DSL Advances

Several methods use existing inside telephone wiring as the physical medium for home networking. The most popular method was developed by the HomePNA, and is discussed in the next paragraph. Alternatively, some or all of the telephone wall jacks in a home may be replaced by new wall jacks containing active data repeaters connecting to the existing inside wiring. With this method, each active wall jack supports one point-to-point physical-layer connection to the wiring hub (in a star-wired topology) or two point-to-point physical-layer connections to each neighboring wall jacks (in a chain-wired topology). The active-jack method can attain high-data performance, but the installation of new wall jacks is too demanding for self-installation by most home owners . A third method is a LAN using the inside telephone wire pairs not used for telephone service. The spare wire pairs rarely have adequate balance, impedance, and low enough noise for use by traditional LAN technology. Furthermore, many homes do not have spare inside telephone wire pairs that are actually connected to most wall jacks.

With HomePNA, information appliances (such as PCs) are easily interconnected via a LAN that uses the existing inside telephone wiring. Each information appliance connects to a low-cost HomePNA adapter which then plugs into a standard telephone wall jack (see Figure 12.6).

The very same inside wire pair that carries traditional telephone service throughout the home also carries the HomePNA data modulated into a different frequency band (4 to 10 MHz for version 2.0 of HomePNA) (see Figure 12.7). HomePNA devices connect to pins three and four of the telephone wall jack; this is line one for multiline wired homes. More important, this is the only telephone pair for homes wired for only one line. Thus, HomePNA connects to the one pair that is surely present and working in virtually all homes. In a remarkable triple-use of the inside telephone wires, the same pair of wires can also carry ADSL signals within the 25 kHz to 1.1 MHz frequency band.

Up to twenty-five information appliances within a building are easily IP (Internet protocol) networked simply by connecting the USB (universal serial bus) or 10baseT Ethernet interface of each information appliance to a low cost HomePNA adapter that plugs into a nearby telephone wall jack. HomePNA version 1.0 provides an aggregate LAN bit rate of up to 1 Mb/s, and HomePNA version 2.0 provides up to 10 Mb/s. The actual maximum LAN throughput may be less for a home with very long wiring runs, poor quality wire, or very noisy environments (such as near a radio transmitter).

HomePNA versions 1.0 and 2.0 are designed to interwork with each other. Version 1.0 uses pulse position modulation in the 5.5 MHz to 9.5 MHz frequency band. Version 2.0 uses a more complex method: trellis -coded frequency diversity quadrature amplitude modulation (FDQAM), which modulates the same data into two separate frequency bands within the 4.0 to 10 MHz band. It is expected that most transmission impairments would not severely impair both frequency bands. Adaptive equalization is also used to reduce the impairments from the home wiring. HomePNA version 2.0 has largely superceded version 1.0 in the marketplace . For typical inside applications, the packet throughput of HomePNA 2.0 (up to 7 Mb/s) is up to three times the packet throughput realized for wireless LANs such as IEEE 802.11b (often 2 to 3 Mb/s).

HomePNA was designed to overcome the challenges of transmitting high speed data over the various conditions found in home telephone wiring:

• Wire types ranging from CAT5 to quad wire and flat wire (not twisted)

• Various wiring topologies: star, tree, ring (see Figure 12.8)

• Bridged taps within the home

• Noise from many sources: answering machines, AM radio, electric motors, light dimmers, fluorescent lights

• Line impedance changes due to phones being on-hook or off-hook

Based upon HomePNA field studies, it is estimated that HomePNA will operate reliably at the rated maximum bit rate upon initial installation in more than 90 percent of U.S. homes, at a reduced bit rate for about 5 percent of homes, and not work reliably for about 5% of homes. Many of the cases of trouble with HomePNA are due to noise from telephony equipment (fax, modem, caller-ID, and answering machine), which can be resolved by placing an in-line low-pass filter in series with the telephony device. On rare occasions, HomePNA causes audible noise in telephone receivers; this too can be resolved by placing an in-line low-pass filter in series with the telephone. Another source of trouble is the home telephone wiring being either too long or having too many long bridged taps. As a rule, the maximum wire distance between HomePNA devices is 500 ft. Inside telephone wiring often takes indirect routes so that two outlets that appear to be only 50 ft apart could be hundreds of feet apart in the wiring.

HomePNA offers the lowest cost home LAN solution using existing wiring ($38 as of the time of this printing). However, it does not work well in all homes, and there may not be a phone wall jack accessible at some of the locations where an information appliance resides.

The frequency bands used by HomePNA and VDSL overlap to a large degree. Analysis of HomePNA and VDSL located at the same premises indicate that both systems would be greatly impaired unless the two systems use separate inside wiring and a low-pass filter was placed at the entrance to the premises to prevent the HomePNA signals from exiting to the outside wiring. This would require installation by a skilled technician. There is a possibility for adverse interference between nearby customers sharing the same distribution cable. One customer with VDSL and a neighbor with HomePNA could experience interference problems. This could be especially acute for an MDU (multiple dwelling unit, such as an apartment building). Radio frequency egress from HomePNA may be a concern in some countries due to radiation from poorly balanced inside wiring. Security is another concern for HomePNA; in rare instances a neighbor in the same telephone distribution cable could pick up enough signal crosstalk to monitor or possibly send unauthorized messages.

HomePNA is supported by a large number of equipment and silicon vendors , including many PC- related companies. Further information may be found at www.homepna.org. The ITU SG15 is developing an international standard based on HomePNA version 2.0 under the name G.pnt (phone network transceivers). A framework ITU Recommendation based on HomePNA 2.0, G.989.1 was approved in 2001. ITU G.989.1 provides a specification for the frequency band and other basic aspects; approval of a more complete specification is expected in 2002.