The number of people using computers, and the types of things computers are used for, seems to increase every day. Computers may soon become as common in day to day life as telephones and televisions. Even today, a typical household or small business is likely to have more than one computer. Networking computers together provides a number of advantages. For instance, even within a household or small business, networked computers can share memory space and share access to resources such as printers and internet connections. Unfortunately, traditional networking approaches have been either prohibitively expense or prohibitively complex to implement for the average household or small business user.
Recently, new products have been specifically targeted for household and small business networking. One approach to these smaller networks uses existing telephone wiring or power line wiring to connect two or more computers within, for instance, a house or office. Most houses these days have telephone jacks in more than one room. Usually, all of the telephone jacks are connected to one phone line leading into the house. If you plug one computer into a telephone jack in one room and you plug another computer into another telephone jack in another room, the telephone wiring provides the network medium, or the physical connection between the computers over which data can be sent and received. Power lines can be used in much the same way if, for instance, two wall outlets are on the same electrical circuit.
Of course, providing the physical network connection is just the first step. Getting the computers to talk to one another over the network connection can be pretty challenging. One basic aspect of network communications is deciding which machine gets to send data over the network and when. Generally, machines can “listen” to data traffic on a network and send data when there is a break in the traffic. If, however, more than one computer is waiting for a break in traffic, more than one computer may send data at the same time when a break is detected. This situation is a “data collision.” The data sent by one computer becomes mingled with the data sent by another computer, rendering all of the data unreadable.
In order to stop a data collision, you first need to detect the data collision. One example of a traditional networking approach to data collision detection is used in Ethernet networks. Whenever a machine transmits data onto an Ethernet network, the machine also “listens” to the network. That is, the machine sends data and listens for a reflection of the data back from the network. If the machine “hears” something coming back from the network other than the reflection of what the machine sent, the machine assumes that the unexpected reflection is due to another machine sending data into the network at the same time causing a data collision in the network. In which case, the machine may stop sending data, wait for a period of time, and then try again.
In order to accurately predict what the reflection of a signal will look like, the physical network medium must have known and predictable characteristics. If the characteristics of the physical medium of a network are unknown, or if the characteristics can vary drastically from one network to another, a reflected signal cannot be accurately predicted. In which case, a traditional approach to collision detection, such as the one described above for Ethernet networks, cannot be relied upon for unknown, or “harsh,” network environments.
Telephone wiring or power line wiring are examples of harsh network media. The characteristics of these media can vary drastically from house to house, and business to business. Telephone and power line wiring were not designed for network data transmission. Building codes vary from region to region and over time. Wiring may deteriorate over time. Wiring in one house may be 50 years old and wiring in a house right next door may be brand new. Some wiring, particularly older wiring, may be susceptible to environmental changes, like temperature and humidity. Given all the possible variations in characteristics of physical media in harsh networks, signal reflections from these network media cannot be accurately predicted, rendering traditional data collision detection un-usable, and therefore limiting the ability to network devices using such media.
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