Off-the-shelf products are available from both Intel and Compaq. The technology for 10 Mbps is part of HomePNA's 2.0 standard. Near-100 Mbps capability is promised with future developments.

The core technology for HomePNA was developed by Tut Systems and is called Time Modulation Line Coding Method. In essence, it is multibit modulation with adaptive circuitry that adjusts for varying noise on the line. Within each adapter, both receiver and transmitter continually monitor line conditions. The receiver circuit adjusts for noise levels, and the transmitter varies its output signal strength.

What installers must know

There are two areas I suggest cable installers understand when they interface a HomePNA network to a cable system: what exactly it means to share a high-speed data connection to the cable network, and what might need to be done to avoid interference between circuit-switched cable telephony return frequencies and frequencies used by the HomePNA.

Typically, the software included with networking kits sets up the networked computers for shared Internet access. One of the computers on the network must contain the network interface card (NIC) that connects to the cable modem, and the others communicate with the Internet through the network.

While access sharing is thus part of the network, simultaneous access is another scenario. To control network load, the cable operator or the Internet service provider (ISP) may constrain access to one device at a time.

Some services, such as AOL, allow simultaneous access to the Internet, but restrict users on the same account to one-at-a-time access to AOL-specific services. A cable installer who hooks up high-speed data as a replacement for

dialup service will need to understand the company's policy for simultaneous Internet access.

As far as connecting a HomePNA system to a circuit-switched hybrid fiber/coax (HFC) telephony system, the degree to which the HomePNA's signals are isolated from the cable telephony system will depend on the vendor of the telephony system. While HomePNA blocks telephony frequencies from the data network, the reverse is not true. The sampling rate used in typical cable telephony analog-to-digital (A/D) conversion will not "see" HomePNA frequencies on the telephone line.

However, there could be some RF interference between input and output of the telephony network interface unit (NIU). (This is the box on the outside of the house, not the adapter that plugs into the phone outlet.) If excessive return frequency hopping becomes a problem when you hook up to a home with a HomePNA network, it might be worth installing a filter that blocks the HomePNA frequencies between the NIU and the rest of the subscriber's wiring.

Phone wire alternatives

In some cases, a homeowner may elect to dedicate some of the telephone wiring in his residence to the data network, rather than share it with regular phone service.

In this case, the home network can be a true Ethernet, operating independently of the phone system, except for dialup data access. With this arrangement, the cable installer needs to know that the subscriber may need to use a router to interface cable's high-speed data service to the in-home network. In general, network interfaces do not allow connection to two Ethernet networks at the same time.

For more information

In-home networks are generating a lot of interest in the cable industry. CableLabs has established a working group similar to PacketCable to look at the standards and interfaces to cable systems. The Society of Cable Telecommunications Engineers is discussing the topic in an issue of DigiPoints, which can be accessed at its Web site, www.scte.org. In addition, the SCTE Emerging Technologies conference in Anaheim in January will include papers on the future of this interesting technology.

Justin Junkus is president of KnowledgeLink, a consulting and training firm specializing in the cable telecommunications industry. To discuss this topic further, or to find out more about KnowledgeLink, you may e-mail him at .

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