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The telephone companies who are competing with each other to bring ATM and ADSL technology online are closemouthed about the status of development of the technologies. Other than their “life is a glorious parade and arent we proud to be a part of it” marketing info, little can be gleaned from their research and development departments. However, I can imagine how ADSL was discovered. Someone, somewhere, connected a device to a telephone line, swept the spectrum with a signal, and looked at the response. Obvious to the engineers who performed this mundane task must have been the ability of that telephone line to transmit and receive analog signals in a band of frequencies much higher than the 4 KHz upper limit imposed by the local office equipment filters. Also, advances in digital signal processing in a noisy environment have contributed significantly to the ability to utilize the telephone lines for digital transmission far above the original 4 KHz. Due to the frequency response of telephone lines high above 4 KHz, we can have ADSL, and the other flavors of DSL technology.

There exist several reasons why DSL technology is limited in the distance signals can be transported. Attenuation, or signal loss, caused by the physical resistance of the medium (wire) to electron current flow, cross-coupling of signals from line to line (cross-talk), and termination impedance (like a rubber ball hitting a brick wall, the signal bounces back) influence just how far any signal of a particular frequency can travel before its level becomes so low that it cannot be distinguished from the ever present noise on the line. See Figure 9-3. Just like a car after it has freely rolled down a hill and levels out, the signal level gradually "slows down" over distance until the signal can no longer be distinguished from the ever present line noise. When the ratio of the signal to the noise becomes less than 10dB, the signal begins to become "noisy" (listen to an AM station almost out of range). As the signal continues to decrease in amplitude, it finally becomes unusable, then indistinguishable from the noise.


Figure 9-3.   Simplified signal-noise relationship

Twisted pair telephone lines coming out from the central office in the local loop may be bundled into large cables of several thousands of twisted pair wires. As the wires are spread across the distribution area (from the central office to your home), they are bundled into 50-pair cables. Of course, along the way there are intermediate distribution equipment the wires are routed through, resulting in any individual wire having to be spliced numerous times. It is estimated by one source that the average U.S. subscriber line has 22 splices from the central office to the premises. Each splice adds a little attenuation to the signal and the opportunity for cross-talk. Cross-talk is the result of a signal coupling from its assigned or desired line into an adjacent line. Cross-talk is unwanted interference in the adjacent wire and reduces the signal level in the desired line. Unwanted signal coupling increases as a function of increasing frequency and increasing line length. Ergo, at some intersection of frequency and line length the desired signal becomes undistinguishable from the noise. Perhaps the intersection nearest your home? Anyway, for the curious, the telephone folks found out long ago that twisting the two copper wires as they traveled from the central office to your premises greatly reduced the amount of cross-talk between pairs. So, they could bundle more wires together with less cross-talk. Now you know why the world is wired with twisted pairs, and you just thought the world was twisted because it was "wired."

Signal Category Frequency (Hertz) Distance (feet)
DS-1/T1 1.544 Mbps 18,000
E1 2.048 Mbps 16,000
DS-2 6.312 Mbps 12,000
E2 9.448 Mbps 9,000
1/4 STS-1 12.960 Mbps 4,000
1/2 STS-1 25.920 Mbps 3,000
STS-1 51.840 Mbps 1,000

Table 9-1. Subscriber line practical frequency limitations

With splices, cable bundles, intervening distribution equipment, the length of the twisted pair, and the resultant attenuation of the signal as it travels down the copper line, there is a maximum practical distance limitation that the ADSL signal can be transmitted in one direction only. The distance specified in Table 9-1 is the maximum distance an ADSL signal can be transmitted, assuming 24 gauge twisted pair copper lines. Attenuation of the signal is caused by the physical resistance of the wire to the moving electrons that comprise the electronic signal. The table is valid only when there is a maximum of two subscriber telephones connected to the line at the customer premises. If there are more telephones connected, the distance goes down. Also, connecting other customer premises equipment, such as recording equipment, will reduce the effective distance the ADSL signal can be transmitted. However, the ADSL modem will act as the interface between the local office and the CPE, providing the isolation between the local office and the CPE. The local office equipment will see only the ADSL modem as the line load. Therefore, the ADSL modem makes it possible to connect televisions, computers, etc., to the line via the modem connection.


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